Top 20 System Design Interview Questions and Answers
Top 20 System Design Interview Questions and Answers Work through these 20 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
Kafka consumer groups, offsets, and delivery semantics?
@KafkaListener with AckMode.MANUAL — ack after DB save. Consumer group order-service scales to partition count max. Idempotency key in message header dedup table. Monitor...
Redis caching patterns: cache-aside vs write-through?
@Cacheable("users") on findById — Redis cache-aside via Spring Data Redis. Update user: @CacheEvict. TTL 15m + jitter. Hot product page: lock on miss with Redisson. Do not...
Explain CAP theorem in the context of microservices.
When a network split isolates nodes, you cannot have both perfect consistency and every node answering. Example: order service during partition — either reject writes (CP) or...
What is the difference between ArrayList and LinkedList?
ArrayList: dynamic array, great for get(i) and iteration, amortized append. LinkedList: node chain, costly random access, only wins for frequent insert/remove in middle with...
How would you design a rate-limited REST API?
API gateway or filter applies token bucket per API key — Redis stores tokens with TTL. Exceeding limit returns 429 + Retry-After. Different tiers per plan. Log rejections for...
What is circuit breaker pattern?
When payment service errors exceed 50% in 10s window, breaker opens — return fallback immediately for 30s. Half-open lets one request through to test recovery. Pair with...
Explain microservices vs monolith trade-offs.
Start monolith until team or scale pain justifies split. Microservices enable independent deploy and tech diversity but add network failures, observability burden, and data...
What is an ExecutorService and why use it?
Instead of new Thread per task, submit Runnable to fixed pool sized to CPU/IO profile. Handle Future results and exceptions. On shutdown hook call shutdown() and...
What are virtual threads in Java 21 and when should you use them?
Virtual threads excel when code blocks on IO — HTTP clients, JDBC, file reads — allowing one platform thread to serve thousands of concurrent requests. For CPU-heavy parallel...
How do CompletableFuture and virtual threads compare for concurrency?
Use virtual threads when business logic is naturally blocking — call DB, call HTTP, return. Use CompletableFuture when composing multiple independent async results with...
What are weak references and WeakHashMap use cases?
Attach metadata to objects without extending their lifetime — canonical mapping while object lives. When key GCs, entry drops. Do not rely on timing. For API response caching...
What is Hibernate second-level cache and when enable it?
Enable for ProductCategory, Currency — READ_ONLY, rarely changed. spring.jpa.properties.hibernate.cache.use_second_level_cache=true with Caffeine region factory. Avoid on...
What are idempotency keys and how implement them in REST APIs?
Client sends Idempotency-Key: uuid on POST /payments. Server Redis SETNX key, process, store response JSON. Retry with same key returns 201 + original body without double...
Saga pattern — choreography vs orchestration?
Order saga: reserve inventory → charge payment → ship. Failure at payment triggers release inventory compensation. Orchestration clearer for complex flows with visibility....
Explain the transactional outbox pattern.
OrderService @Transactional saves Order and OutboxEvent(type=ORDER_CREATED, payload=json). OutboxProcessor @Scheduled polls FOR UPDATE SKIP LOCKED, sends to Kafka, updates...
When accept eventual consistency and how communicate it to clients?
Checkout returns 202 Accepted with orderId — poll or webhook when confirmed. Inventory display may lag seconds — show "approximate stock." Payment ledger uses strong...
What is the Backend-for-Frontend (BFF) pattern?
Mobile BFF /mobile/home aggregates user profile + feed + notifications in one round trip. Web BFF different field selection. BFF handles OAuth BFF cookie pattern. Domain...
Database sharding strategies for high-scale Java services?
Shard by tenant_id mod 32 — RoutingDataSource lookup. User-scoped queries single shard. Global admin reports from CQRS read replica or Elasticsearch. Plan shard key early —...
When and why use CQRS (Command Query Responsibility Segregation)?
E-commerce: write Order aggregate to PostgreSQL; project OrderSummaryView to Elasticsearch for search/filter dashboard. Accept projection lag. Not for simple User CRUD. Combine...
Load shedding and backpressure in microservices?
Tomcat max queue + reject policy returns 503 with Retry-After. WebClient timeout + circuit breaker stops calling slow dependency. Kafka consumer pause partitions when...
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