activitypub-federation

ActivityPub federation enables server-to-server communication through HTTP POST delivery of activities to actor inboxes. Servers resolve recipient addresses, verify content origin via HTTP signatures, and process activities through inbox forwarding and optional shared inbox optimization to synchronize state across the fediverse.

ActivityPub's three core building blocks are Actors (users or services with inbox/outbox collections), Activities (actions like Create or Announce), and Objects (content items like notes or images). Each has required properties defining structure, relationships, and behavior in federated networks.

Implement standard actor structures with required collections (Inbox, Outbox, Followers, Following), support public addressing for visibility, deliver activities via HTTP POST with correct content types, resolve recipients, discover inboxes, verify origin signatures, and optionally optimize with shared inbox endpoints.

ActivityPub implementations must verify content origin and validate HTTP Signatures to prevent spoofing and unauthorized activity injection. Proper signature verification ensures only legitimate federated servers can deliver activities to your actor's inbox.

ActivityPub supports protocol extensions through Fediverse Enhancement Proposals (FEPs). Extensions allow custom properties and behaviors while maintaining compatibility with standard ActivityPub actors and activities across federated servers.

When inbox or outbox processing fails, debugging requires examining server-to-server messaging, recipient resolution, activity delivery logs, and HTTP signature validation. Understanding these mechanics helps identify whether failures stem from malformed activities, missing collections, or security verification issues.