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Java implementation of a lightweight distributed task scheduler based on Redis with support for Jedis, Lettuce and Spring Data Redis
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README.md

redis-scheduler

Distributed Scheduler using Redis for Java applications.

What is this?

redis-scheduler is a Java implementation of a distributed scheduler using Redis. It has the following features:

  • Useable in a distributed environment: Uses Redis transactions for effectively preventing a task to be run on multiple instances of the same application.
  • Lightweight: Uses a single thread.
  • Configurable polling: Polling delay can be configured to tweak execution precision (at the cost of performance)
  • Multiple schedulers support: You can create multiple schedulers in the same logical application if you need to.
  • Support for multiple client libraries: Drivers exist for Jedis, Lettuce and Spring Data's RedisTemplate

High level concepts

Scheduled Task

A scheduled task is a job that you need to execute in the future at a particular time. In redis-scheduler, a task is represented solely by an arbitrary string identifier that has no particular meaning to the library. It's your application that has to make sense of this identifier.

Scheduler

RedisTaskScheduler: This interface is where you submit your tasks for future execution. Once submitted, a task will only be executed at or after the trigger time you provide.

TaskTriggerListener interface

This is the main interface you must implement to actually run the tasks once they are due for execution. The library will call the taskTriggered method for each task that is due for execution.

Building the project

mvn package

Maven dependency

This artifact is published on Maven Central:

<dependency>
    <groupId>com.github.davidmarquis</groupId>
    <artifactId>redis-scheduler</artifactId>
    <version>3.0.0</version>
</dependency>

You'll need to add one of the specific dependencies to use the different available drivers:

To use with Lettuce:

<dependency>
    <groupId>io.lettuce</groupId>
    <artifactId>lettuce-core</artifactId>
    <version>5.0.3.RELEASE</version>
</dependency>

To use with Jedis:

<dependency>
    <groupId>redis.clients</groupId>
    <artifactId>jedis</artifactId>
    <version>2.9.0</version>
</dependency>

To use with Spring Data Redis:

<dependency>
    <groupId>org.springframework.data</groupId>
    <artifactId>spring-data-redis</artifactId>
    <version>1.8.11.RELEASE</version>
</dependency>

Usage with Lettuce

The scheduler must be instantiated with the LettuceDriver:

RedisClient client = RedisClient.create(RedisURI.create("localhost", 6379));
RedisTaskScheduler scheduler = new RedisTaskScheduler(new LettuceDriver(client), new YourTaskTriggerListener());

scheduler.start();

Usage with Jedis

The scheduler must be instantiated with the JedisDriver:

JedisPool pool = new JedisPool("localhost", 6379);
RedisTaskScheduler scheduler = new RedisTaskScheduler(new JedisDriver(pool), new YourTaskTriggerListener());

scheduler.start();

Usage with Spring

First declare the base beans for Redis connectivity (if not already done in your project). This part can be different for your project.

<bean id="jedisConnectionFactory" class="org.springframework.data.redis.connection.jedis.JedisConnectionFactory">
    <property name="hostName" value="localhost"/>
    <property name="port" value="6379"/>
</bean>

<bean id="redisTemplate" class="org.springframework.data.redis.core.RedisTemplate">
    <property name="connectionFactory" ref="jedisConnectionFactory"/>
    <property name="keySerializer">
        <bean class="org.springframework.data.redis.serializer.StringRedisSerializer"/>
    </property>
</bean>

<bean id="springTemplateDriver" class="com.github.davidmarquis.redisscheduler.drivers.spring.RedisTemplateDriver">
    <constructor-arg name="redisTemplate" ref="redisTemplate"/>
</bean>

Finally, declare the scheduler instance:

<bean id="scheduler" class="com.github.davidmarquis.redisscheduler.RedisTaskScheduler">
    <constructor-arg name="driver" ref="springTemplateDriver"/>
    <constructor-arg name="listener">
        <bean class="your.own.implementation.of.TaskTriggerListener"/>
    </constructor-arg>
</bean>

As noted above, RedisTaskScheduler expects an implementation of the TaskTriggerListener interface which it will notify when a task is due for execution. You must implement this interface yourself and provide it to the scheduler as a constructor argument.

See the the test Spring context in test/resources/application-context-test.xml for a complete working example of the setup.

Scheduling a task in the future

scheduler.schedule("mytask", new GregorianCalendar(2015, Calendar.JANUARY, 1, 4, 45, 0));

This would schedule a task with ID "mytask" to be run at 4:45AM on January 1st 2015.

Be notified once a task is due for execution

public class MyTaskTriggerListener implements TaskTriggerListener {
    public void taskTriggered(String taskId) {
        System.out.printf("Task %s is due for execution.", taskId);
    }
}

Customizing polling delay

By default, polling delay is set to a few seconds (see implementation RedisTaskScheduler for actual value). If you need your tasks to be triggered with more precision, decrease the polling delay using the pollingDelayMillis attribute of RedisTaskScheduler:

In Java:

scheduler.setPollingDelayMillis(500);

With Spring:

<bean id="scheduler" class="com.github.davidmarquis.redisscheduler.RedisTaskScheduler">
    <property name="pollingDelayMillis" value="500"/>
</bean>

Increasing polling delay comes with a cost: higher load on Redis and your connection. Try to find the best balance for your needs.

Retry polling when a Redis connection error happens

Retries can be configured using the maxRetriesOnConnectionFailure property on RedisTaskScheduler:

In Java:

scheduler.setMaxRetriesOnConnectionFailure(5);

With Spring:

<bean id="scheduler" class="com.github.davidmarquis.redisscheduler.RedisTaskScheduler">
    <property name="maxRetriesOnConnectionFailure" value="5"/>
</bean>

After the specified number of retries, the polling thread will stop and log an error.

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