Enhanced cron scheduling that's configurable at runtime across your entire cluster.

DynamicCron can configure cron scheduling before boot or during runtime, globally, with scheduling guarantees and per-entry timezone overrides. It's an ideal solution for applications that can't miss a cron job or must dynamically start and manage scheduled jobs at runtime.

Using and Configuring

To begin using DynamicCron, add the module to your list of Oban plugins in config.exs:

config :my_app, Oban,
  plugins: [Oban.Pro.Plugins.DynamicCron]
  ...

By itself, without providing a crontab or dynamically inserting cron entries, the plugin doesn't have anything to schedule. To get scheduling started, provide a list of {cron, worker} or {cron, worker, options} tuples to the plugin. The syntax is identical to Oban's built in :crontab option, which means you can copy an existing standard :crontab list into the plugin's :crontab.

plugins: [{
  Oban.Pro.Plugins.DynamicCron,
  crontab: [
    {"* * * * *", MyApp.MinuteJob},
    {"0 * * * *", MyApp.HourlyJob, queue: :scheduled},
    {"0 0 * * *", MyApp.DailyJob, max_attempts: 1},
    {"0 12 * * MON", MyApp.MondayWorker, tags: ["scheduled"]},
    {"@daily", MyApp.AnotherDailyWorker}
  ]
}]

Now, when dynamic cron initializes, it will persist those cron entries to the database and start scheduling them according to their CRON expression. The plugin's crontab format is nearly identical to Oban's standard crontab, with a few important enhancements we'll look at soon.

Each of the crontab entries are persisted to the database and referenced globally, by all the other connected Oban instances. That allows us to insert, update, or delete cron entries at any time. In fact, changing the schedule or options of an entry in the crontab provided to the plugin will automatically update the persisted entry. To demonstrate, let's modify the MinuteJob we specified so that it runs every other minute in the :scheduled queue:

crontab: [
  {"*/2 * * * *", MyApp.MinuteJob, queue: :scheduled},
  ...
]

Now it isn't really a "minute job" any more, and the name is no longer suitable. However, we didn't provide a name for the entry and it's using the module name instead. To provide more flexibility we can add a :name overrride, then we can update the worker's name as well:

crontab: [
  {"*/2 * * * *", MyApp.FrequentJob, name: "frequent", queue: :scheduled},
  ...
]

All entries are referenced by name, which defaults to the worker's name and must be unique. You may define the same worker multiple times as long as you provide a name override:

crontab: [
  {"*/3 * * * *", MyApp.BasicJob, name: "client-1", args: %{client_id: 1}},
  {"*/3 * * * *", MyApp.BasicJob, name: "client-2", args: %{client_id: 2}},
  ...
]

To temporarily disable scheduling jobs you can set the paused flag:

crontab: [
  {"* * * * *", MyApp.BasicJob, paused: true},
  ...
]

To resume the job you must supply paused: false (or use update/2 to resume it manually), simply removing the paused option will have no effect.

crontab: [
  {"* * * * *", MyApp.BasicJob, paused: false},
  ...
]

It is also possible to delete a persisted entry during initialization by passing the :delete option:

crontab: [
  {"* * * * *", MyApp.MinuteJob, delete: true},
  ...
]

One or more entries can be deleted this way. Deleting entries is idempotent, nothing will happen if no matching entry can be found.

Automatic Synchronization

Synchronizing persisted entries manually requires two deploys: one to flag it with deleted: true and another to clean up the entry entirely. That extra step isn't ideal for applications that don't insert or delete jobs at runtime.

To delete entries that are no longer listed in the crontab automatically set the sync_mode option to :automatic:

[
  sync_mode: :automatic,
  crontab: [
    {"0 * * * *", MyApp.BasicJob},
    {"0 0 * * *", MyApp.OtherJob}
  ]
]

To remove unwanted entries, simply delete them from the crontab:

 crontab: [
   {"0 * * * *", MyApp.BasicJob},
-  {"0 0 * * *", MyApp.OtherJob}
 ]

With :automatic sync, the entry for MyApp.OtherJob will be deleted on the next deployment.

Scheduling Guarantees

Depending on an application's restart timing or as the result of unexpected downtime, a job's scheduled insert period may be missed. To compensate, enable guaranteed mode for the entire crontab or on an individual bases.

Here's an example of enabling guaranteed insertion for the entire crontab:

[
  guaranteed: true,
  crontab: [
    {"0 * * * *", MyApp.HourlyJob},
    {"0 0 * * *", MyApp.DailyJob},
    {"0 0 1 * *", MyApp.MonthlyJob},
    {"0 0 1 */2 *", MyApp.BiMonthlyJob}
  ]
]

Guaranteed mode may be enabled or disabled for individual jobs instead if it's a poor fit for the entire crontab:

[
  crontab: [
    {"@daily", MyApp.DailyJob, guaranteed: true}
    {"@monthly", MyApp.MonthlyJob, guaranteed: false}
  ]
]

Guaranteed insertion is enforced by cron name and it's durable across option changes. Changing an entry's expression or timezone will clear any prior insertions and reset the guarantee.

Overriding the Timezone

Without any configuration the default timezone is Etc/UTC. You can override that for all cron entries by passing a timezone option to the plugin:

plugins: [{
  Oban.Pro.Plugins.DynamicCron,
  timezone: "America/Chicago",
  # ...

You can also override the timezone for individual entries by passing it as an option to the crontab list or to DynamicCron.insert/1:

DynamicCron.insert([
  {"0 0 * * *", MyApp.Pinger, name: "oslo", timezone: "Europe/Oslo"},
  {"0 0 * * *", MyApp.Pinger, name: "chicago", timezone: "America/Chicago"},
  {"0 0 * * *", MyApp.Pinger, name: "zagreb", timezone: "Europe/Zagreb"}
])

Runtime Updates

Dynamic cron entries are persisted to the database, making it easy to manipulate them through typical CRUD operations. The DynamicCron plugin provides convenience functions to simplify working those operations.

The insert/1 function takes a list of one or more tuples with the same {expression, worker} or {expression, worker, options} format as the plugin's crontab option:

DynamicCron.insert([
  {"0 0 * * *", MyApp.GenericWorker},
  {"* * * * *", MyApp.ClientWorker, name: "client-1", args: %{client_id: 1}},
  {"* * * * *", MyApp.ClientWorker, name: "client-2", args: %{client_id: 2}},
  {"* * * * *", MyApp.ClientWorker, name: "client-3", args: %{client_id: 3}}
])

Be aware that insert/1 acts like an "upsert", making it possible to modify existing entries if the worker or name matches. Still, it is better to use update/2 to make targeted updates.

Isolation and Namespacing

All DynamicCron functions have an alternate clause that accepts an Oban instance name as the first argument. This is in line with base Oban functions such as Oban.insert/2, which allow you to seamlessly work with multiple Oban instances and across multiple database prefixes. For example, you can use all/1 to list all cron entries for the instance named ObanPrivate:

entries = DynamicCron.all(ObanPrivate)

Likewise, to insert a new entry using the configuration associated with the ObanPrivate instance:

{:ok, _} = DynamicCron.insert(ObanPrivate, [{"* * * * *", PrivateWorker}])

Instrumenting with Telemetry

The DynamicCron plugin adds the following metadata to the [:oban, :plugin, :stop] event:

• :jobs - a list of jobs that were inserted into the database