Mock interviews

How a mock works

1. TimedAbout 3 minutes a question, with a visible clock you can pause.
2. Out loudAnswer as you would in the room. Nothing is recorded.
3. Follow-upsThen the follow-up questions an interviewer would ask.
4. RubricTick the key points you covered; that is your score.

Mock sets (34)

• Transactions, isolation and locking mock 1

  Transactions, isolation and locking · 5 questions · about 25 min

  • Isolation levels in PostgreSQL
  • The lost update problem
  • SELECT FOR UPDATE and SKIP LOCKED
  • Optimistic vs pessimistic locking
  • Deadlocks and lock ordering

  Start this mock on the Transactions, isolation and locking page

• Transactions, isolation and locking mock 2

  Transactions, isolation and locking · 5 questions · about 25 min

  • Why long transactions hurt
  • Errors inside a transaction
  • Where transactions begin and end in code
  • Write skew under REPEATABLE READ
  • Retrying serialization failures

  Start this mock on the Transactions, isolation and locking page

• SQL, indexing and PostgreSQL mock 1

  SQL, indexing and PostgreSQL · 5 questions · about 25 min

  • Column order in a composite index
  • Reading EXPLAIN ANALYZE
  • Partial and expression indexes
  • Upserts with ON CONFLICT
  • OFFSET vs keyset pagination

  Start this mock on the SQL, indexing and PostgreSQL page

• SQL, indexing and PostgreSQL mock 2

  SQL, indexing and PostgreSQL · 5 questions · about 25 min

  • NULL, three-valued logic and NOT IN
  • Window functions
  • Normalization and when to denormalize
  • Connection pooling and PgBouncer
  • Index-only scans and INCLUDE

  Start this mock on the SQL, indexing and PostgreSQL page

• Distributed systems mock 1

  Distributed systems · 5 questions · about 25 min

  • What CAP actually says
  • Leader-follower replication and lag
  • Consistent hashing
  • Retries with exponential backoff and jitter
  • Timeouts and deadline propagation

  Start this mock on the Distributed systems page

• Distributed systems mock 2

  Distributed systems · 5 questions · about 25 min

  • Circuit breakers and bulkheads
  • Consistency models from strong to eventual
  • Quorum reads and writes
  • Consensus and Raft
  • Leader election, leases and fencing tokens

  Start this mock on the Distributed systems page

• Distributed systems mock 3

  Distributed systems · 5 questions · about 25 min

  • Locks in Redis: when they're safe
  • Sharding: range, hash and directory
  • At-most-once, at-least-once and 'exactly once'
  • The transactional outbox pattern
  • Sagas: orchestration vs choreography

  Start this mock on the Distributed systems page

• API design: REST, HTTP, GraphQL and gRPC mock 1

  API design: REST, HTTP, GraphQL and gRPC · 5 questions · about 25 min

  • Idempotency keys for POST
  • ETags and conditional requests
  • Paginating API responses
  • Versioning an API
  • Which API changes are breaking

  Start this mock on the API design: REST, HTTP, GraphQL and gRPC page

• API design: REST, HTTP, GraphQL and gRPC mock 2

  API design: REST, HTTP, GraphQL and gRPC · 5 questions · about 25 min

  • Rate limiting with a token bucket
  • When GraphQL fits and what it costs
  • gRPC vs REST
  • Designing reliable webhooks
  • Long-running operations: 202 and polling

  Start this mock on the API design: REST, HTTP, GraphQL and gRPC page

• API design: REST, HTTP, GraphQL and gRPC mock 3

  API design: REST, HTTP, GraphQL and gRPC · 5 questions · about 25 min

  • PATCH: merge patch vs JSON Patch
  • Uploading large files
  • Polling, SSE and WebSockets
  • Batch and bulk endpoints
  • What an API gateway does

  Start this mock on the API design: REST, HTTP, GraphQL and gRPC page

• API design: REST, HTTP, GraphQL and gRPC mock 4

  API design: REST, HTTP, GraphQL and gRPC · 5 questions · about 25 min

  • Designing REST resources
  • Safe and idempotent HTTP methods
  • Choosing HTTP status codes
  • A consistent error format
  • 429, Retry-After and rate-limit headers

  Start this mock on the API design: REST, HTTP, GraphQL and gRPC page

• Caching and Redis mock 1

  Caching and Redis · 5 questions · about 25 min

  • Write-through, write-behind and write-around
  • Cache stampede on a hot key
  • Spreading expiries with TTL jitter
  • Eviction: LRU, LFU and maxmemory
  • HTTP caching headers

  Start this mock on the Caching and Redis page

• Caching and Redis mock 2

  Caching and Redis · 5 questions · about 25 min

  • What to put behind a CDN
  • Choosing Redis data structures
  • Cache penetration and negative caching
  • Redis durability: RDB, AOF and when Redis is a database
  • Keeping a cache consistent with the database

  Start this mock on the Caching and Redis page

• Queues, Kafka and event-driven systems mock 1

  Queues, Kafka and event-driven systems · 5 questions · about 25 min

  • Background job queues
  • Kafka partitions, keys and ordering
  • Consumer groups and rebalancing
  • Keeping messages in order
  • Dead-letter queues and poison messages

  Start this mock on the Queues, Kafka and event-driven systems page

• Queues, Kafka and event-driven systems mock 2

  Queues, Kafka and event-driven systems · 5 questions · about 25 min

  • Backpressure when consumers fall behind
  • Kafka or RabbitMQ?
  • Scheduled jobs that run once
  • Monitoring consumer lag
  • Committing offsets: at-most-once vs at-least-once

  Start this mock on the Queues, Kafka and event-driven systems page

• NoSQL and choosing a database mock 1

  NoSQL and choosing a database · 5 questions · about 25 min

  • Document store or relational database?
  • JSONB in PostgreSQL
  • Partition keys in DynamoDB-style stores
  • Search with Elasticsearch or OpenSearch
  • Storing time-series data

  Start this mock on the NoSQL and choosing a database page

• NoSQL and choosing a database mock 2

  NoSQL and choosing a database · 5 questions · about 25 min

  • When a graph database helps
  • Embedding vs referencing in MongoDB
  • Choosing a database for a new service
  • Wide-column stores like Cassandra
  • Object storage for files

  Start this mock on the NoSQL and choosing a database page

• Concurrency in Python services mock 1

  Concurrency in Python services · 5 questions · about 25 min

  • The GIL, threads and processes
  • How asyncio works
  • Blocking calls inside async code
  • Race conditions in async code
  • Limiting concurrency with semaphores

  Start this mock on the Concurrency in Python services page

• Concurrency in Python services mock 2

  Concurrency in Python services · 5 questions · about 25 min

  • WSGI vs ASGI servers
  • Request-scoped context with contextvars
  • Producer-consumer with asyncio.Queue
  • Deadlocks with application locks
  • Cancellation and cleanup in asyncio

  Start this mock on the Concurrency in Python services page

• Authentication and authorization mock 1

  Authentication and authorization · 5 questions · about 25 min

  • Validating a JWT properly
  • Access tokens, refresh tokens and rotation
  • OAuth 2.0 authorization code with PKCE
  • OpenID Connect and ID tokens
  • MFA and TOTP codes

  Start this mock on the Authentication and authorization page

• Authentication and authorization mock 2

  Authentication and authorization · 5 questions · about 25 min

  • API keys done right
  • RBAC, ABAC and resource ownership
  • Revoking tokens and sessions
  • Authenticating service-to-service calls
  • Server sessions vs stateless tokens

  Start this mock on the Authentication and authorization page

• Backend security mock 1

  Backend security · 5 questions · about 25 min

  • Server-side request forgery (SSRF)
  • Managing secrets
  • Signing requests and webhooks with HMAC
  • Credential stuffing and brute force
  • Encryption in transit and at rest

  Start this mock on the Backend security page

• Backend security mock 2

  Backend security · 5 questions · about 25 min

  • Dependency and supply-chain security
  • Broken object level authorization (BOLA)
  • SQL injection and parameterised queries
  • CSRF and how cookies cause it
  • XSS from a backend's point of view

  Start this mock on the Backend security page

• Performance and scaling mock 1

  Performance and scaling · 5 questions · about 25 min

  • Load balancing strategies
  • Capacity with Little's law
  • Finding where time goes
  • Scaling reads with replicas
  • Batching database writes

  Start this mock on the Performance and scaling page

• Performance and scaling mock 2

  Performance and scaling · 5 questions · about 25 min

  • Autoscaling services
  • Load testing a backend
  • Memory growth in Python services
  • Hot keys and hot partitions
  • Why percentiles, not averages

  Start this mock on the Performance and scaling page

• Observability and operations mock 1

  Observability and operations · 5 questions · about 25 min

  • Metrics and label cardinality
  • Distributed tracing
  • SLOs, error budgets and alerts
  • Running an incident
  • Blameless postmortems

  Start this mock on the Observability and operations page

• Observability and operations mock 2

  Observability and operations · 5 questions · about 25 min

  • Audit logs
  • Structured logging
  • Liveness and readiness checks
  • RED and USE dashboards
  • Keeping secrets and personal data out of logs

  Start this mock on the Observability and operations page

• Testing backends mock 1

  Testing backends · 5 questions · about 25 min

  • Integration tests with a real database
  • Contract testing between services
  • Property-based testing
  • Testing database migrations
  • Chaos experiments and failure testing

  Start this mock on the Testing backends page

• Testing backends mock 2

  Testing backends · 5 questions · about 25 min

  • The test pyramid for a backend
  • Fakes, stubs and mocks
  • Testing code that uses time and randomness
  • Flaky tests
  • Test data and fixtures

  Start this mock on the Testing backends page

• Cloud, deploys and infrastructure mock 1

  Cloud, deploys and infrastructure · 5 questions · about 25 min

  • Expand and contract changes
  • Graceful shutdown
  • Kubernetes basics for backend developers
  • Blue-green and canary releases
  • Feature flags on the server

  Start this mock on the Cloud, deploys and infrastructure page

• Cloud, deploys and infrastructure mock 2

  Cloud, deploys and infrastructure · 5 questions · about 25 min

  • Infrastructure as code
  • When serverless fits
  • Backups, RPO, RTO and multi-region
  • Containers and images
  • Configuration across environments

  Start this mock on the Cloud, deploys and infrastructure page

• System design mock 1

  System design · 5 questions · about 25 min

  • Monolith or microservices
  • Design a URL shortener
  • Design search autocomplete
  • Design file uploads and storage
  • Design a news feed

  Start this mock on the System design page

• System design mock 2

  System design · 5 questions · about 25 min

  • Design a chat backend
  • Design a payment flow
  • Design a distributed rate limiter
  • Back-of-the-envelope estimates
  • Layering a backend service

  Start this mock on the System design page

• Backends for mobile apps mock 1

  Backends for mobile apps · 5 questions · about 25 min

  • Sending push notifications at scale
  • Backend for frontend (BFF)
  • Verifying in-app purchases on the server
  • API performance on mobile networks
  • An API for offline sync

  Start this mock on the Backends for mobile apps page

Behavioral round