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Multi-Region and Failover Routing Guardrails

🧭 Quick Return to Map

You are in a sub-page of Cloud_Serverless.
To reorient, go back here:

Think of this page as a desk within a ward.
If you need the full triage and all prescriptions, return to the Emergency Room lobby.

Keep latency low, fail over safely, and avoid split-brain state while your RAG, queues, and streams operate across regions. This page gives a compact playbook that you can paste into any load balancer or edge router policy.

When to use this page

  • Users in different geos see different answers or stale citations.
  • Region outage triggers duplicate tool calls or replayed webhooks.
  • Vector writes happen in one region while reads route to another.
  • Canary in a single region is green yet the global cutover fails.
  • DNS or anycast flips cut long-running streams.

Open these first

Acceptance targets

  • p95 latency improves for local users or stays within ten percent of single-region baseline.
  • No increase in 5xx at failover or failback.
  • Idempotency dedupe rate ≥ 99.9 percent on all write paths during failover windows.
  • For RAG: ΔS(question, retrieved) drift ≤ 0.03 across regions, λ remains convergent on two seeds.
  • Vector index hash identical per region before routing users. INDEX_HASH matches on probes.

Fix in 60 seconds

  1. Pin by header, not only DNS Add X-Region-Pin: {region} and X-Release: {rev}. Edge selects nearest healthy region unless a pin is present. Synthetic probes always include the pin.

  2. Fence writes with idempotency keys Compute sha256(source_id + revision + index_hash + partition) and drop duplicates in the KV. Keep the fence for failover_window plus 24 hours.

  3. Replicate index and blobs before traffic Block user routing until INDEX_HASH equals across regions and blob manifests match.

  4. Graceful streams Sticky route long-lived connections. Drain the old region for N seconds. Do not cut an active stream at the router.

  5. Health with contract checks Health is green only if schema_rev, model_tag, index_hash and secret versions match. Pure 200 is not sufficient.

Routing patterns that work

  • Active-active with sticky reads Reads route to nearest healthy region and stay sticky for the session. Writes go to the region that owns the partition. Use a queue to replicate to others.

  • Active-passive for stateful writers All writes go to primary. Secondary serves read-only. Promote only after index and blob parity plus a clean queue tail.

  • Geo-partitioned stores Partition by tenant or namespace. Keep retrieval within the same partition and region. Cross-partition requests require a join step with explicit contracts.

Typical breakpoints → exact fix

  • Wrong snippet in far region despite high similarity Index or metric differs. Compare INDEX_HASH and analyzer settings. Rebuild and verify a small gold set before routing traffic. Open: Retrieval Playbook, Embedding ≠ Semantic

  • Duplicate webhooks during regional flip Retry plus DNS cutover replays the same event. Use the idempotency fence and a replay TTL beyond the failover window. Open: Bootstrap Ordering

  • Split-brain memory or tool cache Agent memory writes with no version pins cross regions. Namespace by tenant, mem_rev, and region. Open: Multi-Agent Problems

  • Streams cut at failover Router does not support drain. Pin streams with a cookie or header, then change the default path only for new connections. Open: Timeouts and Streaming Limits

  • Cost spikes after enabling global anycast Cold starts increase in remote regions. Raise min instances or provisioned concurrency selectively on hot routes. Open: Cold Start and Concurrency

Minimal recipes you can copy

A) Region pinning contract

Request headers
- X-Region-Pin: us-east-1 | eu-west-1 | ap-southeast-1
- X-Release: r2025-08-27
- X-Index-Hash: a1b2c3
- X-Schema-Rev: sc-12
Router rule
- If pin present and region healthy → route pinned
- Else pick nearest healthy with matching {schema_rev, index_hash, model_tag}
- Sticky cookie for streaming connections

B) Failover gate

Gate conditions before user traffic
- INDEX_HASH equal across regions
- Blob manifest parity
- Health probes return {schema_rev, model_tag, secrets_rev} exact match
- Synthetic RAG probes: ΔS drift ≤ 0.03 on k=10 questions
- Dedupe KV warm and reachable in both regions

C) Vector replication note

Replication
- Prefer periodic rebuild from source texts per region
- If log shipping: checkpoint offsets, verify analyzer parity
- After topology change: run gold-set eval and lock reranker order
Refs: retrieval-playbook, reindex-migration

Observability you must add

  • Split metrics by region, release_id, and revision.
  • Health includes schema_rev, index_hash, model_tag, secrets_rev.
  • Dedupe hit rate, queue age, replay counts per region.
  • ΔS and λ on a fixed probe set, per region.
  • Stream drain success count at flips.

Verification

  • Probe set stable within acceptance targets.
  • No duplicate side effects in the failover window.
  • p95 improves for local users or remains flat.
  • Queue age does not spike at promotion.

When to escalate

  • Persistent ΔS drift across regions after rebuild. Re-embed with the same analyzer and metric, then re-run the gold set.
  • Dedupe misses during outage replay. Increase KV TTL and ensure consistent hashing across regions.
  • Health green yet errors rise. Add contract checks to the probe and block routing when versions disagree.

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