In software architecture, a messaging pattern is a network-oriented architectural pattern which describes how two different parts of a message passing system connect and communicate with each other.
In telecommunications, a message exchange pattern (MEP) describes the pattern of messages required by a communications protocol to establish or use a communication channel. There are two major message exchange patterns -- a request-response pattern, and a one-way pattern. For example, HTTP is a request-response pattern protocol, and UDP is a one-way pattern.
The term "Message Exchange Pattern" has a specific meaning within the Simple Object Access protocol (SOAP). SOAP MEP types include:
- In-Only: This is equivalent to one-way. A standard one-way messaging exchange where the consumer sends a message to the provider that provides only a status response.
- Robust In-Only: This pattern is for reliable one-way message exchanges. The consumer initiates with a message to which the provider responds with status. If the response is a status, the exchange is complete, but if the response is a fault, the consumer must respond with a status.
- In-Out: This is equivalent to request-response. A standard two-way message exchange where the consumer initiates with a message, the provider responds with a message or fault and the consumer responds with a status.
- In-Optional-Out: A standard two-way message exchange where the provider's response is optional.
- Out-Only: The reverse of In-Only. It primarily supports event notification. It cannot trigger a fault message.
- Robust Out-Only: Similar to the out-only pattern, except it can trigger a fault message. The outbound message initiates the transmission.
- Out-In: The reverse of In-Out. The provider transmits the request and initiates the exchange.
- Out-Optional-In: The reverse of In-Optional-Out. The service produces an outbound message. The incoming message is optional ("Optional-in").
The ØMQ message queueing library provides so-called sockets (a kind of generalization over the traditional IP and Unix sockets) which require indicating a messaging pattern to be used, and are optimized for each pattern. The basic ØMQ patterns are:
- Request-reply connects a set of clients to a set of services. This is a remote procedure call and task distribution pattern.
- Publish-subscribe connects a set of publishers to a set of subscribers. This is a data distribution pattern.
- Push-pull connects nodes in a fan-out / fan-in pattern that can have multiple steps, and loops. This is a parallel task distribution and collection pattern.
- Exclusive pair connects two sockets in an exclusive pair. This is a low-level pattern for specific, advanced use cases.
Each pattern defines a particular network topology. Request-reply defines so-called "service bus", publish-subscribe defines "data distribution tree", push-pull defines "parallelised pipeline". All the patterns are deliberately designed in such a way as to be infinitely scalable and thus usable on Internet scale.