Repair with snapshots

An event log cannot grow forever. At some point you drop old entries, restart the process, or deploy new code. Clients need a snapshot that says “start here” with a known position.

The stream carries deltas. The snapshot route carries truth.

Snapshot carries continuation

{
  "incidents": [
    { "id": 41, "status": "monitoring" },
    { "id": 42, "status": "resolved" }
  ],
  "latestEventId": "evt_90",
  "generatedAt": "2026-09-08T15:00:00.000Z"
}

After a server restart, a stale client loads this response, repaints the board, and opens SSE with Last-Event-ID: evt_90. Events after evt_90 apply on top. Events before it are irrelevant.

When to discard local state

Document triggers that wipe the projection:

• resync.required on the stream
• snapshot latestEventId behind the client’s last applied id
• schema version bump the client no longer understands

async function resync() {
  const snapshot = await fetch('/api/incidents').then(r => r.json())
  replaceBoard(snapshot.incidents)
  lastEventId = snapshot.latestEventId
  reconnectStream(lastEventId)
}

Bounded replay plus snapshot

We keep five hundred events in memory for the tutorial. Production might keep fewer. When replay fails, fall back once, not in a loop.

Restart the server, expire the replay window, and recover a client that slept through the outage. The final board must match a fresh GET /api/incidents call.

Consistency note

The snapshot must reflect the same rules as streamed events. If the stream hides resolved incidents after seven days, the snapshot should too. Otherwise resync paints a different board than live updates.

Include generatedAt in the snapshot JSON so the UI can show freshness even before the live channel reconnects.

Treat snapshots as cheap. Regenerate them on a schedule if replay retention is small. Clients prefer one fast GET over waiting for a long replay chain.

Try this on your own project: delete your in-memory replay buffer, hit the snapshot route, and prove a client catches up using snapshot plus later events only.