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# DSL: Ash.Notifier.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`

```elixir
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:

```elixir
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.

```elixir
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`:

```elixir
"1:updated:50"
"1:updated"
"org_1:updated:50"
"org_1:updated"
```

## Custom Delimiters

It's possible to change the default delimiter used when generating topics. This is useful when working with message brokers
like RabbitMQ, which rely on a different set of delimiters for routing.


```elixir
pub_sub do
  delimiter "."
end
```

## 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)}`


## pub_sub
A section for configuring how resource actions are published over pubsub

See `Ash.Notifier.PubSub` and the [Notifiers](/documentation/topics/notifiers.md) guide for more.


### Nested DSLs
 * [publish](#pub_sub-publish)
 * [publish_all](#pub_sub-publish_all)


### Examples
```
pub_sub do
  module MyEndpoint
  prefix "post"

  publish :destroy, ["foo", :id]
  publish :update, ["bar", :name] event: "name_change"
  publish_all :create, "created"
end

```


### Options

| Name | Type | Default | Docs |
|------|------|---------|------|
| [`module`](#pub_sub-module){: #pub_sub-module .spark-required} | `atom` |  | The module to call `broadcast/3` on e.g module.broadcast(topic, event, message). |
| [`prefix`](#pub_sub-prefix){: #pub_sub-prefix } | `String.t` |  | A prefix for all pubsub messages, e.g `users`. A message with `created` would be published as `users:created` |
| [`delimiter`](#pub_sub-delimiter){: #pub_sub-delimiter } | `String.t` |  | A delimiter for building topics. Default is a colon (:) |
| [`broadcast_type`](#pub_sub-broadcast_type){: #pub_sub-broadcast_type } | `:notification \| :phoenix_broadcast \| :broadcast` | `:notification` | What shape the event payloads will be in. See |
| [`name`](#pub_sub-name){: #pub_sub-name } | `atom` |  | A named pub sub to pass as the first argument to broadcast. |


## pub_sub.publish
```elixir
publish action, topic
```


Configure a given action to publish its results over a given topic.

See `Ash.Notifier.PubSub` and the [Notifiers](/documentation/topics/notifiers.md) guides for more.


### Examples
```
publish :create, "created"
```

```
publish :assign, "assigned"

```


### Arguments

| Name | Type | Default | Docs |
|------|------|---------|------|
| [`action`](#pub_sub-publish-action){: #pub_sub-publish-action .spark-required} | `atom` |  | The name of the action that should be published |
| [`topic`](#pub_sub-publish-topic){: #pub_sub-publish-topic .spark-required} | `any` |  | The topic to publish |
### Options

| Name | Type | Default | Docs |
|------|------|---------|------|
| [`previous_values?`](#pub_sub-publish-previous_values?){: #pub_sub-publish-previous_values? } | `boolean` | `false` | Whether or not to publish messages with both the new values and the old values for referencing changed attributes |
| [`event`](#pub_sub-publish-event){: #pub_sub-publish-event } | `String.t` |  | The name of the event to publish. Defaults to the action name |
| [`dispatcher`](#pub_sub-publish-dispatcher){: #pub_sub-publish-dispatcher } | `atom` |  | The module to use as a dispatcher. If none is set, the pubsub module provided is used. |


### Introspection

Target: `Ash.Notifier.PubSub.Publication`

## pub_sub.publish_all
```elixir
publish_all type, topic
```


Works just like `publish`, except that it takes a type
and publishes all actions of that type

See `Ash.Notifier.PubSub` and the [Notifiers](/documentation/topics/notifiers.md) guides for more.


### Examples
```
publish_all :create, "created"
```


### Arguments

| Name | Type | Default | Docs |
|------|------|---------|------|
| [`type`](#pub_sub-publish_all-type){: #pub_sub-publish_all-type } | `:create \| :update \| :destroy` |  | Publish on all actions of a given type |
| [`topic`](#pub_sub-publish_all-topic){: #pub_sub-publish_all-topic .spark-required} | `any` |  | The topic to publish |
### Options

| Name | Type | Default | Docs |
|------|------|---------|------|
| [`action`](#pub_sub-publish_all-action){: #pub_sub-publish_all-action } | `atom` |  | The name of the action that should be published |
| [`previous_values?`](#pub_sub-publish_all-previous_values?){: #pub_sub-publish_all-previous_values? } | `boolean` | `false` | Whether or not to publish messages with both the new values and the old values for referencing changed attributes |
| [`event`](#pub_sub-publish_all-event){: #pub_sub-publish_all-event } | `String.t` |  | The name of the event to publish. Defaults to the action name |
| [`dispatcher`](#pub_sub-publish_all-dispatcher){: #pub_sub-publish_all-dispatcher } | `atom` |  | The module to use as a dispatcher. If none is set, the pubsub module provided is used. |


### Introspection

Target: `Ash.Notifier.PubSub.Publication`


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