Microservices Interview Questions and Answers

CAP states that during a network partition a distributed system must choose between consistency (every read sees the latest write) and availability (every request gets a non-error response). Since partitions are unavoidable, the real choice is CP or AP.

• CP systems refuse writes or reads rather than risk divergence; examples include systems using consensus such as etcd or ZooKeeper, and relational databases with strong quorum.
• AP systems keep serving and reconcile later; examples include Cassandra and DynamoDB, which favour availability with eventual consistency.

Partition occurs -> choose: reject (CP) or serve stale (AP)

PACELC extends this: even without a partition, systems trade latency against consistency. In microservices, apply the choice per operation: money movements may demand consistency, while a product view counter can be eventually consistent. Design compensating flows, idempotency and conflict resolution where you accept AP, and avoid assuming a globally consistent clock or snapshot.

Two-phase commit is rarely used in microservices because it needs a coordinating transaction manager, holds locks across services, blocks on coordinator failure and hurts availability. Instead, prefer an eventual-consistency design:

• Saga: model the workflow as local transactions with compensations (orchestrated or choreographed). Each step commits independently and failures trigger undo actions.
• Outbox pattern: write the business row and an event row in one local transaction, then publish the event asynchronously. This avoids the dual-write problem where a crash after the DB commit but before the publish loses the event.
• Idempotent consumers: deduplicate by message id or use upserts so retries and duplicate deliveries are harmless.
• Reconciliation: background jobs compare states and repair drift, and reads may use a read model projected from events.

BEGIN;
INSERT INTO orders ...;
INSERT INTO outbox (event_type, payload) VALUES ('order.created', :json);
COMMIT;

Choose compensations that are reversible or harmless, and make every step idempotent, because distributed atomicity is replaced by retries and repair.