Requirements & constraints (clarify): zero session loss, minimal downtime (minutes), maintain sticky sessions (client affinity), support reads/writes in multi-region eventually, observable verification, safe rollback if sessions diverge.
High-level approach: migrate from single-region stateful service to a multi-region architecture that preserves affinity by moving session state to a globally-replicated, strongly-consistent session store during a staged cutover (active-passive → active-active if desired). Use canary traffic, no-destructive toggles, and instrumentation for verification.
Step-by-step migration plan
- Prep & infra
- Provision a global session store that supports geo-replication and controlled consistency (e.g., Redis Enterprise with active-geo, CockroachDB/Spanner for strongly consistent rows, or DynamoDB global tables). Ensure encryption, capacity and latency SLAs.
- Add a session API layer (lightweight service) that abstracts session read/write semantics from apps. Implement optimistic conflict resolution metadata (last-write-ts, origin-region).
- Session replication strategy
- Start with active-passive: asynchronously replicate sessions from primary region to global store, but keep primary as write master. Use change-data-capture (CDC) or Redis replication to populate the global store. Set session TTLs same as current.
- Ensure replication preserves write-order (use per-session monotonic counters or timestamps).
- Client affinity (sticky sessions)
- Keep existing load-balancer affinity during early phases. Introduce a sticky session cookie that contains session-id + region tag. Implement shared cookie semantics so both regions can identify session location.
- Add intelligent routing in edge LB / CDN to prefer the region encoded in cookie; if missing, use geo-DNS to direct to nearest region.
- Canary cutover (small %)
- Enable the new session API in one secondary region but keep writes routed to primary. Begin serving read-only sessions from secondary for a small fraction (1-5%) of users via load-balancer weights.
- Verify no session loss and read latency/consistency metrics.
- Dual-write trial
- For a larger canary cohort, enable dual-write: application writes session to both primary and global store (idempotent), reads prefer local if present else fallback to global. Instrument write success/failure counters.
- Monitor divergence metrics: per-session last-write-ts differences, conflicting values.
- Full cutover to multi-region (active-passive → active-active)
- If dual-write stable and divergence negligible, shift LB weights to route more traffic to secondaries. Transition to active-active only after you implement conflict resolution policy:
- Prefer last-write-ts, or
- Use CRDTs for mergeable session data
- For non-mergeable critical keys, pin writes to session’s “home region” via cookie until explicit migration.
- Fallback behavior
- On read miss or replication lag:
- Query global store as fallback with higher timeout.
- If still missing, route request to primary region (synchronously proxy) rather than fail user action.
- For write failure in secondary, queue write and retry; if queue grows beyond threshold, reject with 503 or route to primary.
- Verification & observability
- Metrics: session read/write latency, replication lag histogram, divergence rate (sessions with different values across regions), session loss rate, user error rates (4xx/5xx), SLOs for session availability.
- Traces: distributed traces showing session read path.
- Live checks: synthetic users performing session workflows across regions.
- Runbook prechecks: replication health, cluster CPU/memory, LB config.
- Rollback & divergence remediation
- If sessions diverge or unacceptable errors:
- Immediate rollback option A (fast): stop writes in secondary (flip feature flag), revert LB affinity to primary-only, allow secondary to serve read-only until drained. This guarantees no further divergence.
- Rollback option B (if partial region fail): failover traffic back to primary via LB and DNS with low TTL; keep global store as warm replica.
- Reconciliation:
- For small divergence: run a diff job that merges per-session using deterministic policy (last-write-ts, or preferring primary). For complex conflicts, mark sessions for user re-auth or session reset with notice.
- For large divergence: freeze writes in all regions, promote single authoritative copy (primary), replay or overwrite using logs, then resume dual-write after validation.
- Always have automated scripts to revert LB weight, disable dual-write, and purge orphaned session cookies.
- Post-migration cleanup & improvements
- Remove temporary proxying and queues, reduce TTLs if needed.
- Consider moving to active-active with strong consistency store or CRDTs long-term.
- Document runbook and automate failure drills.
Key operational controls & checks
- Feature flags for: dual-write, read-from-global, write-to-secondary, LB weights.
- Safety thresholds to auto-rollback: replication lag > X ms, divergence rate > Y%, error-rate spike.
- Communication plan: notify stakeholders and customers if session resets unavoidable.
This plan prioritizes no session loss via authoritative replication and staged dual-write, preserves client affinity via cookie+LB routing, defines clear fallback and monitoring, and provides deterministic rollback and reconciliation procedures if sessions diverge.
