AMQP-CPP is a C++ library for communicating with a RabbitMQ message broker. The library can be used to parse incoming data from a RabbitMQ server, and to generate frames that can be sent to a RabbitMQ server.
Unlike all other AMQP libraries, this AMQP-CPP library does not make a connection to RabbitMQ by itself, nor does it create sockets and/or performs IO operations. As a user of this library, you first need to set up a socket connection to RabbitMQ by yourself, and implement a certain interface that you pass to the AMQP-CPP library and that the library will use for IO operations.
This architecture makes the library extremely flexible: it does not rely on operating system specific IO calls, and it can be easily integrated into any event loop. It is fully asynchronous and does not do any blocking (system) calls, so it can be used in high performance applications without the need for threads.
The AMQP-CPP library uses C++11 features, so if you intend use it, please make sure that your compiler is up-to-date and supports C++11.
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As we mentioned above, the library does not do any IO by itself, and you need to pass an object to the library that the library can use for that. So, before you can even start using the library, you first you need to create a class that extends from the ConnectionHandler base class. This is a class with a number of methods that are called by the library every time it wants to send out data, or when it needs to inform you that an error occured.
#include <amqpcpp.h>
class MyConnectionHandler : public AMQP::ConnectionHandler
{
/**
* Method that is called by the AMQP library every time it has data
* available that should be sent to RabbitMQ.
* @param connection pointer to the main connection object
* @param data memory buffer with the data that should be sent to RabbitMQ
* @param size size of the buffer
*/
virtual void onData(AMQP::Connection *connection, const char *data, size_t size)
{
// @todo
// Add your own implementation, for example by doing a call to the
// send() system call. But be aware that the send() call may not
// send all data at once, so you also need to take care of buffering
// the bytes that could not immediately be sent, and try to send
// them again when the socket becomes writable again
}
/**
* Method that is called by the AMQP library when the login attempt
* succeeded. After this method has been called, the connection is ready
* to use.
* @param connection The connection that can now be used
*/
virtual void onConnected(AMQP::Connection *connection)
{
// @todo
// add your own implementation, for example by creating a channel
// instance, and start publishing or consuming
}
/**
* Method that is called by the AMQP library when a fatal error occurs
* on the connection, for example because data received from RabbitMQ
* could not be recognized.
* @param connection The connection on which the error occured
* @param message A human readable error message
*/
virtual void onError(AMQP::Connection *connection, const char *message)
{
// @todo
// add your own implementation, for example by reporting the error
// to the user of your program, log the error, and destruct the
// connection object because it is no longer in a usable state
}
/**
* Method that is called when the connection was closed. This is the
* counter part of a call to Connection::close() and it confirms that the
* connection was correctly closed.
*
* @param connection The connection that was closed and that is now unusable
*/
virtual void onClosed(AMQP::Connection *connection) {}
};After you've implemented the ConnectionHandler class, you can start using the library by creating a Connection object, and one or more Channel objects:
// create an instance of your own connection handler
MyConnectionHandler myHandler;
// create a AMQP connection object
AMQP::Connection connection(&myHandler, Login("guest","guest"), "/");
// and create a channel
AMQP::Channel channel(&connection);
// use the channel object to call the AMQP method you like
channel.declareExchange("my-exchange", AMQP::fanout);
channel.declareQueue("my-queue");
channel.bindQueue("my-exchange", "my-queue");A number of remarks about the example above. First you may have noticed that we've created all objects on the stack. You are of course also free to create them on the heap with the C++ operator 'new'. That works just as well, and is in real life code probably more useful as you normally want to keep your handlers, connection and channel objects around for a much longer time.
But more importantly, you can see in the example above that we have created the channel object directly after we made the connection object, and we also started declaring exchanges and queues right away. However, under the hood, a handshake protocol is executed between the server and the client when the Connection object is first created. During this handshake procedure it is not permitted to send other instructions (like opening a channel or declaring a queue). It would therefore have been better if we had first waited for the connection to be ready (implement the MyConnectionHandler::onConnected() method), and create the channel object only then. But this is not strictly necessary. The methods that are called before the handshake is completed are cached by the AMQP library and will be executed the moment the handshake is completed and the connection becomes ready for use.
The ConnectionHandler class has a method onData() that is called by the library every time that it wants to send out data. We've explained that it is up to you to implement that method. But what about data in the other direction? How does the library receive data back from RabbitMQ?
The AMQP-CPP library does not do any IO by itself and it is therefore of course also not possible for the library to receive data from a socket. It is again up to you to do this. If, for example, you notice in your event loop that the socket that is connected with the RabbitMQ server becomes readable, you should read out that socket (for example by using the recv() system call), and pass the received bytes to the AMQP-CPP library. This is done by calling the parse() method in the Connection object.
The Connection::parse() method gets two parameters, a pointer to a buffer of data received from RabbitMQ, and a parameter that holds the size of this buffer. The code snippet below comes from the Connection.h C++ header file.
/**
* Parse data that was recevied from RabbitMQ
*
* Every time that data comes in from RabbitMQ, you should call this method to parse
* the incoming data, and let it handle by the AMQP-CPP library. This method returns
* the number of bytes that were processed.
*
* If not all bytes could be processed because it only contained a partial frame,
* you should call this same method later on when more data is available. The
* AMQP-CPP library does not do any buffering, so it is up to the caller to ensure
* that the old data is also passed in that later call.
*
* @param buffer buffer to decode
* @param size size of the buffer to decode
* @return number of bytes that were processed
*/
size_t parse(char *buffer, size_t size)
{
return _implementation.parse(buffer, size);
}You should do all the book keeping for the buffer yourselves. If you for example call the Connection::parse() method with a buffer of 100 bytes, and the method returns that only 60 bytes were processed, you should later call the method again, with a buffer filled with the remaining 40 bytes. If the method returns 0, you should make a new call to parse() when more data is available, with a buffer that contains both the old data, and the new data.
In the example we created a channel object. A channel is a virtual connection over a single TCP connection, and it is possible to create many channels that all use the same TCP connection.
AMQP instructions are always sent over a channel, so before you can send the first command to the RabbitMQ server, you first need a channel object. The channel object has many methods to send instructions to the RabbitMQ server. It for example has methods to declare queues and exchanges, to bind and unbind them, and to publish and consume messages. You can best take a look at the channel.h C++ header file for a list of all available methods. Every method in it is well documented.
The constructor of the Channel object accepts one parameter: the connection object. Unlike the connection it does not accept a handler. Instead of a handler object, (almost) every method of the Channel class returns an instance of the 'Deferred' class. This object can be used to install handlers that will be called in case of success or failure.
For example, if you call the channel.declareExchange() method, the AMQP-CPP library will send a message to the RabbitMQ message broker to ask it to declare the queue. However, because all operations in the library are asynchronous, the declareExchange() method can not return 'true' or 'false' to inform you whether the operation was succesful or not. Only after a while, after the instruction has reached the RabbitMQ server, and the confirmation from the server has been sent back to the client, the library can report the result of the declareExchange() call.
To prevent any blocking calls, the channel.declareExchange() method returns a 'Deferred' result object, on which you can set callback functions that will be called when the operation succeeds or fails.
// create a channel
Channel myChannel(&connection);
// declare an exchange, and install callbacks for success and failure
myChannel.declareExchange("my-exchange")
.onSuccess([]() {
// by now the exchange is created
})
.onError([](const char *message) {
// something went wrong creating the exchange
});As you can see in the above example, we call the declareExchange() method, and treat its return value as an object, on which we immediately install a lambda callback function to handle success, and to handle failure.
Installing the callback methods is optional. If you're not interested in the result of an operation, you do not have to install a callback for it. Next to the onSuccess() and onError() callbacks that can be installed, you can also install a onFinalize() method that gets called directly after the onSuccess() and onError() methods, and that can be used to set a callback that should run in either case: when the operation succeeds or when it fails.
The signature for the onError() method is always the same: it gets one parameter with a human readable error message. The onSuccess() function has a different signature depending on the method that you call. Most onSuccess() functions (like the one we showed for the declareExchange() method) do not get any parameters at all. Some specific onSuccess callbacks receive extra parameters with additional information.
As explained, most channel methods return a 'Deferred' object on which you can install callbacks using the Deferred::onError() and Deferred::onSuccess() methods.
The callbacks that you install on a Deferred object, only apply to one specific operation. If you want to install a generic error callback for the entire channel, you can so so by using the Channel::onError() method. Next to the Channel::onError() method, you can also install a callback to be notified when the channel is ready for sending the first instruction to RabbitMQ.
// create a channel
Channel myChannel(&connection);
// install a generic channel-error handler that will be called for every
// error that occurs on the channel
myChannel.onError([](const char *message) {
// report error
std::cout << "channel error: " << message << std::endl;
});
// install a generic callback that will be called when the channel is ready
// for sending the first instruction
myChannel.onReady([]() {
// send the first instructions (like publishing messages)
});In theory, you should wait for the onReady() callback to be called before you send any other instructions over the channel. In practice however, the AMQP library caches all instructions that were sent too early, so that you can use the channel object right after it was constructed.
It is important to realize that any error that occurs on a channel, will invalidate the entire channel,. including all subsequent instructions that were already sent over it. This means that if you call multiple methods in a row, and the first method fails, all subsequent methods will not be executed either:
Channel myChannel(&connection);
myChannel.declareQueue("my-queue");
myChannel.declareExchange("my-exchange");If the first declareQueue() call fails in the example above, the second myChannel.declareExchange() method will not be executed, even when this second instruction was already sent to the server. The second instruction will be ignored by the RabbitMQ server because the channel was already in an invalid state after the first failure.
You can overcome this by using multiple channels:
Channel channel1(&connection);
Channel channel2(&connection);
channel1.declareQueue("my-queue");
channel2.declareExchange("my-exchange");Now, if an error occurs with declaring the queue, it will not have consequences for the other call. But this comes at a small price: setting up the extra channel requires and extra instruction to be sent to the RabbitMQ server, so some extra bytes are sent over the network, and some additional resources in both the client application and the RabbitMQ server are used (although this is all very limited).
Let's take a closer look at one method in the Channel object to explain two other concepts of this AMQP-CPP library: flags and tables. The method that we will be looking at is the Channel::declareQueue() method - but we could've picked a different method too because flags and tables are used by many methods.
/**
* Declare a queue
*
* If you do not supply a name, a name will be assigned by the server.
*
* The flags can be a combination of the following values:
*
* - durable queue survives a broker restart
* - autodelete queue is automatically removed when all connected consumers are gone
* - passive only check if the queue exist
* - exclusive the queue only exists for this connection, and is automatically removed when connection is gone
*
* @param name name of the queue
* @param flags combination of flags
* @param arguments optional arguments
*/
DeferredQueue &declareQueue(const std::string &name, int flags, const Table &arguments);
DeferredQueue &declareQueue(const std::string &name, const Table &arguments);
DeferredQueue &declareQueue(const std::string &name, int flags = 0);
DeferredQueue &declareQueue(int flags, const Table &arguments);
DeferredQueue &declareQueue(const Table &arguments);
DeferredQueue &declareQueue(int flags = 0);As you can see, the method comes in many forms, and it is up to you to choose the one that is most appropriate. We now take a look at the most complete one, the method with three parameters.
All above methods returns a 'DeferredQueue' object. The DeferredQueue class extends from the AMQP::Deferred class and allows you to install a more powerful onSuccess() callback function. The 'onSuccess' method for the declareQueue() function gets three arguments:
// create a custom callback
auto callback = [](const std::string &name, int msgcount, int consumercount) {
// @todo add your own implementation
}
// declare the queue, and install the callback that is called on success
channel.declareQueue("myQueue").onSuccess(callback);Just like many others methods in the Channel class, the declareQueue() method accept an integer parameter named 'flags'. This is a variable in which you can set method-specific options, by summing up all the options that are described in the documentation above the method. If you for example want to create a durable, auto-deleted queue, you can pass in the value AMQP::durable + AMQP::autodelete.
The declareQueue() method also accepts a parameter named 'arguments', which is of type Table. This Table object can be used as an associative array to send additional options to RabbitMQ, that are often custom RabbitMQ extensions to the AMQP standard. For a list of all supported arguments, take a look at the documentation on the RabbitMQ website. With every new RabbitMQ release more features, and supported arguments are added.
The Table class is a very powerful class that enables you to build complicated, deeply nested structures full of strings, arrays and even other tables. In reality, you only need strings and integers.
// custom options that are passed to the declareQueue call
AMQP::Table arguments;
arguments["x-dead-letter-exchange"] = "some-exchange";
arguments["x-message-ttl"] = 3600 * 1000;
arguments["x-expires"] = 7200 * 1000;
// declare the queue
channel.declareQueue("my-queue-name", AMQP::durable + AMQP::autodelete, arguments);Publishing messages is easy, and the Channel class has a list of methods that can all be used for it. The most simple one takes three arguments: the name of the exchange to publish to, the routing key to use, and the actual message that you're publishing - all these parameters are standard C++ strings.
More extended versions of the publish() method exist that accept additional arguments, and that enable you to publish entire Envelope objects. An envelope is an object that contains the message plus a list of optional meta information like the content-type, content-encoding, priority, expire time and more. None of these meta fields are interpreted by this library, and also the RabbitMQ ignores most of them, but the AMQP protocol defines them, and they are free for you to use. For an extensive list of the fields that are supported, take a look at the MetaData.h header file (MetaData is the base class for Envelope). You should also check the RabbitMQ documentation to find out if an envelope header is interpreted by the RabbitMQ server (at the time of this writing, only the expire time is being used).
The following snippet is copied from the Channel.h header file and lists all available publish() methods. As you can see, you can call the publish() method in almost any form:
/**
* Publish a message to an exchange
*
* The following flags can be used
*
* - mandatory if set, an unroutable message will be sent back to
* the client (currently not supported)
*
* - immediate if set, a message that could not immediately be consumed
* is returned to the client (currently not supported)
*
* If either of the two flags is set, and the message could not immediately
* be published, the message is returned by the server to the client. However,
* at this moment in time, the AMQP-CPP library does not support catching
* such returned messages.
*
* @param exchange the exchange to publish to
* @param routingkey the routing key
* @param flags optional flags (see above)
* @param envelope the full envelope to send
* @param message the message to send
* @param size size of the message
*/
bool publish(const std::string &exchange, const std::string &routingKey, int flags, const AMQP::Envelope &envelope);
bool publish(const std::string &exchange, const std::string &routingKey, const AMQP::Envelope &envelope);
bool publish(const std::string &exchange, const std::string &routingKey, int flags, const std::string &message);
bool publish(const std::string &exchange, const std::string &routingKey, const std::string &message);
bool publish(const std::string &exchange, const std::string &routingKey, int flags, const char *message, size_t size);
bool publish(const std::string &exchange, const std::string &routingKey, const char *message, size_t size);Published messages are normally not confirmed by the server, and the RabbitMQ will not send a report back to inform us whether the message was succesfully published or not. Therefore the publish method does also not return a Deferred object.
As long as no error is reported via the Channel::onError() method, you can safely assume that your messages were delivered.
This can of course be a problem when you are publishing many messages. If you get an error halfway through there is no way to know for sure how many messages made it to the broker and how many should be republished. If this is important, you can wrap the publish commands inside a transaction. In this case, if an error occurs, the transaction is automatically rolled back by RabbitMQ and none of the messages are actually published.
// start a transaction
channel.startTransaction();
// publish a number of messages
channel.publish("my-exchange", "my-key", "my first message");
channel.publish("my-exchange", "my-key", "another message");
// commit the transactions, and set up callbacks that are called when
// the transaction was successful or not
channel.commitTransaction()
.onSuccess([]() {
// all messages were successfully published
})
.onError([]() {
// none of the messages were published
// now we have to do it all over again
});Fetching messages from RabbitMQ is called consuming, and can be started by calling the method Channel::consume(). After you've called this method, RabbitMQ starts delivering messages to you.
Just like the publish() method that we just described, the consume() method also comes in many forms. The first parameter is always the name of the queue you like to consume from. The subsequent parameters are an optional consumer tag, flags and a table with custom arguments. The first additional parameter, the consumer tag, is nothing more than a string identifier that you can use when you want to stop consuming.
The full documentation from the C++ Channel.h headerfile looks like this:
/**
* Tell the RabbitMQ server that we're ready to consume messages
*
* After this method is called, RabbitMQ starts delivering messages to the client
* application. The consume tag is a string identifier that will be passed to
* each received message, so that you can associate incoming messages with a
* consumer. If you do not specify a consumer tag, the server will assign one
* for you.
*
* The following flags are supported:
*
* - nolocal if set, messages published on this channel are
* not also consumed
*
* - noack if set, consumed messages do not have to be acked,
* this happens automatically
*
* - exclusive request exclusive access, only this consumer can
* access the queue
*
* The callback registered with DeferredConsumer::onSuccess() will be called when the
* consumer has started.
*
* @param queue the queue from which you want to consume
* @param tag a consumer tag that will be associated with this consume operation
* @param flags additional flags
* @param arguments additional arguments
* @return bool
*/
DeferredConsumer &consume(const std::string &queue, const std::string &tag, int flags, const AMQP::Table &arguments);
DeferredConsumer &consume(const std::string &queue, const std::string &tag, int flags = 0);
DeferredConsumer &consume(const std::string &queue, const std::string &tag, const AMQP::Table &arguments);
DeferredConsumer &consume(const std::string &queue, int flags, const AMQP::Table &arguments);
DeferredConsumer &consume(const std::string &queue, int flags = 0);
DeferredConsumer &consume(const std::string &queue, const AMQP::Table &arguments);As you can see, the consume method returns a DeferredConsumer. This object is a regular Deferred, with additions. The onSuccess() method of a DeferredConsumer is slightly different than the onSuccess() method of a regular Deferred object: one extra parameter will be supplied to your callback function with the consumer tag.
The onSuccess() callback will be called when the consume operation has started, but not when messages are actually consumed. For this you will have to install a different callback, using the onReceived() method.
// callback function that is called when the consume operation starts
auto startCb = [](const std::string &consumertag) {
std::cout << "consume operation started" << std::endl;
};
// callback function that is called when the consume operation failed
auto errorCb = [](const char *message) {
std::cout << "consume operation failed" << std::endl;
}
// callback operation when a message was received
auto messageCb = [&channel](const AMQP::Message &message, uint64_t deliveryTag, bool redelivered) {
std::cout << "message received" << std::endl;
// acknowledge the message
channel.ack(deliveryTag);
}
// start consuming from the queue, and install the callbacks
channel.consume("my-queue")
.onSuccess(startCb)
.onError(errorCb)
.onReceived(messageCb);
The Message object holds all information of the delivered message: the actual content, all meta information from the envelope (in fact, the Message class is derived from the Envelope class), and even the name of the exchange and the routing key that were used when the message was originally published. For a full list of all information in the Message class, you best have a look at the message.h, envelope.h and metadata.h header files.
Another important parameter to the onReceived() method is the deliveryTag parameter. This is a unique identifier that you need to acknowledge an incoming message. RabbitMQ only removes the message after it has been acknowledged, so that if your application crashes while it was busy processing the message, the message does not get lost but remains in the queue. But this means that after you've processed the message, you must inform RabbitMQ about it by calling the Channel:ack() method. This method is very simple and takes in its simplest form only one parameter: the deliveryTag of the message.
Consuming messages is a continuous process. RabbitMQ keeps sending messages, until you stop the consumer, which can be done by calling the Channel::cancel() method. If you close the channel, or the entire TCP connection, consuming also stops.
RabbitMQ throttles the number of messages that are delivered to you, to prevent that your application is flooded with messages from the queue, and to spread out the messages over multiple consumers. This is done with a setting called quality-of-service (QOS). The QOS setting is a numeric value which holds the number of unacknowledged messages that you are allowed to have. RabbitMQ stops sending additional messages when the number of unacknowledges messages has reached this limit, and only sends additional messages when an earlier message gets acknowledged. To change the QOS, you can simple call Channel::setQos().
Almost all AMQP features have been implemented. But the following things might need additional attention:
- ability to set up secure connections (or is this fully done on the IO level)
- login with other protocols than login/password
- publish confirms
- returned messages
We also need to add more safety checks so that strange or invalid data from RabbitMQ does not break the library (although in reality RabbitMQ only sends valid data). Also, when we now receive an answer from RabbitMQ that does not match the request that we sent before, we do not report an error (this is also an issue that only occurs in theory).
It would be nice to have sample implementations for the ConnectionHandler class that can be directly plugged into libev, libevent and libuv event loops.
For performance reasons, we need to investigate if we can limit the number of times an incoming or outgoing messages is copied.