Throttle

The Throttler Pattern allows you to ensure that a specific endpoint does not get overloaded, or that we don’t exceed an agreed SLA with some external service.

Options

The Throttle EIP supports 6 options which are listed below:

Name Description Default Type

correlationExpression

The expression used to calculate the correlation key to use for throttle grouping. The Exchange which has the same correlation key is throttled together.

ExpressionSubElementDefinition

executorServiceRef

To use a custom thread pool (ScheduledExecutorService) by the throttler.

String

timePeriodMillis

Sets the time period during which the maximum request count is valid for

1000

String

asyncDelayed

Enables asynchronous delay which means the thread will not block while delaying.

false

Boolean

callerRunsWhenRejected

Whether or not the caller should run the task when it was rejected by the thread pool. Is by default true

true

Boolean

rejectExecution

Whether or not throttler throws the ThrottlerRejectedExecutionException when the exchange exceeds the request limit Is by default false

false

Boolean

Samples

from("seda:a")
  .throttle(3).timePeriodMillis(10000)
  .to("log:result", "mock:result");

So the above example will throttle messages all messages received on seda:a before being sent to mock:result ensuring that a maximum of 3 messages are sent in any 10 second window. Note that since timePeriodMillis defaults to 1000 milliseconds, just setting the maximumRequestsPerPeriod has the effect of setting the maximum number of requests per second. So to throttle requests at 100 requests per second between two endpoints, it would look more like this…​

from("seda:a")
  .throttle(100)
  .to("seda:b");

For further examples of this pattern in use you could look at the junit test case.

And an example in XML

<route>
  <from uri="seda:a"/>
  <!-- throttle 3 messages per 10 sec -->
  <throttle timePeriodMillis="10000">
    <constant>3</constant>
    <to uri="log:result"/>
    <to uri="mock:result"/>
  </throttle>
</route>

Dynamically changing maximum requests per period

Since we use an Expression you can adjust this value at runtime, for example you can provide a header with the value. At runtime Camel evaluates the expression and converts the result to a java.lang.Long type. In the example below we use a header from the message to determine the maximum requests per period. If the header is absent, then the Throttler uses the old value. So that allows you to only provide a header if the value is to be changed:

<route>
  <from uri="direct:expressionHeader"/>
  <throttle timePeriodMillis="500">
    <!-- use a header to determine how many messages to throttle per 0.5 sec -->
    <header>throttleValue</header>
    <to uri="log:result"/>
    <to uri="mock:result"/>
  </throttle>
</route>

Asynchronous delaying

You can let the Throttler use non blocking asynchronous delaying, which means Camel will use a scheduler to schedule a task to be executed in the future. The task will then continue routing. This allows the caller thread to not block and be able to service other messages, etc.

from("seda:a")
  .throttle(100).asyncDelayed()
  .to("seda:b");
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