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Read Consistency Strategy: Adds hub region header for LastCommittedWriteRegion strategy. #5815

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Read Consistency Strategy: Adds hub region header for LastCommittedWriteRegion strategy. #5815

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b5e72b2
Add hub region header for read consistency strategy
aavasthy Apr 28, 2026
57e2e02
Only setting hub region header for LastCommittedWriteRegion ReadConsi…
aavasthy Apr 29, 2026
cddf8f1
Merge branch 'master' into users/aavasthy/ReadConsistencyWithHubRegio…
aavasthy Apr 29, 2026
b009087
Update tests
aavasthy Apr 29, 2026
a24b765
merge with master
aavasthy Apr 29, 2026
8db9406
Merge branch 'master' into users/aavasthy/ReadConsistencyWithHubRegio…
aavasthy Apr 29, 2026
5bcbbaa
Update contracts and fix tests
aavasthy Apr 30, 2026
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11 changes: 11 additions & 0 deletions Microsoft.Azure.Cosmos/src/Handler/RequestInvokerHandler.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -509,6 +509,9 @@ private async Task ValidateAndSetConsistencyLevelAsync(RequestMessage requestMes

/// <summary>
/// Validate and set the ReadConsistencyStrategy header.
/// When the strategy is LastCommittedWriteRegion and the operation is a read,
/// also set the hub region processing header so the backend routes the request
/// to the hub (write) region.
/// </summary>
private Task ValidateAndSetReadConsistencyStrategyAsync(RequestMessage requestMessage)
{
Expand All @@ -529,6 +532,14 @@ private Task ValidateAndSetReadConsistencyStrategyAsync(RequestMessage requestMe
requestMessage.Headers.Set(
HttpConstants.HttpHeaders.ReadConsistencyStrategy,
readConsistencyStrategy.Value.ToString());

if (readConsistencyStrategy.Value == Cosmos.ReadConsistencyStrategy.LastCommittedWriteRegion
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@xinlian12 xinlian12 Apr 30, 2026

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🟡 Recommendation — Design: Consider interaction with CrossRegionHedgingAvailabilityStrategy

When CrossRegionHedgingAvailabilityStrategy and LastCommittedWriteRegion are both active, the hedging strategy clones the request (including the ShouldProcessOnlyInHubRegion header) and sends copies to multiple regions in parallel. Non-hub regions will return 403/3, which IsFinalResult() does NOT treat as final — so each hedged copy retries internally, generating cascading 403/3 traffic.

Concrete scenario: 3-region account with hedging enabled + LastCommittedWriteRegion read → 3 parallel requests → 2 get 403/3 and retry → generates ~5+ backend requests for one logical read, with wasted RUs and added latency.

Why it matters: The combination is semantically contradictory — hedging spreads across regions, but hub-only routing rejects all non-hub regions. This isn't a correctness bug (it eventually converges), but it's wasteful and could confuse diagnostics.

Suggestion: Either:

  1. Add a check in ShouldHedge to skip hedging when ShouldProcessOnlyInHubRegion is set, or
  2. Document this as an unsupported combination, or
  3. At minimum, add a code comment noting this interaction for future maintainers.

⚠️ AI-generated review — may be incorrect. Agree? → resolve the conversation. Disagree? → reply with your reasoning.

&& OperationTypeExtensions.IsReadOperation(requestMessage.OperationType))
{
requestMessage.Headers.Set(
HttpConstants.HttpHeaders.ShouldProcessOnlyInHubRegion,
bool.TrueString);
}
}

return Task.CompletedTask;
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8 changes: 7 additions & 1 deletion Microsoft.Azure.Cosmos/src/Resource/Settings/ReadConsistencyStrategy.cs
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Expand Up @@ -48,6 +48,12 @@ enum ReadConsistencyStrategy
/// Quorum read with GCLSN barrier - returns the latest version across all regions.
/// Only valid for accounts configured with Strong consistency.
/// </summary>
GlobalStrong = 4
GlobalStrong = 4,

/// <summary>
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/// Returns the latest committed version from the hub (write) region, ensuring reads
/// reflect the most recent writes regardless of which region the client is connected to.
/// </summary>
LastCommittedWriteRegion = 5
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}
}
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