Restrict Cross-Shard Atomic Commit Path to Only Modified Shards

[harshit-gangal]

Description

Currently, VTGate tracks per-shard transaction state in the session to identify if DMLs modified any underlying database. However, during cross-shard transactions, all participating shards (including read-only ones) are part of the atomic commit path.

This task involves:

• Ensuring that only modified shards are part of the cross-shard commit path using the atomic commit flow.
• Keeping read-only shards on the direct commit path to prevent unnecessary inclusion in atomic commit operations.

Impact

By restricting the atomic commit path to modified shards only, this change will:

• Improve efficiency by minimizing the number of shards involved in the two-phase commit.
• Reduce overhead on read-only shards that do not need atomic commit guarantees.
• Potentially reduce cross-shard transactions to single-shard ACID if only one shard is modified.

Related

• PR: Track Per Shard Transaction in VTGate Session
• RFC: Atomic Distributed Transactions