Senior/Staff System Design Interviews: API Gateways

Whether a senior/staff system design interview ends up meriting an offer often depends on how well one understands the components that support service security and reliability. In the world of modern software development, API gateways have become a vital component for building scalable, secure, and efficient microservices architectures. Whether you are a seasoned engineer or a technical enthusiast, understanding API gateways is essential as they play a significant role in system design and real-world development scenarios. In this blog post, we’ll explore what API gateways are, how they work under the hood, their implementation details, and the important technical trade-offs to consider when incorporating them into your software systems.

What is an API Gateway?

An API gateway acts as an intermediary between clients and the underlying microservices in a microservices-based architecture. It serves as the single entry point for all incoming requests from clients, providing a unified interface for multiple services. API gateways offer several key benefits:

  • Service Aggregation: API gateways can consolidate multiple microservices into a single API, reducing the client-side complexity.
  • Load Balancing: API gateways can distribute incoming requests across multiple instances of a microservice, ensuring even load distribution and better resource utilization. (Load-balancing technologies often get mentioned when talking about gateways). Check out a deeper explanation of load-balancing techniques here.
  • Authentication and Authorization: API gateways handle authentication and authorization tasks, ensuring that only authenticated and authorized requests reach the underlying microservices. (Concepts here include TLS termination, OAuth 2.0, validation of JSON Web Tokens (JWTs), Cross-Origin Resource Sharing (CORS) and a lot more!)
  • Caching: API gateways can cache responses to reduce the load on the backend services and improve response times.
  • Monitoring and Analytics: API gateways offer insights into the traffic flowing through the system, providing valuable data for monitoring and analytics purposes.

How do API Gateways work?

Under the Hood, API gateways typically perform several essential tasks to ensure the smooth flow of requests and responses, some of which follow directly from the benefits above:

  • Routing: Upon receiving a request, the API gateway examines the request and determines the appropriate microservice to handle it based on the defined routing rules.
  • Load Balancing: If the API gateway finds multiple instances of the target microservice, it utilizes load balancing algorithms to distribute the incoming requests evenly across those instances.
  • Authentication and Authorization: API gateways enforce security policies, validating API keys, tokens, or user credentials to ensure that only authenticated and authorized requests proceed.
  • Request Transformation: In some cases, the incoming request may not match the expected format of the underlying microservice. The API gateway can transform the request payload and headers accordingly.
  • Response Aggregation: When a client request requires data from multiple microservices, the API gateway can aggregate the responses and present a unified response to the client.


When implementing an API gateway, several technical considerations must be taken into account:

  • Choosing the Right Technology: API gateways can be implemented using various technologies, including Nginx and HAProxy. Nginx is often preferred for its rich feature set and versatility, while HAProxy excels in load balancing and high availability scenarios.
  • Security and Rate Limiting: Implementing robust security mechanisms, rate limiting, and throttling is vital to protect the underlying microservices from potential attacks and excessive traffic.
  • Logging and Monitoring: Extensive logging and monitoring are critical for diagnosing issues and gaining insights into the gateway’s performance and usage patterns.
  • Service Discovery: API gateways may use service discovery mechanisms like Consul or etcd to locate microservice instances dynamically. Consul excels in robust service discovery, offering registration, discovery, and health checks. It also provides a distributed key-value store. On the other hand, etcd is a simpler, widely used as Kubernetes’ primary data store, but requires more configuration for service discovery. (Consul could be used for large-scale environments with multiple data centers, and etcd for more straightforward use-cases and Kubernetes-based setups). Both can be used in production.

Technical Trade-offs

While API gateways offer numerous advantages, they also come with certain trade-offs that engineers must carefully consider:

  • Increased Complexity: Introducing an API gateway adds a new layer of complexity to the system, requiring additional maintenance and monitoring.
  • Single Point of Failure: If the API gateway fails, it can disrupt the entire system’s functionality. Implementing redundancy and failover mechanisms is crucial to mitigate this risk.
  • Performance Overhead: API gateways can introduce additional latency due to request processing, especially when aggregating responses from multiple microservices.
  • Versioning: Handling versioning of APIs can be challenging, especially when multiple microservices have different release cycles. Several types of versioning exist, including URL-based versioning (i.e. prefixing /v1/ or /v2, etc. in the URL for a REST request) and header-based versioning (where the info determining the API to use is in the request header… and many others).

API gateways have emerged as a fundamental component of modern microservices-based architectures, providing essential benefits like service aggregation, load balancing, and security enforcement. By understanding how API gateways work under the hood, considering the technical implementation details, and being aware of the trade-offs involved, engineers can wield this powerful tool to build scalable, robust, and secure software solutions.

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