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Back-Pressure and MaxConcurrency

Under heavy load, Mule runtime engine (Mule) might not immediately have the resources available to process a specific event. This issue might occur because all threads are busy and the current flow’s concurrency is exceeded. When this occurs, Mule logs the following message: Flow 'flowName' is unable to accept new events at this time
This message is known as back-pressure. Mule back-pressure actions are specific to each connector’s source.

When a producer component outputs information at a higher rate than a consumer component can process it, you can use the maxConcurrency parameter to set the maximum number of concurrent messages that a flow can process at any given time, whether the components are internal or external, flows or sources, and so on.

Although Mule applies back-pressure on its own, the maxConcurrency parameter helps to control the flow performance, especially when a backend server may affect when having high concurrency. See how to set the parameter in the following example:

<flow name="perf-test-flow" maxConcurrency="${flow.max.concurrency}">

Similarly, you can limit the number of records that can be processed in a batch job by setting the maxConcurrency parameter as in the following example:

<batch:job jobName="test-batch" maxConcurrency="${batch.max.concurrency}">

By default, the Flow and Async scopes do not set a maxConcurrency limit, but the Batch Job component limits concurrency to twice the number of available cores. The capacity of your system also limits concurrency.

Components with Max Concurrency Settings

The following components provide a max concurrency setting:

  • Flow scope

  • Scatter-Gather router

  • Async scope

  • Batch Job scope

  • Parallel For Each scope


Unless a lower concurrency is required to artificially trigger a back-pressure event, use the default settings, and do not change the max concurrency value.