AMQP Performatives and The Link Protocol
The basic unit of data in AMQP is a frame. There are nine AMQP frame bodies defined that are used to initiate, control and tear down the transfer of messages between two peers. These are:
- open
- begin
- attach
- transfer
- flow
- disposition
- detach
- end
- close
The link protocol is at the heart of AMQP.
An attach frame body is sent to initiate a new link; a detach to tear down a link. Links may be established in order to receive or send messages.
Messages are sent over an established link using the transfer frame. Messages on a link flow in only one direction.
Transfers are subject to a credit based flow control scheme, managed using flow frames. This allows a process to protect itself from being overwhelmed by too large a volume of messages or more simply to allow a subscribing link to pull messages as and when desired.
Each transferred message must eventually be settled. Settlement ensures that the sender and receiver agree on the state of the transfer, providing reliability guarantees. Changes in state and settlement for a transfer (or set of transfers) are communicated between the peers using the disposition frame. Various reliability guarantees can be enforced this way: at-most-once, at-least-once and exactly-once.
Multiple links, in both directions, can be grouped together in a session. A session is a bidirectional, sequential conversation between two peers that is initiated with a begin frame and terminated with an end frame. A connection between two peers can have multiple sessions multiplexed over it, each logically independent. Connections are initiated with an open frame in which the sending peer's capabilities are expressed, and terminated with a close frame.
Read more about this topic: Advanced Message Queuing Protocol