Maelstrom

Cron

A cron event source activates your component at a timed interval specified in the schedule field. When a cron event occurs, a HTTP request is made to your component on the path specified.

This allows you to run periodic jobs without an additional scheduler.

In a cluster with more than one node, a single node is responsible for triggering cron events at any time.

The cron system is implementing using robfig/cron, so full documentation on the different schedule string formats is available in that package’s documentation.

This library is invoked with default options which enables the “standard” cron parser, which expects the date patterns to have 5 fields starting with minute. Quartz style cron rules (which support second granularity) are not currently supported.

NOTE: Make sure to use quotes around the schedule value to avoid YAML parsing issues with * characters.

Shorthand “crontab” support

Since some projects have a large number of scheduled jobs, a more compact “crontab” format is supported in maelstrom.yml files. Each line is parsed and added to the event source list. You may combine the crontab field with standard eventsources fields.

Each cron entry created from the crontab block has these attributes:

  • Event source name: <component name>-cron-<index> (zero based)
  • HTTP method: GET
  • No HTTP data or headers

Examples

Example 1: Run every hour - make GET request 

components:
  mywebapp:
    image: coopernurse/go-hello-http
    eventsources:
      backup_db_hourly:
        cron:
          schedule: "@every 1h"
          http:
            method: GET
            path: /jobs/backup_db

Example 2: Make a POST request nightly with custom headers 

components:
  mywebapp:
    image: coopernurse/go-hello-http
    eventsources:
      some_job_nightly:
        cron:
          schedule: "30 * * * *"
          http:
            method: POST
            path: /jobs/some_job
            data: '{"key": "value", "key2", "value2}'
            headers:
              - name: Content-Type
                value: application/json

Example 3: Define 3 cron rules via the crontab block 

components:
  mywebapp:
    image: coopernurse/go-hello-http
    # Important: use a | (not >) to define the crontab multi-line block
    crontab: |
        # lines starting with hash are ignored
        @every 30m    /cron/job1
        # second job:
        30 * * * *    /cron/job2
        * */2 * * *   /cron/job3
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