Overview

Define the internal API and key model for a conflict detection module that is separate from the storage engine, as part of the SPM-4946 prototype. This header is the seam that the detector implementations, commit-path integration, staged writes, and log-and-apply work all build on.

Background

Today WiredTiger detects write-write conflicts while writes are staged. The proposed design stages writes in the RecoveryUnit and asks an in-memory detector at commit time. The detector must never miss a conflict. It may report false conflicts. The approach is modeled on the Aurora DSQL adjudicator. The existing POC on branch conflict-detector-poc (src/mongo/db/repl/adjudicator.h) is reference material only.

Scope of Work

• Define ConflictKey: (ident, keyspace kind [record store / index], KeyString or RecordId bytes), plus a stable hash.
• Define WriteSet: how it is built from RecoveryUnit staged ops, and dedup.
• Define ConflictDetector::tryCommit(writeSet, readTs, reserveSlot) -> optional<OpTime>:
  • "No conflict" means the oplog slot is reserved and the keys are recorded at the commit timestamp.
  • Commit timestamps are recorded in the order oplog slots are reserved.
  • reserveSlot is not called when there is a conflict.
  • The floor timestamp, for step-up, restart, and GC.
• Sketch (do not implement) where read-set hooks for serializable and for the optimistic read-your-writes check would attach.

Acceptance Criteria

• Header(s) in the project branch, with doc comments stating the contract.
• Findings recorded on this ticket: API trade-offs, open questions, and recommendations for the follow-on production milestone.

Technical Notes

• Prototype quality; not intended for commit to master.
• Out of scope: concrete detector implementations and commit-path wiring. Those are follow-on tickets in this epic.