ShedLock

ShedLock does one and only thing. It makes sure your scheduled tasks ar executed at most once at the same time. If a task is being executed on one node, it acquires a lock which prevents execution of the same task from another node (or thread). Please note, that if one task is already being executed on one node, execution on other nodes does not wait, it is simply skipped.

Currently, only Spring scheduled tasks coordinated through Mongo, JDBC database or ZooKeeper are supported. More scheduling and coordination mechanisms and expected in the future.

Feedback and pull-requests welcome!

ShedLock is not a distributed scheduler

Please note that ShedLock is not and will never be full-fledged scheduler, it's just a lock. If you need a distributed scheduler, please use another project. ShedLock is designed to be used in situations where you have scheduled tasks that are not ready to be executed in parallel, but can be safely executed repeatedly. For example if the task is fetching records from a database, processing them and marking them as processed at the end without using any transaction. In such case ShedLock may be right for you.

Usage

Import project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-spring</artifactId>
    <version>0.7.0</version>
</dependency>

Annotate your scheduled tasks

import net.javacrumbs.shedlock.core.SchedulerLock;

...

@Scheduled(...)
@SchedulerLock(name = "scheduledTaskName")
public void scheduledTask() {
   // do something
}

The @SchedulerLock annotation has several purposes. First of all, only annotated methods are locked, the library ignores all other scheduled tasks. You also have to specify the name for the lock. Only one tasks with the same name can be executed at the same time.

You can also set lockAtMostFor attribute which specifies how long the lock should be kept in case the executing node dies. This is just a fallback, under normal circumstances the lock is released as soon the tasks finishes.

Lastly, you can set lockAtLeastFor attribute which specifies minimum amount of time for which the lock should be kept. Its main purpose is to prevent execution from multiple nodes in case of really short tasks and clock difference between the nodes.

Example

Let's say you have a task which you execute every 15 minutes and which usually takes few minutes to run. Moreover, you want to execute it at most once per 15 minutes. In such case, you can configure it like this

import net.javacrumbs.shedlock.core.SchedulerLock;

...
private static final FOURTEEN_MIN = 14 * 60 * 1000;
...

@Scheduled(cron = "0 */15 * * * *")
@SchedulerLock(name = "scheduledTaskName", lockAtMostFor = FOURTEEN_MIN, lockAtLeastFor = FOURTEEN_MIN)
public void scheduledTask() {
   // do something
}

By setting lockAtMostFor we make sure that the lock is released even if the node dies and by setting lockAtLeastFor we make sure it's not executed more than once in fifteen minutes. Please note that if the task takes longer than 15 minutes, it will be executed again.

Configure the task scheduler

Now we need to integrate the library into Spring. It's done by wrapping standard Spring task scheduler.

import net.javacrumbs.shedlock.spring.SpringLockableTaskSchedulerFactory;

...

@Bean
public TaskScheduler taskScheduler(LockProvider lockProvider) {
    int poolSize = 10;
    return SpringLockableTaskSchedulerFactory.newLockableTaskScheduler(poolSize, lockProvider);
}

Or if you already have an instance of ScheduledExecutorService

@Bean
public TaskScheduler taskScheduler(ScheduledExecutorService executorService, LockProvider lockProvider) {
    return SpringLockableTaskSchedulerFactory.newLockableTaskScheduler(executorService, lockProvider, Duration.of(10, MINUTES));
}

Configure LockProvider

Mongo

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-mongo</artifactId>
    <version>0.8.0</version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.mongo.MongoLockProvider;

...

@Bean
public LockProvider lockProvider(MongoClient mongo) {
    return new MongoLockProvider(mongo, "databaseName");
}

Please note that MongoDB integration requires Mongo >= 2.4 and mongo-java-driver >= 3.4.0

JdbcTemplate

Create the table

CREATE TABLE shedlock(
    name VARCHAR(64), 
    lock_until TIMESTAMP(3) NULL, 
    locked_at TIMESTAMP(3) NULL, 
    locked_by  VARCHAR(255), 
    PRIMARY KEY (name)
) 

script for MS SQL is here

Add dependency

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-jdbc-template</artifactId>
    <version>0.9.0</version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.jdbctemplate.JdbcTemplateLockProvider;

...

@Bean
public LockProvider lockProvider(DataSource dataSource) {
    return new JdbcTemplateLockProvider(dataSource);
}

Tested with MySql, Postgres and HSQLDB

Plain JDBC

For those who do not want to use jdbc-template, there is plain JDBC lock provider. Just import

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-jdbc</artifactId>
    <version>0.9.0</version>
</dependency>

and configure

import net.javacrumbs.shedlock.provider.jdbc.JdbcLockProvider;

...

@Bean
public LockProvider lockProvider(DataSource dataSource) {
    return new JdbcLockProvider(dataSource);
}

the rest is the same as with JdbcTemplate lock provider.

ZooKeeper (using Curator)

Import

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-zookeeper-curator</artifactId>
    <version>0.9.0</version>
</dependency>

and configure

@Bean
public LockProvider lockProvider(org.apache.curator.framework.CuratorFramework client) {
    return new ZookeeperCuratorLockProvider(client);
}

By default, ephemeral nodes for locks will be created under /shedlock node.

Spring XML configuration

If you are using Spring XML config, use this configuration

<!-- Wrap the original scheduler -->
<bean id="scheduler" class="net.javacrumbs.shedlock.spring.SpringLockableTaskSchedulerFactory" factory-method="newLockableTaskScheduler">
    <constructor-arg>
        <!-- The original scheduler -->
        <task:scheduler id="sch" pool-size="10"/>
    </constructor-arg>
    <constructor-arg ref="lockProvider"/>
</bean>


<!-- Your task(s) without change -->
<task:scheduled-tasks scheduler="scheduler">
    <task:scheduled ref="task" method="run" fixed-delay="1" fixed-rate="1"/>
</task:scheduled-tasks>

Annotate scheduler method(s)

@SchedulerLock(name = "taskName")
public void run() {

}

Requirements and dependencies

1. Java 8
2. slf4j-api

Change log

0.9.0

• Support for @SchedulerLock annotations on proxied classes

0.8.0

• LockableTaskScheduler made AutoClosable so it's closed upon Spring shutdown

0.7.0

• Support for lockAtLeastFor

0.6.0

• Possible to configure defaultLockFor time so it does not have to be repeated in every annotation

0.5.0

• ZooKeeper nodes created under /shedlock by default

0.4.1

• JdbcLockProvider insert does not fail on DataIntegrityViolationException

0.4.0

• Extracted LockingTaskExecutor
• LockManager.executeIfNotLocked renamed to executeWithLock
• Default table name in JDBC lock providers

0.3.0

• @ShedlulerLock.name made obligatory
• @ShedlulerLock.lockForMillis renamed to lockAtMostFor
• Adding plain JDBC LockProvider
• Adding ZooKeepr LockProvider