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Luke Sun

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Two-Phase Commit (2PC)

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Two-Phase Commit (2PC)

Introduction: The “All or Nothing” Wedding

Imagine a wedding. The priest asks the bride and groom: “Do you take…?”

  • If both say “Yes,” the marriage is committed.
  • If one says “No” (or faints), the ceremony is aborted for both.

Two-Phase Commit (2PC) is exactly this. In a distributed system where a single transaction spans multiple databases (e.g., deducting money from Bank A and adding it to Bank B), we need a way to ensure they both agree. We use a Coordinator to manage the process.

What Problem does it solve?

  • Input: A transaction involving multiple nodes.
  • Output: Atomic outcome (Commit or Abort).
  • The Promise: Consistency. No node will commit if another node fails.

How it Works

Phase 1: Prepare (Voting)

  1. The Coordinator sends a “Prepare” message to all nodes (Participants).
  2. Each node executes the transaction locally but does not commit. It locks the resources.
  3. Each node votes: “Yes” (ready) or “No” (error).

Phase 2: Commit (Execution)

  1. If everyone voted “Yes”: The Coordinator sends a “Commit” message. Everyone makes the changes permanent.
  2. If anyone voted “No” (or timed out): The Coordinator sends a “Rollback” message. Everyone undoes their local changes.

Typical Business Scenarios

  • ✅ Distributed Databases: Relational databases (like Postgres or MySQL) use 2PC for XA transactions across different physical machines.

  • ✅ Legacy Enterprise Systems: Older banking and ERP systems rely heavily on 2PC to keep data synchronized.

  • ❌ High Availability: 2PC is blocking. If the Coordinator crashes during Phase 2, the other nodes are left hanging with locked resources. They can’t move forward or backward.

  • ❌ High Performance: Because it holds locks for a long time (two round-trips), 2PC is slow. Modern systems often prefer Sagas or TCC (Try-Confirm-Cancel) for better scalability.

Performance & Complexity

  • Latency: High (requires 2 rounds of network communication).
  • Fault Tolerance: Weak. It is a blocking protocol.

Summary

"2PC is the 'Distributed Wedding'. It ensures atomicity by asking everyone to vote before anyone makes a final decision. It's the standard for correctness, but it's slow and fragile."