[DB-1951] Treat writes as idempotent if all events were previously written, even with check-only streams

[timothycoleman]

An idempotent multi-stream write retry should succeed even if a
check-only stream's expected version no longer matches. The events are
already written and the check-only stream's state may have changed
since, ultimately we resend the same response to the client as they
should have received the first time they sent the request.

Previously, a check-only stream (failing or not) caused the entire retry
to fail with WrongExpectedVersion.

[linear]

DB-1951 Adjust idempotency for check-only streams

[qodo-code-review]

Review Summary by Qodo

Treat idempotent writes as successful with failing check-only streams

🐞 Bug fix

Walkthroughs

Description

• Add EventCount field to CommitCheckResult to distinguish check-only streams
• Treat idempotent writes as successful even if check-only streams fail version checks
• Update verification logic to only require streams with events to be idempotent
• Remove HasEventsToWrite from WritePreparesStreamInfo struct

Diagram

flowchart LR
  A["Multi-stream write retry"] --> B["Check all streams"]
  B --> C["Streams with events"]
  B --> D["Check-only streams"]
  C --> E["All idempotent?"]
  D --> F["Version mismatch?"]
  E -->|Yes| G["Success: AlreadyCommitted"]
  E -->|No| H["Failure: WrongExpectedVersion"]
  F -->|Yes| G
  F -->|No| H

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File Changes

1. src/KurrentDB.Core/Services/Storage/ReaderIndex/CommitCheckResult.cs ✨ Enhancement +10/-3

Add EventCount field to distinguish check-only streams

• Add EventCount parameter to constructor to track number of events being written
• Add EventCount property to store the event count
• Add HasEventsToWrite property that returns true when EventCount > 0
• Update constructor parameter order to place EventCount before EventStreamId
• Add XML documentation explaining the purpose of each constructor parameter

src/KurrentDB.Core/Services/Storage/ReaderIndex/CommitCheckResult.cs

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2. src/KurrentDB.Core/Services/Storage/ReaderIndex/IndexWriter.cs ✨ Enhancement +18/-18

Update CommitCheckResult instantiations with EventCount

• Update all CommitCheckResult constructor calls to include EventCount parameter
• Pass eventIds.Length as the event count in all check commit methods
• Pass 0 for event count when returning error results
• Maintain consistent parameter ordering across all result instantiations

src/KurrentDB.Core/Services/Storage/ReaderIndex/IndexWriter.cs

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3. src/KurrentDB.Core/Services/Storage/StorageWriterService.cs 🐞 Bug fix +36/-17

Refactor idempotency verification for check-only streams

• Remove HasEventsToWrite field from WritePreparesStreamInfo struct
• Use checkResult.HasEventsToWrite property instead of struct field
• Update VerifyCommitChecks to count streams with events separately
• Allow idempotent check-only streams without throwing exception
• Modify idempotency success condition to only require streams with events to be idempotent
• Return EventNumber.CheckOnlyFirst/Last for check-only streams in idempotent responses

src/KurrentDB.Core/Services/Storage/StorageWriterService.cs

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View more (2)

4. src/KurrentDB.Core.Tests/Services/Storage/FakeIndexWriter.cs 🧪 Tests +2/-2

Update FakeIndexWriter with EventCount parameter

• Update CheckCommitStartingAt to pass 0 as event count parameter
• Update CheckCommit to pass eventIds.Length as event count parameter
• Align with new CommitCheckResult constructor signature

src/KurrentDB.Core.Tests/Services/Storage/FakeIndexWriter.cs

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5. src/KurrentDB.Core.XUnit.Tests/TransactionLog/MultiStreamWrites/MultiStreamWritesTests.cs 🧪 Tests +18/-18

Update idempotency test expectations for check-only streams

• Update test expectations for WithFailedCheck idempotency kind from failure to success
• Update test expectations for WithSuccessfulCheck idempotency kind from failure to success
• Update test comments to reflect new behavior where check-only streams don't cause failures
• Apply changes across all applicable test cases with different expected versions and stream states

src/KurrentDB.Core.XUnit.Tests/TransactionLog/MultiStreamWrites/MultiStreamWritesTests.cs

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[Copilot]

Pull request overview

This PR updates multi-stream write idempotency handling so that a retry can still succeed (returning an idempotent/AlreadyCommitted response) even when one or more check-only streams fail their expected-version checks, as long as all streams that actually had events to write were previously written.

Changes:

• Add EventCount (and derived HasEventsToWrite) to CommitCheckResult so “check-only” vs “write” streams are carried through commit checking.
• Update StorageWriterService.VerifyCommitChecks to treat retries as idempotent when all event-writing streams are idempotent, ignoring failures on check-only streams in that scenario.
• Update multi-stream write tests to expect success for idempotent retries that include check-only streams.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/KurrentDB.Core/Services/Storage/StorageWriterService.cs	Adjusts commit-check verification and response shaping to ignore check-only failures when all write-streams are idempotent.
src/KurrentDB.Core/Services/Storage/ReaderIndex/IndexWriter.cs	Threads event-count through commit-check results so callers can identify check-only streams.
src/KurrentDB.Core/Services/Storage/ReaderIndex/CommitCheckResult.cs	Extends commit-check result with EventCount/HasEventsToWrite.
src/KurrentDB.Core.XUnit.Tests/TransactionLog/MultiStreamWrites/MultiStreamWritesTests.cs	Updates expectations so idempotent retries with check-only streams succeed.
src/KurrentDB.Core.Tests/Services/Storage/FakeIndexWriter.cs	Updates test fake to match the new CommitCheckResult constructor signature.

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[github-actions]

@timothycoleman 👉 Created pull request targeting release/v26.0: #5577