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Chapter 338. VM Component
Available as of Camel version 1.1
The vm: component provides asynchronous SEDA behavior, exchanging messages on a BlockingQueue and invoking consumers in a separate thread pool.
This component differs from the Seda component in that VM supports communication across CamelContext instances - so you can use this mechanism to communicate across web applications (provided that camel-core.jar is on the system/boot classpath).
VM is an extension to the Seda component.
338.1. URI format
vm:queueName[?options]
Where queueName can be any string to uniquely identify the endpoint within the JVM (or at least within the classloader that loaded camel-core.jar)
You can append query options to the URI in the following format: ?option=value&option=value&…
An exactly identical VM endpoint URI must be used for both the producer and the consumer endpoint. Otherwise, Camel will create a second VM endpoint despite that the queueName portion of the URI is identical. For example:
from("direct:foo").to("vm:bar?concurrentConsumers=5");
from("vm:bar?concurrentConsumers=5").to("file://output");Notice that we have to use the full URI, including options in both the producer and consumer.
In Camel 2.4 this has been fixed so that only the queue name must match. Using the queue name bar, we could rewrite the previous exmple as follows:
from("direct:foo").to("vm:bar");
from("vm:bar?concurrentConsumers=5").to("file://output");338.2. Options
The VM component supports 4 options which are listed below.
| Name | Description | Default | Type |
|---|---|---|---|
| queueSize (advanced) | Sets the default maximum capacity of the SEDA queue (i.e., the number of messages it can hold). | int | |
| concurrentConsumers (consumer) | Sets the default number of concurrent threads processing exchanges. | 1 | int |
| defaultQueueFactory (advanced) | Sets the default queue factory. | Exchange> | |
| resolveProperty Placeholders (advanced) | Whether the component should resolve property placeholders on itself when starting. Only properties which are of String type can use property placeholders. | true | boolean |
The VM endpoint is configured using URI syntax:
vm:name
with the following path and query parameters:
338.2.1. Path Parameters (1 parameters):
| Name | Description | Default | Type |
|---|---|---|---|
| name | Required Name of queue | String |
338.2.2. Query Parameters (16 parameters):
| Name | Description | Default | Type |
|---|---|---|---|
| size (common) | The maximum capacity of the SEDA queue (i.e., the number of messages it can hold). | 2147483647 | int |
| bridgeErrorHandler (consumer) | Allows for bridging the consumer to the Camel routing Error Handler, which mean any exceptions occurred while the consumer is trying to pickup incoming messages, or the likes, will now be processed as a message and handled by the routing Error Handler. By default the consumer will use the org.apache.camel.spi.ExceptionHandler to deal with exceptions, that will be logged at WARN/ERROR level and ignored. | false | boolean |
| concurrentConsumers (consumer) | Number of concurrent threads processing exchanges. | 1 | int |
| exceptionHandler (consumer) | To let the consumer use a custom ExceptionHandler. Notice if the option bridgeErrorHandler is enabled then this options is not in use. By default the consumer will deal with exceptions, that will be logged at WARN/ERROR level and ignored. | ExceptionHandler | |
| exchangePattern (consumer) | Sets the default exchange pattern when creating an exchange. | ExchangePattern | |
| limitConcurrentConsumers (consumer) | Whether to limit the number of concurrentConsumers to the maximum of 500. By default, an exception will be thrown if an endpoint is configured with a greater number. You can disable that check by turning this option off. | true | boolean |
| multipleConsumers (consumer) | Specifies whether multiple consumers are allowed. If enabled, you can use SEDA for 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. | false | boolean |
| pollTimeout (consumer) | The timeout used when polling. When a timeout occurs, the consumer can check whether it is allowed to continue running. Setting a lower value allows the consumer to react more quickly upon shutdown. | 1000 | int |
| purgeWhenStopping (consumer) | Whether to purge the task queue when stopping the consumer/route. This allows to stop faster, as any pending messages on the queue is discarded. | false | boolean |
| blockWhenFull (producer) | Whether a thread that sends messages to a full SEDA queue will block until the queue’s capacity is no longer exhausted. By default, an exception will be thrown stating that the queue is full. By enabling this option, the calling thread will instead block and wait until the message can be accepted. | false | boolean |
| discardIfNoConsumers (producer) | Whether the producer should discard the message (do not add the message to the queue), when sending to a queue with no active consumers. Only one of the options discardIfNoConsumers and failIfNoConsumers can be enabled at the same time. | false | boolean |
| failIfNoConsumers (producer) | Whether the producer should fail by throwing an exception, when sending to a queue with no active consumers. Only one of the options discardIfNoConsumers and failIfNoConsumers can be enabled at the same time. | false | boolean |
| timeout (producer) | Timeout (in milliseconds) before a SEDA producer will stop waiting for an asynchronous task to complete. You can disable timeout by using 0 or a negative value. | 30000 | long |
| waitForTaskToComplete (producer) | 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 Request Reply based. The default option is IfReplyExpected. | IfReplyExpected | WaitForTaskToComplete |
| queue (advanced) | Define the queue instance which will be used by the endpoint. This option is only for rare use-cases where you want to use a custom queue instance. | BlockingQueue | |
| synchronous (advanced) | Sets whether synchronous processing should be strictly used, or Camel is allowed to use asynchronous processing (if supported). | false | boolean |
See the Seda component for options and other important usage details as the same rules apply to the Vm component.
338.3. Samples
In the route below we send exchanges across CamelContext instances to a VM queue named order.email:
from("direct:in").bean(MyOrderBean.class).to("vm:order.email");
And then we receive exchanges in some other Camel context (such as deployed in another .war application):
from("vm:order.email").bean(MyOrderEmailSender.class);