Write tutorial for core functionality

Explain core commands, pubsub, and reply types.

Write tutorial for core functionality
Explain core commands, pubsub, and reply types.
Hayk Martirosyan
1 parent e07c208e
Showing 2 changed files with 195 additions and 27 deletions
README.md
examples/basic.cpp
@@ -40,6 +40,198 @@ Local Redis server, TCP connection:
 Results are comparable to that of an average laptop. On a high-end machine,
 `speed_test_async_multi` usually tops 1,000,000 commands/s.
+## Tutorial
+This section introduces the main features of redox. Look in the `examples/` for more inspiration.
+
+#### Hello world
+Here is the simplest possible redox program:
+
+    #include <iostream>
+    #include <redox.hpp>
+    
+    using namespace std;
+    using namespace redox;
+    
+    int main(int argc, char* argv[]) {
+    
+      Redox rdx = {"localhost", 6379};
+      if(!rdx.connect()) return 1;
+    
+      rdx.set("hello", "world!");
+      cout << "Hello, " << rdx.get("hello") << endl;
+    
+      rdx.disconnect();
+      return 0;
+    }
+
+Compile and run:
+
+    $ g++ hello.cpp -o hello -std=c++11 -lredox -lev -lhiredis
+    $ ./hello
+    Hello, world!
+
+This example is synchronous, in the sense that the commands don't return until
+a reply is received from the server. 
+
+#### Asynchronous commands
+In a high-performance application, we don't want to wait for a reply, but instead
+do other work.
+The `command` method accepts a Redis command and a callback to be invoked when a reply is received.
+    
+    rdx.command<string>("GET hello", [](Command<string>& c) {
+      if(c.ok()) {
+        cout << "Hello, async " << c.reply() << endl;
+      } else {
+        cerr << "Command has error code " << c.status() << endl;
+      }
+    });
+
+This statement tells redox to run the command `GET hello`. The `<string>` template
+parameter means that we want the reply to be put into a string and that we expect
+the server to respond with something that can be put into a string. The full list
+of reply types is listed in this document and covers convenient access to anything
+returned from the Redis protocol.
+
+The second argument is a callback function that accepts a reference to a Command object
+of the requested reply type. The Command object contains the reply and any error
+information. If `c.ok()` is true, the expected reply is accessed from
+`c.reply()` (a string in this case). If `c.ok()` is false, then the error
+code is given by `c.status()`, which can report an error or nil reply, a reply of
+the wrong type, a send error, etc. The callback is guaranteed to be invoked
+exactly once, and the memory for the Command object is freed automatically once
+the callback returns.
+
+Here is a simple example of running `GET hello` asynchronously ten times:
+
+    Redox rdx; // Localhost by default
+    if(!rdx.connect()) return 1; // Block until connected
+
+    auto got_reply = [](Command<string>& c) {
+      if(!c.ok()) return;
+      cout << c.cmd() << ": " << c.reply() << endl;
+    };
+
+    for(int i = 0; i < 10; i++) rdx.command<string>("GET hello", got_reply);
+
+    // Do useful work
+    this_thread::sleep_for(chrono::milliseconds(10));
+
+    rdx.disconnect(); // Block until disconnected
+
+The `.command()` method returns immediately, so this program doesn't wait for a reply
+from the server - it just pauses for ten milliseconds and then shuts down. If we want to
+shut down after we get all replies, we could do something like this:
+
+    Redox rdx;
+    if(!rdx.connect()) return 1;
+    
+    int total = 10; // Number of commands to run
+    atomic_int count(0); // Number of replies expected
+    auto got_reply = [&](Command<string>& c) {
+      count++;
+      if(c.ok()) cout << c.cmd() << " #" << count << ": " << c.reply() << endl;
+      if(count == total) rdx.stop(); // Signal to shut down
+    };
+    
+    for(int i = 0; i < total; i++) rdx.command<string>("GET hello", got_reply);
+    
+    // Do useful work
+    
+    rdx.wait(); // Block until shut down complete
+
+This example tracks of how how many replies are received and signals the Redox
+instance to stop once they all process. We use an `std::atomic_int` to be safe
+because the callback is invoked from a separate thread. The `stop()` method
+signals Redox to shut down its event loop and disconnect from Redis. The `wait()`
+method blocks until `stop()` has been called and everything is brought down.
+The `disconnect()` method used earlier is just a call to `stop()` and then a 
+call to `wait()`.
+
+#### Synchronous commands
+Redox implements synchronous commands by running asynchronous commands and waiting
+on them with condition variables. That way, we can reap the benefits of pipelining
+between synchronous commands in different threads. The `commandSync` method provides
+a similar API to `command`, but instead of a callback returns a Command object when
+a reply is received.
+
+    Command<string>& c = rdx.commandSync<string>("GET hello");
+    if(c.ok()) cout << c.cmd() << ": " << c.reply() << endl;
+    c.free();
+
+When using synchronous commands, the user is responsible for freeing the memory of
+the Command object by calling `c.free()`. The `c.cmd()` method just returns the
+command string (`GET hello` in this case).
+
+#### Looping and delayed commands
+We often want to run commands on regular invervals. Redox provides the `commandLoop`
+method to accomplish this. It is easier to use and more efficient than running individual
+commands in a loop, because it only creates a single Command object.
+`commandLoop` takes a command string, a callback, and an interval (in seconds)
+to repeat the command. It then runs the command on the given interval until the user
+calls `c.free()`.
+
+    Command<string>& cmd = rdx.commandLoop<string>("GET hello", [](Command<string>& c) {
+      if(c.ok()) cout << c.cmd() << ": " << c.reply() << endl;
+    }, 0.1);
+    
+    this_thread::sleep_for(chrono::seconds(1));
+    cmd.free();
+    rdx.disconnect();
+
+Finally, `commandDelayed` runs a command after a specified delay (in seconds). It does
+not return a command object, because the memory is automatically freed after the callback
+is invoked.
+
+    rdx.commandDelayed<string>("GET hello", [](Command<string>& c) {
+      if(c.ok()) cout << c.cmd() << ": " << c.reply() << endl;
+    }, 1);
+    this_thread::sleep_for(chrono::seconds(2));
+
+#### Convenience methods
+The four methods `command`, `commandSync`, `commandLoop`, and `commandDelayed` form
+the core of Redox's functionality. There are convenience methods provided that are
+simple wrappers over the core methods. Some examples of those are `.get()`, `.set()`,
+`.del()`, and `.publish()`. These methods are nice because they return simple values,
+and there are no Command objects or template parameters. However, they make strong
+assumptions about how to deal with errors (ignore or throw exceptions), and since
+their implementations are a few lines of code it is often easier to create custom
+convenience methods for your application.
+
+#### Publisher / Subscriber
+Redox provides an API for the pub/sub functionality of Redis. Publishing is done just like
+any other command using a Redox instance. There is a separate Subscriber class that
+receives messages and provides subscribe/unsubscribe and psubscribe/punsubscribe methods.
+
+    Redox rdx; Subscriber sub;
+    if(!rdx.connect() || !sub.connect()) return 1;
+    
+    sub.subscribe("hello", [](const string& topic, const string& msg) {
+      cout << topic << ": " << msg << endl;
+    });
+    
+    for(int i = 0; i < 10; i++) {
+      rdx.publish("hello", "this is a pubsub message");
+      this_thread::sleep_for(chrono::milliseconds(500));
+    }
+    
+    sub.disconnect(); rdx.disconnect();
+
+## Reply types
+These the available template parameters in redox and the Redis
+[return types](http://redis.io/topics/protocol) they can hold.
+If a given command returns an incompatible type you will get
+a `WRONG_TYPE` or `NIL_REPLY` status.
+
+ * `<redisReply*>`: All reply types, returns the hiredis struct directly
+ * `<char*>`: Simple Strings, Bulk Strings
+ * `<std::string>`: Simple Strings, Bulk Strings
+ * `<long long int>`: Integers
+ * `<int>`: Integers (careful about overflow, `long long int` recommended)
+ * `<std::nullptr_t>`: Null Bulk Strings, any other receiving a nil reply will get a NIL_REPLY status
+ * `<std::vector<std::string>>`: Arrays of Simple Strings or Bulk Strings (in received order)
+ * `<std::set<std::string>>`: Arrays of Simple Strings or Bulk Strings (in sorted order)
+ * `<std::unordered_set<std::string>>`: Arrays of Simple Strings or Bulk Strings (in no order)
+ 
 ## Installation
 Instructions provided are for Ubuntu, but all components are platform-independent.
@@ -72,27 +264,3 @@ then:
     cmake -Dtests=ON ..
     make test_redox
     ./test_redox
-
-## Tutorial
-Here is a hello world program for redox:
-
-    #include <iostream>
-    #include "redox.hpp"
-    
-    int main(int argc, char* argv[]) {
-    
-      redox::Redox rdx = {"localhost", 6379};
-      if(!rdx.connect()) return 1;
-    
-      rdx.set("hello", "world!");
-      std::cout << "Hello, " << rdx.get("hello") << std::endl;
-    
-      rdx.disconnect();
-      return 0;
-    }
-
-Compile and run:
-
-    $ g++ hello.cpp -o hello -std=c++11 -lredox -lev -lhiredis
-    $ ./hello
-    Hello, world!
@@ -3,15 +3,16 @@
 */
 #include <iostream>
-#include "redox.hpp"
+#include <redox.hpp>
 using namespace std;
 using redox::Redox;
 using redox::Command;
+using redox::Subscriber;
 int main(int argc, char* argv[]) {
-  Redox rdx = {"localhost", 6379, nullptr, cout, redox::log::Info}; // Initialize Redox
+  Redox rdx = {"localhost", 6379}; // Initialize Redox
   if(!rdx.connect()) return 1; // Start the event loop
@@ -22,6 +23,5 @@ int main(int argc, char* argv[]) {
   cout << "key = \"occupation\", value = \"" << rdx.get("occupation") << "\"" << endl;
-  rdx.disconnect();
   return 0;
 }