Polling Consumer

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.

In Java:

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

The ConsumerTemplate (discussed below) is also available.

There are 3 main polling methods on PollingConsumer

EventDrivenPollingConsumer Options

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

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 this:

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.

First we setup our route as:

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:

• PollingConsumer
• Scheduled Polling Components
  • ScheduledPollConsumer
  • Scheduler
  • Atom
  • Beanstalk
  • File
  • FTP
  • hbase
  • iBATIS
  • JPA
  • Mail
  • MyBatis
  • Quartz
  • SNMP
  • AWS-S3
  • AWS-SQS

ScheduledPollConsumer Options

The ScheduledPollConsumer supports the following options:

Using backoff to let the Scheduler be Less Aggressive

Available as of 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&passowrd=secret?delete=true&delay=5s&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

Available as of 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 Quartz2, 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 3 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 3 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();

Notice: 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

If you would like to use this EIP Pattern then please read the Getting Started, you may also find the Architecture useful particularly the description of Endpoint and URIs. Then you could try out some of the Examples first before trying this pattern out.

See Also

• POJO Consuming
• Batch Consumer