= Creating Message Sources with the Mule SDK
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:keywords: mule, sdk, sources, listener, triggers

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

[options="header"]
|===
|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.
|===

== 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:

[source,java,linenums]
----
@Alias("listener")
@EmitsResponse
public class HttpListener extends Source<InputStream, HttpRequestAttributes> {}
----

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

== Next Step

* <<sources-parameters#, Defining Parameters in a Message Source>>