Scheduler Module

• 1. Introduction
  • 1.1. Activating
  • 1.2. Type
• 2. Java vs. XQuery Jobs
  • 2.1. Introduction
  • 2.2. XQuery Jobs
  • 2.3. Java Jobs
• 3. Periodic
  • 3.1. Introduction
• 4. Cron
  • 4.1. Introduction
  • 4.2. Format
  • 4.3. Special characters
  • 4.4. Examples
  • 4.5. Notes

1. Introduction

Quartz is a full-featured, open source job scheduling system that has been integrated with eXist. Quartz can be used to create simple or complex schedules for executing tens, hundreds, or even tens-of-thousands of jobs. The Quartz Scheduler includes many enterprise-class features, such as JTA transactions and clustering.

1.1. Activating

There are two type of uses of the Quartz Scheduler within eXist. The first is always active and is invoked at system startup. The second is the XQuery function module for initiating and managing scheduled jobs. This second use is not activated out-of-the-box. It has to be activated through the conf.xml file.

System Startup

The jobs that are initiated at system startup are invoked by the settings within the <scheduler> tag in the conf.xml file. There is a full set of instructions with example jobs (that are commented out) in the file.

XQuery Function Module

The scheduler XQuery function module is activated by uncommenting the following in conf.xml:

<module class="org.exist.xquery.modules.scheduler.SchedulerModule"
         uri="http://exist-db.org/xquery/scheduler" />

Once the scheduler XQuery function module is active, then XQuery code can be written to invoke and manage user type jobs.

1.2. Type

The type of the job to schedule. Must be either "startup", "system" or "user".

2. Java vs. XQuery Jobs

2.1. Introduction

There are two types of jobs that can be scheduled. They are Java and XQuery. Java jobs can be startup, system or user. XQuery jobs can only be user jobs.

2.2. XQuery Jobs

If the job is written in XQuery (not suitable for startup or system jobs) then this should be a path to an XQuery stored in the database. e.g. /db/myCollection/myJob.xql

XQuery job's will be launched under the guest account initially, although the running XQuery may switch permissions through calls to xmldb:login().

2.3. Java Jobs

A Java job that is to be invoked from the XQuery function module has to be extended from the org.exist.scheduler.UserJavaJob class. A startup or system Java job that is invoked from the conf.xml file implements org.exist.storage.SystemTask.

package org.exist.scheduler;

import java.util.Map;

import org.exist.storage.BrokerPool;

public class TestJob extends UserJavaJob {

    private String jobName = this.getClass().getName();
    
	public void execute(BrokerPool brokerpool, Map params) throws JobException {
		
		System.out.println("****** TEST JOB EXECUTED ******");

	}

	public String getName() {
		return jobName;
	}

    public void setName(String name) {
        this.jobName = name;
    }
}
                

3. Periodic

3.1. Introduction

A scheduled job can be run on a periodic basis. When the job is scheduled, it can be specified to run every n milliseconds. There is the additional option to start after x milliseconds and to only be repeated y times after the initial execution.

4. Cron

4.1. Introduction

cron is a UNIX tool that has been around for a long time, so its scheduling capabilities are powerful and proven. The CronTrigger class is based on the scheduling capabilities of cron.

CronTrigger uses "cron expressions", which are able to create firing schedules such as: "At 8:00am every Monday through Friday" or "At 1:30am every last Friday of the month".

Cron expressions are powerful, but can be pretty confusing. This tutorial aims to take some of the mystery out of creating a cron expression, giving users a resource which they can visit before having to ask in a forum or mailing list.

4.2. Format

A cron expression is a string comprised of 6 or 7 fields separated by white space. Fields can contain any of the allowed values, along with various combinations of the allowed special characters for that field. The fields are as follows:

So cron expressions can be as simple as this: * * * * ? *

or more complex, like this: 0 0/5 14,18,3-39,52 ? JAN,MAR,SEP MON-FRI 2002-2010

4.3. Special characters

• * ("all values") - used to select all values within a field. For example, "*" in the minute field means "every minute".
• ? ("no specific value") - useful when you need to specify something in one of the two fields in which the character is allowed, but not the other. For example, if I want my trigger to fire on a particular day of the month (say, the 10th), but don't care what day of the week that happens to be, I would put "10" in the day-of-month field, and "?" in the day-of-week field. See the examples below for clarification.
• - - used to specify ranges. For example, "10-12" in the hour field means "the hours 10, 11 and 12".
• , - used to specify additional values. For example, "MON,WED,FRI" in the day-of-week field means "the days Monday, Wednesday, and Friday".
• / - used to specify increments. For example, "0/15" in the seconds field means "the seconds 0, 15, 30, and 45". And "5/15" in the seconds field means "the seconds 5, 20, 35, and 50". You can also specify '/' after the '' character - in this case '' is equivalent to having '0' before the '/'. '1/3' in the day-of-month field means "fire every 3 days starting on the first day of the month".
• L ("last") - has different meaning in each of the two fields in which it is allowed. For example, the value "L" in the day-of-month field means "the last day of the month" - day 31 for January, day 28 for February on non-leap years. If used in the day-of-week field by itself, it simply means "7" or "SAT". But if used in the day-of-week field after another value, it means "the last xxx day of the month" - for example "6L" means "the last friday of the month". When using the 'L' option, it is important not to specify lists, or ranges of values, as you'll get confusing results.
• W ("weekday") - used to specify the weekday (Monday-Friday) nearest the given day. As an example, if you were to specify "15W" as the value for the day-of-month field, the meaning is: "the nearest weekday to the 15th of the month". So if the 15th is a Saturday, the trigger will fire on Friday the 14th. If the 15th is a Sunday, the trigger will fire on Monday the 16th. If the 15th is a Tuesday, then it will fire on Tuesday the 15th. However if you specify "1W" as the value for day-of-month, and the 1st is a Saturday, the trigger will fire on Monday the 3rd, as it will not 'jump' over the boundary of a month's days. The 'W' character can only be specified when the day-of-month is a single day, not a range or list of days. The 'L' and 'W' characters can also be combined in the day-of-month field to yield 'LW', which translates to "last weekday of the month".
• # - used to specify "the nth" XXX day of the month. For example, the value of "6#3" in the day-of-week field means "the third Friday of the month" (day 6 = Friday and "#3" = the 3rd one in the month). Other examples: "2#1" = the first Monday of the month and "4#5" = the fifth Wednesday of the month. Note that if you specify "#5" and there is not 5 of the given day-of-week in the month, then no firing will occur that month. The legal characters and the names of months and days of the week are not case sensitive. MON is the same as mon.

4.4. Examples

Here are some full examples:

Pay attention to the effects of '?' and '*' in the day-of-week and day-of-month fields!

4.5. Notes

• Support for specifying both a day-of-week and a day-of-month value is not complete (you must currently use the '?' character in one of these fields).
• Be careful when setting fire times between mid-night and 1:00 AM - "daylight savings" can cause a skip or a repeat depending on whether the time moves back or jumps forward.