System Design Roadmap 03 - Application Layer and Services

Application layer responsibility

Hosts business logic, validates requests, coordinates data access, emits events, enforces authorization, and returns APIs or rendered responses.

Stateless service

A service that does not keep user/session state in local memory between requests. Statelessness makes horizontal scaling and failover much easier.

Microservices

Small independently deployable services aligned to business capabilities. They improve ownership and independent scaling but add network, data, testing, and operational complexity.

Monolith vs microservices

A monolith is simpler to develop, test, deploy, and debug early. Microservices help when team/product boundaries and scaling needs justify distributed-system overhead.

Service discovery

A mechanism for finding healthy service instances dynamically. It can be client-side, server-side, DNS-based, or registry-based.

Service registry

A database of service instances and health metadata. Instances register on startup and deregister on shutdown or failed health checks.

API gateway

A front door for APIs that can route, authenticate, rate limit, aggregate, transform, and log requests. Avoid stuffing too much business logic into it.

Backends for Frontend

Separate backend/API layers tailored to different clients such as web, iOS, Android, or partner APIs. Useful when clients need different shapes or release cadences.

Sidecar pattern

Run helper functionality beside an app instance, often for proxies, logging, configuration, or service mesh behavior. It keeps cross-cutting concerns out of app code.

Ambassador pattern

A helper proxy handles outbound network concerns such as retries, discovery, routing, or TLS on behalf of the app.

Anti-corruption layer

A translation layer that protects a domain model from legacy systems or external models. It prevents incompatible concepts from leaking inward.

Strangler Fig pattern

Gradually replace a legacy system by routing new functionality to new services while old parts keep running until retired.

External configuration store

Centralizes config outside application binaries so settings can change safely across environments. Needs versioning, access control, and rollback.

Gateway routing

Route requests to services based on URL, host, headers, tenant, or version. Useful for APIs, migrations, and multi-service front doors.

Gateway offloading

Move shared concerns like TLS termination, auth checks, compression, or rate limits to a gateway so services stay focused.

Gateway aggregation

Combine calls to multiple backend services into one client-facing response. Useful for reducing client round trips but can create gateway bottlenecks.

Compute resource consolidation

Pack workloads efficiently to improve utilization and cost, while keeping isolation and noisy-neighbor risk under control.