Distributed Systems Primitives • Distributed Transactions (2PC, Saga)Medium⏱️ ~3 min
Choosing Between 2PC, Saga, and Alternative Approaches
Deciding between Two-Phase Commit, Saga, or alternative approaches requires evaluating consistency requirements, latency tolerance, failure domain boundaries, and operational complexity against your specific use case. The first question is whether you can avoid distributed transactions entirely by redesigning to localize invariants within a single partition or service. For example, instead of coordinating inventory across multiple warehouses in a transaction, partition inventory by region and allow each region to manage its own stock independently. If you must span services, determine whether you can tolerate eventual consistency and visible intermediate states. If invariants can be temporarily violated and compensated later (such as overselling inventory with后续 automated apology and refunds), a saga is usually the better choice for availability and throughput.
Prefer Two-Phase Commit when you have a small number of participants (ideally fewer than five), operations are short lived (milliseconds to low seconds), participants are within a single region or connected by low latency links (sub 10 millisecond round-trip times), and business invariants absolutely cannot be violated even momentarily. Financial ledger transfers between tightly controlled internal datastores are a classic 2PC use case: atomicity is non-negotiable, participants are co-located, and the business accepts the latency and availability trade-offs. Systems like Google Spanner or Amazon DynamoDB Transactions encapsulate 2PC complexity and provide ACID guarantees when you can accept regional (DynamoDB) or global (Spanner) commit latencies in the tens to hundreds of milliseconds.
Prefer Saga when processes are long running (seconds to days, such as order fulfillment or loan approval), span multiple teams or external services, must remain operational during network partitions, or when latency requirements are tight and you cannot afford synchronous cross service coordination in the hot path. Saga orchestration (Uber Cadence/Temporal, Netflix Conductor, AWS Step Functions) is appropriate when you want centralized visibility, durable timers, and explicit compensation logic; expect tens to hundreds of milliseconds orchestration overhead per step. Choreography is suitable when you want to avoid a single point of failure and can invest in robust event correlation and bounded context design. Consider alternative patterns such as Try-Confirm/Cancel (TCC) for reservations with explicit confirmation, conflict-free replicated data types (CRDTs) for eventually consistent counters or sets, or pre-authorization with later capture (common in payment processing) to decouple decision making from commitment.
💡 Key Takeaways
•First option: redesign to avoid distributed transactions by partitioning invariants within single services or using conflict-free replicated data types; this eliminates coordination overhead and failure modes entirely.
•Prefer 2PC for fewer than five participants, sub-second operations, single region or sub 10 millisecond round-trip time links, and invariants that cannot be temporarily violated (e.g., financial ledgers, tightly controlled internal systems).
•Prefer Saga for long running processes (seconds to days), cross team or external service boundaries, high availability requirements during partitions, and scenarios where eventual consistency and compensation are acceptable (e.g., order fulfillment, booking workflows).
•2PC availability is approximately the product of participant availabilities; two services at 99.5% each yield 99.0% end to end, making 2PC unsuitable for five nines availability targets without expensive replication and failover.
•Saga orchestration adds tens to hundreds of milliseconds per state transition in managed engines; acceptable for business processes but not for hot paths requiring single digit millisecond latencies (use local transactions or caching instead).
•Alternative patterns: Try-Confirm/Cancel for explicit reservation and confirmation, pre-authorization with later capture for payments, conflict-free replicated data types for counters, and escrow services to centralize contention points without distributed locks.
📌 Examples
Financial ledger transfer (prefer 2PC): moving money between internal accounts requires atomicity and cannot tolerate even momentary inconsistency; use 2PC or a system like Spanner that encapsulates it, accepting tens of milliseconds commit latency and reduced availability during coordinator failures.
E-commerce order fulfillment (prefer Saga): reserve inventory, authorize payment, schedule shipment, send confirmation; process spans seconds to minutes, crosses multiple services, and can tolerate eventual consistency with compensation if shipment fails after payment authorized.
Global content distribution (avoid distributed transactions): partition content by region; each region independently manages its catalog and availability; use asynchronous replication and CRDTs for eventual consistency across regions, eliminating need for cross region coordination.
Hotel booking (Try-Confirm/Cancel): try phase reserves room tentatively (hold with expiration), confirm phase captures payment and finalizes reservation, cancel phase releases hold; this pattern decouples reservation from payment commitment and avoids full saga compensation complexity for simple two-phase interactions.