2PC vs Saga: Atomic Commit vs Compensating Transactions
Overview Both patterns coordinate a transaction that spans multiple services or databases, but they resolve the coordination problem in opposite ways. 2PC locks every participant until a coordinator confirms all can commit, guaranteeing atomicity at the cost of blocking; Saga lets each step commit immediately and unwinds failures afterward with compensating actions, trading strict atomicity for availability and throughput. Comparison Diagram 2PCSagaCoordinatorP1P2P3prepare / vote / commitlockedlockedlockedall-or-nothing, blocking until ackT1T2T3local commit, local commitcompensatecompensateindependent commits, compensating rollbacks Comparison Table Aspect 2PC Saga Initiation Coordinator sends a prepare request to all participants at once First service runs its local transaction and triggers the next step Commit decision Coordinator waits for every vote, then issues a single atomic commit or abort No central decision; each step commits independently as it finishes Resource locking Participants hold locks from prepare until the commit acknowledgment arrives No cross-step locks; each local transaction commits and releases immediately Failure handling Coordinator aborts and tells all participants to roll back the uncommitted work Already-committed steps are undone via explicit compensating transactions Atomicity guarantee True all-or-nothing atomicity across every participant No real atomicity; intermediate states are visible until compensations finish Coordination dependency Single coordinator is a synchronous, blocking point of failure Runs via choreography or a lightweight orchestrator that doesn’t hold locks Latency and throughput Higher latency and lower throughput from synchronous cross-service locking Lower per-step latency and higher throughput since nothing blocks across services Implementation effort Relies on XA-compliant resources and a transaction manager Requires custom compensating logic and step/state tracking per operation Key Differences 2PC provides true atomicity across services, while Saga only approximates it through compensations after the fact 2PC holds locks on every participant until the coordinator commits; Saga commits each step locally with no cross-service locking Saga failures require explicit compensating transactions; 2PC failures simply abort the still-uncommitted transaction 2PC depends on a synchronous coordinator that all participants must trust and wait on; Saga can run via choreography or a non-blocking orchestrator 2PC trades throughput for consistency, while Saga trades strict consistency for availability at scale When to Use Each 2PC ...