View Source 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, "~> 0.2.4"}
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:
- you can set
config :ash_oban, authorize?: false
(easiest, reverts to old behavior, but not recommended) - 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:
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.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.