# Agent handoff: migrate session storage to Redis

## Objective

Move `acme/shop-web` session storage from the in-process memory store to Redis so sessions survive deploys and work across multiple instances. Keep the public cookie format unchanged.

## Repository and branch

- Repo: `github.com/acme/shop-web`
- Branch: `chore/redis-sessions` (pushed)
- Base: `main` at `4f2c9a1`

## Relevant commit

`b71e0d3` — "Add Redis session store behind SESSION_STORE flag". Everything below describes the tree at this commit. If `git log -1` on the branch shows a different hash, this document is stale.

## Completed changes

- Added `lib/session/redis-store.ts` implementing the `SessionStore` interface (get, set, touch, destroy) with a 7-day TTL.
- `lib/session/index.ts` picks the store from `SESSION_STORE=memory|redis` (default `memory`, so nothing changes until the flag flips).
- `docker-compose.yml` gains a `redis:7-alpine` service for local dev.
- `.env.example` documents `REDIS_URL` and `SESSION_STORE`.

## Files affected

```
lib/session/redis-store.ts      (new)
lib/session/index.ts            (modified)
lib/session/memory-store.ts     (unchanged, still default)
docker-compose.yml              (modified)
.env.example                    (modified)
tests/session/redis-store.test.ts (new)
```

## Commands run and results

```
npm test -- session         → 14 passed, 0 failed
npm run type-check          → clean
npm run lint                → 1 warning (unused import in memory-store.ts, pre-existing)
docker compose up redis     → healthy on :6379
SESSION_STORE=redis npm run dev → login/logout works; cart persists across a server restart
```

## Unresolved problems

1. **Session touch on every request** doubles Redis round-trips. Left as is; measure before optimizing.
2. `tests/session/redis-store.test.ts` needs a live Redis. It is skipped in CI (`describe.skipIf(!process.env.REDIS_URL)`). CI has no Redis service yet.
3. Not tested: concurrent logins from two instances. Needs the staging environment.

## Decisions and constraints

- Cookie name and signing stay identical, so a rollout does not log anyone out.
- No session data migration: memory-store sessions are lost on the first deploy with `SESSION_STORE=redis`. Product agreed (deploy at low traffic, Tuesday 04:00 UTC).
- Do not add `ioredis`; the project already uses `redis@4`.

## Next steps

1. Add a Redis service to `.github/workflows/ci.yml` and unskip the store test.
2. Set `REDIS_URL` and `SESSION_STORE=redis` in staging; run the two-instance login check.
3. Open the PR from `chore/redis-sessions` → `main` with this document as the description; request review from the platform team.
4. After merge: flip the flag in production during the agreed window, watch `session_store_errors` for one hour.