Communication¶

Evolve API contracts, deliver real-time updates, and route traffic to the right target — while old and new consumers run at the same time.

This section covers how services and clients talk to each other, end to end: the API contracts they agree on, how updates reach a client in real time, and how traffic actually gets routed and delivered once a request leaves the client. The transport and network layer underneath all of this — TCP/IP, TLS, HTTP evolution, VPCs — lives in Infrastructure → Network.

API contracts and evolution¶

#	Topic	Practice outcome
01	API Gateway	Centralize edge policy without centralizing domain logic.
02	REST Design at Scale	Keep resource contracts predictable under growth.
03	GraphQL Federation	Define schema ownership and composition checks.
04	gRPC and Streaming	Operate long-lived and backpressured streams.
05	Versioning and Deprecation	Migrate consumers with measurable exit conditions.
06	Pagination and Filtering	Preserve stable traversal under changing data.
07	Idempotency and Retries	Prevent duplicate effects across failures.
08	Webhooks	Deliver signed events with retry and replay controls.
09	Backends for Frontends	Give client experiences focused composition boundaries.

Real-time and long-lived connections¶

#	Topic	Practice outcome
10	WebSockets	Operate a bidirectional connection safely.
11	Long Polling and Streaming	Match delivery style to update behavior.

Traffic routing and delivery¶

Each of these is a topic cluster with its own set of subtopics — open the cluster's README for the full breakdown.

#	Cluster	Focus
12	Load Balancers	Route traffic only to capable targets and prove failure detection and recovery behave as designed — 7 subtopics from L4/L7 routing to global server load balancing.
13	Content Delivery Networks	Control which responses are cached, where they're served, and how stale or unsafe content is removed — 5 subtopics from pull/push CDN to cache invalidation.
14	Domain Name System	Resolve a name deliberately, predict cache behavior, and roll out record changes without guessing — 5 subtopics from resolution flow to GeoDNS/anycast.

Practice loop¶

Choose one contract change, list every active consumer, deploy a compatible intermediate state, and define the telemetry that proves the old contract can be removed. For the routing and delivery clusters, use each cluster's own practice loop — the loop differs by what you're actually changing (a cache key, a health check, a DNS record).