Get started

Flagr is an open-source Go service for feature flags, A/B tests, and dynamic configuration.

Your app calls POST /api/v1/evaluation with an entityID and optional entityContext. Flagr returns a variantKey and optional variantAttachment JSON. One round trip. Sticky for a stable entity. No per-user state store.

You can run it against SQLite (local demo), MySQL, or Postgres, or as an eval-only sidecar fed from a JSON file in Git. Same flag can be a kill switch today, an experiment tomorrow, and a runtime config knob the day after, without redeploying the app.

Hard rules (eval vs exposure, segment stop, blank vs stream, recording gates, cache lag): Behavioral contracts. HTTP copy-paste: Integration guide. Just want something running? Use the demo below.

Quick demo

docker pull ghcr.io/openflagr/flagr
docker run -it -p 18000:18000 ghcr.io/openflagr/flagr

open http://localhost:18000

No install? Hit the hosted demo at try-flagr.onrender.com (cold start possible):

curl -sS -X POST https://try-flagr.onrender.com/api/v1/evaluation \
  -H 'content-type: application/json' \
  -d '{
    "entityID": "127",
    "entityType": "user",
    "entityContext": { "state": "NY" },
    "flagID": 1,
    "enableDebug": true
  }'

With enableDebug: true, the response includes a segment walk so you can see how a constraint like state == "NY" becomes a variant.

What Flagr does

One evaluation primitive covers several jobs. Use the map below as a routing table, not a marketing feature list.

Capability	Where to read more
Feature flags, rollouts, kill switches	Overview, Use cases
Browser-friendly eval (GET ?json=)	Use cases: GET evaluation
Time / header targeting (@ts, @http_*)	Built-in context injection
A/B tests + trustworthy denominators	Exposure logging, Data recorders
Runtime config on variants	Use cases: dynamic configuration
GitOps / eval-only JSON	JSON flag source
Deploy, DB, auth, recorders	Self-hosting, Environment variables

To clone an existing flag (segments, variants, tags), use POST /api/v1/flags/{id}/duplicate or Duplicate Flag in the UI (#724).

Deploy

The demo above is local SQLite. Production (MySQL/Postgres, Compose, Kubernetes, TLS) is in Self-hosting. Every env knob lives in Environment variables. The struct in pkg/config/env.go is the source of truth.

Develop Flagr

Clone the repo, then from the root:

make build
make start   # backend :18000 + UI dev :8080
make test

Contributor layout, OpenAPI regen, and test conventions: Contributing. Docs: make serve-docs → http://127.0.0.1:8081/flagr/ ; production: make build-docs → docs/.vitepress/dist.