diff --git a/CMakeLists.txt b/CMakeLists.txt
index 759c073..3657202 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -50,6 +50,10 @@ add_executable(FlashMQ
     enums.h
     threadlocalutils.h
     flashmq_plugin.h
+    retainedmessagesdb.h
+    persistencefile.h
+    sessionsandsubscriptionsdb.h
+    qospacketqueue.h
 
     mainapp.cpp
     main.cpp
@@ -81,6 +85,10 @@ add_executable(FlashMQ
     acltree.cpp
     threadlocalutils.cpp
     flashmq_plugin.cpp
+    retainedmessagesdb.cpp
+    persistencefile.cpp
+    sessionsandsubscriptionsdb.cpp
+    qospacketqueue.cpp
 
     )
 
diff --git a/FlashMQTests/FlashMQTests.pro b/FlashMQTests/FlashMQTests.pro
index 1c48319..450323e 100644
--- a/FlashMQTests/FlashMQTests.pro
+++ b/FlashMQTests/FlashMQTests.pro
@@ -42,6 +42,10 @@ SOURCES +=  tst_maintests.cpp \
     ../acltree.cpp \
     ../threadlocalutils.cpp \
     ../flashmq_plugin.cpp \
+    ../retainedmessagesdb.cpp \
+    ../persistencefile.cpp \
+    ../sessionsandsubscriptionsdb.cpp \
+    ../qospacketqueue.cpp \
     mainappthread.cpp \
     twoclienttestcontext.cpp
 
@@ -77,6 +81,10 @@ HEADERS += \
     ../acltree.h \
     ../threadlocalutils.h \
     ../flashmq_plugin.h \
+    ../retainedmessagesdb.h \
+    ../persistencefile.h \
+    ../sessionsandsubscriptionsdb.h \
+    ../qospacketqueue.h \
     mainappthread.h \
     twoclienttestcontext.h
 
diff --git a/FlashMQTests/tst_maintests.cpp b/FlashMQTests/tst_maintests.cpp
index 14294a9..ef1866a 100644
--- a/FlashMQTests/tst_maintests.cpp
+++ b/FlashMQTests/tst_maintests.cpp
@@ -21,12 +21,18 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 #include <QtQmqtt/qmqtt.h>
 #include <QScopedPointer>
 #include <QHostInfo>
+#include <list>
+#include <unordered_map>
 
 #include "cirbuf.h"
 #include "mainapp.h"
 #include "mainappthread.h"
 #include "twoclienttestcontext.h"
 #include "threadlocalutils.h"
+#include "retainedmessagesdb.h"
+#include "sessionsandsubscriptionsdb.h"
+#include "session.h"
+#include "threaddata.h"
 
 // Dumb Qt version gives warnings when comparing uint with number literal.
 template <typename T1, typename T2>
@@ -88,6 +94,12 @@ private slots:
 
     void testTopicsMatch();
 
+    void testRetainedMessageDB();
+    void testRetainedMessageDBNotPresent();
+    void testRetainedMessageDBEmptyList();
+
+    void testSavingSessions();
+
 };
 
 MainTests::MainTests()
@@ -822,6 +834,221 @@ void MainTests::testTopicsMatch()
 
 }
 
+void MainTests::testRetainedMessageDB()
+{
+    try
+    {
+        std::string longpayload = getSecureRandomString(65537);
+        std::string longTopic = formatString("one/two/%s", getSecureRandomString(4000).c_str());
+
+        std::vector<RetainedMessage> messages;
+        messages.emplace_back("one/two/three", "payload", 0);
+        messages.emplace_back("one/two/wer", "payload", 1);
+        messages.emplace_back("one/e/wer", "payload", 1);
+        messages.emplace_back("one/wee/wer", "asdfasdfasdf", 1);
+        messages.emplace_back("one/two/wer", "µsdf", 1);
+        messages.emplace_back("/boe/bah", longpayload, 1);
+        messages.emplace_back("one/two/wer", "paylasdfaoad", 1);
+        messages.emplace_back("one/two/wer", "payload", 1);
+        messages.emplace_back(longTopic, "payload", 1);
+        messages.emplace_back(longTopic, longpayload, 1);
+        messages.emplace_back("one", "µsdf", 1);
+        messages.emplace_back("/boe", longpayload, 1);
+        messages.emplace_back("one", "µsdf", 1);
+        messages.emplace_back("", "foremptytopic", 0);
+
+        RetainedMessagesDB db("/tmp/flashmqtests_retained.db");
+        db.openWrite();
+        db.saveData(messages);
+        db.closeFile();
+
+        RetainedMessagesDB db2("/tmp/flashmqtests_retained.db");
+        db2.openRead();
+        std::list<RetainedMessage> messagesLoaded = db2.readData();
+        db2.closeFile();
+
+        QCOMPARE(messages.size(), messagesLoaded.size());
+
+        auto itOrg = messages.begin();
+        auto itLoaded = messagesLoaded.begin();
+        while (itOrg != messages.end() && itLoaded != messagesLoaded.end())
+        {
+            RetainedMessage &one = *itOrg;
+            RetainedMessage &two = *itLoaded;
+
+            // Comparing the fields because the RetainedMessage class has an == operator that only looks at topic.
+            QCOMPARE(one.topic, two.topic);
+            QCOMPARE(one.payload, two.payload);
+            QCOMPARE(one.qos, two.qos);
+
+            itOrg++;
+            itLoaded++;
+        }
+    }
+    catch (std::exception &ex)
+    {
+        QVERIFY2(false, ex.what());
+    }
+}
+
+void MainTests::testRetainedMessageDBNotPresent()
+{
+    try
+    {
+        RetainedMessagesDB db2("/tmp/flashmqtests_asdfasdfasdf.db");
+        db2.openRead();
+        std::list<RetainedMessage> messagesLoaded = db2.readData();
+        db2.closeFile();
+
+        MYCASTCOMPARE(messagesLoaded.size(), 0);
+
+        QVERIFY2(false, "We should have run into an exception.");
+    }
+    catch (PersistenceFileCantBeOpened &ex)
+    {
+        QVERIFY(true);
+    }
+    catch (std::exception &ex)
+    {
+        QVERIFY2(false, ex.what());
+    }
+}
+
+void MainTests::testRetainedMessageDBEmptyList()
+{
+    try
+    {
+        std::vector<RetainedMessage> messages;
+
+        RetainedMessagesDB db("/tmp/flashmqtests_retained.db");
+        db.openWrite();
+        db.saveData(messages);
+        db.closeFile();
+
+        RetainedMessagesDB db2("/tmp/flashmqtests_retained.db");
+        db2.openRead();
+        std::list<RetainedMessage> messagesLoaded = db2.readData();
+        db2.closeFile();
+
+        MYCASTCOMPARE(messages.size(), messagesLoaded.size());
+        MYCASTCOMPARE(messages.size(), 0);
+    }
+    catch (std::exception &ex)
+    {
+        QVERIFY2(false, ex.what());
+    }
+}
+
+void MainTests::testSavingSessions()
+{
+    try
+    {
+        std::shared_ptr<Settings> settings(new Settings());
+        std::shared_ptr<SubscriptionStore> store(new SubscriptionStore());
+        std::shared_ptr<ThreadData> t(new ThreadData(0, store, settings));
+
+        std::shared_ptr<Client> c1(new Client(0, t, nullptr, false, nullptr, settings, false));
+        c1->setClientProperties(ProtocolVersion::Mqtt311, "c1", "user1", true, 60, false);
+        store->registerClientAndKickExistingOne(c1);
+        c1->getSession()->touch();
+        c1->getSession()->addIncomingQoS2MessageId(2);
+        c1->getSession()->addIncomingQoS2MessageId(3);
+
+        std::shared_ptr<Client> c2(new Client(0, t, nullptr, false, nullptr, settings, false));
+        c2->setClientProperties(ProtocolVersion::Mqtt311, "c2", "user2", true, 60, false);
+        store->registerClientAndKickExistingOne(c2);
+        c2->getSession()->touch();
+        c2->getSession()->addOutgoingQoS2MessageId(55);
+        c2->getSession()->addOutgoingQoS2MessageId(66);
+
+        const std::string topic1 = "one/two/three";
+        std::vector<std::string> subtopics;
+        splitTopic(topic1, subtopics);
+        store->addSubscription(c1, topic1, subtopics, 0);
+
+        const std::string topic2 = "four/five/six";
+        splitTopic(topic2, subtopics);
+        store->addSubscription(c2, topic2, subtopics, 0);
+        store->addSubscription(c1, topic2, subtopics, 0);
+
+        const std::string topic3 = "";
+        splitTopic(topic3, subtopics);
+        store->addSubscription(c2, topic3, subtopics, 0);
+
+        const std::string topic4 = "#";
+        splitTopic(topic4, subtopics);
+        store->addSubscription(c2, topic4, subtopics, 0);
+
+        uint64_t count = 0;
+
+        Publish publish("a/b/c", "Hello Barry", 1);
+
+        std::shared_ptr<Session> c1ses = c1->getSession();
+        c1.reset();
+        c1ses->writePacket(publish, 1, false, count);
+
+        store->saveSessionsAndSubscriptions("/tmp/flashmqtests_sessions.db");
+
+        std::shared_ptr<SubscriptionStore> store2(new SubscriptionStore());
+        store2->loadSessionsAndSubscriptions("/tmp/flashmqtests_sessions.db");
+
+        MYCASTCOMPARE(store->sessionsById.size(), 2);
+        MYCASTCOMPARE(store2->sessionsById.size(), 2);
+
+        for (auto &pair : store->sessionsById)
+        {
+            std::shared_ptr<Session> &ses = pair.second;
+            std::shared_ptr<Session> &ses2 = store2->sessionsById[pair.first];
+            QCOMPARE(pair.first, ses2->getClientId());
+
+            QCOMPARE(ses->username, ses2->username);
+            QCOMPARE(ses->client_id, ses2->client_id);
+            QCOMPARE(ses->incomingQoS2MessageIds, ses2->incomingQoS2MessageIds);
+            QCOMPARE(ses->outgoingQoS2MessageIds, ses2->outgoingQoS2MessageIds);
+            QCOMPARE(ses->nextPacketId, ses2->nextPacketId);
+        }
+
+
+        std::unordered_map<std::string, std::list<SubscriptionForSerializing>> store1Subscriptions;
+        store->getSubscriptions(&store->root, "", true, store1Subscriptions);
+
+        std::unordered_map<std::string, std::list<SubscriptionForSerializing>> store2Subscriptions;
+        store->getSubscriptions(&store->root, "", true, store2Subscriptions);
+
+        MYCASTCOMPARE(store1Subscriptions.size(), 4);
+        MYCASTCOMPARE(store2Subscriptions.size(), 4);
+
+        for(auto &pair : store1Subscriptions)
+        {
+            std::list<SubscriptionForSerializing> &subscList1 = pair.second;
+            std::list<SubscriptionForSerializing> &subscList2 = store2Subscriptions[pair.first];
+
+            QCOMPARE(subscList1.size(), subscList2.size());
+
+            auto subs1It = subscList1.begin();
+            auto subs2It = subscList2.begin();
+
+            while (subs1It != subscList1.end())
+            {
+                SubscriptionForSerializing &one = *subs1It;
+                SubscriptionForSerializing &two = *subs2It;
+                QCOMPARE(one.clientId, two.clientId);
+                QCOMPARE(one.qos, two.qos);
+
+                subs1It++;
+                subs2It++;
+            }
+
+        }
+
+
+    }
+    catch (std::exception &ex)
+    {
+        QVERIFY2(false, ex.what());
+    }
+}
+
 QTEST_GUILESS_MAIN(MainTests)
 
 #include "tst_maintests.moc"
diff --git a/configfileparser.cpp b/configfileparser.cpp
index b99810e..9211f17 100644
--- a/configfileparser.cpp
+++ b/configfileparser.cpp
@@ -107,6 +107,7 @@ ConfigFileParser::ConfigFileParser(const std::string &path) :
     validKeys.insert("allow_anonymous");
     validKeys.insert("rlimit_nofile");
     validKeys.insert("expire_sessions_after_seconds");
+    validKeys.insert("storage_dir");
 
     validListenKeys.insert("port");
     validListenKeys.insert("protocol");
@@ -412,6 +413,14 @@ void ConfigFileParser::loadFile(bool test)
                     }
                     tmpSettings->authPluginTimerPeriod = newVal;
                 }
+
+                if (key == "storage_dir")
+                {
+                    std::string newPath = value;
+                    rtrim(newPath, '/');
+                    checkWritableDir<ConfigFileException>(newPath);
+                    tmpSettings->storageDir = newPath;
+                }
             }
         }
         catch (std::invalid_argument &ex) // catch for the stoi()
diff --git a/mainapp.cpp b/mainapp.cpp
index de05ac7..de4f00e 100644
--- a/mainapp.cpp
+++ b/mainapp.cpp
@@ -202,6 +202,15 @@ MainApp::MainApp(const std::string &configFilePath) :
         auto fAuthPluginPeriodicEvent = std::bind(&MainApp::queueAuthPluginPeriodicEventAllThreads, this);
         timer.addCallback(fAuthPluginPeriodicEvent, settings->authPluginTimerPeriod*1000, "Auth plugin periodic event.");
     }
+
+    if (!settings->storageDir.empty())
+    {
+        subscriptionStore->loadRetainedMessages(settings->getRetainedMessagesDBFile());
+        subscriptionStore->loadSessionsAndSubscriptions(settings->getSessionsDBFile());
+    }
+
+    auto fSaveState = std::bind(&MainApp::saveState, this);
+    timer.addCallback(fSaveState, 900000, "Save state.");
 }
 
 MainApp::~MainApp()
@@ -369,6 +378,25 @@ void MainApp::publishStat(const std::string &topic, uint64_t n)
     subscriptionStore->setRetainedMessage(topic, subtopics, payload, 0);
 }
 
+void MainApp::saveState()
+{
+    try
+    {
+        if (!settings->storageDir.empty())
+        {
+            const std::string retainedDBPath = settings->getRetainedMessagesDBFile();
+            subscriptionStore->saveRetainedMessages(retainedDBPath);
+
+            const std::string sessionsDBPath = settings->getSessionsDBFile();
+            subscriptionStore->saveSessionsAndSubscriptions(sessionsDBPath);
+        }
+    }
+    catch(std::exception &ex)
+    {
+        logger->logf(LOG_ERR, "Error saving state: %s", ex.what());
+    }
+}
+
 void MainApp::initMainApp(int argc, char *argv[])
 {
     if (instance != nullptr)
@@ -659,6 +687,8 @@ void MainApp::start()
     {
         thread->waitForQuit();
     }
+
+    saveState();
 }
 
 void MainApp::quit()
diff --git a/mainapp.h b/mainapp.h
index 2a00475..7a31509 100644
--- a/mainapp.h
+++ b/mainapp.h
@@ -84,6 +84,7 @@ class MainApp
     void setFuzzFile(const std::string &fuzzFilePath);
     void publishStatsOnDollarTopic();
     void publishStat(const std::string &topic, uint64_t n);
+    void saveState();
 
     MainApp(const std::string &configFilePath);
 public:
diff --git a/mqttpacket.cpp b/mqttpacket.cpp
index 19817d7..e69c310 100644
--- a/mqttpacket.cpp
+++ b/mqttpacket.cpp
@@ -47,8 +47,11 @@ MqttPacket::MqttPacket(CirBuf &buf, size_t packet_len, size_t fixed_header_lengt
     pos += fixed_header_length;
 }
 
-// This is easier than using the copy constructor publically, because then I have to keep maintaining a functioning copy constructor.
-// Returning shared pointer because that's typically how we need it; we only need to copy it if we pass it around as shared resource.
+/**
+ * @brief MqttPacket::getCopy (using default copy constructor and resetting some selected fields) is easier than using the copy constructor
+ *        publically, because then I have to keep maintaining a functioning copy constructor for each new field I add.
+ * @return a shared pointer because that's typically how we need it; we only need to copy it if we pass it around as shared resource.
+ */
 std::shared_ptr<MqttPacket> MqttPacket::getCopy() const
 {
     std::shared_ptr<MqttPacket> copyPacket(new MqttPacket(*this));
@@ -129,6 +132,9 @@ MqttPacket::MqttPacket(const Publish &publish) :
         writeBytes(zero, 2);
     }
 
+    payloadStart = pos;
+    payloadLen = publish.payload.length();
+
     writeBytes(publish.payload.c_str(), publish.payload.length());
     calculateRemainingLength();
 }
@@ -546,13 +552,14 @@ void MqttPacket::handlePublish()
         }
     }
 
+    payloadLen = remainingAfterPos();
+    payloadStart = pos;
+
     if (sender->getThreadData()->authentication.aclCheck(sender->getClientId(), sender->getUsername(), topic, *subtopics, AclAccess::write, qos, retain) == AuthResult::success)
     {
         if (retain)
         {
-            size_t payload_length = remainingAfterPos();
-            std::string payload(readBytes(payload_length), payload_length);
-
+            std::string payload(readBytes(payloadLen), payloadLen);
             sender->getThreadData()->getSubscriptionStore()->setRetainedMessage(topic, *subtopics, payload, qos);
         }
 
@@ -679,6 +686,23 @@ size_t MqttPacket::getTotalMemoryFootprint()
     return bites.size() + sizeof(MqttPacket);
 }
 
+/**
+ * @brief MqttPacket::getPayloadCopy takes part of the vector of bytes and returns it as a string.
+ * @return
+ *
+ * It's necessary sometimes, but it's against FlashMQ's concept of not parsing the payload. Normally, you can just write out
+ * the whole byte array that is a packet to subscribers. No need to copy and such.
+ *
+ * I created it for saving QoS packages in the db file.
+ */
+std::string MqttPacket::getPayloadCopy()
+{
+    assert(payloadStart > 0);
+    assert(pos <= bites.size());
+    std::string payload(&bites[payloadStart], payloadLen);
+    return payload;
+}
+
 size_t MqttPacket::getSizeIncludingNonPresentHeader() const
 {
     size_t total = bites.size();
diff --git a/mqttpacket.h b/mqttpacket.h
index 6c8ddb8..9b0d7a5 100644
--- a/mqttpacket.h
+++ b/mqttpacket.h
@@ -59,6 +59,8 @@ class MqttPacket
     size_t packet_id_pos = 0;
     uint16_t packet_id = 0;
     ProtocolVersion protocolVersion = ProtocolVersion::None;
+    size_t payloadStart = 0;
+    size_t payloadLen = 0;
     Logger *logger = Logger::getInstance();
 
     char *readBytes(size_t length);
@@ -116,6 +118,7 @@ public:
     uint16_t getPacketId() const;
     void setDuplicate();
     size_t getTotalMemoryFootprint();
+    std::string getPayloadCopy();
 };
 
 #endif // MQTTPACKET_H
diff --git a/persistencefile.cpp b/persistencefile.cpp
new file mode 100644
index 0000000..03ed4f7
--- /dev/null
+++ b/persistencefile.cpp
@@ -0,0 +1,320 @@
+/*
+This file is part of FlashMQ (https://www.flashmq.org)
+Copyright (C) 2021 Wiebe Cazemier
+
+FlashMQ is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, version 3.
+
+FlashMQ is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public
+License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#include "persistencefile.h"
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <exception>
+#include <stdexcept>
+#include <stdio.h>
+#include <cstring>
+
+#include "utils.h"
+#include "logger.h"
+
+PersistenceFile::PersistenceFile(const std::string &filePath) :
+    digestContext(EVP_MD_CTX_new()),
+    buf(1024*1024)
+{
+    if (!filePath.empty() && filePath[filePath.size() - 1] == '/')
+        throw std::runtime_error("Target file can't contain trailing slash.");
+
+    this->filePath = filePath;
+    this->filePathTemp = formatString("%s.newfile.%s", filePath.c_str(), getSecureRandomString(8).c_str());
+    this->filePathCorrupt = formatString("%s.corrupt.%s", filePath.c_str(), getSecureRandomString(8).c_str());
+}
+
+PersistenceFile::~PersistenceFile()
+{
+    closeFile();
+
+    if (digestContext)
+    {
+        EVP_MD_CTX_free(digestContext);
+    }
+}
+
+/**
+ * @brief RetainedMessagesDB::hashFile hashes the data after the headers and writes the hash in the header. Uses SHA512.
+ *
+ */
+void PersistenceFile::writeCheck(const void *ptr, size_t size, size_t n, FILE *s)
+{
+    if (fwrite(ptr, size, n, s) != n)
+    {
+        throw std::runtime_error(formatString("Error writing: %s", strerror(errno)));
+    }
+}
+
+ssize_t PersistenceFile::readCheck(void *ptr, size_t size, size_t n, FILE *stream)
+{
+    size_t nread = fread(ptr, size, n, stream);
+
+    if (nread != n)
+    {
+        if (feof(f))
+            return -1;
+
+        throw std::runtime_error(formatString("Error reading: %s", strerror(errno)));
+    }
+
+    return nread;
+}
+
+void PersistenceFile::hashFile()
+{
+    logger->logf(LOG_DEBUG, "Calculating and saving hash of '%s'.", filePath.c_str());
+
+    fseek(f, TOTAL_HEADER_SIZE, SEEK_SET);
+
+    unsigned int output_len = 0;
+    unsigned char md_value[EVP_MAX_MD_SIZE];
+    std::memset(md_value, 0, EVP_MAX_MD_SIZE);
+
+    EVP_MD_CTX_reset(digestContext);
+    EVP_DigestInit_ex(digestContext, sha512, NULL);
+
+    while (!feof(f))
+    {
+        size_t n = fread(buf.data(), 1, buf.size(), f);
+        EVP_DigestUpdate(digestContext, buf.data(), n);
+    }
+
+    EVP_DigestFinal_ex(digestContext, md_value, &output_len);
+
+    if (output_len != HASH_SIZE)
+        throw std::runtime_error("Impossible: calculated hash size wrong length");
+
+    fseek(f, MAGIC_STRING_LENGH, SEEK_SET);
+
+    writeCheck(md_value, output_len, 1, f);
+    fflush(f);
+}
+
+void PersistenceFile::verifyHash()
+{
+    fseek(f, 0, SEEK_END);
+    const size_t size = ftell(f);
+
+    if (size < TOTAL_HEADER_SIZE)
+        throw std::runtime_error(formatString("File '%s' is too small for it even to contain a header.", filePath.c_str()));
+
+    unsigned char md_from_disk[HASH_SIZE];
+    std::memset(md_from_disk, 0, HASH_SIZE);
+
+    fseek(f, MAGIC_STRING_LENGH, SEEK_SET);
+    fread(md_from_disk, 1, HASH_SIZE, f);
+
+    unsigned int output_len = 0;
+    unsigned char md_value[EVP_MAX_MD_SIZE];
+    std::memset(md_value, 0, EVP_MAX_MD_SIZE);
+
+    EVP_MD_CTX_reset(digestContext);
+    EVP_DigestInit_ex(digestContext, sha512, NULL);
+
+    while (!feof(f))
+    {
+        size_t n = fread(buf.data(), 1, buf.size(), f);
+        EVP_DigestUpdate(digestContext, buf.data(), n);
+    }
+
+    EVP_DigestFinal_ex(digestContext, md_value, &output_len);
+
+    if (output_len != HASH_SIZE)
+        throw std::runtime_error("Impossible: calculated hash size wrong length");
+
+    if (std::memcmp(md_from_disk, md_value, output_len) != 0)
+    {
+        fclose(f);
+        f = nullptr;
+
+        if (rename(filePath.c_str(), filePathCorrupt.c_str()) == 0)
+        {
+            throw std::runtime_error(formatString("File '%s' is corrupt: hash mismatch. Moved aside to '%s'.", filePath.c_str(), filePathCorrupt.c_str()));
+        }
+        else
+        {
+            throw std::runtime_error(formatString("File '%s' is corrupt: hash mismatch. Tried to move aside, but that failed: '%s'.",
+                                                  filePath.c_str(), strerror(errno)));
+        }
+    }
+
+    logger->logf(LOG_DEBUG, "Hash of '%s' correct", filePath.c_str());
+}
+
+/**
+ * @brief PersistenceFile::makeSureBufSize grows the buffer if n is bigger.
+ * @param n in bytes.
+ *
+ * Remember that when you're dealing with fields that are sized in MQTT by 16 bit ints, like topic paths, the buffer will always be big enough, because it's 1 MB.
+ */
+void PersistenceFile::makeSureBufSize(size_t n)
+{
+    if (n > buf.size())
+        buf.resize(n);
+}
+
+void PersistenceFile::writeInt64(const int64_t val)
+{
+    unsigned char buf[8];
+
+    // Write big-endian
+    int shift = 56;
+    int i = 0;
+    while (shift >= 0)
+    {
+        unsigned char wantedByte = val >> shift;
+        buf[i++] = wantedByte;
+        shift -= 8;
+    }
+    writeCheck(buf, 1, 8, f);
+}
+
+void PersistenceFile::writeUint32(const uint32_t val)
+{
+    unsigned char buf[4];
+
+    // Write big-endian
+    int shift = 24;
+    int i = 0;
+    while (shift >= 0)
+    {
+        unsigned char wantedByte = val >> shift;
+        buf[i++] = wantedByte;
+        shift -= 8;
+    }
+    writeCheck(buf, 1, 4, f);
+}
+
+void PersistenceFile::writeUint16(const uint16_t val)
+{
+    unsigned char buf[2];
+
+    // Write big-endian
+    int shift = 8;
+    int i = 0;
+    while (shift >= 0)
+    {
+        unsigned char wantedByte = val >> shift;
+        buf[i++] = wantedByte;
+        shift -= 8;
+    }
+    writeCheck(buf, 1, 2, f);
+}
+
+int64_t PersistenceFile::readInt64(bool &eofFound)
+{
+    if (readCheck(buf.data(), 1, 8, f) < 0)
+        eofFound = true;
+
+    unsigned char *buf_ = reinterpret_cast<unsigned char *>(buf.data());
+    const uint64_t val1 = ((buf_[0]) << 24) | ((buf_[1]) << 16) | ((buf_[2]) << 8) | (buf_[3]);
+    const uint64_t val2 = ((buf_[4]) << 24) | ((buf_[5]) << 16) | ((buf_[6]) << 8) | (buf_[7]);
+    const int64_t val = (val1 << 32) | val2;
+    return val;
+}
+
+uint32_t PersistenceFile::readUint32(bool &eofFound)
+{
+    if (readCheck(buf.data(), 1, 4, f) < 0)
+        eofFound = true;
+
+    uint32_t val;
+    unsigned char *buf_ = reinterpret_cast<unsigned char *>(buf.data());
+    val = ((buf_[0]) << 24) | ((buf_[1]) << 16) | ((buf_[2]) << 8) | (buf_[3]);
+    return val;
+}
+
+uint16_t PersistenceFile::readUint16(bool &eofFound)
+{
+    if (readCheck(buf.data(), 1, 2, f) < 0)
+        eofFound = true;
+
+    uint16_t val;
+    unsigned char *buf_ = reinterpret_cast<unsigned char *>(buf.data());
+    val = ((buf_[0]) << 8) | (buf_[1]);
+    return val;
+}
+
+/**
+ * @brief RetainedMessagesDB::openWrite doesn't explicitely name a file version (v1, etc), because we always write the current definition.
+ */
+void PersistenceFile::openWrite(const std::string &versionString)
+{
+    if (openMode != FileMode::unknown)
+        throw std::runtime_error("File is already open.");
+
+    f = fopen(filePathTemp.c_str(), "w+b");
+
+    if (f == nullptr)
+    {
+        throw std::runtime_error(formatString("Can't open '%s': %s", filePathTemp.c_str(), strerror(errno)));
+    }
+
+    openMode = FileMode::write;
+
+    writeCheck(buf.data(), 1, MAGIC_STRING_LENGH, f);
+    rewind(f);
+    writeCheck(versionString.c_str(), 1, versionString.length(), f);
+    fseek(f, MAGIC_STRING_LENGH, SEEK_SET);
+    writeCheck(buf.data(), 1, HASH_SIZE, f);
+}
+
+void PersistenceFile::openRead()
+{
+    if (openMode != FileMode::unknown)
+        throw std::runtime_error("File is already open.");
+
+    f = fopen(filePath.c_str(), "rb");
+
+    if (f == nullptr)
+        throw PersistenceFileCantBeOpened(formatString("Can't open '%s': %s.", filePath.c_str(), strerror(errno)).c_str());
+
+    openMode = FileMode::read;
+
+    verifyHash();
+    rewind(f);
+
+    fread(buf.data(), 1, MAGIC_STRING_LENGH, f);
+    detectedVersionString = std::string(buf.data(), strlen(buf.data()));
+
+    fseek(f, TOTAL_HEADER_SIZE, SEEK_SET);
+}
+
+void PersistenceFile::closeFile()
+{
+    if (!f)
+        return;
+
+    if (openMode == FileMode::write)
+        hashFile();
+
+    if (f != nullptr)
+    {
+        fclose(f);
+        f = nullptr;
+    }
+
+    if (openMode == FileMode::write && !filePathTemp.empty() && ! filePath.empty())
+    {
+        if (rename(filePathTemp.c_str(), filePath.c_str()) < 0)
+            throw std::runtime_error(formatString("Saving '%s' failed: rename of temp file to target failed with: %s", filePath.c_str(), strerror(errno)));
+    }
+}
diff --git a/persistencefile.h b/persistencefile.h
new file mode 100644
index 0000000..6000d01
--- /dev/null
+++ b/persistencefile.h
@@ -0,0 +1,92 @@
+/*
+This file is part of FlashMQ (https://www.flashmq.org)
+Copyright (C) 2021 Wiebe Cazemier
+
+FlashMQ is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, version 3.
+
+FlashMQ is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public
+License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#ifndef PERSISTENCEFILE_H
+#define PERSISTENCEFILE_H
+
+#include <vector>
+#include <list>
+#include <string>
+#include <stdio.h>
+#include <openssl/evp.h>
+#include <stdexcept>
+#include <cstring>
+
+#include "logger.h"
+
+#define MAGIC_STRING_LENGH 32
+#define HASH_SIZE 64
+#define TOTAL_HEADER_SIZE (MAGIC_STRING_LENGH + HASH_SIZE)
+
+/**
+ * @brief The PersistenceFileCantBeOpened class should be thrown when a non-fatal file-not-found error happens.
+ */
+class PersistenceFileCantBeOpened : public std::runtime_error
+{
+public:
+    PersistenceFileCantBeOpened(const std::string &msg) : std::runtime_error(msg) {}
+};
+
+class PersistenceFile
+{
+    std::string filePath;
+    std::string filePathTemp;
+    std::string filePathCorrupt;
+
+    EVP_MD_CTX *digestContext = nullptr;
+    const EVP_MD *sha512 = EVP_sha512();
+
+    void hashFile();
+    void verifyHash();
+
+protected:
+    enum class FileMode
+    {
+        unknown,
+        read,
+        write
+    };
+
+    FILE *f = nullptr;
+    std::vector<char> buf;
+    FileMode openMode = FileMode::unknown;
+    std::string detectedVersionString;
+
+    Logger *logger = Logger::getInstance();
+
+    void makeSureBufSize(size_t n);
+
+    void writeCheck(const void *__restrict __ptr, size_t __size, size_t __n, FILE *__restrict __s);
+    ssize_t readCheck(void *__restrict ptr, size_t size, size_t n, FILE *__restrict stream);
+
+    void writeInt64(const int64_t val);
+    void writeUint32(const uint32_t val);
+    void writeUint16(const uint16_t val);
+    int64_t readInt64(bool &eofFound);
+    uint32_t readUint32(bool &eofFound);
+    uint16_t readUint16(bool &eofFound);
+
+public:
+    PersistenceFile(const std::string &filePath);
+    ~PersistenceFile();
+
+    void openWrite(const std::string &versionString);
+    void openRead();
+    void closeFile();
+};
+
+#endif // PERSISTENCEFILE_H
diff --git a/qospacketqueue.cpp b/qospacketqueue.cpp
new file mode 100644
index 0000000..696bedd
--- /dev/null
+++ b/qospacketqueue.cpp
@@ -0,0 +1,77 @@
+#include "qospacketqueue.h"
+
+#include "cassert"
+
+#include "mqttpacket.h"
+
+void QoSPacketQueue::erase(const uint16_t packet_id)
+{
+    auto it = queue.begin();
+    auto end = queue.end();
+    while (it != end)
+    {
+        std::shared_ptr<MqttPacket> &p = *it;
+        if (p->getPacketId() == packet_id)
+        {
+            size_t mem = p->getTotalMemoryFootprint();
+            qosQueueBytes -= mem;
+            assert(qosQueueBytes >= 0);
+            if (qosQueueBytes < 0) // Should not happen, but correcting a hypothetical bug is fine for this purpose.
+                qosQueueBytes = 0;
+
+            queue.erase(it);
+
+            break;
+        }
+
+        it++;
+    }
+}
+
+size_t QoSPacketQueue::size() const
+{
+    return queue.size();
+}
+
+size_t QoSPacketQueue::getByteSize() const
+{
+    return qosQueueBytes;
+}
+
+/**
+ * @brief QoSPacketQueue::queuePacket makes a copy of the packet because it has state for the receiver in question.
+ * @param p
+ * @param id
+ * @return the packet copy.
+ */
+std::shared_ptr<MqttPacket> QoSPacketQueue::queuePacket(const MqttPacket &p, uint16_t id)
+{
+    assert(p.getQos() > 0);
+
+    std::shared_ptr<MqttPacket> copyPacket = p.getCopy();
+    copyPacket->setPacketId(id);
+    queue.push_back(copyPacket);
+    qosQueueBytes += copyPacket->getTotalMemoryFootprint();
+    return copyPacket;
+}
+
+std::shared_ptr<MqttPacket> QoSPacketQueue::queuePacket(const Publish &pub, uint16_t id)
+{
+    assert(pub.qos > 0);
+
+    std::shared_ptr<MqttPacket> copyPacket(new MqttPacket(pub));
+    copyPacket->setPacketId(id);
+    queue.push_back(copyPacket);
+    qosQueueBytes += copyPacket->getTotalMemoryFootprint();
+    return copyPacket;
+}
+
+std::list<std::shared_ptr<MqttPacket>>::const_iterator QoSPacketQueue::begin() const
+{
+    return queue.cbegin();
+}
+
+std::list<std::shared_ptr<MqttPacket>>::const_iterator QoSPacketQueue::end() const
+{
+    return queue.cend();
+}
diff --git a/qospacketqueue.h b/qospacketqueue.h
new file mode 100644
index 0000000..f8b18d1
--- /dev/null
+++ b/qospacketqueue.h
@@ -0,0 +1,25 @@
+#ifndef QOSPACKETQUEUE_H
+#define QOSPACKETQUEUE_H
+
+#include "list"
+
+#include "forward_declarations.h"
+#include "types.h"
+
+class QoSPacketQueue
+{
+    std::list<std::shared_ptr<MqttPacket>> queue; // Using list because it's easiest to maintain order [MQTT-4.6.0-6]
+    ssize_t qosQueueBytes = 0;
+
+public:
+    void erase(const uint16_t packet_id);
+    size_t size() const;
+    size_t getByteSize() const;
+    std::shared_ptr<MqttPacket> queuePacket(const MqttPacket &p, uint16_t id);
+    std::shared_ptr<MqttPacket> queuePacket(const Publish &pub, uint16_t id);
+
+    std::list<std::shared_ptr<MqttPacket>>::const_iterator begin() const;
+    std::list<std::shared_ptr<MqttPacket>>::const_iterator end() const;
+};
+
+#endif // QOSPACKETQUEUE_H
diff --git a/retainedmessage.cpp b/retainedmessage.cpp
index 29647d1..c01f9e2 100644
--- a/retainedmessage.cpp
+++ b/retainedmessage.cpp
@@ -34,3 +34,8 @@ bool RetainedMessage::empty() const
 {
     return payload.empty();
 }
+
+uint32_t RetainedMessage::getSize() const
+{
+    return topic.length() + payload.length() + 1;
+}
diff --git a/retainedmessage.h b/retainedmessage.h
index 01491b0..97b5369 100644
--- a/retainedmessage.h
+++ b/retainedmessage.h
@@ -30,6 +30,7 @@ struct RetainedMessage
 
     bool operator==(const RetainedMessage &rhs) const;
     bool empty() const;
+    uint32_t getSize() const;
 };
 
 namespace std {
diff --git a/retainedmessagesdb.cpp b/retainedmessagesdb.cpp
new file mode 100644
index 0000000..70ae9b0
--- /dev/null
+++ b/retainedmessagesdb.cpp
@@ -0,0 +1,146 @@
+/*
+This file is part of FlashMQ (https://www.flashmq.org)
+Copyright (C) 2021 Wiebe Cazemier
+
+FlashMQ is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, version 3.
+
+FlashMQ is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public
+License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <exception>
+#include <stdexcept>
+#include <stdio.h>
+#include <cstring>
+
+#include "retainedmessagesdb.h"
+#include "utils.h"
+#include "logger.h"
+
+RetainedMessagesDB::RetainedMessagesDB(const std::string &filePath) : PersistenceFile(filePath)
+{
+
+}
+
+void RetainedMessagesDB::openWrite()
+{
+    PersistenceFile::openWrite(MAGIC_STRING_V1);
+}
+
+void RetainedMessagesDB::openRead()
+{
+    PersistenceFile::openRead();
+
+    if (detectedVersionString == MAGIC_STRING_V1)
+        readVersion = ReadVersion::v1;
+    else
+        throw std::runtime_error("Unknown file version.");
+}
+
+/**
+ * @brief RetainedMessagesDB::writeRowHeader writes two 32 bit integers: topic size and payload size.
+ * @param rm
+ *
+ * So, the header per message is 8 bytes long.
+ *
+ * It writes no information about the length of the QoS value, because that is always one.
+ */
+void RetainedMessagesDB::writeRowHeader(const RetainedMessage &rm)
+{
+    writeUint32(rm.topic.size());
+    writeUint32(rm.payload.size());
+}
+
+RetainedMessagesDB::RowHeader RetainedMessagesDB::readRowHeaderV1(bool &eofFound)
+{
+    RetainedMessagesDB::RowHeader result;
+
+    result.topicLen = readUint32(eofFound);
+    result.payloadLen = readUint32(eofFound);
+
+    return  result;
+}
+
+/**
+ * @brief RetainedMessagesDB::saveData doesn't explicitely name a file version (v1, etc), because we always write the current definition.
+ * @param messages
+ */
+void RetainedMessagesDB::saveData(const std::vector<RetainedMessage> &messages)
+{
+    if (!f)
+        return;
+
+    char reserved[RESERVED_SPACE_RETAINED_DB_V1];
+    std::memset(reserved, 0, RESERVED_SPACE_RETAINED_DB_V1);
+
+    char qos = 0;
+    for (const RetainedMessage &rm : messages)
+    {
+        logger->logf(LOG_DEBUG, "Saving retained message for topic '%s' QoS %d.", rm.topic.c_str(), rm.qos);
+
+        writeRowHeader(rm);
+        qos = rm.qos;
+        writeCheck(&qos, 1, 1, f);
+        writeCheck(reserved, 1, RESERVED_SPACE_RETAINED_DB_V1, f);
+        writeCheck(rm.topic.c_str(), 1, rm.topic.length(), f);
+        writeCheck(rm.payload.c_str(), 1, rm.payload.length(), f);
+    }
+
+    fflush(f);
+}
+
+std::list<RetainedMessage> RetainedMessagesDB::readData()
+{
+    std::list<RetainedMessage> defaultResult;
+
+    if (!f)
+        return defaultResult;
+
+    if (readVersion == ReadVersion::v1)
+        return readDataV1();
+
+    return defaultResult;
+}
+
+std::list<RetainedMessage> RetainedMessagesDB::readDataV1()
+{
+    std::list<RetainedMessage> messages;
+
+    while (!feof(f))
+    {
+        bool eofFound = false;
+        RetainedMessagesDB::RowHeader header = readRowHeaderV1(eofFound);
+
+        if (eofFound)
+            continue;
+
+        makeSureBufSize(header.payloadLen);
+
+        readCheck(buf.data(), 1, 1, f);
+        char qos = buf[0];
+        fseek(f, RESERVED_SPACE_RETAINED_DB_V1, SEEK_CUR);
+
+        readCheck(buf.data(), 1, header.topicLen, f);
+        std::string topic(buf.data(), header.topicLen);
+
+        readCheck(buf.data(), 1, header.payloadLen, f);
+        std::string payload(buf.data(), header.payloadLen);
+
+        RetainedMessage msg(topic, payload, qos);
+        logger->logf(LOG_DEBUG, "Loading retained message for topic '%s' QoS %d.", msg.topic.c_str(), msg.qos);
+        messages.push_back(std::move(msg));
+    }
+
+    return messages;
+}
diff --git a/retainedmessagesdb.h b/retainedmessagesdb.h
new file mode 100644
index 0000000..7b2299d
--- /dev/null
+++ b/retainedmessagesdb.h
@@ -0,0 +1,71 @@
+/*
+This file is part of FlashMQ (https://www.flashmq.org)
+Copyright (C) 2021 Wiebe Cazemier
+
+FlashMQ is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, version 3.
+
+FlashMQ is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public
+License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#ifndef RETAINEDMESSAGESDB_H
+#define RETAINEDMESSAGESDB_H
+
+#include "persistencefile.h"
+#include "retainedmessage.h"
+
+#include "logger.h"
+
+#define MAGIC_STRING_V1 "FlashMQRetainedDBv1"
+#define ROW_HEADER_SIZE 8
+#define RESERVED_SPACE_RETAINED_DB_V1 31
+
+/**
+ * @brief The RetainedMessagesDB class saves and loads the retained messages.
+ *
+ * The DB looks like, from the top:
+ *
+ * MAGIC_STRING_LENGH bytes file header
+ * HASH_SIZE SHA512
+ * [MESSAGES]
+ *
+ * Each message has a row header, which is 8 bytes. See writeRowHeader().
+ *
+ */
+class RetainedMessagesDB : public PersistenceFile
+{
+    enum class ReadVersion
+    {
+        unknown,
+        v1
+    };
+
+    struct RowHeader
+    {
+        uint32_t topicLen = 0;
+        uint32_t payloadLen = 0;
+    };
+
+    ReadVersion readVersion = ReadVersion::unknown;
+
+    void writeRowHeader(const RetainedMessage &rm);
+    RowHeader readRowHeaderV1(bool &eofFound);
+    std::list<RetainedMessage> readDataV1();
+public:
+    RetainedMessagesDB(const std::string &filePath);
+
+    void openWrite();
+    void openRead();
+
+    void saveData(const std::vector<RetainedMessage> &messages);
+    std::list<RetainedMessage> readData();
+};
+
+#endif // RETAINEDMESSAGESDB_H
diff --git a/rwlockguard.cpp b/rwlockguard.cpp
index 15bae1c..c25c324 100644
--- a/rwlockguard.cpp
+++ b/rwlockguard.cpp
@@ -32,7 +32,15 @@ RWLockGuard::~RWLockGuard()
 
 void RWLockGuard::wrlock()
 {
-    if (pthread_rwlock_wrlock(rwlock) != 0)
+    const int rc = pthread_rwlock_wrlock(rwlock);
+
+    if (rc == EDEADLK)
+    {
+        rwlock = nullptr;
+        return;
+    }
+
+    if (rc != 0)
         throw std::runtime_error("wrlock failed.");
 }
 
diff --git a/session.cpp b/session.cpp
index 3b328f8..e0fc9fe 100644
--- a/session.cpp
+++ b/session.cpp
@@ -20,16 +20,75 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 #include "session.h"
 #include "client.h"
 
+std::chrono::time_point<std::chrono::steady_clock> appStartTime = std::chrono::steady_clock::now();
+
 Session::Session()
 {
 
 }
 
+int64_t Session::getProgramStartedAtUnixTimestamp()
+{
+    auto secondsSinceEpoch = std::chrono::duration_cast<std::chrono::seconds>(std::chrono::system_clock::now().time_since_epoch()).count();
+    const std::chrono::seconds age = std::chrono::duration_cast<std::chrono::seconds>(std::chrono::steady_clock::now() - appStartTime);
+    int64_t result = secondsSinceEpoch - age.count();
+    return result;
+}
+
+void Session::setProgramStartedAtUnixTimestamp(const int64_t unix_timestamp)
+{
+    auto secondsSinceEpoch = std::chrono::duration_cast<std::chrono::seconds>(std::chrono::system_clock::now().time_since_epoch());
+    const std::chrono::seconds _unix_timestamp = std::chrono::seconds(unix_timestamp);
+    const std::chrono::seconds age_in_s = secondsSinceEpoch - _unix_timestamp;
+    appStartTime = std::chrono::steady_clock::now() - age_in_s;
+}
+
+
+int64_t Session::getSessionRelativeAgeInMs() const
+{
+    const std::chrono::milliseconds sessionAge = std::chrono::duration_cast<std::chrono::milliseconds>(lastTouched - appStartTime);
+    const int64_t sInMs = sessionAge.count();
+    return sInMs;
+}
+
+void Session::setSessionTouch(int64_t ageInMs)
+{
+    std::chrono::milliseconds ms(ageInMs);
+    std::chrono::time_point<std::chrono::steady_clock> point = appStartTime + ms;
+    lastTouched = point;
+}
+
+/**
+ * @brief Session::Session copy constructor. Was created for session storing, and is explicitely kept private, to avoid making accidental copies.
+ * @param other
+ *
+ * Because it was created for session storing, the fields we're copying are the fields being stored.
+ */
+Session::Session(const Session &other)
+{
+    this->username = other.username;
+    this->client_id = other.client_id;
+    this->incomingQoS2MessageIds = other.incomingQoS2MessageIds;
+    this->outgoingQoS2MessageIds = other.outgoingQoS2MessageIds;
+    this->nextPacketId = other.nextPacketId;
+    this->lastTouched = other.lastTouched;
+
+    // To be fully correct, we should copy the individual packets, but copying sessions is only done for saving them, and I know
+    // that no member of MqttPacket changes in the QoS process, so we can just keep the shared pointer to the original.
+    this->qosPacketQueue = other.qosPacketQueue;
+}
+
 Session::~Session()
 {
     logger->logf(LOG_DEBUG, "Session %s is being destroyed.", getClientId().c_str());
 }
 
+std::unique_ptr<Session> Session::getCopy() const
+{
+    std::unique_ptr<Session> s(new Session(*this));
+    return s;
+}
+
 bool Session::clientDisconnected() const
 {
     return client.expired();
@@ -45,7 +104,6 @@ void Session::assignActiveConnection(std::shared_ptr<Client> &client)
     this->client = client;
     this->client_id = client->getClientId();
     this->username = client->getUsername();
-    this->thread = client->getThreadData();
 }
 
 /**
@@ -60,7 +118,9 @@ void Session::writePacket(const MqttPacket &packet, char max_qos, bool retain, u
     assert(max_qos <= 2);
     const char qos = std::min<char>(packet.getQos(), max_qos);
 
-    if (thread->authentication.aclCheck(client_id, username, packet.getTopic(), *packet.getSubtopics(), AclAccess::read, qos, retain) == AuthResult::success)
+    assert(packet.getSender());
+    Authentication &auth = packet.getSender()->getThreadData()->authentication;
+    if (auth.aclCheck(client_id, username, packet.getTopic(), *packet.getSubtopics(), AclAccess::read, qos, retain) == AuthResult::success)
     {
         if (qos == 0)
         {
@@ -73,11 +133,10 @@ void Session::writePacket(const MqttPacket &packet, char max_qos, bool retain, u
         }
         else if (qos > 0)
         {
-            std::shared_ptr<MqttPacket> copyPacket = packet.getCopy();
             std::unique_lock<std::mutex> locker(qosQueueMutex);
 
             const size_t totalQosPacketsInTransit = qosPacketQueue.size() + incomingQoS2MessageIds.size() + outgoingQoS2MessageIds.size();
-            if (totalQosPacketsInTransit >= MAX_QOS_MSG_PENDING_PER_CLIENT || (qosQueueBytes >= MAX_QOS_BYTES_PENDING_PER_CLIENT && qosPacketQueue.size() > 0))
+            if (totalQosPacketsInTransit >= MAX_QOS_MSG_PENDING_PER_CLIENT || (qosPacketQueue.getByteSize() >= MAX_QOS_BYTES_PENDING_PER_CLIENT && qosPacketQueue.size() > 0))
             {
                 logger->logf(LOG_WARNING, "Dropping QoS message for client '%s', because its QoS buffers were full.", client_id.c_str());
                 return;
@@ -86,13 +145,7 @@ void Session::writePacket(const MqttPacket &packet, char max_qos, bool retain, u
             if (nextPacketId == 0)
                 nextPacketId++;
 
-            const uint16_t pid = nextPacketId;
-            copyPacket->setPacketId(pid);
-            QueuedQosPacket p;
-            p.packet = copyPacket;
-            p.id = pid;
-            qosPacketQueue.push_back(p);
-            qosQueueBytes += copyPacket->getTotalMemoryFootprint();
+            std::shared_ptr<MqttPacket> copyPacket = qosPacketQueue.queuePacket(packet, nextPacketId);
             locker.unlock();
 
             if (!clientDisconnected())
@@ -115,27 +168,7 @@ void Session::clearQosMessage(uint16_t packet_id)
 #endif
 
     std::lock_guard<std::mutex> locker(qosQueueMutex);
-
-    auto it = qosPacketQueue.begin();
-    auto end = qosPacketQueue.end();
-    while (it != end)
-    {
-        QueuedQosPacket &p = *it;
-        if (p.id == packet_id)
-        {
-            size_t mem = p.packet->getTotalMemoryFootprint();
-            qosQueueBytes -= mem;
-            assert(qosQueueBytes >= 0);
-            if (qosQueueBytes < 0) // Should not happen, but correcting a hypothetical bug is fine for this purpose.
-                qosQueueBytes = 0;
-
-            qosPacketQueue.erase(it);
-
-            break;
-        }
-
-        it++;
-    }
+    qosPacketQueue.erase(packet_id);
 }
 
 // [MQTT-4.4.0-1]: "When a Client reconnects with CleanSession set to 0, both the Client and Server MUST re-send any
@@ -152,10 +185,10 @@ uint64_t Session::sendPendingQosMessages()
     {
         std::shared_ptr<Client> c = makeSharedClient();
         std::lock_guard<std::mutex> locker(qosQueueMutex);
-        for (QueuedQosPacket &qosMessage : qosPacketQueue)
+        for (const std::shared_ptr<MqttPacket> &qosMessage : qosPacketQueue)
         {
-            c->writeMqttPacketAndBlameThisClient(*qosMessage.packet.get(), qosMessage.packet->getQos());
-            qosMessage.packet->setDuplicate(); // Any dealings with this packet from here will be a duplicate.
+            c->writeMqttPacketAndBlameThisClient(*qosMessage.get(), qosMessage->getQos());
+            qosMessage->setDuplicate(); // Any dealings with this packet from here will be a duplicate.
             count++;
         }
 
diff --git a/session.h b/session.h
index c0faad3..ec2770e 100644
--- a/session.h
+++ b/session.h
@@ -25,38 +25,46 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 
 #include "forward_declarations.h"
 #include "logger.h"
+#include "sessionsandsubscriptionsdb.h"
+#include "qospacketqueue.h"
 
 // TODO make settings. But, num of packets can't exceed 65536, because the counter is 16 bit.
 #define MAX_QOS_MSG_PENDING_PER_CLIENT 32
 #define MAX_QOS_BYTES_PENDING_PER_CLIENT 4096
 
-struct QueuedQosPacket
-{
-    uint16_t id;
-    std::shared_ptr<MqttPacket> packet;
-};
-
 class Session
 {
+#ifdef TESTING
+    friend class MainTests;
+#endif
+
+    friend class SessionsAndSubscriptionsDB;
+
     std::weak_ptr<Client> client;
-    std::shared_ptr<ThreadData> thread;
     std::string client_id;
     std::string username;
-    std::list<QueuedQosPacket> qosPacketQueue; // Using list because it's easiest to maintain order [MQTT-4.6.0-6]
+    QoSPacketQueue qosPacketQueue;
     std::set<uint16_t> incomingQoS2MessageIds;
     std::set<uint16_t> outgoingQoS2MessageIds;
     std::mutex qosQueueMutex;
     uint16_t nextPacketId = 0;
-    ssize_t qosQueueBytes = 0;
-    std::chrono::time_point<std::chrono::steady_clock> lastTouched;
+    std::chrono::time_point<std::chrono::steady_clock> lastTouched = std::chrono::steady_clock::now();
     Logger *logger = Logger::getInstance();
+    int64_t getSessionRelativeAgeInMs() const;
+    void setSessionTouch(int64_t ageInMs);
 
+    Session(const Session &other);
 public:
     Session();
-    Session(const Session &other) = delete;
+
     Session(Session &&other) = delete;
     ~Session();
 
+    static int64_t getProgramStartedAtUnixTimestamp();
+    static void setProgramStartedAtUnixTimestamp(const int64_t unix_timestamp);
+
+    std::unique_ptr<Session> getCopy() const;
+
     const std::string &getClientId() const { return client_id; }
     bool clientDisconnected() const;
     std::shared_ptr<Client> makeSharedClient() const;
@@ -74,7 +82,6 @@ public:
 
     void addOutgoingQoS2MessageId(uint16_t packet_id);
     void removeOutgoingQoS2MessageId(u_int16_t packet_id);
-
 };
 
 #endif // SESSION_H
diff --git a/sessionsandsubscriptionsdb.cpp b/sessionsandsubscriptionsdb.cpp
new file mode 100644
index 0000000..b9b2bf5
--- /dev/null
+++ b/sessionsandsubscriptionsdb.cpp
@@ -0,0 +1,299 @@
+/*
+This file is part of FlashMQ (https://www.flashmq.org)
+Copyright (C) 2021 Wiebe Cazemier
+
+FlashMQ is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, version 3.
+
+FlashMQ is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public
+License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#include "sessionsandsubscriptionsdb.h"
+#include "mqttpacket.h"
+
+#include "cassert"
+
+SubscriptionForSerializing::SubscriptionForSerializing(const std::string &clientId, char qos) :
+    clientId(clientId),
+    qos(qos)
+{
+
+}
+
+SubscriptionForSerializing::SubscriptionForSerializing(const std::string &&clientId, char qos) :
+    clientId(clientId),
+    qos(qos)
+{
+
+}
+
+SessionsAndSubscriptionsDB::SessionsAndSubscriptionsDB(const std::string &filePath) : PersistenceFile(filePath)
+{
+
+}
+
+void SessionsAndSubscriptionsDB::openWrite()
+{
+    PersistenceFile::openWrite(MAGIC_STRING_SESSION_FILE_V1);
+}
+
+void SessionsAndSubscriptionsDB::openRead()
+{
+    PersistenceFile::openRead();
+
+    if (detectedVersionString == MAGIC_STRING_SESSION_FILE_V1)
+        readVersion = ReadVersion::v1;
+    else
+        throw std::runtime_error("Unknown file version.");
+}
+
+SessionsAndSubscriptionsResult SessionsAndSubscriptionsDB::readDataV1()
+{
+    SessionsAndSubscriptionsResult result;
+
+    while (!feof(f))
+    {
+        bool eofFound = false;
+
+        const int64_t programStartAge = readInt64(eofFound);
+        if (eofFound)
+            continue;
+
+        logger->logf(LOG_DEBUG, "Setting first app start time to timestamp %ld", programStartAge);
+        Session::setProgramStartedAtUnixTimestamp(programStartAge);
+
+        const uint32_t nrOfSessions = readUint32(eofFound);
+
+        if (eofFound)
+            continue;
+
+        std::vector<char> reserved(RESERVED_SPACE_SESSIONS_DB_V1);
+
+        for (uint32_t i = 0; i < nrOfSessions; i++)
+        {
+            readCheck(buf.data(), 1, RESERVED_SPACE_SESSIONS_DB_V1, f);
+
+            uint32_t usernameLength = readUint32(eofFound);
+            readCheck(buf.data(), 1, usernameLength, f);
+            std::string username(buf.data(), usernameLength);
+
+            uint32_t clientIdLength = readUint32(eofFound);
+            readCheck(buf.data(), 1, clientIdLength, f);
+            std::string clientId(buf.data(), clientIdLength);
+
+            std::shared_ptr<Session> ses(new Session());
+            result.sessions.push_back(ses);
+            ses->username = username;
+            ses->client_id = clientId;
+
+            logger->logf(LOG_DEBUG, "Loading session '%s'.", ses->getClientId().c_str());
+
+            const uint32_t nrOfQueuedQoSPackets = readUint32(eofFound);
+            for (uint32_t i = 0; i < nrOfQueuedQoSPackets; i++)
+            {
+                const uint16_t id = readUint16(eofFound);
+                const uint32_t topicSize = readUint32(eofFound);
+                const uint32_t payloadSize = readUint32(eofFound);
+
+                assert(id > 0);
+
+                readCheck(buf.data(), 1, 1, f);
+                const unsigned char qos = buf[0];
+
+                readCheck(buf.data(), 1, topicSize, f);
+                const std::string topic(buf.data(), topicSize);
+
+                makeSureBufSize(payloadSize);
+                readCheck(buf.data(), 1, payloadSize, f);
+                const std::string payload(buf.data(), payloadSize);
+
+                Publish pub(topic, payload, qos);
+                logger->logf(LOG_DEBUG, "Loaded QoS %d message for topic '%s'.", pub.qos, pub.topic.c_str());
+                ses->qosPacketQueue.queuePacket(pub, id);
+            }
+
+            const uint32_t nrOfIncomingPacketIds = readUint32(eofFound);
+            for (uint32_t i = 0; i < nrOfIncomingPacketIds; i++)
+            {
+                uint16_t id = readUint16(eofFound);
+                assert(id > 0);
+                logger->logf(LOG_DEBUG, "Loaded incomming QoS2 message id %d.", id);
+                ses->incomingQoS2MessageIds.insert(id);
+            }
+
+            const uint32_t nrOfOutgoingPacketIds = readUint32(eofFound);
+            for (uint32_t i = 0; i < nrOfOutgoingPacketIds; i++)
+            {
+                uint16_t id = readUint16(eofFound);
+                assert(id > 0);
+                logger->logf(LOG_DEBUG, "Loaded outgoing QoS2 message id %d.", id);
+                ses->outgoingQoS2MessageIds.insert(id);
+            }
+
+            const uint16_t nextPacketId = readUint16(eofFound);
+            logger->logf(LOG_DEBUG, "Loaded next packetid %d.", ses->nextPacketId);
+            ses->nextPacketId = nextPacketId;
+
+            int64_t sessionAge = readInt64(eofFound);
+            logger->logf(LOG_DEBUG, "Loaded session age: %ld ms.", sessionAge);
+            ses->setSessionTouch(sessionAge);
+        }
+
+        const uint32_t nrOfSubscriptions = readUint32(eofFound);
+        for (uint32_t i = 0; i < nrOfSubscriptions; i++)
+        {
+            const uint32_t topicLength = readUint32(eofFound);
+            readCheck(buf.data(), 1, topicLength, f);
+            const std::string topic(buf.data(), topicLength);
+
+            logger->logf(LOG_DEBUG, "Loading subscriptions to topic '%s'.", topic.c_str());
+
+            const uint32_t nrOfClientIds = readUint32(eofFound);
+
+            for (uint32_t i = 0; i < nrOfClientIds; i++)
+            {
+                const uint32_t clientIdLength = readUint32(eofFound);
+                readCheck(buf.data(), 1, clientIdLength, f);
+                const std::string clientId(buf.data(), clientIdLength);
+
+                char qos;
+                readCheck(&qos, 1, 1, f);
+
+                logger->logf(LOG_DEBUG, "Saving session '%s' subscription to '%s' QoS %d.", clientId.c_str(), topic.c_str(), qos);
+
+                SubscriptionForSerializing sub(std::move(clientId), qos);
+                result.subscriptions[topic].push_back(std::move(sub));
+            }
+
+        }
+    }
+
+    return result;
+}
+
+void SessionsAndSubscriptionsDB::writeRowHeader()
+{
+
+}
+
+void SessionsAndSubscriptionsDB::saveData(const std::list<std::unique_ptr<Session>> &sessions, const std::unordered_map<std::string, std::list<SubscriptionForSerializing>> &subscriptions)
+{
+    if (!f)
+        return;
+
+    char reserved[RESERVED_SPACE_SESSIONS_DB_V1];
+    std::memset(reserved, 0, RESERVED_SPACE_SESSIONS_DB_V1);
+
+    const int64_t start_stamp = Session::getProgramStartedAtUnixTimestamp();
+    logger->logf(LOG_DEBUG, "Saving program first start time stamp as %ld", start_stamp);
+    writeInt64(start_stamp);
+
+    writeUint32(sessions.size());
+
+    for (const std::unique_ptr<Session> &ses : sessions)
+    {
+        logger->logf(LOG_DEBUG, "Saving session '%s'.", ses->getClientId().c_str());
+
+        writeRowHeader();
+
+        writeCheck(reserved, 1, RESERVED_SPACE_SESSIONS_DB_V1, f);
+
+        writeUint32(ses->username.length());
+        writeCheck(ses->username.c_str(), 1, ses->username.length(), f);
+
+        writeUint32(ses->client_id.length());
+        writeCheck(ses->client_id.c_str(), 1, ses->client_id.length(), f);
+
+        const size_t qosPacketsExpected = ses->qosPacketQueue.size();
+        size_t qosPacketsCounted = 0;
+        writeUint32(qosPacketsExpected);
+
+        for (const std::shared_ptr<MqttPacket> &p: ses->qosPacketQueue)
+        {
+            logger->logf(LOG_DEBUG, "Saving QoS %d message for topic '%s'.", p->getQos(), p->getTopic().c_str());
+
+            qosPacketsCounted++;
+
+            writeUint16(p->getPacketId());
+
+            writeUint32(p->getTopic().length());
+            std::string payload = p->getPayloadCopy();
+            writeUint32(payload.size());
+
+            const char qos = p->getQos();
+            writeCheck(&qos, 1, 1, f);
+
+            writeCheck(p->getTopic().c_str(), 1, p->getTopic().length(), f);
+            writeCheck(payload.c_str(), 1, payload.length(), f);
+        }
+
+        assert(qosPacketsExpected == qosPacketsCounted);
+
+        writeUint32(ses->incomingQoS2MessageIds.size());
+        for (uint16_t id : ses->incomingQoS2MessageIds)
+        {
+            logger->logf(LOG_DEBUG, "Writing incomming QoS2 message id %d.", id);
+            writeUint16(id);
+        }
+
+        writeUint32(ses->outgoingQoS2MessageIds.size());
+        for (uint16_t id : ses->outgoingQoS2MessageIds)
+        {
+            logger->logf(LOG_DEBUG, "Writing outgoing QoS2 message id %d.", id);
+            writeUint16(id);
+        }
+
+        logger->logf(LOG_DEBUG, "Writing next packetid %d.", ses->nextPacketId);
+        writeUint16(ses->nextPacketId);
+
+        const int64_t sInMs = ses->getSessionRelativeAgeInMs();
+        logger->logf(LOG_DEBUG, "Writing session age: %ld ms.", sInMs);
+        writeInt64(sInMs);
+    }
+
+    writeUint32(subscriptions.size());
+
+    for (auto &pair : subscriptions)
+    {
+        const std::string &topic = pair.first;
+        const std::list<SubscriptionForSerializing> &subscriptions = pair.second;
+
+        logger->logf(LOG_DEBUG, "Writing subscriptions to topic '%s'.", topic.c_str());
+
+        writeUint32(topic.size());
+        writeCheck(topic.c_str(), 1, topic.size(), f);
+
+        writeUint32(subscriptions.size());
+
+        for (const SubscriptionForSerializing &subscription : subscriptions)
+        {
+            logger->logf(LOG_DEBUG, "Saving session '%s' subscription to '%s' QoS %d.", subscription.clientId.c_str(), topic.c_str(), subscription.qos);
+
+            writeUint32(subscription.clientId.size());
+            writeCheck(subscription.clientId.c_str(), 1, subscription.clientId.size(), f);
+            writeCheck(&subscription.qos, 1, 1, f);
+        }
+    }
+
+    fflush(f);
+}
+
+SessionsAndSubscriptionsResult SessionsAndSubscriptionsDB::readData()
+{
+    SessionsAndSubscriptionsResult defaultResult;
+
+    if (!f)
+        return defaultResult;
+
+    if (readVersion == ReadVersion::v1)
+        return readDataV1();
+
+    return defaultResult;
+}
diff --git a/sessionsandsubscriptionsdb.h b/sessionsandsubscriptionsdb.h
new file mode 100644
index 0000000..15a242e
--- /dev/null
+++ b/sessionsandsubscriptionsdb.h
@@ -0,0 +1,71 @@
+/*
+This file is part of FlashMQ (https://www.flashmq.org)
+Copyright (C) 2021 Wiebe Cazemier
+
+FlashMQ is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as
+published by the Free Software Foundation, version 3.
+
+FlashMQ is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public
+License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#ifndef SESSIONSANDSUBSCRIPTIONSDB_H
+#define SESSIONSANDSUBSCRIPTIONSDB_H
+
+#include <list>
+#include <memory>
+
+#include "persistencefile.h"
+#include "session.h"
+
+#define MAGIC_STRING_SESSION_FILE_V1 "FlashMQRetainedDBv1"
+#define RESERVED_SPACE_SESSIONS_DB_V1 32
+
+/**
+ * @brief The SubscriptionForSerializing struct contains the fields we're interested in when saving a subscription.
+ */
+struct SubscriptionForSerializing
+{
+    const std::string clientId;
+    const char qos = 0;
+
+    SubscriptionForSerializing(const std::string &clientId, char qos);
+    SubscriptionForSerializing(const std::string &&clientId, char qos);
+};
+
+struct SessionsAndSubscriptionsResult
+{
+    std::list<std::shared_ptr<Session>> sessions;
+    std::unordered_map<std::string, std::list<SubscriptionForSerializing>> subscriptions;
+};
+
+
+class SessionsAndSubscriptionsDB : public PersistenceFile
+{
+    enum class ReadVersion
+    {
+        unknown,
+        v1
+    };
+
+    ReadVersion readVersion = ReadVersion::unknown;
+
+    SessionsAndSubscriptionsResult readDataV1();
+    void writeRowHeader();
+public:
+    SessionsAndSubscriptionsDB(const std::string &filePath);
+
+    void openWrite();
+    void openRead();
+
+    void saveData(const std::list<std::unique_ptr<Session>> &sessions, const std::unordered_map<std::string, std::list<SubscriptionForSerializing>> &subscriptions);
+    SessionsAndSubscriptionsResult readData();
+};
+
+#endif // SESSIONSANDSUBSCRIPTIONSDB_H
diff --git a/settings.cpp b/settings.cpp
index 49b175e..629ea52 100644
--- a/settings.cpp
+++ b/settings.cpp
@@ -16,7 +16,7 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 */
 
 #include "settings.h"
-
+#include "utils.h"
 
 AuthOptCompatWrap &Settings::getAuthOptsCompat()
 {
@@ -27,3 +27,21 @@ std::unordered_map<std::string, std::string> &Settings::getFlashmqAuthPluginOpts
 {
     return this->flashmqAuthPluginOpts;
 }
+
+std::string Settings::getRetainedMessagesDBFile() const
+{
+    if (storageDir.empty())
+        return "";
+
+    std::string path = formatString("%s/%s", storageDir.c_str(), "retained.db");
+    return path;
+}
+
+std::string Settings::getSessionsDBFile() const
+{
+    if (storageDir.empty())
+        return "";
+
+    std::string path = formatString("%s/%s", storageDir.c_str(), "sessions.db");
+    return path;
+}
diff --git a/settings.h b/settings.h
index d79e486..ecc627a 100644
--- a/settings.h
+++ b/settings.h
@@ -53,10 +53,14 @@ public:
     int rlimitNoFile = 1000000;
     uint64_t expireSessionsAfterSeconds = 1209600;
     int authPluginTimerPeriod = 60;
+    std::string storageDir;
     std::list<std::shared_ptr<Listener>> listeners; // Default one is created later, when none are defined.
 
     AuthOptCompatWrap &getAuthOptsCompat();
     std::unordered_map<std::string, std::string> &getFlashmqAuthPluginOpts();
+
+    std::string getRetainedMessagesDBFile() const;
+    std::string getSessionsDBFile() const;
 };
 
 #endif // SETTINGS_H
diff --git a/subscriptionstore.cpp b/subscriptionstore.cpp
index e216228..6628ebb 100644
--- a/subscriptionstore.cpp
+++ b/subscriptionstore.cpp
@@ -20,7 +20,7 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 #include "cassert"
 
 #include "rwlockguard.h"
-
+#include "retainedmessagesdb.h"
 
 SubscriptionNode::SubscriptionNode(const std::string &subtopic) :
     subtopic(subtopic)
@@ -33,6 +33,11 @@ std::vector<Subscription> &SubscriptionNode::getSubscribers()
     return subscribers;
 }
 
+const std::string &SubscriptionNode::getSubtopic() const
+{
+    return subtopic;
+}
+
 void SubscriptionNode::addSubscriber(const std::shared_ptr<Session> &subscriber, char qos)
 {
     Subscription sub;
@@ -84,15 +89,20 @@ SubscriptionStore::SubscriptionStore() :
 
 }
 
-void SubscriptionStore::addSubscription(std::shared_ptr<Client> &client, const std::string &topic, const std::vector<std::string> &subtopics, char qos)
+/**
+ * @brief SubscriptionStore::getDeepestNode gets the node in the tree walking the path of 'the/subscription/topic/path', making new nodes as required.
+ * @param topic
+ * @param subtopics
+ * @return
+ *
+ * caller is responsible for locking.
+ */
+SubscriptionNode *SubscriptionStore::getDeepestNode(const std::string &topic, const std::vector<std::string> &subtopics)
 {
     SubscriptionNode *deepestNode = &root;
     if (topic.length() > 0 && topic[0] == '$')
         deepestNode = &rootDollar;
 
-    RWLockGuard lock_guard(&subscriptionsRwlock);
-    lock_guard.wrlock();
-
     for(const std::string &subtopic : subtopics)
     {
         std::unique_ptr<SubscriptionNode> *selectedChildren = nullptr;
@@ -114,6 +124,15 @@ void SubscriptionStore::addSubscription(std::shared_ptr<Client> &client, const s
     }
 
     assert(deepestNode);
+    return deepestNode;
+}
+
+void SubscriptionStore::addSubscription(std::shared_ptr<Client> &client, const std::string &topic, const std::vector<std::string> &subtopics, char qos)
+{
+    RWLockGuard lock_guard(&subscriptionsRwlock);
+    lock_guard.wrlock();
+
+    SubscriptionNode *deepestNode = getDeepestNode(topic, subtopics);
 
     if (deepestNode)
     {
@@ -494,6 +513,183 @@ int64_t SubscriptionStore::getRetainedMessageCount() const
     return retainedMessageCount;
 }
 
+void SubscriptionStore::getRetainedMessages(RetainedMessageNode *this_node, std::vector<RetainedMessage> &outputList) const
+{
+    for(const RetainedMessage &rm : this_node->retainedMessages)
+    {
+        outputList.push_back(rm);
+    }
+
+    for(auto &pair : this_node->children)
+    {
+        const std::unique_ptr<RetainedMessageNode> &child = pair.second;
+        getRetainedMessages(child.get(), outputList);
+    }
+}
+
+/**
+ * @brief SubscriptionStore::getSubscriptions
+ * @param this_node
+ * @param composedTopic
+ * @param root bool. Every subtopic is concatenated with a '/', but not the first topic to 'root'. The root is a bit weird, virtual, so it needs different treatment.
+ * @param outputList
+ */
+void SubscriptionStore::getSubscriptions(SubscriptionNode *this_node, const std::string &composedTopic, bool root,
+                                         std::unordered_map<std::string, std::list<SubscriptionForSerializing>> &outputList) const
+{
+    for (const Subscription &node : this_node->getSubscribers())
+    {
+        if (!node.sessionGone())
+        {
+            SubscriptionForSerializing sub(node.session.lock()->getClientId(), node.qos);
+            outputList[composedTopic].push_back(sub);
+        }
+    }
+
+    for (auto &pair : this_node->children)
+    {
+        SubscriptionNode *node = pair.second.get();
+        const std::string topicAtNextLevel = root ? pair.first : composedTopic + "/" + pair.first;
+        getSubscriptions(node, topicAtNextLevel, false, outputList);
+    }
+
+    if (this_node->childrenPlus)
+    {
+        const std::string topicAtNextLevel = root ? "+" : composedTopic + "/+";
+        getSubscriptions(this_node->childrenPlus.get(), topicAtNextLevel, false, outputList);
+    }
+
+    if (this_node->childrenPound)
+    {
+        const std::string topicAtNextLevel = root ? "#" : composedTopic + "/#";
+        getSubscriptions(this_node->childrenPound.get(), topicAtNextLevel, false, outputList);
+    }
+}
+
+void SubscriptionStore::saveRetainedMessages(const std::string &filePath)
+{
+    logger->logf(LOG_INFO, "Saving retained messages to '%s'", filePath.c_str());
+
+    std::vector<RetainedMessage> result;
+    result.reserve(retainedMessageCount);
+
+    // Create the list of messages under lock, and unlock right after.
+    RWLockGuard locker(&retainedMessagesRwlock);
+    locker.rdlock();
+    getRetainedMessages(&retainedMessagesRoot, result);
+    locker.unlock();
+
+    logger->logf(LOG_DEBUG, "Collected %ld retained messages to save.", result.size());
+
+    // Then do the IO without locking the threads.
+    RetainedMessagesDB db(filePath);
+    db.openWrite();
+    db.saveData(result);
+}
+
+void SubscriptionStore::loadRetainedMessages(const std::string &filePath)
+{
+    try
+    {
+        logger->logf(LOG_INFO, "Loading '%s'", filePath.c_str());
+
+        RetainedMessagesDB db(filePath);
+        db.openRead();
+        std::list<RetainedMessage> messages = db.readData();
+
+        RWLockGuard locker(&retainedMessagesRwlock);
+        locker.wrlock();
+
+        std::vector<std::string> subtopics;
+        for (const RetainedMessage &rm : messages)
+        {
+            splitTopic(rm.topic, subtopics);
+            setRetainedMessage(rm.topic, subtopics, rm.payload, rm.qos);
+        }
+    }
+    catch (PersistenceFileCantBeOpened &ex)
+    {
+        logger->logf(LOG_WARNING, "File '%s' is not there (yet)", filePath.c_str());
+    }
+}
+
+void SubscriptionStore::saveSessionsAndSubscriptions(const std::string &filePath)
+{
+    logger->logf(LOG_INFO, "Saving sessions and subscriptions to '%s'", filePath.c_str());
+
+    RWLockGuard lock_guard(&subscriptionsRwlock);
+    lock_guard.wrlock();
+
+    // First copy the sessions...
+
+    std::list<std::unique_ptr<Session>> sessionCopies;
+
+    for (const auto &pair : sessionsByIdConst)
+    {
+        const Session &org = *pair.second.get();
+        sessionCopies.push_back(org.getCopy());
+    }
+
+    std::unordered_map<std::string, std::list<SubscriptionForSerializing>> subscriptionCopies;
+    getSubscriptions(&root, "", true, subscriptionCopies);
+
+    lock_guard.unlock();
+
+    // Then write the copies to disk, after having released the lock
+
+    logger->logf(LOG_DEBUG, "Collected %ld sessions and %ld subscriptions to save.", sessionCopies.size(), subscriptionCopies.size());
+
+    SessionsAndSubscriptionsDB db(filePath);
+    db.openWrite();
+    db.saveData(sessionCopies, subscriptionCopies);
+}
+
+void SubscriptionStore::loadSessionsAndSubscriptions(const std::string &filePath)
+{
+    try
+    {
+        logger->logf(LOG_INFO, "Loading '%s'", filePath.c_str());
+
+        SessionsAndSubscriptionsDB db(filePath);
+        db.openRead();
+        SessionsAndSubscriptionsResult loadedData = db.readData();
+
+        RWLockGuard locker(&subscriptionsRwlock);
+        locker.wrlock();
+
+        for (std::shared_ptr<Session> &session : loadedData.sessions)
+        {
+            sessionsById[session->getClientId()] = session;
+        }
+
+        std::vector<std::string> subtopics;
+
+        for (auto &pair : loadedData.subscriptions)
+        {
+            const std::string &topic = pair.first;
+            const std::list<SubscriptionForSerializing> &subs = pair.second;
+
+            for (const SubscriptionForSerializing &sub : subs)
+            {
+                splitTopic(topic, subtopics);
+                SubscriptionNode *subscriptionNode = getDeepestNode(topic, subtopics);
+
+                auto session_it = sessionsByIdConst.find(sub.clientId);
+                if (session_it != sessionsByIdConst.end())
+                {
+                    const std::shared_ptr<Session> &ses = session_it->second;
+                    subscriptionNode->addSubscriber(ses, sub.qos);
+                }
+
+            }
+        }
+    }
+    catch (PersistenceFileCantBeOpened &ex)
+    {
+        logger->logf(LOG_WARNING, "File '%s' is not there (yet)", filePath.c_str());
+    }
+}
+
 // QoS is not used in the comparision. This means you upgrade your QoS by subscribing again. The
 // specs don't specify what to do there.
 bool Subscription::operator==(const Subscription &rhs) const
diff --git a/subscriptionstore.h b/subscriptionstore.h
index 35416b4..93e3dc6 100644
--- a/subscriptionstore.h
+++ b/subscriptionstore.h
@@ -53,6 +53,7 @@ public:
     SubscriptionNode(SubscriptionNode &&node) = delete;
 
     std::vector<Subscription> &getSubscribers();
+    const std::string &getSubtopic() const;
     void addSubscriber(const std::shared_ptr<Session> &subscriber, char qos);
     void removeSubscriber(const std::shared_ptr<Session> &subscriber);
     std::unordered_map<std::string, std::unique_ptr<SubscriptionNode>> children;
@@ -77,6 +78,10 @@ class RetainedMessageNode
 
 class SubscriptionStore
 {
+#ifdef TESTING
+    friend class MainTests;
+#endif
+
     SubscriptionNode root;
     SubscriptionNode rootDollar;
     pthread_rwlock_t subscriptionsRwlock = PTHREAD_RWLOCK_INITIALIZER;
@@ -93,7 +98,11 @@ class SubscriptionStore
     void publishNonRecursively(const MqttPacket &packet, const std::vector<Subscription> &subscribers, uint64_t &count) const;
     void publishRecursively(std::vector<std::string>::const_iterator cur_subtopic_it, std::vector<std::string>::const_iterator end,
                             SubscriptionNode *this_node, const MqttPacket &packet, uint64_t &count) const;
+    void getRetainedMessages(RetainedMessageNode *this_node, std::vector<RetainedMessage> &outputList) const;
+    void getSubscriptions(SubscriptionNode *this_node, const std::string &composedTopic, bool root,
+                          std::unordered_map<std::string, std::list<SubscriptionForSerializing>> &outputList) const;
 
+    SubscriptionNode *getDeepestNode(const std::string &topic, const std::vector<std::string> &subtopics);
 public:
     SubscriptionStore();
 
@@ -103,7 +112,6 @@ public:
     bool sessionPresent(const std::string &clientid);
 
     void queuePacketAtSubscribers(const std::vector<std::string> &subtopics, const MqttPacket &packet, bool dollar = false);
-    uint64_t giveClientRetainedMessages(const std::shared_ptr<Session> &ses, const std::string &subscribe_topic, char max_qos);
     void giveClientRetainedMessagesRecursively(std::vector<std::string>::const_iterator cur_subtopic_it, std::vector<std::string>::const_iterator end,
                                                RetainedMessageNode *this_node, char max_qos, const std::shared_ptr<Session> &ses,
                                                bool poundMode, uint64_t &count) const;
@@ -114,6 +122,12 @@ public:
     void removeExpiredSessionsClients(int expireSessionsAfterSeconds);
 
     int64_t getRetainedMessageCount() const;
+
+    void saveRetainedMessages(const std::string &filePath);
+    void loadRetainedMessages(const std::string &filePath);
+
+    void saveSessionsAndSubscriptions(const std::string &filePath);
+    void loadSessionsAndSubscriptions(const std::string &filePath);
 };
 
 #endif // SUBSCRIPTIONSTORE_H
diff --git a/utils.cpp b/utils.cpp
index 79f78fc..575d328 100644
--- a/utils.cpp
+++ b/utils.cpp
@@ -289,6 +289,14 @@ void trim(std::string &s)
     rtrim(s);
 }
 
+std::string &rtrim(std::string &s, unsigned char c)
+{
+    s.erase(std::find_if(s.rbegin(), s.rend(), [=](unsigned char ch) {
+        return (c != ch);
+    }).base(), s.end());
+    return s;
+}
+
 bool startsWith(const std::string &s, const std::string &needle)
 {
     return s.find(needle) == 0;
diff --git a/utils.h b/utils.h
index 0a83fa3..70c8352 100644
--- a/utils.h
+++ b/utils.h
@@ -28,6 +28,8 @@ License along with FlashMQ. If not, see <https://www.gnu.org/licenses/>.
 #include <openssl/evp.h>
 #include <memory>
 #include <arpa/inet.h>
+#include "unistd.h"
+#include "sys/stat.h"
 
 #include "cirbuf.h"
 #include "bindaddr.h"
@@ -64,6 +66,7 @@ void ltrim(std::string &s);
 void rtrim(std::string &s);
 void trim(std::string &s);
 bool startsWith(const std::string &s, const std::string &needle);
+std::string &rtrim(std::string &s, unsigned char c);
 
 std::string getSecureRandomString(const ssize_t len);
 std::string str_tolower(std::string s);
@@ -92,5 +95,32 @@ ssize_t getFileSize(const std::string &path);
 
 std::string sockaddrToString(struct sockaddr *addr);
 
+template<typename ex> void checkWritableDir(const std::string &path)
+{
+    if (path.empty())
+        throw ex("Dir path to check is an empty string.");
+
+    if (access(path.c_str(), W_OK) != 0)
+    {
+        std::string msg = formatString("Path '%s' is not there or not writable", path.c_str());
+        throw ex(msg);
+    }
+
+    struct stat statbuf;
+    memset(&statbuf, 0, sizeof(struct stat));
+    if (stat(path.c_str(), &statbuf) < 0)
+    {
+        // We checked for W_OK above, so this shouldn't happen.
+        std::string msg = formatString("Error getting information about '%s'.", path.c_str());
+        throw ex(msg);
+    }
+
+    if (!S_ISDIR(statbuf.st_mode))
+    {
+        std::string msg = formatString("Path '%s' is not a directory.", path.c_str());
+        throw ex(msg);
+    }
+}
+
 
 #endif // UTILS_H