APIs
An API is a contract that lets client and server change independently. Covers HTTP methods, status codes, and why breaking the contract breaks clients.
API Gateway
An API gateway centralizes auth, rate limiting, and routing in front of many backend services, and becomes a single point of failure if not made redundant.
REST vs GraphQL
REST exposes fixed-shape resources per URL; GraphQL lets clients query exactly the fields they need. The sharpest difference: which one caches for free.
WebSockets
WebSockets upgrade one HTTP request into a persistent, full-duplex connection. Covers why polling isn't enough and the statefulness cost you take on.
Webhooks
A webhook flips the API call around: instead of asking, you get a POST the moment something happens. Covers retries, duplicates, and signature checks.
Idempotency
An idempotent operation has the same effect run once or a hundred times. It's what makes retrying a failed request safe instead of dangerous.
Rate Limiting
Rate limiting caps how many requests a client can make and rejects the rest. Covers token bucket, sliding window, and where to enforce the limit.
API Design
Good API design means modeling resources as nouns, versioning before you need to, and paginating anything unbounded — habits, not one clever rule.