Courses/Distributed Systems

Distributed Systems

CS3.401

Prof. Kishore Kothapalli•Monsoon 2025-26•4 credits

Revision Notes/Unit 8 — Distributed Transactions, 2PC & 3PC

Unit 8 — Distributed Transactions, 2PC & 3PC

ACID across multiple sites. Two-Phase Commit (2PC): Prepare + Decide phases; log records; recovery rules; the BLOCKING problem when coordinator crashes after all <ready>. Three-Phase Commit (3PC): adds a PRE-COMMIT phase to break blocking; requires no-partition assumption (rarely used in practice).

ACID + 2PC + 3PC + Blocking & In-Doubt States

16 min

A distributed transaction touches data at multiple sites. ACID requires that it either commits at ALL sites or aborts at ALL sites — never inconsistent partial outcomes. **2PC** is the textbook protocol: a coordinator gathers votes, then broadcasts the decision. Its weakness — and the entire reason 3PC exists — is the **blocking problem**: if the coordinator crashes after participants have voted but before announcing the decision, participants hold all their locks and wait indefinitely.