Overview {#overview_page}

Qpid Proton's concepts and capabilities closely match those of its wire protocol, AMQP. See the Qpid AMQP page and the AMQP 1.0 spec for more information.

Key concepts

A proton::message has a body (the main content), application properties where you can store additional data, and specific properties defined by AMQP.

Messages are transferred over links. The sending end of a link is a proton::sender, and the receiving end is a proton::receiver. Links have a source and target address, as explained below.

Links are grouped in a proton::session. Messages in the same session are sent sequentially, while those on different sessions can be interleaved. A large message being sent on one session does not block messages being sent on another session.

Sessions belong to a proton::connection. If you don't need multiple sessions, a connection will create links directly using a default session.

A proton::delivery represents the transfer of a message and allows the receiver to accept or reject it. The sender can use a proton::tracker to track the status of a sent message and find out if it was accepted.

A delivery is settled when both ends are done with it. Different settlement methods give different levels of reliability: at-most-once, at-least-once, and exactly-once. See below.

The anatomy of a Proton application

proton::container is the top-level object in a Proton application. A client uses proton::container::connect() to establish connections. A server uses proton::container::listen() to accept connections.

Proton is an event-driven API. You implement a subclass of proton::messaging_handler and override functions to handle AMQP events, such as on_container_open() or on_message(). Each connection is associated with a handler for its events. proton::container::run() polls all connections and listeners and dispatches events to your handlers.

A message body can be a string or byte sequence encoded any way you like. However, AMQP also provides standard, interoperable encodings for basic data types and structures such as maps and lists. You can use this encoding for your message bodies via proton::value and proton::scalar, which convert C++ types to their AMQP equivalents.

Sources and targets

Every link has two addresses, source and target. The most common pattern for using these addresses is as follows.

When a client creates a receiver link, it sets the source address. This means “I want to receive messages from this source”. This is often referred to as “subscribing” to the source. When a client creates a sender link, it sets the target address. This means “I want to send to this target”.

In the case of a broker, the source or target usually refers to a queue or topic. In general they can refer to any AMQP-capable node.

In the request-response pattern, a request message carries a reply-to address for the response message. This can be any AMQP address, but it is often useful to create a temporary address for the response message. The client creates a receiver with no source address and the dynamic flag set. The server generates a unique source address for the receiver, which is discarded when the link closes. The client uses this source address as the reply-to when it sends the request, so the response is delivered to the client's receiver.

The @ref server_direct.cpp example shows how to implement a request-response server.

Delivery guarantees

Proton offers three levels of message delivery guarantee: at-most-once, at-least-once, and exactly-once.

For at-most-once, the sender settles the message as soon as it sends it. If the connection is lost before the message is received by the receiver, the message will not be delivered.

For at-least-once, the receiver accepts and settles the message on receipt. If the connection is lost before the sender is informed of the settlement, then the delivery is considered in-doubt and should be retried. This will ensure it eventually gets delivered (provided of course the connection and link can be reestablished). It may mean that it is delivered multiple times, however.

Finally, for exactly-once, the receiver accepts the message but doesn't settle it. The sender settles once it is aware that the receiver accepted it. In this way the receiver retains knowledge of an accepted message until it is sure the sender knows it has been accepted. If the connection is lost before settlement, the receiver informs the sender of all the unsettled deliveries it knows about, and from this the sender can deduce which need to be redelivered. The sender likewise informs the receiver which deliveries it knows about, from which the receiver can deduce which have already been settled.