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=== Scheduler

Apache Karaf provides an optional Scheduler which provides a Service Listener which listens for Runnable Services and schedules their execution, based  on the service properties.

This Scheduler implementation uses the Quartz Scheduler library to understand cron-like expressions.

==== Installation

To enable the Apache Karaf Scheduler, you just have to install the `scheduler` feature:

----
karaf@root()> feature:install scheduler
----

The `scheduler` feature automatically installs the `scheduler` command group, too:

----
scheduler:list
scheduler:schedule
scheduler:unschedule
scheduler:reschedule
----

==== Configuration

All jobs allow configuration using service properties:

.Scheduler properties
[width="100%",cols="3,2,10",options="header"]
|=========================================================
|Property |Default |Description

|Scheduler.PROPERTY_SCHEDULER_PERIOD | - |
Defines the period for a job. The period is expressed in seconds. This property needs to be of type Long.

|Scheduler.PROPERTY_SCHEDULER_TIMES | -1 |
Defines the number of times the job is executed. -1 means infinite.

|Scheduler.PROPERTY_SCHEDULER_IMMEDIATE |false |
Define if a periodically job should be scheduled immediate.
Default is to not startup immediate, the job is started the first time after the period has expired.
This property needs to be of type Boolean.

|Scheduler.PROPERTY_SCHEDULER_EXPRESSION | - |
Define the cron expression for a job. Must be a Quartz compatible expression.

|Scheduler.PROPERTY_SCHEDULER_CONCURRENT | - |
Define if the job can be run concurrently.

|Scheduler.PROPERTY_SCHEDULER_NAME |-  |
Define the job name.

|=========================================================


==== Schedule a new Job using the Whitebox-Service paradigm (recommended)
This example uses Declarative Services to register a Service of Type "org.apache.karaf.scheduler.Job" so that it is recognized by the Scheduler Service.

Alternatively, jobs can be registered as type "Runnable" in a more API neutral way. In this case you won't get the "JobContext" information though.
----
@Component(immediate = true, property = {
        Scheduler.PROPERTY_SCHEDULER_EXPRESSION + "=0 0/10 * * * ?",
} )
public class SchedulerPing implements Job {

    @Override
    public void execute(JobContext context) {
        // ..
    }
}
----

This will register a Job with the WhiteboxHandler. You can verify that the job is registered:

----
karaf@root()> scheduler:list
Name                   │ Schedule
───────────────────────┼─────────────────────
Registered Service.185 │ cron(0 0/10 * * * ?)
----

The Karaf scheduler can also schedule `Runnable` service.

For instance, if you have the following bean:

```
@Component(immediate = true, property = {
    "scheduler.period:Long=60",
    "scheduler.concurrent:Boolean=false",
    "scheduler.name=PingJob"
  }
)
public class PingThread implements Runnable {

  @Override
  public void run() {
    // ..
  }

}
```

This will register a job for the thread (runnable):

----
karaf@root()> scheduler:list
Name                       │ Schedule
───────────────────────────┼──────────────────────────────────────────
PingJob.126                │ at(2017-11-22T15:37:17.103+01:00, -1, 10)
----

It's also possible to pass the number of execution of the scheduler job using the `scheduler.times` service property:

```
@Component(immediate = true, property = {
    "scheduler.period:Long=60",
    "scheduler.times:Integer=5",
    "scheduler.concurrent:Boolean=false",
    "scheduler.name=PingJob"
  }
)
public class PingThread implements Runnable {

  @Override
  public void run() {
    // ..
  }

}
```

==== Schedule a new Job using the Gogo Shell

----
karaf@root()> scheduler:schedule --help
DESCRIPTION
        scheduler:schedule

	Schedule a script execution

SYNTAX
        scheduler:schedule [options] script

ARGUMENTS
        script
                The script to schedule

OPTIONS
        --at
                Absolute date in ISO format (ex: 2014-05-13T13:56:45)
        --concurrent
                Should jobs run concurrently or not (defaults to false)
        --period
                Time during executions (in seconds)
        --times
                Number of times this job should be executed
                (defaults to -1)
        --cron
                The cron expression
        --help
                Display this help message
        --name
                Name of this job

----

==== Schedule a new Job using the Scheduler Service

Recommendation: Before using this low level api for registering jobs, consider using the whitebox approach instead.

----
..
import org.apache.karaf.scheduler.Scheduler;

@Component
public class Demo {

  @Reference Scheduler scheduler;

  public void useScheduler()
  {
    schedule(new MyJob(), scheduler.EXPR("0 0/10 * * * ?"));
  }

  class MyJob implements Job {
    ..
  }

}

----

==== Update scheduling of an existing job

You can change the scheduling of an existing job using `scheduler:reschedule` command.

This command works as the schedule command (using the same `at`, `period`, `cron`, ... options) but taking the job name
as argument (as given by the `scheduler:list` command).