ConfigCat

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Build me feature flags with edge delivery, replacing ConfigCat.

The distinguishing claim is availability: the flag values keep being served when everything else is down. That is achievable without a vendor, and this is how.

STACK
- Node 20+ with Fastify for the control plane
- SQLite through better-sqlite3, WAL mode
- A static JSON file published to object storage behind a delivery network, which is the serving path
- Caddy in front of the control plane

THE TRICK WORTH COPYING
- **The serving path is a static file, not an API.** On every change, render the whole flag set for an environment to JSON, write it to object storage, and let the network serve it
- Now the availability of your flags is the availability of a delivery network, and your control plane can be offline for an hour without any consequence
- Clients fetch it with an ETag every few seconds. No connection to keep open, no server to scale

THE DATA MODEL
- flags, rules, environments as usual: key, kind, default, ordered conditions, rollout percent
- publications: id, environment_id, sha256, published_at, actor — every published file recorded, **so a rollback is republishing a previous hash rather than reconstructing state**
- changes: append-only, before and after, who and when

EVALUATION IN THE CLIENT
- The rules travel with the file and the SDK evaluates locally. Zero latency and no round trip per flag
- Deterministic bucketing from a hash of flag key plus user id, so somebody stays in the same bucket
- **This means your targeting rules are public if the file is.** Keep the file behind a secret path or an authenticated fetch, and never name an unreleased product in a flag key

SAFETY
- The SDK caches the last good file on disk and uses it if the fetch fails. Never block start-up on it
- A staged publish: percentage, then everybody, with the previous hash one click away
- Expiry dates on temporary flags and a loud list of the expired ones

THE DASHBOARD
- A list, a toggle per environment, and a rule editor with a preview: paste a user and see what they would get. That preview is what makes non-developers trust it
- Every change requires a note in production

WHAT MATTERS MOST
Publishing a static file rather than serving an API. It gives you the availability the paid product charges for, and it costs almost nothing.