Skip to content

Scheduled Cron Tasks

New

Introduced in 0.23.0

cron:list <app> [--format json|stdout]  # List scheduled cron tasks for an app
cron:report [<app>] [<flag>]            # Display report about an app
cron:resume <app> <cron_id>             # Resume a cron task
cron:run <app> <cron_id> [--detach]     # Run a cron task on the fly
cron:set [--global|<app>] <key> <value> # Set or clear a cron property for an app
cron:suspend <app> <cron_id>            # Suspend a cron task

Usage

Dokku Managed Cron

Dokku automates scheduled dokku run commands via it's app.json cron integration.

Specifying commands

The app.json file for a given app can define a special cron key that contains a list of commands to run on given schedules. The following is a simple example app.json that effectively runs the command dokku run $APP npm run send-email once a day:

{
  "cron": [
    {
      "command": "npm run send-email",
      "schedule": "@daily"
    }
  ]
}

A cron task takes the following properties:

  • command: A command to be run within the built app image. Specified commands can also be Procfile entries.
  • maintenance: A boolean value that decides whether the cron task is in maintenance and therefore executable or not.
  • schedule: A cron-compatible scheduling definition upon which to run the command. Seconds are generally not supported.
  • concurrency_policy: A string (default: allow), that controls whether the cron task can be run concurrently with another invocation of itself. Valid options are allow (allow concurrency), forbid (exit the new cron task if there is an existing one), replace (delete any existing cron task and start the new one).

Zero or more cron tasks can be specified per app. Cron tasks are validated after the build artifact is created but before the app is deployed, and the cron schedule is updated during the post-deploy phase.

Cron tasks can run for a maximum of 24 hours via the docker-local scheduler, after which they are reaped from the system.

See the app.json location documentation for more information on where to place your app.json file.

Task Environment

When running scheduled cron tasks, there are a few items to be aware of:

  • Scheduled cron tasks are performed within the app environment available at runtime. If the app image does not exist, the command may fail to execute.
  • Schedules are performed on the hosting server's timezone, which is typically UTC.
  • At this time, only the PATH and SHELL environment variables are specified in the cron template.
    • A MAILTO value can be set via the cron:set command.
    • A MAILFROM value can be set via the cron:set command.
  • Each scheduled task is executed within a one-off run container, and thus inherit any docker-options specified for run containers. Resources are never shared between scheduled tasks.
  • Scheduled cron tasks are supported on a per-scheduler basis, and are currently only implemented by the docker-local scheduler.
  • Tasks for all apps managed by the docker-local scheduler are written to a single crontab file owned by the dokku user. The dokku user's crontab should be considered reserved for this purpose.

Changing cron management settings

The cron plugin provides a number of settings that can be used to managed deployments on a per-app basis. The following table outlines ones not covered elsewhere:

Name Description Level Global Default
mailfrom Sets the MAILFROM variable in a cron file for cron reporting Global-only empty string
maintenance Whether to have cron running for the app or not. App-only false
mailto Sets the MAILTO variable in a cron file for cron reporting Global-only empty string

All settings can be set via the cron:set command. Using maintenance as an example:

dokku cron:set node-js-app maintenance true

The default value may be set by passing an empty value for the option in question:

dokku cron:set node-js-app maintenance

If a property can be set globally - such as mailto, use the --global flag. If not set for an app, the global value will apply if it exists.

dokku cron:set --global maintenance true

The global default value may be set by passing an empty value for the option.

dokku cron:set --global maintenance

Listing Cron tasks

Cron tasks for an app can be listed via the cron:list command. This command takes an app argument.

dokku cron:list node-js-app

ID                                    Schedule   Command
cGhwPT09cGhwIHRlc3QucGhwPT09QGRhaWx5  @daily     node index.js
cGhwPT09dHJ1ZT09PSogKiAqICogKg==      * * * * *  true

The output can also be displayed in json format:

dokku cron:list node-js-app --format json

[{"id":"cGhwPT09cGhwIHRlc3QucGhwPT09QGRhaWx5","app":"node-js-app","command":"node index.js","schedule":"@daily"}]

To fetch global tasks, use the --global flag:

dokku cron:list --global

ID                            Schedule  Command
5cruaotm4yzzpnjlsdunblj8qyjp  @daily    /bin/true

Suspending and resuming a specific cron task

Cron tasks can be suspended to temporarily prevent them from running, and later resumed to re-enable them. This is useful for maintenance or debugging purposes.

To suspend a specific cron task, use the cron:suspend command with the app name and cron ID:

dokku cron:suspend node-js-app cGhwPT09cGhwIHRlc3QucGhwPT09QGRhaWx5

A suspended task will not execute according to its schedule. You can verify a task is suspended by checking the Maintenance column in the cron:list output, which will show true (task) for suspended tasks.

To resume a suspended cron task, use the cron:resume command:

dokku cron:resume node-js-app cGhwPT09cGhwIHRlc3QucGhwPT09QGRhaWx5

Once resumed, the task will execute according to its schedule again. The cron ID can be retrieved from the cron:list output.

Executing a cron task on the fly

Cron tasks can be invoked via the cron:run command. This command takes an app argument and a cron id (retrievable from cron:list output).

dokku cron:run node-js-app cGhwPT09cGhwIHRlc3QucGhwPT09QGRhaWx5

By default, the task is run in an attached container - as supported by the scheduler. To run in a background detached container, specify the --detach flag:

dokku cron:run node-js-app cGhwPT09cGhwIHRlc3QucGhwPT09QGRhaWx5 --detach

All one-off cron executions have their containers terminated after invocation.

Displaying reports

You can get a report about the cron configuration for apps using the cron:report command:

dokku cron:report

=====> node-js-app cron information
       Cron task count:               2
=====> python-sample cron information
       Cron task count:               0
=====> ruby-sample cron information
       Cron task count:               10

You can run the command for a specific app also.

dokku cron:report node-js-app

=====> node-js-app cron information
       Cron task count:               2

You can pass flags which will output only the value of the specific information you want. For example:

dokku cron:report node-js-app --cron-task-count

Self Managed Cron

Warning

Self-managed cron tasks should be considered advanced usage. While the instructions are available, users are highly encouraged to use the built-in scheduled cron task support unless absolutely necessary.

Some installations may require more fine-grained control over cron usage. The following are advanced instructions for configuring cron.

Using run for cron tasks

You can always use a one-off container to run an app task:

dokku run node-js-app some-command

For tasks that should not be interrupted, run is the preferred method of handling cron tasks, as the container will continue running even during a deploy or scaling event. The trade-off is that there will be an increase in memory usage if there are multiple concurrent tasks running.

Using enter for cron tasks

Your Procfile can have the following entry:

cron: sleep infinity

With the cron process scaled to 1:

dokku ps:scale node-js-app cron=1

You can now run all your commands in that container:

dokku enter node-js-app cron some-command

Note that you can also run multiple commands at the same time to reduce memory usage, though that may result in polluting the container environment.

For tasks that will properly resume, you should use the above method, as running tasks will be interrupted during deploys and scaling events, and subsequent commands will always run with the latest container. Note that if you scale the cron container down, this may interrupt proper running of the task.

General cron recommendations

Regularly scheduled tasks can be a bit of a pain with Dokku. The following are general recommendations to follow to help ensure successful task runs.

  • Use the dokku user in your cron task.
    • If you do not, the dokku binary will attempt to execute with sudo, and your cron run with fail with sudo: no tty present and no askpass program specified.
  • Add a MAILTO environment variable to ship cron emails to yourself.
  • Add a PATH environment variable or specify the full path to binaries on the host.
  • Add a SHELL environment variable to specify Bash when running commands.
  • Keep your cron tasks in time-sorted order.
  • Keep your server time in UTC so you don't need to translate daylight savings time when reading the cronfile.
  • Run tasks at the lowest traffic times if possible.
  • Use cron to trigger jobs, not run them. Use a real queuing system such as rabbitmq to actually process jobs.
  • Try to keep tasks quiet so that mails only send on errors.
  • Do not silence standard error or standard out. If you silence the former, you will miss failures. Silencing the latter means you should actually make app changes to handle log levels.
  • Use a service such as Dead Man's Snitch to verify that cron tasks completed successfully.
  • Add lots of comments to your cronfile, including what a task is doing, so that you don't spend time deciphering the file later.
  • Place your cronfiles in a pattern such as /etc/cron.d/APP.
  • Do not use non-ASCII characters in your cronfile names. cron is finicky.
  • Remember to have trailing newlines in your cronfile! cron is finicky.

The following is a sample cronfile that you can use for your apps:

# server cron jobs
MAILTO="mail@dokku.me"
PATH=/usr/local/bin:/usr/bin:/bin
SHELL=/bin/bash

# m   h   dom mon dow   username command
# *   *   *   *   *     dokku    command to be executed
# -   -   -   -   -
# |   |   |   |   |
# |   |   |   |   +----- day of week (0 - 6) (Sunday=0)
# |   |   |   +------- month (1 - 12)
# |   |   +--------- day of month (1 - 31)
# |   +----------- hour (0 - 23)
# +----------- min (0 - 59)

### HIGH TRAFFIC TIME IS B/W 00:00 - 04:00 AND 14:00 - 23:59
### RUN YOUR TASKS FROM 04:00 - 14:00
### KEEP SORTED IN TIME ORDER

### PLACE ALL CRON TASKS BELOW

# removes unresponsive users from the subscriber list to decrease bounce rates
0 0 * * * dokku dokku run node-js-app some-command

# sends out our email alerts to users
0 1 * * * dokku dokku ps:scale node-js-app cron=1 && dokku enter node-js-app cron some-other-command && dokku ps:scale node-js-app cron=0

### PLACE ALL CRON TASKS ABOVE, DO NOT REMOVE THE WHITESPACE AFTER THIS LINE