MVCC conflict detection engine for Bedrock's optimistic concurrency control system.

The Resolver detects read-write and write-write conflicts by maintaining an interval tree that tracks which key ranges were written at which versions. It processes transaction batches from Commit Proxies and returns lists of conflicting transaction indices to abort.

Resolvers start in running mode and are immediately ready to process transactions. They handle out-of-order transactions through a version-indexed waiting queue that ensures consistent conflict detection regardless of network timing variations.

Metadata Distribution

The Resolver also acts as a distribution point for system metadata mutations (keys with \xFF prefix). Each request includes metadata mutations per transaction plus the calling commit proxy's stable identity (its server pid, not the per-batch finalization task pid); the response includes an exact metadata window (last_served, last_version] computed resolver-side from the calling proxy's served floor (FDB's per-proxy lastVersion). Consecutive windows to one proxy tile exactly: the proxy applies them in batch order and asserts each window's from_version equals its applied version.

Converged verdicts (sharded resolvers)

With sharded resolvers each resolver only sees its own shard's conflict ranges, so a locally-committed transaction can still be aborted globally (by another shard's resolver). Metadata therefore travels with its LOCAL verdict: every resolver receives every batch's metadata_per_tx, records each metadata-carrying transaction's mutations together with its own verdict, and window entries carry {mutations, committed?} pairs. The proxy ANDs the verdicts positionally across all resolvers' windows - a conflict anywhere vetoes; a resolver holding none of a transaction's ranges contributes a trivially-true verdict - so the AND is exactly the global verdict, and no resolver ever needs to know it. This is FoundationDB's stateMutations relay (each resolver records local committed; the consuming proxy ANDs across resolvers in applyMetadataEffect).