Camel supports implementing the Polling Consumer from the EIP patterns using the PollingConsumer interface which can be created via the Endpoint.createPollingConsumer() method.
So in your Java code you can do
ConsumerTemplate (discussed below) is also available.
There are 3 main polling methods on PollingConsumer
Waits until a message is available and then returns it; potentially blocking forever
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
Attempts to receive a message exchange immediately without waiting and returning null if a message exchange is not available yet
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 3 operations above, but also including convenient methods for returning the body, etc
The example from above using ConsumerTemplate is:
Or to extract and get the body you can do:
And you can provide the body type as a parameter and have it returned as the type:
You get hold of a
ConsumerTemplate from the
CamelContext with the
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:
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.
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:
And then we have out logic in our bean:
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:
The ScheduledPollConsumer supports the following options:
org.apache.camel.PollingConsumerPollingStrategy allowing you to provide your custom implementation to control error handling usually occurred during the
poll operation before an Exchange have been created and being 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.
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.
Whether the scheduler should be auto started.
Milliseconds before the first poll starts.
Milliseconds before the next poll.
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 onwards the default value is
time unit for
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.
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.
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.
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 Quartz2, and Spring based which supports CRON expressions. Notice: If using a custom scheduler then the options for
scheduledExecutorService may not be in use. Use the text
quartz2 to refer to use the Quartz2 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 Quartz2 page for an example.
Camel 2.12: To configure additional properties when using a custom
scheduler or any of the Quartz2, Spring based scheduler.
Camel 2.12: To let the scheduled polling consumer backoff 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
backoffErrorThreshold must also be configured.
Camel 2.12: The number of subsequent idle polls that should happen before the
backoffMultipler should kick-in.
Camel 2.12: The number of subsequent error polls (failed due some error) that should happen before the
backoffMultipler should kick-in.
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 onwards 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
For example to let a FTP consumer backoff if its becoming idle for a while you can do:
In this example, the FTP consumer will poll for new FTP files evert 5th second. But if it has been idle for 5 attempts in a row, then it will
backoff 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 backoff 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 backoff 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
Controlling the error handling 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
void begin(Consumer consumer, Endpoint endpoint)
- begin (Camel 2.3)
boolean begin(Consumer consumer, Endpoint endpoint)
void commit(Consumer consumer, Endpoint endpoint)
- commit (Camel 2.6)
void commit(Consumer consumer, Endpoint endpoint, int polledMessages)
boolean rollback(Consumer consumer, Endpoint endpoint, int retryCounter, Exception e) throws Exception
In Camel 2.3 onwards 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 onwards 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:
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:
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
Configuring an Endpoint to use
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
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.