How does directed reads improve read latency in multi-region Spanner?

By default, some read paths in a multi-region instance can end up routed toward the leader region even when a closer replica could have served them, because the client isn't explicitly telling Spanner where it prefers to read from. Directed reads let the client specify a preferred replica type (read-write or read-only) and a preferred region or set of regions for a given request.

// Client option example (conceptual)
options.directedReadOptions = {
  includeReplicas: {
    replicaSelections: [{ location: "us-east4", type: "READ_ONLY" }]
  }
};

With that hint, Spanner routes the read to a nearby read-only replica instead of the leader whenever the request's consistency requirements (like bounded staleness) allow it, cutting the network hop that would otherwise cross to a distant leader region. This is most valuable for read-heavy services with a geographically distributed user base, where consistently steering reads to the nearest healthy replica meaningfully reduces tail latency without changing the transaction's correctness guarantees.

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