Introduction

In distributed systems and modern databases, maintaining data consistency is critical. This post introduces the concept of transactions, explains ACID properties, and dives into the Two-Phase Commit (2PC) protocol to ensure reliable distributed transactions.

What is a Transaction?

A transaction is a sequence of operations performed as a single unit of work. Transactions are designed to be atomic, consistent, isolated, and durable (ACID):

  • Atomic: All operations succeed or none.
  • Consistent: Database moves from one valid state to another.
  • Isolated: Concurrent transactions don’t interfere.
  • Durable: Changes are permanent once committed.

Challenges in Distributed Systems

  • Multiple databases or nodes can be involved.
  • Partial failures can leave the system in an inconsistent state.
  • Simple commit/rollback is not enough when operations span multiple systems.

Two-Phase Commit (2PC)

The Two-Phase Commit protocol solves these problems by coordinating distributed transactions in two phases:

  1. Prepare Phase:

    • The coordinator asks all participants if they are ready to commit.
    • Each participant responds with Yes (ready) or No (abort).

  2. Commit Phase:

    • If all participants said Yes, the coordinator sends a commit command.
    • If any participant said No, the coordinator sends an abort command.

Example Workflow

Coordinator -> Participant1: Prepare
Coordinator -> Participant2: Prepare
Participant1 -> Coordinator: Yes
Participant2 -> Coordinator: Yes
Coordinator -> Participant1: Commit
Coordinator -> Participant2: Commit