Polling Consumer

Camel supports implementing the Polling Consumer from the EIP patterns using the PollingConsumer interface which can be created via the Endpoint.createPollingConsumer() method.

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In Java:

javaEndpoint endpoint = context.getEndpoint("activemq:my.queue"); PollingConsumer consumer = endpoint.createPollingConsumer(); Exchange exchange = consumer.receive();

The ConsumerTemplate (discussed below) is also available.

There are three main polling methods on PollingConsumer

Method name Description

PollingConsumer.receive()

Waits until a message is available and then returns it; potentially blocking forever

PollingConsumer.receive(long)

Attempts to receive a message exchange, waiting up to the given timeout and returning null if no message exchange could be received within the time available

PollingConsumer.receiveNoWait()

Attempts to receive a message exchange immediately without waiting and returning null if a message exchange is not available yet

EventDrivenPollingConsumer Options

The EventDrivePollingConsumer (the default implementation) supports the following options:

Option Default Description

pollingConsumerQueueSize

1000

Camel 2.14/2.13.1/2.12.4: The queue size for the internal hand-off queue between the polling consumer, and producers sending data into the queue.

pollingConsumerBlockWhenFull

true

Camel 2.14/2.13.1/2.12/4: Whether to block any producer if the internal queue is full.

pollingConsumerBlockTimeout

0

Camel 2.16: To use a timeout (in milliseconds) when the producer is blocked if the internal queue is full. If the value is 0 or negative then no timeout is in use. If a timeout is triggered then a ExchangeTimedOutException is thrown.

Notice that some Camel Components has their own implementation of PollingConsumer and therefore do not support the options above.

You can configure these options in endpoints URIs, such as shown below:

Endpoint endpoint =
context.getEndpoint("file:inbox?pollingConsumerQueueSize=50");
PollingConsumer consumer = endpoint.createPollingConsumer();
Exchange exchange = consumer.receive(5000);

ConsumerTemplate

The ConsumerTemplate is a template much like Spring’s JmsTemplate or JdbcTemplate supporting the Polling Consumer EIP. With the template you can consume Exchanges from an Endpoint. The template supports the three operations listed above. However, it also includes convenient methods for returning the body, etc consumeBody.

Example:

Exchange exchange = consumerTemplate.receive("activemq:my.queue");

Or to extract and get the body you can do:

Object body = consumerTemplate.receiveBody("activemq:my.queue");

And you can provide the body type as a parameter and have it returned as the type:

String body = consumerTemplate.receiveBody("activemq:my.queue", String.class);

You get hold of a ConsumerTemplate from the CamelContext with the createConsumerTemplate operation:

ConsumerTemplate consumer = context.createConsumerTemplate();

Using ConsumerTemplate with Spring DSL

With the Spring DSL we can declare the consumer in the CamelContext with the consumerTemplate tag, just like the ProducerTemplate. The example below illustrates

  <camelContext xmlns="http://camel.apache.org/schema/spring">
    <!-- define a producer template -->
    <template id="producer"/>
    <!-- define a consumer template -->
    <consumerTemplate id="consumer"/>
    <!-- define endpoint -->
    <endpoint id="result" uri="mock:result"/>

    <route>
      <from uri="seda:foo"/>
      <to uri="ref:result"/>
    </route>
  </camelContext>

Then we can get leverage Spring to inject the ConsumerTemplate in our java class. The code below is part of an unit test but it shows how the consumer and producer can work together.

@ContextConfiguration
public class SpringConsumerTemplateTest extends SpringRunWithTestSupport {

    @Autowired
    private ProducerTemplate producer;

    @Autowired
    private ConsumerTemplate consumer;

    @EndpointInject("ref:result")
    private MockEndpoint mock;

    @Test
    public void testConsumeTemplate() throws Exception {
        // we expect Hello World received in our mock endpoint
        mock.expectedBodiesReceived("Hello World");

        // we use the producer template to send a message to the seda:start endpoint
        producer.sendBody("seda:start", "Hello World");

        // we consume the body from seda:start
        String body = consumer.receiveBody("seda:start", String.class);
        assertEquals("Hello World", body);

        // and then we send the body again to seda:foo so it will be routed to the mock
        // endpoint so our unit test can complete
        producer.sendBody("seda:foo", body);

        // assert mock received the body
        mock.assertIsSatisfied();
    }

}

Timer Based Polling Consumer

In this sample we use a Timer to schedule a route to be started every 5th second and invoke our bean MyCoolBean where we implement the business logic for the Polling Consumer. Here we want to consume all messages from a JMS queue, process the message and send them to the next queue.

Scheduled Poll Components

Quite a few inbound Camel endpoints use a scheduled poll pattern to receive messages and push them through the Camel processing routes. That is to say externally from the client the endpoint appears to use an Event Driven Consumer but internally a scheduled poll is used to monitor some kind of state or resource and then fire message exchanges.

Since this a such a common pattern, polling components can extend the ScheduledPollConsumer base class which makes it simpler to implement this pattern. There is also the Quartz Component which provides scheduled delivery of messages using the Quartz enterprise scheduler.

For more details see:

ScheduledPollConsumer Options

The ScheduledPollConsumer supports the following options:

confluenceTableSmall

Option Default Description

backoffErrorThreshold

0

Camel 2.12: The number of subsequent error polls (failed due some error) that should happen before the backoffMultipler should kick-in.

backoffIdleThreshold

0

Camel 2.12: The number of subsequent idle polls that should happen before the backoffMultipler should kick-in.

backoffMultiplier

0

Camel 2.12: To let the scheduled polling consumer back-off if there has been a number of subsequent idles/errors in a row. The multiplier is then the number of polls that will be skipped before the next actual attempt is happening again. When this option is in use then backoffIdleThreshold and/or backoffErrorThreshold must also be configured.

delay

500

Milliseconds before the next poll.

greedy

false

Camel 2.10.6/2.11.1: If greedy is enabled, then the ScheduledPollConsumer will run immediately again, if the previous run polled 1 or more messages.

initialDelay

1000

Milliseconds before the first poll starts.

pollStrategy

A pluggable org.apache.camel.PollingConsumerPollingStrategy allowing you to provide your custom implementation to control error handling usually occurred during the poll operation *before* an Exchange has been created and routed in Camel. In other words the error occurred while the polling was gathering information, for instance access to a file network failed so Camel cannot access it to scan for files.

The default implementation will log the caused exception at WARN level and ignore it.

runLoggingLevel

TRACE

Camel 2.8: The consumer logs a start/complete log line when it polls. This option allows you to configure the logging level for that.

scheduledExecutorService

null

Camel 2.10: Allows for configuring a custom/shared thread pool to use for the consumer. By default each consumer has its own single threaded thread pool. This option allows you to share a thread pool among multiple consumers.

scheduler

null

Camel 2.12: Allow to plugin a custom org.apache.camel.spi.ScheduledPollConsumerScheduler to use as the scheduler for firing when the polling consumer runs. The default implementation uses the ScheduledExecutorService and there is a Quartz, and Spring based which supports CRON expressions. Notice: If using a custom scheduler then the options for initialDelay, useFixedDelay, timeUnit and scheduledExecutorService may not be in use. Use the text quartz to refer to use the Quartz scheduler; and use the text spring to use the Spring based; and use the text #myScheduler to refer to a custom scheduler by its id in the Registry.

See Quartz page for an example.

scheduler.xxx

null

Camel 2.12: To configure additional properties when using a custom scheduler or any of the Quartz, Spring based scheduler.

sendEmptyMessageWhenIdle

false

Camel 2.9: If the polling consumer did not poll any files, you can enable this option to send an empty message (no body) instead.

startScheduler

true

Whether the scheduler should be auto started.

timeUnit

TimeUnit.MILLISECONDS

Time unit for initialDelay and delay options.

useFixedDelay

Controls if fixed delay or fixed rate is used. See ScheduledExecutorService in JDK for details. In Camel 2.7.x or older the default value is false.

From Camel 2.8: the default value is true.

Using backoff to Let the Scheduler be Less Aggressive

Since Camel 2.12

The scheduled Polling Consumer is by default static by using the same poll frequency whether or not there is messages to pickup or not.

From Camel 2.12: you can configure the scheduled Polling Consumer to be more dynamic by using backoff. This allows the scheduler to skip N number of polls when it becomes idle, or there has been X number of errors in a row. See more details in the table above for the backoffXXX options.

For example to let a FTP consumer back-off if its becoming idle for a while you can do:

from("ftp://myserver?username=foo&password=secret&delete=true&delay=5000&backoffMultiplier=6&backoffIdleThreshold=5")
.to("bean:processFile");

In this example, the FTP consumer will poll for new FTP files every 5th second. But if it has been idle for 5 attempts in a row, then it will back-off using a multiplier of 6, which means it will now poll every 5 x 6 = 30th second instead. When the consumer eventually pickup a file, then the back-off will reset, and the consumer will go back and poll every 5th second again.

Camel will log at DEBUG level using org.apache.camel.impl.ScheduledPollConsumer when back-off is kicking-in.

About Error Handling and Scheduled Polling Consumers

ScheduledPollConsumer is scheduled based and its run method is invoked periodically based on schedule settings. But errors can also occur when a poll is being executed. For instance if Camel should poll a file network, and this network resource is not available then a java.io.IOException could occur. As this error happens before any Exchange has been created and prepared for routing, then the regular Error handling in Camel does not apply. So what does the consumer do then? Well the exception is propagated back to the run method where its handled. Camel will by default log the exception at WARN level and then ignore it. At next schedule the error could have been resolved and thus being able to poll the endpoint successfully.

Using a Custom Scheduler

Since Camel 2.12:

The SPI interface org.apache.camel.spi.ScheduledPollConsumerScheduler allows to implement a custom scheduler to control when the Polling Consumer runs. The default implementation is based on the JDKs ScheduledExecutorService with a single thread in the thread pool. There is a CRON based implementation in the Quartz, and Spring components.

For an example of developing and using a custom scheduler, see the unit test org.apache.camel.component.file.FileConsumerCustomSchedulerTest from the source code in camel-core.

Error Handling When Using PollingConsumerPollStrategy

org.apache.camel.PollingConsumerPollStrategy is a pluggable strategy that you can configure on the ScheduledPollConsumer. The default implementation org.apache.camel.impl.DefaultPollingConsumerPollStrategy will log the caused exception at WARN level and then ignore this issue.

The strategy interface provides the following three methods:

  • begin

    • void begin(Consumer consumer, Endpoint endpoint)

  • begin (Camel 2.3)

    • boolean begin(Consumer consumer, Endpoint endpoint)

  • commit

    • void commit(Consumer consumer, Endpoint endpoint)

  • commit (Camel 2.6)

    • void commit(Consumer consumer, Endpoint endpoint, int polledMessages)

  • rollback

    • boolean rollback(Consumer consumer, Endpoint endpoint, int retryCounter, Exception e) throws Exception

In Camel 2.3: the begin method returns a boolean which indicates whether or not to skipping polling. So you can implement your custom logic and return false if you do not want to poll this time.

In Camel 2.6: the commit method has an additional parameter containing the number of message that was actually polled. For example if there was no messages polled, the value would be zero, and you can react accordingly.

The most interesting is the rollback as it allows you do handle the caused exception and decide what to do.

For instance if we want to provide a retry feature to a scheduled consumer we can implement the PollingConsumerPollStrategy method and put the retry logic in the rollback method. Lets just retry up till three times:

public boolean rollback(Consumer consumer, Endpoint endpoint, int retryCounter, Exception e) throws Exception {
    if (retryCounter < 3) {
        // return true to tell Camel that it should retry the poll immediately
        return true;
    }
    // okay we give up do not retry anymore
    return false;
}

Notice that we are given the Consumer as a parameter. We could use this to restart the consumer as we can invoke stop and start:

// error occurred lets restart the consumer, that could maybe
resolve the issue consumer.stop();
consumer.start();

Note: if you implement the begin operation make sure to avoid throwing exceptions as in such a case the poll operation is not invoked and Camel will invoke the rollback directly.

Configuring an Endpoint to Use PollingConsumerPollStrategy

To configure an Endpoint to use a custom PollingConsumerPollStrategy you use the option pollStrategy. For example in the file consumer below we want to use our custom strategy defined in the Registry with the bean id myPoll:

from("file://inbox/?pollStrategy=#myPoll").to("activemq:queue:inbox")

Using This Pattern

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