Deep Dive into MySQL 8.0.33 Two‑Phase Commit: Source‑Code Analysis

MySQL's commit command triggers a two‑phase commit (Prepare and Commit). This article analyzes the implementation in MySQL 8.0.33, showing the complete logic flow and the underlying source code.

Overall Logic

The process consists of a Prepare phase followed by a Commit phase, each divided into several sub‑stages that coordinate binlog handling, GTID generation, redo‑log flushing, and synchronization.

Prepare Phase

1. Binlog Prepare

Retrieves the largest sequence number timestamp from the previous transaction.

2. InnoDB Prepare

Set transaction state to prepared .

Release GAP locks for RC and lower isolation levels.

Change the undo‑log segment state from TRX_UNDO_ACTIVE to TRX_UNDO_PREPARED .

Write the transaction XID into the undo log.

Commit Phase

1. Stage 0

Ensures commit order for replica instances.

2. Flush Stage

Flush redo logs according to innodb_flush_log_at_trx_commit .

Clear BINLOG_FLUSH_STAGE and COMMIT_ORDER_FLUSH_STAGE queues.

Write prepare state redo logs based on the same flush parameter.

Generate GTID via get_server_sidno() and Gtid_state::get_automatic_gno() .

Flush binlog caches (stmt, trx) and update last_committed , sequence_number , and GTID log events.

Register flushed binlog file/position with semi‑synchronous replication plugins.

If sync_binlog != 1 , update binlog position and broadcast an update signal.

3. Sync Stage

Wait according to sync_binlog and invoke fsync() to persist the binlog file.

If sync_binlog == 1 , update binlog position and broadcast the update signal.

4. Commit Stage

Execute after_sync hook (for semi‑sync replication).

Update global m_max_committed_transaction and initialize the transaction’s sequence number.

Binlog layer commit (no‑op).

Storage‑engine commit, which includes: Allocate update undo segment for persistent GTID. Update table update_time in the data dictionary. Allocate mini‑transaction handle and buffer. Set undo state (e.g., TRX_UNDO_TO_FREE for inserts, TRX_UNDO_TO_PURGE for updates). Append undo‑log header to the history list. Record binlog position in the system‑transaction table. Close MVCC read view, persist GTID, release undo logs, and wake background threads (master, purge, page cleaner).

Update the executed GTID group.

Decrease the prepared XID counter after engine commit.

Broadcast m_stage_cond_binlog to wake waiting followers.

Stage Transition Logic

The function change_stage (which calls enroll_for ) manages the movement of threads between stages. The first thread in a queue becomes the leader; subsequent threads become followers and wait on condition variables until the leader finishes its work. Special handling ensures that the binlog flush leader and the commit‑order leader cooperate without deadlock.

Key Code Snippets

int ha_commit_trans(THD *thd, bool all, bool ignore_global_read_lock) {
  // ...
  // Prepare phase
  if (!trn_ctx->no_2pc(trx_scope) && (trn_ctx->rw_ha_count(trx_scope) > 1))
    error = tc_log->prepare(thd, all);
  // Commit phase
  if (error || (error = tc_log->commit(thd, all))) {
    ha_rollback_trans(thd, all);
    error = 1;
    goto end;
  }
}

int MYSQL_BIN_LOG::ordered_commit(THD *thd, bool all, bool skip_commit) {
  // Stage #0: preserve commit order for replicas
  if (Commit_order_manager::wait_for_its_turn_before_flush_stage(thd) ||
      ending_trans(thd, all) ||
      Commit_order_manager::get_rollback_status(thd)) {
    if (Commit_order_manager::wait(thd)) {
      return thd->commit_error;
    }
  }
  // Stage #1: flush to binary log
  if (change_stage(thd, Commit_stage_manager::BINLOG_FLUSH_STAGE, thd, nullptr, &LOCK_log)) {
    return finish_commit(thd);
  }
  // ... flush logic ...
  // Stage #2: sync to disk
  if (change_stage(thd, Commit_stage_manager::SYNC_STAGE, wait_queue, &LOCK_log, &LOCK_sync)) {
    return finish_commit(thd);
  }
  // Stage #3: commit all transactions in order
  if (change_stage(thd, Commit_stage_manager::COMMIT_STAGE, final_queue, &LOCK_sync, &LOCK_commit)) {
    return finish_commit(thd);
  }
  // ... commit logic ...
  return thd->commit_error == THD::CE_COMMIT_ERROR;
}

Keywords

#MySQL #事务 #源码