Feature Flags Rewrite: List of things to get done

[neilkakkar]

Bug Description

There's a lot of things we need to do to make flags on rust production ready. This is a necessary-but-not-sufficient list - will add more things as I (re)discover them.

Verification Criteria

• Audit existing feature flag integration tests to make sure we're covering enough
• Staged rollout plan
  • Rollout percentages
  • Mirror traffic in contour
  • Verification plan for mirrored traffic
  • Decide rollout order

Local dev

1. Migrate hog-rs into main posthog repo, invest in CI to make this seamless.
2. Write RFC for this if necessary (RFC: Bringing back the monorepo for rust services and main app meta#207)
3. Add cross-directory integration tests, so changes to posthog's data model doesn't bork flags service
4. Figure out local dev flow - with using an image when not developing the flags/capture repo, and without for developing.

Business logic

1. Add geoIP maxmind DB, query data for IP addresses
2. Handle token validation + distinct id parsing
3. Add basic matching logic
4. Connect to pg to fetch person properties
5. Handle early access feature properties
6. Handle multivariate flags
7. Handle variant overrides
8. Handle evaluation reasons
9. Handle billing - adding info to redis
10. Setup populating and fetching token/team/flags from redis
11. Handle experience continuity + write path for flags
12. Handle group properties
13. Copy all tests from main repo
14. ensure flag responses are consistent irrespective of internal handling of hashes
15. Handle payloads
16. Handle optimisation for not going to db if flags can be evaluated locally.
17. Handle dynamic cohorts property matching (at rust level)
18. Handle static cohorts property matching (at db level)
19. [Not necessary for first production deploy, but necessary to deprecate old functionality]: Add additional endpoints for internal calling - bulk evaluation, evaluation reason.

Production readiness

13. Replicate prometheus metrics
14. Setup DDOS protection rate limiting
15. Handle db healthchecks / timeouts
16. Setup env vars for controlling sampling for billing / skipping write path / adjusting rate limits
17. Setup sentry error reporting
18. Setup profiling the endpoint
19. Add liveness/readiness checks

Timeline