System Type: Web Service

Patterns, failure modes, and anti-patterns for request/response web services.

API Patterns

REST

When to use. Public APIs, browser-facing services, CRUD-heavy domains, when discoverability and cacheability matter. When to avoid. Highly relational data with many nested queries (N+1 fetches). Real-time bidirectional communication. High-throughput internal service-to-service calls where payload efficiency matters. Key decisions. Resource naming, versioning strategy (URL vs header), pagination approach, error format.

GraphQL

When to use. Multiple client types needing different data shapes from the same backend. Complex, nested data relationships. When frontend teams need to iterate independently from backend. When to avoid. Simple CRUD APIs. Server-to-server communication. When caching at the HTTP layer is important (GraphQL's POST-based model breaks HTTP caching). When the team doesn't have GraphQL operational expertise. Key decisions. Schema-first vs code-first, query complexity limits, N+1 resolution strategy (DataLoader pattern), authorization model.

gRPC

When to use. Internal service-to-service communication. When payload size and serialization speed matter. When you want strongly-typed contracts with code generation. Streaming use cases. When to avoid. Browser clients (requires grpc-web proxy). When human readability of requests matters for debugging. When the team lacks protobuf experience. Public APIs (tooling ecosystem is smaller). Key decisions. Proto file organization, backward compatibility discipline, deadline propagation, load balancing (L7 required for HTTP/2).

Scaling Patterns

Horizontal scaling. Add more instances behind a load balancer. Requires stateless services (or externalized state). The default approach for web services. Watch for: session affinity requirements, connection pool exhaustion at the database, cache consistency across instances.

Vertical scaling. Bigger machines. Simpler than horizontal but has a ceiling. Right for: databases, in-memory workloads, and when horizontal scaling's coordination cost exceeds the performance benefit.

Autoscaling. Scale instance count based on metrics (CPU, request rate, queue depth). Essential for variable load. Watch for: cold start latency, scaling lag, minimum instance counts for availability, cost runaway from misconfigured scaling policies.

CDN and edge caching. Serve static and cacheable dynamic content from edge locations. Dramatically reduces latency and origin load. Watch for: cache invalidation complexity, cache poisoning, TTL tuning, varying content by headers (accept-language, authorization).

Read replicas. Offload read traffic from the primary database. Watch for: replication lag causing stale reads, read-after-write consistency requirements, connection routing complexity.

Data Layer Patterns

RDBMS (PostgreSQL, MySQL). Default choice for structured, relational data. Strong consistency, ACID transactions, mature tooling. Scales vertically well; horizontal scaling requires sharding (hard) or read replicas (easier).

Document stores (MongoDB, DynamoDB). When data is naturally document-shaped, schema varies per record, or you need horizontal scaling without sharding complexity. Watch for: lack of joins, transaction limitations across documents, query patterns that don't match the data model.

Key-value stores (Redis, Memcached). Caching, session storage, rate limiting, leaderboards. Extremely fast for simple access patterns. Watch for: data loss on restart (unless configured for persistence), memory limits, using it as a primary datastore when it's a cache.

Search engines (Elasticsearch, OpenSearch). Full-text search, log aggregation, analytics on semi-structured data. Watch for: operational complexity, eventual consistency, write amplification, cluster sizing that's hard to change later.

Observability

Structured logging. JSON logs with consistent fields (request_id, user_id, service, timestamp, level). Enable correlation across services. Avoid: unstructured log lines, logging sensitive data, excessive log volume without sampling.

Distributed tracing. Propagate trace IDs across service boundaries to reconstruct request paths. Essential when requests span multiple services. Use OpenTelemetry for vendor-neutral instrumentation.

Metrics. RED method for services (Rate, Errors, Duration). USE method for resources (Utilization, Saturation, Errors). Define SLOs before choosing what to measure.

SLOs and SLIs. Define service level objectives in terms of measurable indicators (latency P99 < 200ms, error rate < 0.1%). SLOs drive alerting, capacity planning, and error budgets. Without SLOs, you're guessing about reliability.

Common Failure Modes

• Cascading failures. One slow service causes callers to queue up, exhausting their resources. Mitigation: timeouts, circuit breakers, bulkheads.
• Connection pool exhaustion. Database or HTTP connection pools fill up under load. Mitigation: pool sizing, connection timeouts, backpressure.
• Thundering herd. Cache expiry causes all instances to hit the backend simultaneously. Mitigation: jittered TTLs, request coalescing, cache warming.
• Retry storms. Clients retry failed requests, multiplying load on an already-stressed system. Mitigation: exponential backoff with jitter, retry budgets, circuit breakers.
• Memory leaks. Gradual memory growth leading to OOM kills. Mitigation: memory limits, health checks, regular restarts (if you can't find the leak).
• Dependency failures. External services go down. Mitigation: timeouts, fallbacks, graceful degradation, feature flags.

Anti-Patterns

• Distributed monolith. Microservices that must deploy together, share databases, or make synchronous calls in long chains. You got the complexity of distribution without the benefits.
• God service. One service that does everything. Split by domain boundary, not by arbitrary size targets.
• Chatty interfaces. Many small API calls where one well-designed call would do. Increases latency, error surface, and complexity.
• Shared mutable state. Multiple services writing to the same database tables. Define ownership or accept the coupling.
• Premature microservices. Splitting into services before understanding domain boundaries. Start with a well-structured monolith; extract services when you have evidence they need independent scaling or deployment.
• Ignoring cold starts. Assuming services are always warm. New deployments, autoscaling events, and restarts all serve cold traffic.