PubSub

Ash includes a builtin notifier to help you publish events over any kind of pub-sub pattern. This is plug and play with Phoenix.PubSub , but could be used with any pubsub pattern.

You configure a module that defines a broadcast/3 function, and then add some “publications” which configure under what conditions an event should be sent and what the topic should be.

For the full DSL spec see Ash.Notifier.PubSub

Debugging PubSub

It can be quite frustrating when setting up pub_sub when everything appears to be set up properly, but you aren’t receiving events. This usually means some kind of mismatch between the event names produced by the resource/config of your publications, and you can use the following flag to display debug information about pub sub events coming from Ash.Notifier.PubSub

config :ash, :pub_sub, debug?: true

Topic Templates

Often you want to include some piece of data in the thing being changed, like the :id attribute. This is done by providing a list as the topic, and using atoms which will be replaced by their corresponding values. They will ultimately be joined with : .

For example:

prefix "user"

publish :create, ["created", :user_id]

This might publish a message to “user:created:1” for example.

For updates, if the field in the template is being changed, a message is sent to both values. So if you change user 1 to user 2 , the same message would be published to user:updated:1 and user:updated:2 . If there are multiple attributes in the template, and they are all being changed, a message is sent for every combination of substitutions.

Important

If the previous value was nil or the field was not selected on the data passed into the action, then a notification is not sent for the previous value.

If the new value is nil then a notification is not sent for the new value.

Template parts

Templates may contain lists, in which case all combinations of values in the list will be used. Add nil to the list if you want to produce a pattern where that entry is omitted.

The atom :_tenant may be used. If the changeset has a tenant set on it, that value will be used, otherwise that combination of values is ignored.

The atom :_pkey may be used. It will be a stringified, concatenation of the primary key fields, or just the primary key if there is only one primary key field.

The atom nil may be used. It only makes sense to use it in the context of a list of alternatives, and adds a pattern where that part is skipped.

publish :updated, [[:team_id, :_tenant], "updated", [:id, nil]]

Would produce the following messages, given a team_id of 1, a tenant of org_1 , and an id of 50 :

"1:updated:50"
"1:updated"
"org_1:updated:50"
"org_1:updated"

Usage with Phoenix

Phoenix expects a specific shape of data to be broadcasted, and since it is so often used with Ash, instead of making you define your own notifier that creates the %Phoenix.Socket.Broadcast struct and publishes it, Ash has an option to do that automatically, via

broadcast_type :phoenix_broadcast

Named Pubsub modules

If you are using a phoenix Endpoint module for pubsub then this is unnecessary. If you want to use a custom pub sub started with something like {Phoenix.PubSub, name: MyName} , then you can provide MyName to here.

Broadcast Types

Configured with broadcast_type .

• :notification just sends the notification
• :phoenix_broadcast sends a %Phoenix.Socket.Broadcast{} (see above)
• :broadcast sends %{topic: (topic), event: (event), notification: (notification)}