To understand what is API Gateway, first we need to know why it is need so much nowadays for big companies.

Why use API Gateway?

Suppose, you have a four services which are Auth Service which handles authentication, Order Service which handles orders, Delivery Service which handles the updates of orders and Payment Service which handles the payments. In the service, various machines are working together and to handle incoming request we have to use load balancers.

Now the question we need to ask is how are you going to connect all these services together in such a way that users who need authentication go to Auth Service and users who want to order go to Order Service. One solution that can be arriving in your mind is, "How about we use Load Balancer to handle the request?". But this is incorrect. We use API Gateway.

What is API Gateway?

API Gateway is an entry point which manages and direct incoming API requests to specific service according to the request. Like, if a new user arrives, API Gateway will direct it to Auth Service.

Formal definition: An API gateway is a server that sits between clients and backend services, acting as a single entry point for all API traffic. It receives every API request, applies cross-cutting policies (authentication, rate limiting, logging), routes the request to the correct backend service, and returns the response to the client. In a microservices architecture, the API gateway eliminates the need for clients to know about or communicate directly with individual services.

How API Gateway work?

Let's take a restaurant analogy to understand how API gateway works.

API Gateway is like a Head Waiter. Head Waiter servers as a liaison between guests and restaurant staffs and is responsible for:

• Greeting: When a customer arrives head waiter is the first one to greet the customers.

• Orders: When customer wants to order a dish, head waiter is the one who take the order and inform the chef about the dish.

• Reservation: Head Waiter check if the customer is reserved or not. The head waiter is responsible for managing reservations and ensuring that tables are allocated efficiently.

• Managing Wait Times: Head Waiter manages the customers during the busy hours by providing the customers with the estimated wait time and offer alternatives.

• Resolving Issues: If any issues or concerns arise during a guest's meal, the head waiter steps in to address them promptly and ensure that the guest is satisfied.

In short, head waiter is a person with multiple talents and responsibility.

An API gateway works in a similar fashion. It acts as the communicator between clients and the many services they may need to access.

API Gateway serves as the middle-man between clients and the many services they may need to access.

Core API Gateway features

Authentication and Authorization

An API Gateway can authenticate the incoming user and authorize which service it is allowed to access.

Rate Limiting

API Gateway also provide rate limiting to prevent malicious users from exploiting the resources of the service.

• Per-consumer limits — Each API key or user gets a separate quota

• Per-route limits — Expensive endpoints (e.g., AI inference) get lower limits

• Global limits — Cap total system throughput

• Dynamic limits — Adjust based on current system load

When a client exceeds the limit, the gateway returns 429 Too Many Requests with a Retry-After header without the request ever reaching your backend.

Load Balancing

The gateway distributes traffic across multiple backend instances using algorithms like:

• Round-robin — Requests cycle through backends sequentially

• Weighted round-robin — More traffic goes to more powerful instances

• Least connections — New requests go to the instance with fewest active connections

• Consistent hashing — Requests from the same client always go to the same instance (useful for caching)

Observability

• Centralized Logging & Monitoring: Provides insights into API usage, errors, and performance metrics.

Production API gateways integrate with monitoring and logging systems:

• Metrics — Request count, latency histograms, error rates per route (Prometheus, Datadog, StatsD)

• Distributed tracing — Inject trace headers for end-to-end request tracing (Jaeger, Zipkin, OpenTelemetry)

• Access logging — Structured logs for every request (Elasticsearch, Splunk, CloudWatch)

• Alerting — Trigger alerts when error rates spike or latency exceeds thresholds

Conclusion

An API Gateway acts as the single entry point that intelligently routes client requests to the appropriate backend services (for example: Auth, Order, Delivery, Payment) while applying cross-cutting policies like authentication, rate limiting, logging, and protocol translation. Unlike a plain load balancer, an API Gateway understands the API semantics and can direct traffic, aggregate responses, offload security concerns, and provide centralized observability.

Key benefits:

• Centralized routing and policy enforcement (security, quotas, CORS, etc.).

• Simplified client surface: clients call one endpoint instead of many services.

• Reduced chattiness: response aggregation and protocol translation (e.g., WebSocket, gRPC, REST).

• Improved observability and control for monitoring, tracing, and rate limiting.

Trade-offs to consider:

• Added latency and potential single point of failure—requires proper scaling and redundancy.

• Operational complexity and additional infrastructure to manage and secure.

• Need to design graceful degradation and versioning strategies.

In short, for microservice-based systems like the example services, an API Gateway provides the routing, security, and management capabilities that a simple load balancer cannot—making it a crucial component for large, distributed applications when used with attention to scalability and resilience.