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@meso @alex Pasted your reply and asked for an explanation a seasoned programmer would understand:

Certainly! As a seasoned programmer, you’re likely familiar with RESTful APIs, HTTP requests, and JSON serialization. ActivityPub is a protocol that uses these concepts to enable decentralized social networking. Let’s dive into the details of how federation works in ActivityPub, including the roles of the inbox and outbox, and how servers communicate with each other.

ActivityPub Federation:

Federation is the process by which servers in the ActivityPub network (known as the Fediverse) communicate and share data. Each server in the network is responsible for hosting user accounts (actors) and their content (objects). Federation allows actors on one server to interact with actors on other servers. For example, a user on server A can follow, like, or reply to a post created by a user on server B. Server Endpoints:

Each server implementing ActivityPub exposes two key endpoints for each actor: the inbox and the outbox. The inbox is an endpoint that receives incoming activities from other actors. It’s essentially an HTTP endpoint that accepts POST requests containing ActivityStreams objects. The outbox is an endpoint that holds activities created by the actor. It’s a collection of activities that the actor has published. Other servers can retrieve activities from an actor’s outbox to stay updated. Inbox and Delivery:

When an actor performs an activity (e.g., creating a post or following another actor), the activity is published to their outbox and delivered to relevant recipients. Recipients can be specified using properties like “to,” “cc,” and “bto.” The server is responsible for delivering the activity to the inboxes of recipients. This is done by making an HTTP POST request to the inbox endpoint of each recipient with the JSON-serialized activity as the payload. The recipient’s server processes the incoming activity, performs necessary actions (e.g., updating the follower list), and may generate new activities in response (e.g., an “Accept” activity for a “Follow” request). Outbox and Retrieval:

Other servers can retrieve activities from an actor’s outbox to stay updated with their actions. The outbox is typically accessible via an HTTP GET request, and it returns a JSON-serialized collection of activities. The outbox may support different types of views, such as an “OrderedCollection” view that provides a paginated list of activities. Example of Activity Delivery:

Actor A on server A creates a “Follow” activity to follow actor B on server B. The activity is added to actor A’s outbox. Server A delivers the “Follow” activity to actor B’s inbox on server B by making an HTTP POST request to the inbox endpoint with the activity payload. Server B processes the incoming activity, updates actor B’s list of followers, and generates an “Accept” activity in response. Server B delivers the “Accept” activity to actor A’s inbox on server A, confirming the follow request. Discovering Actors and Endpoints:

Actors are usually identified by a URI (e.g., “https://example.com/actor/alice”). To interact with an actor, a server needs to discover the actor’s endpoints (inbox, outbox, etc.). This is done by fetching the actor’s URI, which returns an ActivityStreams “Person” (or other actor type) object containing the relevant endpoints and metadata. Overall, ActivityPub federation relies on RESTful principles, HTTP requests, and JSON-LD serialization to enable communication between servers. The inbox and outbox play central roles in receiving and publishing activities, respectively. The protocol is flexible and can be extended to support various social networking use cases.