Monolithic API to API Gateway Blueprint
Place an API gateway in front of a monolithic API to centralize auth, rate limiting, routing, and telemetry. Offload edge logic from the app and use route rules to prepare for strangler-fig service extraction.
What and Why
A monolithic API often handles auth, rate limiting, logging, and routing inside application code, duplicated and inconsistent. An API gateway pulls these cross-cutting concerns to the edge: one place for authentication, throttling, request routing, and telemetry. It is also the foundation for later decomposition, because you can route paths to new services without changing clients.
This blueprint introduces a gateway in front of an existing monolith, offloads edge concerns, and prepares for the strangler-fig extraction of services.
Phases
Assessment. Map all public routes, current auth mechanisms, rate limits, and SLAs. Identify cross-cutting logic embedded in the monolith that the gateway can absorb.
Gateway design. Choose a gateway (managed cloud gateway, NGINX/Kong, or a Kubernetes ingress gateway). Design routing rules, auth integration with the identity provider (OIDC), rate-limit tiers, and a problem+json error contract.
Edge cutover. Place the gateway in front of the monolith with pass-through routing. Validate latency and correctness, then move DNS so all traffic flows through it.
Policy offload. Move authentication/token validation, rate limiting, and request logging from the monolith to the gateway. Remove now-duplicated code from the app.
Decomposition prep. Add route-level rules so a path can be split off to a new microservice later. This sets up the strangler-fig pattern.
Key Risks and Mitigations
- Single point of failure: the gateway now sits in every request path. Run it highly available across zones with health checks and autoscaling.
- Latency overhead: an extra hop adds milliseconds. Keep the gateway close to backends, enable connection reuse/HTTP/2, and load-test before cutover.
- Misconfigured auth: centralizing auth concentrates risk. Validate token scopes, test negative cases, and roll out behind a canary.
Recommended Tooling
Kong, NGINX, or a managed gateway (AWS API Gateway, Azure API Management); Keycloak or another OIDC provider for tokens; Redis for rate-limit counters; Datadog/OpenTelemetry for edge metrics and tracing.
Success Metrics
Measure auth consistency (one enforcement point), edge p95 latency overhead, rate-limit effectiveness, and how quickly the first service can be extracted afterward.
Prerequisites
A documented route inventory, an OIDC-capable identity provider, and an HA deployment target. Agreement on rate-limit tiers and the standard error contract.