db-scheduler

Task-scheduler for Java that was inspired by the need for a clustered java.util.concurrent.ScheduledExecutorService simpler than Quartz.

Features

• Cluster-friendly. Guarantees execution by single scheduler instance.
• Persistent tasks. Requires single database-table for persistence.
• Embeddable. Built to be embedded in existing applications.
• Simple.
• Minimal dependencies. (slf4j)

Getting started

1. Add maven dependency

<dependency>
    <groupId>com.github.kagkarlsson</groupId>
    <artifactId>db-scheduler</artifactId>
    <version>3.0</version>
</dependency>

2. Create the scheduled_tasks table in your database-schema. See table definition for postgresql, oracle or mysql.

3. Instantiate and start the scheduler, which then will start any defined recurring tasks.

RecurringTask<Void> hourlyTask = Tasks.recurring("my-hourly-task", FixedDelay.ofHours(1))
        .execute((inst, ctx) -> {
            System.out.println("Executed!");
        });

final Scheduler scheduler = Scheduler
        .create(dataSource)
        .startTasks(hourlyTask)
        .threads(5)
        .build();

// hourlyTask is automatically scheduled on startup if not already started (i.e. exists in the db)
scheduler.start();

For more examples, continue reading. For details on the inner workings, see How it works.

Examples

Recurring task

Define a recurring task and schedule the task's first execution on start-up using the startTasks builder-method. Upon completion, the task will be re-scheduled according to the defined schedule.

RecurringTask<Void> hourlyTask = Tasks.recurring("my-hourly-task", FixedDelay.ofHours(1))
        .execute((inst, ctx) -> {
            System.out.println("Executed!");
        });

final Scheduler scheduler = Scheduler
        .create(dataSource)
        .startTasks(hourlyTask)
        .threads(5)
        .build();

// hourlyTask is automatically scheduled on startup if not already started (i.e. exists in the db)
scheduler.start();

Ad-hoc tasks / One-time tasks

An instance of an ad-hoc task has a single execution-time some time in the future (i.e. non-recurring). The instance-id must be unique within this task, and may be used to encode some metadata (e.g. an id). For more complex state, custom serializable java objects are supported (as used in the example).

Define a onetime task and start the scheduler:

OneTimeTask<MyTaskData> myAdhocTask = Tasks.oneTime("my-typed-adhoc-task", MyTaskData.class)
        .execute((inst, ctx) -> {
            System.out.println("Executed! Custom data, Id: " + inst.getData().id);
        });

final Scheduler scheduler = Scheduler
        .create(dataSource, myAdhocTask)
        .threads(5)
        .build();

scheduler.start();

... and then at some point (at runtime), an execution is scheduled using the SchedulerClient:

// Schedule the task for execution a certain time in the future and optionally provide custom data for the execution
scheduler.schedule(myAdhocTask.instance("1045", new MyTaskData(1001L)), Instant.now().plusSeconds(5));

Proper shutdown of the scheduler

To avoid unnecessary dead exexutions, it is important to shutdown the scheduler properly, i.e. calling the shutdown method.

final Scheduler scheduler = Scheduler
        .create(dataSource, myAdhocTask)
        .build();

Runtime.getRuntime().addShutdownHook(new Thread() {
    @Override
    public void run() {
        LOG.info("Received shutdown signal.");
        scheduler.stop();
    }
});

scheduler.start();

How it works

A single database table is used to track future task-executions. When a task-execution is due, db-scheduler picks it and executes it. When the execution is done, the Task is consulted to see what should be done. For example, a RecurringTask is typically rescheduled in the future based on its Schedule.

Optimistic locking is used to guarantee that a one and only one scheduler-instance gets to pick a task-execution.

Recurring tasks

The term recurring task is used for tasks that should be run regularly, according to some schedule (see RecurringTask).

When the execution of a recurring task has finished, a Schedule is consulted to determine what the next time for execution should be, and a future task-execution is created for that time (i.e. it is rescheduled). The time chosen will be the nearest time according to the Schedule, but still in the future.

To create the initial execution for a RecurringTask, the scheduler has a method startTasks(...) that takes a list of tasks that should be "started" if they do not already have a future execution. Note: The first execution-time will not be according to the schedule, but simply now().

Ad-hoc tasks

The other type of task has been named ad-hoc task, but is most typically something that should be run once at a certain time in the future, a OneTimeTask.

In addition to encode some data into the instanceIdof a task-execution, it is possible to store arbitrary binary data in a separate field for use at execution-time.

Dead executions

During execution, the scheduler regularly updates a heartbeat-time for the task-execution. If an execution is marked as executing, but is not receiving updates to the heartbeat-time, it will be considered a dead execution after time X. That may for example happen if the JVM running the scheduler suddenly exits.

When a dead execution is found, the Taskis consulted to see what should be done. A dead RecurringTask is typically rescheduled to now().

Things to note / gotchas

• There are no guarantees that all instants in a schedule for a RecurringTask will be executed. The Schedule is consulted after the previous task-execution finishes, and the closest time in the future will be selected for next execution-time. A new type of task may be added in the future to provide such functionality.

• The methods on SchedulerClient (schedule, cancel, reschedule) and the CompletionHandler will run using a new Connectionfrom the DataSourceprovided. To have the action be a part of a transaction, it must be taken care of by the DataSourceprovided, for example using something like Spring's TransactionAwareDataSourceProxy.

• Currently, the precision of db-scheduler is depending on the pollingInterval (default 10s) which specifies how often to look in the table for due executions.

Versions / upgrading

Version 3.0

• New builders for task-creation, making it clearer what the config-options are. (See Tasks class and examples)
• Better default for failure handling for one-time tasks
• Enables recurring tasks to have data

Upgrading to 3.x

• No schema changes
• Task creation are preferrably done through builders in Tasks class

Version 2.0

• Possible to cancel and reschedule executions.
• Optional data can be stored with the execution. Default using Java Serialization.
• Exposing the Executionto the ExecutionHandler.

Upgrading to 2.x

• Add column task_data to the database schema. See table definitions for postgresql, oracle or mysql.

FAQ

Coming