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.
So in your Java code you can do
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
Method name |
Description |
---|---|
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
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 3 operations above, but also including convenient methods for returning the body, etc consumeBody
.
The example from above using ConsumerTemplate is:
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:
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:
- PollingConsumer
- Scheduled Polling Components
ScheduledPollConsumer Options
The ScheduledPollConsumer supports the following options:
Option |
Default |
Description |
---|---|---|
pollStrategy |
A pluggable |
|
sendEmptyMessageWhenIdle |
|
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 |
|
Whether the scheduler should be auto started. |
initialDelay |
|
Initial delay in millis before first poll. |
delay |
|
Delay in millis between polls. |
timeUnit |
|
time unit for |
useFixedDelay |
|
Whether to use fixed delay or fixed rate between polls. |
runLoggingLevel |
|
Logging level when a poll is executed. |
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.
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
- 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 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:
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.