Understanding APIs: What They Are, Use Cases, and When to Use Them
Introduction
In today's interconnected digital world, the term API is widely used across software development, business, and even non-technical domains. APIs, or Application Programming Interfaces, sit at the core of modern software. They let different applications communicate, share data, and use each other's services, which makes them essential for building dynamic, distributed systems.
This guide introduces what an API is, common use cases, when to use them (and when not to), and basic vocabulary. We'll use this foundation in the following milestones to build a complete REST API in Go, step by step.
What is an API?
At its core, an API (Application Programming Interface) is a set of rules that lets one piece of software talk to another. It defines methods and protocols for requesting data, triggering actions, or performing operations.
APIs sit between systems so that components built with different technologies can work together. Those components may run on the same machine or on servers and clouds around the world.
Key Components of an API
| Component | Description |
|---|---|
| Endpoint | The URL or path where the API is called. Each endpoint is like an address where the API waits for requests. |
| Request | What the client sends: a method (e.g. GET, POST), path, optional headers, and sometimes a body. |
| Response | What the API returns, often as JSON or XML, with data or the result of an operation. |
| HTTP Methods | Verbs that describe the action: GET (read), POST (create), PUT (replace), PATCH (partial update), DELETE (remove). |
| Authentication | How callers prove identity; common approaches include API keys, OAuth, and JWT. |
| Rate Limiting | Rules that cap how many requests a client can make in a given time to avoid misuse or overload. |
Common Use Cases for APIs
APIs support many everyday scenarios:
1. Web and Mobile Applications
Web and mobile apps use APIs to talk to backend servers. Logging in, loading a feed, or saving data usually involves one or more API calls. For example:
- A weather app calling a server for the latest forecast.
- A social app posting or fetching user content.
2. Microservices
In a microservices architecture, the system is split into many small services. APIs are how these services talk to each other. Each service handles a focused capability, and APIs let them share information and cooperate.
3. Third-Party Integrations
Businesses often use external services (payments, maps, email, etc.). Those providers expose APIs so you can integrate their functionality without building everything yourself—for example, Stripe for payments or Google Maps for locations.
4. IoT (Internet of Things)
Devices like smart thermostats, cameras, and wearables send and receive data through APIs. APIs are how they report status, receive commands, and sometimes get firmware updates.
5. Automation and Workflows
Tools like Zapier or IFTTT connect apps via APIs so non-developers can automate tasks—e.g. "when a new row is added in a spreadsheet, create a task in a project tool."
6. Data Aggregation
APIs are used to pull data from many sources (internal or external) and combine it for dashboards, reports, or analytics.
7. Cloud and Infrastructure
Cloud providers (AWS, GCP, Azure) offer APIs to create and manage resources—servers, databases, networks—programmatically instead of only through a web UI.
When APIs Shine
APIs are a good fit when you need to:
- Integrate with external systems — Connect to payment gateways, identity providers, or other SaaS.
- Build scalable systems — Separate frontends, backends, and services so each part can scale and evolve independently.
- Support multiple clients — One API can serve web, mobile, and partner integrations from a single backend.
- Expose data or capabilities to partners — Offer a controlled, documented way for others to use your data or features.
When Other Approaches Fit Better
APIs are not always the best tool:
1. Message Queues
For asynchronous work where you don't need an immediate response, message queues (e.g. RabbitMQ, Kafka, SQS) can be better. Requests are enqueued and processed later, which often scales well for high throughput or background jobs.
2. File-Based Exchange
For bulk or batch data (e.g. daily exports to a data warehouse), file-based exchange (CSV, Parquet, etc.) can be simpler than designing many API calls for large payloads.
3. Shared Database Access
When two systems are internal and share the same database, direct DB access can be enough. You lose some encapsulation and isolation, so this is usually a trade-off for simplicity vs. long-term maintainability.
4. Real-Time Communication
For live updates (chat, notifications, collaborative editing), WebSockets or similar protocols keep a long-lived connection open. They're often a better fit than request–response APIs for real-time, bidirectional traffic.
Basic API Vocabulary
| Term | Meaning |
|---|---|
| REST | A style for building APIs over HTTP: resources identified by URLs, operations expressed with standard methods (GET, POST, etc.). |
| JSON | A common format for request and response bodies. Lightweight and easy for both humans and machines to read. |
| Endpoint | A specific URL path (e.g. /api/products) that accepts requests for a resource or action. |
| Idempotent | A request that, when repeated, has the same effect as doing it once (e.g. GET, PUT, DELETE). Important for retries. |
| Rate limiting | Restricting how many requests a client can make per minute/hour to protect the server. |
| Versioning | Supporting multiple API versions (e.g. /v1/, /v2/) so you can change the API without breaking old clients. |
| GraphQL | An alternative to REST where clients request exactly the fields they need, reducing over- and under-fetching. |
What We'll Build in Go
In the next milestones we will build a product catalog API in Go that:
- Serves REST over HTTP.
- Uses JSON for request and response bodies.
- Implements CRUD (Create, Read, Update, Delete) for products.
- Adds a database, validation, error handling, and documentation.
We'll use the standard library and a couple of popular libraries so you see both "vanilla" HTTP and practical patterns used in production Go services.
Next step
Start with Milestone 1 — Your First Go API to set up the project and implement a simple in-memory CRUD API.