components/camel-disruptor/src/main/docs/disruptor.adoc

[[Disruptor-DisruptorComponent]]
Disruptor Component
~~~~~~~~~~~~~~~~~~~

*Available as of Camel 2.12*

The *disruptor:* component provides asynchronous
http://www.eecs.harvard.edu/~mdw/proj/seda/[SEDA] behavior much as the
standard SEDA Component, but utilizes a
https://github.com/LMAX-Exchange/disruptor[Disruptor] instead of a
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/BlockingQueue.html[BlockingQueue]
utilized by the standard link:seda.html[SEDA]. Alternatively, a

*disruptor-vm:* endpoint is supported by this component, providing an
alternative to the standard link:vm.html[VM]. As with the SEDA
component, buffers of the *disruptor:* endpoints are only visible within
a *single* link:camelcontext.html[CamelContext] and no support is
provided for persistence or recovery. The buffers of the
**disruptor-vm:** endpoints also provides support for communication
across CamelContexts instances so you can use this mechanism to
communicate across web applications (provided that *camel-disruptor.jar*
is on the *system/boot* classpath).

The main advantage of choosing to use the Disruptor Component over the
SEDA or the VM Component is performance in use cases where there is high
contention between producer(s) and/or multicasted or concurrent
Consumers. In those cases, significant increases of throughput and
reduction of latency has been observed. Performance in scenarios without
contention is comparable to the SEDA and VM Components.

The Disruptor is implemented with the intention of mimicing the
behaviour and options of the SEDA and VM Components as much as possible.
The main differences with the them are the following:

* The buffer used is always bounded in size (default 1024 exchanges).
* As a the buffer is always bouded, the default behaviour for the
Disruptor is to block while the buffer is full instead of throwing an
exception. This default behaviour may be configured on the component
(see options).
* The Disruptor enpoints don't implement the BrowsableEndpoint
interface. As such, the exchanges currently in the Disruptor can't be
retrieved, only the amount of exchanges.
* The Disruptor requires its consumers (multicasted or otherwise) to be
statically configured. Adding or removing consumers on the fly requires
complete flushing of all pending exchanges in the Disruptor.
* As a result of the reconfiguration: Data sent over a Disruptor is
directly processed and 'gone' if there is at least one consumer, late
joiners only get new exchanges published after they've joined.
* The *pollTimeout* option is not supported by the Disruptor Component.
* When a producer blocks on a full Disruptor, it does not respond to
thread interrupts.

Maven users will need to add the following dependency to their `pom.xml`
for this component:

[source,java]
------------------------------------------------------------
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-disruptor</artifactId>
    <version>x.x.x</version>
    <!-- use the same version as your Camel core version -->
</dependency>
------------------------------------------------------------

[[Disruptor-URIformat]]
URI format
^^^^^^^^^^

[source,java]
-----------------------------
 disruptor:someName[?options]
-----------------------------

or

[source,java]
--------------------------------
 disruptor-vm:someName[?options]
--------------------------------

Where **someName** can be any string that uniquely identifies the
endpoint within the current link:camelcontext.html[CamelContext] (or
across contexts in case of +
 **disruptor-vm:**). +
 You can append query options to the URI in the following format:

[source,java]
------------------------------
  ?option=value&option=value&…
------------------------------

[[Disruptor-Options]]
Options
^^^^^^^

All the following options are valid for both the **disruptor:** and
**disruptor-vm:** components.

[width="100%",cols="10%,10%,80%",options="header",]
|=======================================================================
|Name | Default | Description

|size |1024 |The maximum capacity of the Disruptors ringbuffer. Will be effectively
increased to the nearest power of two. *Notice:* Mind if you use this
option, then its the first endpoint being created with the queue name,
that determines the size. To make sure all endpoints use same size, then
configure the size option on all of them, or the first endpoint being
created.

|bufferSize |  | *Component only:* The maximum default size (capacity of the number of
messages it can hold) of the Disruptors ringbuffer. This option is used
if size is not in use.

|queueSize |  | *Component only:* Additional option to specify the <em>bufferSize</em>
to maintain maximum compatibility with the link:seda.html[SEDA]
Component.

|concurrentConsumers |1 |Number of concurrent threads processing exchanges.

|waitForTaskToComplete |IfReplyExpected |Option to specify whether the caller should wait for the async task to
complete or not before continuing. The following three options are
supported: _Always_, _Never_ or _IfReplyExpected_. The first two values
are self-explanatory. The last value, _IfReplyExpected_, will only wait
if the message is link:request-reply.html[Request Reply] based. See more
information about link:async.html[Async] messaging.

|timeout |30000 |Timeout (in milliseconds) before a producer will stop waiting for an
asynchronous task to complete. See _waitForTaskToComplete_ and
link:async.html[Async] for more details. You can disable timeout by
using 0 or a negative value.

|defaultMultipleConsumers |  | *Component only:* Allows to set the default allowance of multiple
consumers for endpoints created by this component used when
_multipleConsumers_ is not provided.

|multipleConsumers |false |Specifies whether multiple consumers are allowed. If enabled, you can
use Disruptor for http://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern[Publish-Subscribe] messaging.
That is, you can send a message to the SEDA queue and have each consumer
receive a copy of the message. When enabled, this option should be
specified on every consumer endpoint.

|limitConcurrentConsumers |true |Whether to limit the number of concurrentConsumers to the maximum of
500. By default, an exception will be thrown if a Disruptor endpoint is
configured with a greater number. You can disable that check by turning
this option off.

|blockWhenFull |true |Whether a thread that sends messages to a full Disruptor will block
until the ringbuffer's capacity is no longer exhausted. By default, the
calling thread will block and wait until the message can be accepted. By
disabling this option, an exception will be thrown stating that the
queue is full.

|defaultBlockWhenFull | | *Component only:* Allows to set the default producer behaviour when the
ringbuffer is full for endpoints created by this comonent used when
_blockWhenFull_ is not provided.

|waitStrategy |Blocking |Defines the strategy used by consumer threads to wait on new exchanges
to be published. The options allowed are:_Blocking_, _Sleeping_,
_BusySpin_ and _Yielding_. Refer to the section below for more
information on this subject

|defaultWaitStrategy |   | *Component only:* Allows to set the default wait strategy for endpoints
created by this comonent used when _waitStrategy_ is not provided.

|producerType |Multi | Defines the producers allowed on the Disruptor. The options allowed are:
_Multi_ to allow multiple producers and _Single_ to enable certain
optimizations only allowed when one concurrent producer (on one thread
or otherwise synchronized) is active.

|defaultProducerType |  | *Component only:* Allows to set the default producer type for endpoints
created by this comonent used when _producerType_ is not provided.
|=======================================================================

[[Disruptor-Waitstrategies]]
Wait strategies
^^^^^^^^^^^^^^^

The wait strategy effects the type of waiting performed by the consumer
threads that are currently waiting for the next exchange to be
published. The following strategies can be chosen:

[width="100%",cols="10%,45%,45%",options="header",]
|=======================================================================
|Name |Description |Advice

|Blocking | Blocking strategy that uses a lock and condition variable for Consumers
waiting on a barrier. | This strategy can be used when throughput and low-latency are not as
important as CPU resource.

|Sleeping |Sleeping strategy that initially spins, then uses a Thread.yield(), and
eventually for the minimum number of nanos the OS and JVM will allow
while the Consumers are waiting on a barrier. |This strategy is a good compromise between performance and CPU resource.
Latency spikes can occur after quiet periods.

|BusySpin |Busy Spin strategy that uses a busy spin loop for Consumers waiting on a
barrier. |This strategy will use CPU resource to avoid syscalls which can
introduce latency jitter. It is best used when threads can be bound to
specific CPU cores.

|Yielding |Yielding strategy that uses a Thread.yield() for Consumers waiting on a
barrier after an initially spinning. |This strategy is a good compromise between performance and CPU resource
without incurring significant latency spikes.
|=======================================================================

[[Disruptor-UseofRequestReply]]
Use of Request Reply
^^^^^^^^^^^^^^^^^^^^

The Disruptor component supports using link:request-reply.html[Request
Reply], where the caller will wait for the Async route to complete. For
instance:

[source,java]
------------------------------------------------------------------------------
from("mina:tcp://0.0.0.0:9876?textline=true&sync=true").to("disruptor:input");
from("disruptor:input").to("bean:processInput").to("bean:createResponse");
------------------------------------------------------------------------------

In the route above, we have a TCP listener on port 9876 that accepts
incoming requests. The request is routed to the _disruptor:input_
buffer. As it is a link:request-reply.html[Request Reply] message, we
wait for the response. When the consumer on the _disruptor:input_ buffer
is complete, it copies the response to the original message response.

[[Disruptor-Concurrentconsumers]]
Concurrent consumers
^^^^^^^^^^^^^^^^^^^^

By default, the Disruptor endpoint uses a single consumer thread, but
you can configure it to use concurrent consumer threads. So instead of
thread pools you can use:

[source,java]
--------------------------------------------------------------
from("disruptor:stageName?concurrentConsumers=5").process(...)
--------------------------------------------------------------

As for the difference between the two, note a thread pool can
increase/shrink dynamically at runtime depending on load, whereas the
number of concurrent consumers is always fixed and supported by the
Disruptor internally so performance will be higher.

[[Disruptor-Threadpools]]
Thread pools
^^^^^^^^^^^^

Be aware that adding a thread pool to a Disruptor endpoint by doing
something like:

[source,java]
--------------------------------------------------
from("disruptor:stageName").thread(5).process(...)
--------------------------------------------------

Can wind up with adding a normal
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/BlockingQueue.html[BlockingQueue]
to be used in conjunction with the Disruptor, effectively negating part
of the performance gains achieved by using the Disruptor. Instead, it is
advices to directly configure number of threads that process messages on
a Disruptor endpoint using the concurrentConsumers option.

[[Disruptor-Sample]]
Sample
^^^^^^

In the route below we use the Disruptor to send the request to this
async queue to be able to send a fire-and-forget message for further
processing in another thread, and return a constant reply in this thread
to the original caller.

[source,java]
-------------------------------------------------
public void configure() throws Exception {
    from("direct:start")
        // send it to the disruptor that is async
        .to("disruptor:next")
        // return a constant response
        .transform(constant("OK"));

    from("disruptor:next").to("mock:result");
}
-------------------------------------------------

Here we send a Hello World message and expects the reply to be OK.

[source,java]
-----------------------------------------------------------------
Object out = template.requestBody("direct:start", "Hello World");
assertEquals("OK", out);
-----------------------------------------------------------------

The "Hello World" message will be consumed from the Disruptor from
another thread for further processing. Since this is from a unit test,
it will be sent to a mock endpoint where we can do assertions in the
unit test.

[[Disruptor-UsingmultipleConsumers]]
Using multipleConsumers
^^^^^^^^^^^^^^^^^^^^^^^

In this example we have defined two consumers and registered them as
spring beans.

[source,java]
-------------------------------------------------------------------------------------------
<!-- define the consumers as spring beans -->
<bean id="consumer1" class="org.apache.camel.spring.example.FooEventConsumer"/>

<bean id="consumer2" class="org.apache.camel.spring.example.AnotherFooEventConsumer"/>

<camelContext xmlns="http://camel.apache.org/schema/spring">
    <!-- define a shared endpoint which the consumers can refer to instead of using url -->
    <endpoint id="foo" uri="disruptor:foo?multipleConsumers=true"/>
</camelContext>
-------------------------------------------------------------------------------------------

Since we have specified multipleConsumers=true on the Disruptor foo
endpoint we can have those two or more consumers receive their own copy
of the message as a kind of pub-sub style messaging. As the beans are
part of an unit test they simply send the message to a mock endpoint,
but notice how we can use @Consume to consume from the Disruptor.

[source,java]
-------------------------------------------
public class FooEventConsumer {

    @EndpointInject(uri = "mock:result")
    private ProducerTemplate destination;

    @Consume(ref = "foo")
    public void doSomething(String body) {
        destination.sendBody("foo" + body);
    }

}
-------------------------------------------

[[Disruptor-Extractingdisruptorinformation]]
Extracting disruptor information
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If needed, information such as buffer size, etc. can be obtained without
using JMX in this fashion:

[source,java]
--------------------------------------------------------------------
DisruptorEndpoint disruptor = context.getEndpoint("disruptor:xxxx");
int size = disruptor.getBufferSize();
--------------------------------------------------------------------