[[schedule-cron]]
==== `cron` Schedule
A <<trigger-schedule, `schedule`>> trigger that enables you to use a http://unixhelp.ed.ac.uk/CGI/man-cgi?crontab+5[cron] style expression to specify when you want the scheduler to start the watch execution. Watcher uses the cron parser from the http://www.quartz-scheduler.org[Quartz Job Scheduler]. For more information about writing Quartz cron expressions, see the http://www.quartz-scheduler.org/documentation/quartz-1.x/tutorials/crontrigger[Quartz CronTrigger Tutorial]. 

WARNING: While `cron` triggers are super powerful, we recommend using one of the other schedule types if you can, as they
are much more straightforward to configure. If you use `cron`, construct your `cron` expressions with care to be sure you
are actually setting the schedule you want. You can use the <<croneval, `croneval`>> tool to validate your cron expressions and see what the resulting trigger times will be.

===== Cron Expressions

A cron expression is a string of the following form: 
    
    <seconds> <minutes> <hours> <day_of_month> <month> <day_of_week> [year]

All elements are required except for `year`. <<schedule-cron-elements>> shows the valid values for each 
element in a cron expression.

[[schedule-cron-elements]]
.Cron Expression Elements
[options="header"]
|======
| Name           | Required    | Valid Values               | Valid Special Characters
| `seconds`      | yes         | `0`-`59`                   | `,` `-` `*` `/`
| `minutes`      | yes         | `0`-`59`                   | `,` `-` `*` `/`
| `hours`        | yes         | `0`-`23`                   | `,` `-` `*` `/`
| `day_of_month` | yes         | `1`-`31`                   | `,` `-` `*` `/` `?` `L` `W`
| `month`        | yes         | `1`-`12` or `JAN`-`DEC`    | `,` `-` `*` `/`
| `day_of_week`  | yes         | `1`-`7` or `SUN`-`SAT`     | `,` `-` `*` `/` `?` `L` `#`
| `year`         | no          | empty or `1970`-`2099`     | `,` `-` `*` `/`
|======

The special characters you can use in a cron expression are described in <<schedule-cron-special-characters>>.
The names of months and days of the week are not case sensitive. For example, `MON` and `mon` are equivalent.

Be careful when setting trigger times between midnight and 1:00 AM  as daylight savings time changes can 
cause a watch to skip or a repeat depending on whether the time moves back or jumps forward.

NOTE: Currently, you must specify `?` for either the `day_of_week` or `day_of_month`. Explicitly specifying
both values is not supported. 

[[schedule-cron-special-characters]]
.Cron Special Characters
[options="header"]
|======
| Special Character  | Description
| *                  | All values. Selects every possible value for a field. For example, `*` in the `hours` field means "every hour".
| ?                  | No specific value. Use when you don't care what the value is. For example, if you want the schedule to trigger on a particular day of the month, but don't care what day of the week that happens to be, you can specify `?` in the `day_of_week` field. 
| -                  | A range of values (inclusive). Use to separate a minimum and maximum value. For example, if you want
the schedule to trigger every hour between 9:00 AM and 5:00 PM, you could specify  `9-17` in the `hours` field.
| ,                  | Multiple values. Use to separate multiple values for a field. For example, if you want the schedule to trigger every Tueday and Thursday, you could specify `TUE,THU` in the `day_of_week` field.
| /                  | Increment. Use to separate values when specifying a time increment. The first value represents the starting point, and the second value represents the interval. For example, if you want the schedule to trigger every 20 minutes starting at the top of the hour, you could specify `0/20` in the `minutes` field. Similarly, specifying `1/5` in day_of_month field will trigger every 5 days starting on the first day of the month.
| L                  | Last. Use in the `day_of_month` field to mean the last day of the month--day 31 for January, day 28 for February in non-leap years, day 30 for April, and so on. Use alone in the `day_of_week` field in place of `7` or `SAT`, or after a particular day of the week to select the last day of that type in the month. For example `6L` means the last Friday of the month. You can specify
`LW` in the `day_of_month` field to specify the last weekday of the month. Avoid using the `L` option when specifying lists or ranges of values, as the results likely won't be what you expect. 
| W                  | Weekday. Use to specify the weekday (Monday-Friday) nearest the given day. As an example, if you specify `15W` in the `day_of_month` field and the 15th is a Saturday, the schedule will trigger on the 14th. If the 15th is a Sunday, the schedule will trigger on Monday the 16th. If the 15th is a Tuesday, the schedule will trigger on Tuesday the 15th. However if you specify `1W` as the value for `day_of_month`, and the 1st is a Saturday, the schedule will trigger on Monday the 3rd--it won't jump over the month boundary. You can specify `LW` in the `day_of_month` field to specify the last weekday of the month. You can only use the `W` option when the `day_of_month` is a single day--it is not valid when specifying a range or list of days. 
| #                  | Nth XXX day in a month. Use in the `day_of_week` field to specify the nth XXX day of the month. For example, if you specify `6#1`, the schedule will trigger on the first Friday of the month. Note that if you specify `3#5` and there are not 5 Tuesdays in a particular month, the schedule won't trigger that month.
|======

.Setting Daily Triggers
[options="header"]
|======
| Cron Expression             | Description
| `0 5 9 * * ?`               | Trigger at 9:05 AM every day.
| `0 5 9 * * ? 2015`          | Trigger at 9:05 AM every day during the year 2015.
|======

.Restricting Triggers to a Range of Days or Times
[options="header"]
|======
| Cron Expression             | Description
| `0 5 9 ? * MON-FRI`         | Trigger at 9:05 AM Monday through Friday.
| `0 0-5 9 * * ?`             | Trigger every minute starting at 9:00 AM and ending at 9:05 AM every day.
|======

.Setting Interval Triggers
[options="header"]
|======
| Cron&nbsp;Expression&nbsp;  | Description
| `0 0/15 9 * * ?`            | Trigger every 15 minutes starting at 9:00 AM and ending at 9:45 AM every day.
| `0 5 9 1/3 * ?`             | Trigger at 9:05 AM every 3 days every month, starting on the first day of the month.
|======

.Setting Schedules that Trigger on a Particular Day
[options="header"]
|======
| Cron Expression             | Description
| `0 1 4 1 4 ?`               | Trigger every April 1st at 4:01 AM.
| `0 0,30 9 ? 4 WED`          | Trigger at 9:00 AM and at 9:30 AM every Wednesday in the month of April.
| `0 5 9 15 * ?`              | Trigger at 9:05 AM on the 15th day of every month.
| `0 5 9 15W * ?`             | Trigger at 9:05 AM on the nearest weekday to the 15th of every month. 
| `0 5 9 ? * 6#1`             | Trigger at 9:05 AM on the first Friday of every month.
|======

.Setting Triggers Using Last
[options="header"]
|======
| Cron Expression              | Description
| `0 5 9 L * ?`                | Trigger at 9:05 AM on the last day of every month.
| `0 5 9 ? * 2L`               | Trigger at 9:05 AM on the last Monday of every month
| `0 5 9 LW * ?`               | Trigger at 9:05 AM on the last weekday of every month.
|======


===== Configuring a Cron Schedule
To configure a `cron` schedule, you simply specify the cron expression as a string value.
For example, the following snippet configures a `cron` schedule that triggers every day at noon:

[source,json]
--------------------------------------------------
{
  ...
  "trigger" : {
    "schedule" : {
      "cron" : "0 0 12 * * ?"
    }
  }
  ...
}
--------------------------------------------------

[[croneval]]
===== Verifying Cron Expressions

Watcher ships with a `croneval` command line tool that you can use to verify that your cron expressions are
valid and produce the expected results. This tool is
provided in the `$ES_HOME/bin/watcher` directory. 

To verify a cron expression, simply pass it in as a string to `croneval`: 

[source,bash]
--------------------------------------------------
bin/watcher/croneval "0 0/1 * * * ?"
--------------------------------------------------

If the cron expression is valid, `croneval` displays the next 10 times that the schedule will be triggered.

You can specify the  `-c` option to control how many future trigger times are displayed. For example, 
the following command displays the next 20 trigger times. 

[source,bash]
--------------------------------------------------
bin/watcher/croneval "0 0/1 * * * ?" -c 20 
--------------------------------------------------