getting-started-with-ash-oban.md

Getting Started With Ash Oban

AshOban will likely grow to provide many more oban-related features, but for now the primary focus is on "triggers".

A trigger describes an action that is run periodically.

Get familiar with Ash resources

If you haven't already, read the Ash Getting Started Guide, and familiarize yourself with Ash and Ash resources.

Bring in the ash_oban dependency

{:ash_oban, "~> 0.3.3"}

Setup

First, follow the Oban setup guide.

Oban Pro

If you are using Oban Pro, set the following configuration:

config :ash_oban, :pro?, true

Oban Pro lives in a separate hex repository, and therefore we, unfortunately, cannot have an explicit version dependency on it. What this means is that any version you use in hex will technically be accepted, and if you don't have the oban pro package installed and you use the above configuration, you will get compile time errors/warnings.

Setting up AshOban

Next, allow AshOban to alter your configuration in your Application module:

# Replace this
{Oban, your_oban_config}

# With this
{Oban, AshOban.config(Application.fetch_env!(:my_app, :ash_domains), your_oban_config)}
# OR this, to selectively enable AshOban only for specific domains
{Oban, AshOban.config([YourDomain, YourOtherDomain], your_oban_config)}

Usage

Finally, configure your triggers in your resources.

Add the AshOban extension:

use Ash.Resource, domain: MyDomain, extensions: [AshOban]

For example:

oban do
  triggers do
    # add a trigger called `:process`
    trigger :process do
      # this trigger calls the `process` action
      action :process
      # for any record that has `processed != true`
      where expr(processed != true)
      # checking for matches every minute
      scheduler_cron "* * * * *"
      on_error :errored
    end
  end
end

See the DSL documentation for more: AshOban

Handling Errors

Error handling is done by adding an on_error to your trigger. This is an update action that will get the error as an argument called :error. The error will be an Ash error class. These error classes can contain many kinds of errors, so you will need to figure out handling specific errors on your own. Be sure to add the :error argument to the action if you want to receive the error.

This is not foolproof. You want to be sure that your on_error action is as simple as possible, because if an exception is raised during the on_error action, the oban job will fail. If you are relying on your on_error logic to alter the resource to make it no longer apply to a trigger, consider making your action do only that. Then you can add another trigger watching for things in an errored state to do more rich error handling behavior.

Changing Triggers when using Oban Pro

To remove or disable triggers, do not just remove them from your resource. Due to the way that Oban Pro implements cron jobs, if you just remove them from your resource, the cron will attempt to continue scheduling jobs. Instead, set paused true or delete true on the trigger. See the oban docs for more: https://getoban.pro/docs/pro/0.14.1/Oban.Pro.Plugins.DynamicCron.html#module-using-and-configuring

PS: delete true is also indempotent, so there is no issue with deploying with that flag set to true multiple times. After you have deployed once with delete true you can safely delete the trigger.

When not using Oban Pro, all crons are simply loaded on boot time and there is no side effects to simply deleting an unused trigger.

Transactions

AshOban adds two new transaction reasons, as it uses explicit transactions to ensure that each triggered record is properly locked and executed in serially.

%{
  type: :ash_oban_trigger,
  metadata: %{
    resource: Resource,
    trigger: :trigger_name,
    primary_key: %{primary_key_fields: value}
  }
}

and

%{
  type: :ash_oban_trigger_error,
  metadata: %{
    resource: Resource
    trigger: :trigger_name,
    primary_key: %{primary_key_fields: value},
    error: <the error (this will be an ash error class)>
  }
}

Authorizing actions

As of v0.2, authorize?: true is passed into every action that is called. This may be a breaking change for some users that are using policies. There are two ways to get around this:

1. you can set config :ash_oban, authorize?: false (easiest, reverts to old behavior, but not recommended)
2. you can install the bypass at the top of your policies in any resource that you have triggers on that has policies:

policies do
  bypass AshOban.Checks.AshObanInteraction do
    authorize_if always()
  end

  ...the rest of your policies
end

Persisting the actor along with a job

Create a module that is responsible for translating the current user to a value that will be JSON encoded, and for turning that encoded value back into an actor.

defmodule MyApp.AshObanActorPersister do
  use AshOban.PersistActor

  def store(%MyApp.User{id: id}), do: %{"type" => "user", "id" => id}

  def lookup(%{"type" => "user", "id" => id}), do: MyApp.Accounts.get(MyApp.User, id)

  # This allows you to set a default actor
  # in cases where no actor was present
  # when scheduling.
  def lookup(nil), do: {:ok, nil}
end

Then, configure this in application config.

config :ash_oban, :actor_persister, MyApp.AshObanActorPersister

Considerations

There are a few things that are important to keep in mind:

1. The actor could be deleted or otherwise unavailable when you look it up. You very likely want your lookup/1 to return an error in that scenario.

2. The actor can have changed between when the job was scheduled and when the trigger is executing. It can even change across retries. If you are trying to authorize access for a given trigger's update action to a given actor, keep in mind that just because the trigger is running for a given action, does not mean that the conditions that allowed them to originally schedule that action are still true.