Creating Message Sources with the Mule SDK

Operations are components that process a message and generate a result. Message sources are components that receive or generate new messages to be processed by the Mule Runtime.

Examples of message sources:

An HTTP listener
A File watcher
A JMS/AMQP listener that retrieves messages from a queue to which the user previously subscribed
Salesforce Streaming API

Similarities Between an Operation and a Message Source

Operations and message sources have:

Parameters
A return type
Name and Description
Reconnection capabilities

Differences Between an Operation and a Message Source

Operations Message Sources

Operations	Message Sources
Operations process messages.	Sources create messages and push them to a flow.
The lifecycle of operations is tied to the containing flow.	Message sources can be started and stopped independently of the containing flow.
Parameters of operations have the option of accepting expressions (the default).	Sources have a clear definition of which parameters can accept expressions and which cannot. Only parameters that are part of generating a response can accept expressions.
An operation can have a lifecycle, but its parameter values are not available or not needed.	A Message source must have `start()` and `stop()` phases, and they might require access to the parameter values.
A connection is obtained each time the operation is executed.	A connection is obtained each time the message source is started or when reconnection happens.

Operations process messages.

Sources create messages and push them to a flow.

The lifecycle of operations is tied to the containing flow.

Message sources can be started and stopped independently of the containing flow.

Parameters of operations have the option of accepting expressions (the default).

Sources have a clear definition of which parameters can accept expressions and which cannot. Only parameters that are part of generating a response can accept expressions.

An operation can have a lifecycle, but its parameter values are not available or not needed.

A Message source must have start() and stop() phases, and they might require access to the parameter values.

A connection is obtained each time the operation is executed.

A connection is obtained each time the message source is started or when reconnection happens.

Implementing a Message Source through the Mule SDK

The differences listed above make it difficult to define a way to implement a message source in a way 100% consistent with the model defined for operations.

Sources are required to extend the Source class, which takes two generics: one for the type of the generated event payload, the other for the type of the Attributes:

@Alias("listener")
@EmitsResponse
public class HttpListener extends Source<InputStream, HttpRequestAttributes> {}

You can use the @Alias annotation to force a name. Otherwise, the SDK will infer one.

Next Step

Defining Parameters in a Message Source