ACID Properties: Correctness Through Coordination

Definition
ACID: Atomicity (all-or-nothing), Consistency (valid states), Isolation (no interference), Durability (crash survival). These guarantees require coordination that adds latency.
Performance Cost of Coordination
ACID requires coordination before confirming writes. A typical setup writes to a distributed log, waiting for a quorum (majority) to acknowledge. This adds one network round-trip plus disk flush: 10-20ms median, 20-40ms P99 (99th percentile). Cross-region ACID uses two-phase commit (coordinator asks all to prepare, commits if all agree), pushing latencies to 100-200ms.
Availability Trade-off
ACID systems become unavailable during partitions if they cannot reach quorum. When the primary fails, a new one must be elected, blocking writes for 10-30s. This is acceptable when correctness is non-negotiable: money transfers, inventory at checkout, ledgers, entitlements.
Practical Boundaries
Use ACID within boundaries where coordination is cheap (single region, single shard) and relax guarantees across boundaries. Single-shard transactions stay fast (10-40ms); cross-shard either uses distributed transactions (slower) or eventual consistency patterns.

💡 Key Takeaways

✓ACID: Atomicity (all-or-nothing), Consistency (valid states), Isolation (no interference), Durability (crash survival)

✓Quorum writes add 10-20ms median latency within region; cross-region ACID costs 100-200ms due to two-phase commit

✓ACID systems block writes during partitions when quorum unavailable; typical failover is 10-30 seconds

✓Use ACID for correctness-critical operations: financial transfers, inventory at checkout, entitlements

📌 Interview Tips

1When asked about ACID costs, quantify: single-region quorum adds 10-20ms per commit; cross-region adds 100-200ms

2Explain two-phase commit: coordinator asks all to prepare (vote), commits only if all vote yes, aborts if any vote no

3For availability questions, mention failover window: 10-30 seconds of blocked writes during primary election

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