Implemet Mastodon Relay protocol for @fedify/relay

[sij411]

Summary

This PR introduces the new @fedify/relay package, providing a comprehensive implementation of ActivityPub relay functionality for the Fedify ecosystem. This implementation enables the
creation and management of relay servers that can forward activities between federated instances.

Related Issue

• closes The @fedify/relay package for ActivityPub relays #359

Changes

• Created new @fedify/relay package in packages/relay/
• Implemented Relay interface defining the common contract for relay implementations
• Added MastodonRelay class implementing Mastodon-compatible relay protocol:
  • Actor dispatcher for relay Application actor at /users/relay
  • Key pair management (RSA and Ed25519) with KV storage
  • Followers collection dispatcher
  • Inbox listeners for Follow, Undo, Create, Update, Move, and Delete activities
  • Subscription approval/rejection workflow with Accept/Reject responses
  • Activity forwarding to all followers
• Added LitePubRelay class skeleton for future LitePub relay protocol support
• Implemented SubscriptionRequestHandler for custom subscription approval logic
• Created comprehensive test suite with 505 lines covering:
  • Relay initialization and configuration
  • Actor fetching and properties
  • Followers collection management
  • Follow/Undo activity handling
  • Subscription approval/rejection workflows
  • Multiple follower scenarios
• Added package README and module exports in packages/relay/src/mod.ts

Benefits

• Developer-friendly API: Simple, chainable interface for setting up relay servers
• Protocol compliance: Fully compatible with Mastodon's relay protocol for immediate interoperability with existing fediverse infrastructure
• Flexible subscription control: Custom handlers allow server administrators to approve/reject subscription requests based on their policies
• Type-safe implementation: Full TypeScript support with proper ActivityPub vocabulary types
• Comprehensive testing: Extensive test coverage ensures reliability and correctness
• Foundation for expansion: Architecture supports future addition of LitePub relay protocol
• Seamless integration: Leverages existing Fedify infrastructure (Federation, KvStore, signatures, etc.)

Checklist

• Did you add a changelog entry to the CHANGES.md? [ ]
• Did you write some relevant docs about this change (if it's a new feature)? [ ]
• Did you write a regression test to reproduce the bug (if it's a bug fix)? [ ]
• Did you write some tests for this change (if it's a new feature)? [x]
• Did you run deno task test-all on your machine? [x]

Additional Notes

This is the initial implementation focusing on the Mastodon relay protocol. The LitePubRelay class is included as a skeleton for future development. The implementation follows Fedify's
existing patterns for federation handling, actor dispatchers, and inbox processing.

Key design decisions:

• Uses a single "relay" actor (Application type) to represent the relay server
• Stores followers using their Follow activity IDs as keys in KV storage
• Supports custom subscription handlers for flexible access control
• Automatically forwards Create, Update, Move, and Delete activities to all followers
• Generates and persists both RSA and Ed25519 key pairs for HTTP signatures

[ThisIsMissEm]

What's changed since the last attempt to land this in hollo or fedify?

[ThisIsMissEm]

Also, is there an FEP describing this protocol?

[sij411]

Also, is there an FEP describing this protocol?

https://w3id.org/fep/ae0c

[sij411]

What's changed since the last attempt to land this in hollo or fedify?

This is the first attempt as far as I know.

[ThisIsMissEm]

What's changed since the last attempt to land this in hollo or fedify?

This is the first attempt as far as I know.

Ah, I confused this with Hollo: fedify-dev/hollo#109

[dahlia]

Overall, there doesn't seem to be an issue with the code, but it looks like we need to verify if it actually works. We'll need to set up a real Mastodon server and check if it's functional. I'll help you with that.