Chatto

Chatto is a real-time chat application for teams and communities, built as a single Go binary with a SvelteKit frontend. It uses NATS/JetStream as its sole persistent data store — no PostgreSQL, no Redis, no MongoDB. The project is licensed AGPL-3.0-or-later with Apache-2.0 exceptions for the frontend and integration surfaces.

The architecture is event-sourced: all durable domain facts are appended as protobuf-encoded events to a JetStream stream (EVT), with optimistic concurrency control on every write. Thirteen in-memory projections consume the event stream and maintain current state per-process. At startup, a shared replay fans decoded events out to all projectors, avoiding duplicate work.

The API is protobuf-first, with ~100+ ConnectRPC endpoints across five service packages and a real-time WebSocket layer using protobuf binary frames. Permission-only RBAC with hierarchical scope resolution governs access. Per-user encryption keys with crypto-shredding satisfy GDPR deletion requirements without mutating the immutable event log.

Key design choices include multi-instance clients connecting to multiple servers simultaneously, optimistic UI with scoped provisional patches, and elected leader workers for voice-call reconciliation, asset cleanup, and video processing. The project is unusually well-documented for its stage: 48 ADRs, 28 FDRs, a 910-line architecture document, and AGENTS.md guidance for LLM coding agents.

The event-sourcing model was adopted to escape subject-index RAM growth under the prior KV-based approach, cutting subject cardinality from O(messages) to O(aggregates). Acknowledged technical debt includes deferred snapshots (cold starts replay the full event stream) and per-aggregate phased migration from the old CRUD code.