Exchange Pattern

There are two Message Exchange Patterns (MEP) you can use in messaging.

From the Enterprise Integration Patterns they are:

• Event Message (or one-way)
• Request Reply

In Camel, we have an org.apache.camel.ExchangePattern enumeration which can be configured on the exchangePattern property on the Message Exchange indicating if a message exchange is a one way Event Message (InOnly) or a Request Reply message exchange (InOut).

How Exchange Pattern Works

The exchange pattern is a contract between the route and the component endpoint. It tells the component whether the caller expects a reply (InOut) or not (InOnly).

The exchange pattern does not affect how EIPs and processors work within the route. Processors such as setBody, transform, process, and bean always operate on the current message body regardless of the exchange pattern. The pattern only matters when the exchange reaches a producer endpoint (i.e., a to step) that acts on it.

Most components have a fixed exchange pattern — for example, HTTP is always InOut (request/reply) and Kafka is always InOnly (fire-and-forget). For these components the exchange pattern setting has no effect.

The exchange pattern is historically used by a limited set of components that support both InOnly and InOut, such as JMS, SEDA, and AMQP. These components use the pattern to decide between fire-and-forget and request/reply messaging. For example, with JMS InOnly sends a message to a queue without waiting for a reply, while InOut sets up a temporary reply queue (via JMSReplyTo) and waits for a response.

For example to override the default pattern on a JMS endpoint you could use the exchangePattern parameter in the Endpoint URI as shown:

jms:myQueue?exchangePattern=InOut