Replicache

Replicache is a client-side JavaScript library (not a hosted service) that provides instant optimistic mutations, offline support, and real-time sync for collaborative web applications. The model: clients apply mutations immediately (optimistic), Replicache replicates them to the server via a /push endpoint, and the server broadcasts changes to other clients who pull via /pull. Any backend can implement the push/pull protocol. For agents building collaborative tools: Replicache handles the hardest parts of collaborative state (conflict resolution, offline queue, real-time updates) so application code only needs to define mutations and implement push/pull endpoints. Higher integration complexity than managed services (Liveblocks, PartyKit). Confidence is docs-derived.

Client SDK: npm install replicache. const rep = new Replicache({licenseKey, mutators: {addTodo: async (tx, todo) => await tx.put(todo.id, todo)}}). rep.mutate.addTodo({id, text}) applies mutation optimistically. rep.subscribe(q => q.scan({prefix: 'todo/'}), results => setTodos(results)) for reactive queries. Server endpoints (implement in any backend): POST /push receives client mutations, applies them, saves to DB. GET /pull returns server state as patches to client. POST /replicache-poke (optional) server-pushes a poke to connected clients to trigger pull. No Replicache-hosted infrastructure; sync goes through developer's own endpoints.

License key auth: licenseKey in Replicache constructor (validates SDK usage, not user auth). Get license key from replicache.dev. User authentication is handled entirely by the application — the push/pull endpoints use standard session/JWT auth provided by the developer. No Replicache auth server. HTTPS is the developer's responsibility for their endpoints. License key enforcement: SDK validates key format; expired/invalid keys cause sync to stop.

Client: mutations are queued in IndexedDB if offline; replayed when reconnected. Conflict resolution: last-write-wins by default (mutations are applied in sequence determined by the server); custom conflict resolution implementable in the server push handler. Client errors in mutators are caught; failed server push retries automatically with exponential backoff. Server push/pull errors: SDK retries; surface errors via onClientStateNotFound callback. Server implementation reliability is the developer's responsibility. Replicache itself has no hosted infrastructure to fail.

doc.replicache.dev provides a detailed tutorial, API reference, integration guides (Supabase, Cloudflare Workers, Next.js, Express), and architecture documentation explaining the push/pull protocol. Getting started requires implementing push/pull endpoints in your backend (1–4 hours depending on experience). License key required (free for open-source; paid for commercial use). Active community via Replicache Discord. GitHub: rocicorp/replicache. Strong documentation quality; somewhat complex initial setup.