Beyond Last-Second Failures: Exploring the World of Two-Phase Commits and Data Consistency

Here's what could happen without 2PC:

1. You initiate the transfer, and system A deducts the money from your account.
2. Right before system B updates account B, it crashes.
3. System A remains updated, but B doesn't, leaving your data inconsistent.

This "last-second failure" creates an issue - the money is gone from A, but hasn't reached B.

2PC introduces a coordinator that manages the transaction across both systems. Here's how it works in two phases:

Phase 1: Preparing for Commitment

1. The coordinator sends a "prepare" message to both systems (A and B).
2. Each system performs the necessary changes for the transaction without actually committing them. This is like writing a draft but not submitting it yet.
3. Crucially, each system acknowledges the "prepare" message, indicating it's ready to commit.

Phase 2: Committing or Rolling Back

1. The coordinator analyzes these "prepared" responses. If all systems are ready, it sends a "commit" message to each.
2. If any system fails to respond or signals an issue, the coordinator sends a "rollback" message to all, undoing the prepared changes.

Here's how 2PC prevents last-second failures:

• Guaranteed Consistency: Even if a system crashes after acknowledging the "prepare" message (like system B in our example), the coordinator ensures consistency. It either commits or rolls back all changes, preventing data inconsistency.
• Atomicity: The transaction is treated as a single unit. Either all changes occur or none do, ensuring data integrity.

While 2PC solves the last-second failure problem, it has limitations:

• Blocking: While waiting for responses, the transaction can't proceed, potentially impacting performance.
• Single Point of Failure: The coordinator is a single point of failure. If it crashes, the entire process stalls.