Replica-aware routing public beta

ClickHouse Cloud’s replica-aware routing, available in public beta for Enterprise customers, keeps related requests directed to the same replica by consistently hashing a user-provided routing key. This addresses issues in multi-replica deployments where temporary tables, named sessions, local caches, recent writes, or schema changes may exist on one replica before being visible on others. HTTP clients can provide an `X-ClickHouse-Replica-Tag` header, while native clients can use a TLS SNI override that separates the routing hostname from certificate validation. Built using Istio and Envoy, the feature evolved from an impractical subdomain-based approach and avoids using ClickHouse session IDs because they permit only one query at a time. Replica-aware routing can improve read-after-write behavior and cache locality, but it is best-effort rather than guaranteed, as upgrades, restarts, or scaling can remap keys to different replicas; it also directs traffic without providing workload isolation.

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