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RabbitMQ Exchanges

cattle_grid uses topic exchanges. This means that every message has an attached routing key. These have the form of being separated by a dot ., e.g.

incoming.Follow

Types of Exchange

There are three exchanges used by cattle_grid:

  • The RawExchange: Handles incoming and outgoing messages as they are transferred to the Fediverse.
  • The ActivityExchange: You can subscribe to this exchange. Topics are organized by the type of activity, e.g. incoming.Follow and outgoing.Accept. Furthermore, this exchange is meant to contain the methods, e.g. send_message and update_actor.
  • The AccountExchange: This exchange contains the account name as part of the routing key, e.g. receive.Alice.incoming.

Message Flow

In the following, we will denote an exchange using a rounded box with the exchange name followed by the routing key in italics, e.g.

flowchart LR
E(["`Exchange
_routing.key_`"])
S[Something]
E --> S

Square boxes are used to denote other objects. Arrows indicate the message flow.

Incoming messages

We now illustrate what happens when a message is send from an external Fediverse application to cattle_grid.

flowchart LR
R(["`RawExchange
_incoming.*_`"])
A(["`ActivityExchange
_incoming.*_`"])
A2(["`AccountExchange
_receive.Alice.incoming_`"])

    F[Fediverse] --> |"`ActivityPub
POST /inbox`"| R
    R -->|transformer| A
    R -->|transformer| A2
    A ==> YE[Your Extensions]
    A2 ==> YC[Your Client Applications]

The processing of cattle_grid as described in ActivityPub Processing takes place on the RawExchange.

See cattle_grid.extensions.load.build_transformer for how the transformer is being constructed from extensions, and cattle_grid.dependencies.internals.Transformer for how to inject it.

Outgoing messages

flowchart LR
A(["`ActivityExchange
_send_message_`"])
R(["`RawExchange
_outgoing.*_`"])
A2(["`ActivityExchange
_outgoing.*_`"])
AP[Your application]

    AP -->|out_transformer| A
    A --> A2
    A --> R
    R --> F[Fediverse]
    A2 --> YE[Your Extension]

The processing of cattle_grid as described in ActivityPub Processing takes place on the RawExchange.

Fetching objects

flowchart LR
AP[Your application]
A(["Activity Exchange"])
R(["RawExchange"])

    AP --> A
    A --> AP
    A --> R
    R -->|transformer| A
    R -->|GET /resource| F[Fediverse]

Todo

Check if it is really teh position of the transformer, we want.

Serving Content

One should use the out_transformer result. Alternatively, one can store the result of the outgoing.* topic.

Configuration

The names used for the exchange can be configured via

cattle_grid.toml
activity_pub.internal_exchange = "cattle_grid_internal" # RawExchange
activity_pub.exchange = "cattle_grid" # ActivityExchange
activity_pub.account_exchange = "amq.topic" # AccountExchange

where the prescribed values are the default values.

Todo

Harmonize names