7.8. Throttler
Overview
A throttler is a processor that limits the flow rate of incoming messages. You can use this pattern to protect a target endpoint from getting overloaded. In Apache Camel, you can implement the throttler pattern using the
throttle() Java DSL command.
Video demo
There is a video demo of how to implement the throttler pattern at http://vimeo.com/27592682.
Java DSL example
To limit the flow rate to 100 messages per second, define a route as follows:
from("seda:a").throttle(100).to("seda:b");
If necessary, you can customize the time period that governs the flow rate using the
timePeriodMillis() DSL command. For example, to limit the flow rate to 3 messages per 30000 milliseconds, define a route as follows:
from("seda:a").throttle(3).timePeriodMillis(30000).to("mock:result");XML configuration example
The following example shows how to configure the preceding route in XML:
<camelContext id="throttleRoute" xmlns="http://camel.apache.org/schema/spring">
<route>
<from uri="seda:a"/>
<!-- throttle 3 messages per 30 sec -->
<throttle timePeriodMillis="30000">
<constant>3</constant>
<to uri="mock:result"/>
</throttle>
</route>
</camelContext>Dynamically changing maximum requests per period
Available os of Camel 2.8 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:
<camelContext id="throttleRoute" xmlns="http://camel.apache.org/schema/spring">
<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="mock:result"/>
</throttle>
</route>
</camelContext>Asynchronous delaying
The throttler can enable non-blocking asynchronous delaying, which means that Apache Camel schedules a task to be executed in the future. The task is responsible for processing the latter part of the route (after the throttler). This allows the caller thread to unblock and service further incoming messages. For example:
from("seda:a").throttle(100).asyncDelayed().to("seda:b");Options
The
throttle DSL command supports the following options:
| Name | Default Value | Description |
|---|---|---|
maximumRequestsPerPeriod
|
Maximum number of requests per period to throttle. This option must be provided and a positive number. Notice, in the XML DSL, from Camel 2.8 onwards this option is configured using an Expression instead of an attribute. | |
timePeriodMillis
|
1000
|
The time period in millis, in which the throttler will allow at most maximumRequestsPerPeriod number of messages.
|
asyncDelayed
|
false
|
Camel 2.4: If enabled then any messages which is delayed happens asynchronously using a scheduled thread pool. |
executorServiceRef
|
Camel 2.4: Refers to a custom Thread Pool to be used if asyncDelay has been enabled.
|
|
callerRunsWhenRejected
|
true
|
Camel 2.4: Is used if asyncDelayed was enabled. This controls if the caller thread should execute the task if the thread pool rejected the task.
|
