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To build an API, define the resource and operations it must expose, write down the request and response contract, implement a small set of HTTP routes, then test, secure, document, deploy, and monitor them. This guide uses a to-do API to make the process concrete; the design principles apply across languages and frameworks.
What an API does—and what you need to decide first
An API (application programming interface) lets one program request data or actions from another through an agreed interface. A web API commonly uses HTTP: a client sends a method, URL, headers, and possibly a body; the server returns a status code, headers, and usually a body.
Before choosing a framework, state the problem in user terms. For a to-do service, users need to list tasks, retrieve one task, create one, edit one, and delete one. Those tasks are the API’s resources and operations. Sketch how resources relate, identify who may access them, and decide what data a client is allowed to read or change. Postman’s API guide also recommends identifying resources and relationships before building models and routes: Postman’s API learning guide.
Start with a small, consistent resource
Use a plural resource name such as /api/todoitems. A REST-style design uses HTTP methods to express the operation rather than inventing a different URL for every action:
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| Method and path | Purpose | Typical result |
|---|---|---|
GET /api/todoitems |
List tasks | 200 OK with a JSON array |
GET /api/todoitems/{id} |
Fetch one task | 200 OK, or 404 Not Found |
POST /api/todoitems |
Create a task | 201 Created with the new resource |
PUT /api/todoitems/{id} |
Replace or update a task | 204 No Content, or 404 Not Found |
DELETE /api/todoitems/{id} |
Delete a task | 204 No Content, or 404 Not Found |
This route shape follows Microsoft’s ASP.NET Core Todo API tutorial: Create a Minimal API with ASP.NET Core. Status codes are part of the contract: clients need to distinguish a successful empty response from a missing resource or invalid request.
Design the API contract before implementation
Write down the contract that clients can rely on: paths, methods, fields, types, validation rules, success responses, error responses, and authentication requirements. Decide whether an update is a full replacement (PUT) or a partial change (often PATCH); do not make clients guess. Define how identifiers are represented and what happens for an unknown identifier.
OpenAPI provides a machine-readable description of endpoints, models, and authentication schemes. Google Cloud describes design-first API development as using OpenAPI as a blueprint before implementation: Google Cloud’s design-first API development guide. Even if you start with a short written contract, keeping the specification aligned with the running service helps client developers and catches disagreements early.
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For example, a task might have an integer id, a string title, and a Boolean isComplete. Decide whether missing optional fields have defaults, whether unknown fields are rejected, and how validation failures are reported. Use a consistent JSON content type and avoid returning internal exception details to clients. The exact schema depends on your use case; the important part is to specify it and test it as part of the contract.
Choose a framework style: minimal APIs or controllers
For ASP.NET Core, a Minimal API defines routes with relatively little ceremony. Microsoft describes Minimal APIs as designed to create HTTP APIs with minimal dependencies. Controllers organize endpoints into controller classes and are a more structured option when an application grows. Neither style changes the need for a sound contract, input validation, security, and testing.
Rank #2
- Used Book in Good Condition
| Consideration | Minimal APIs | Controller-based APIs |
|---|---|---|
| Framework ceremony | Fewer structures to get a small HTTP service running. | More explicit controller and action organization. |
| Files and dependencies | Often a compact starting point. | Useful when a project benefits from grouping actions and models into a fuller structure. |
| Cross-cutting features | Can support application-wide features; plan their organization as the service expands. | A familiar structured path for shared filters and conventions. |
| Growing models and persistence | Can remain appropriate, but route organization needs deliberate care as complexity grows. | Often a natural fit when a larger project has many models, actions, and conventions. |
| Team familiarity | Choose it if the team is comfortable with route-handler organization. | Choose it if the team already works effectively with controllers. |
Microsoft documents both approaches in its Minimal API tutorial and controller-based Web API tutorial. A small service can begin with the lighter style; project structure should follow the team and the features the API actually needs, not a rule that every API must use one approach.
Build one working slice in ASP.NET Core
The following example follows Microsoft’s Minimal API tutorial pattern and demonstrates the route shape with an in-memory list. It is a learning scaffold, not durable storage: data disappears when the process stops, and it is not suitable as a production database. For a production service, replace the list with persistent storage and add the security, validation, and operational controls described below. The code uses the standard ASP.NET Core tutorial project and its Todo model; follow Microsoft’s tutorial for the complete project setup and current framework-specific prerequisites: Microsoft Learn: Minimal API tutorial.
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();
var items = new List<Todo>();
var nextId = 1;
app.MapGet("/api/todoitems", () => items);
app.MapGet("/api/todoitems/{id:int}", (int id) =>
items.FirstOrDefault(item => item.Id == id) is Todo item
? Results.Ok(item)
: Results.NotFound());
app.MapPost("/api/todoitems", (Todo input) =>
{
var item = new Todo
{
Id = nextId++,
Name = input.Name,
IsComplete = input.IsComplete
};
items.Add(item);
return Results.Created($"/api/todoitems/{item.Id}", item);
});
app.MapPut("/api/todoitems/{id:int}", (int id, Todo input) =>
{
var item = items.FirstOrDefault(item => item.Id == id);
if (item is null) return Results.NotFound();
item.Name = input.Name;
item.IsComplete = input.IsComplete;
return Results.NoContent();
});
app.MapDelete("/api/todoitems/{id:int}", (int id) =>
{
var item = items.FirstOrDefault(item => item.Id == id);
if (item is null) return Results.NotFound();
items.Remove(item);
return Results.NoContent();
});
app.Run();
class Todo
{
public int Id { get; set; }
public string Name { get; set; } = "";
public bool IsComplete { get; set; }
}The endpoints make the resource operations explicit. The create handler returns a location for the new task; the update and delete handlers return no content when successful. Before exposing an implementation like this to real users, validate incoming fields, protect routes as needed, and use a database with appropriate concurrency behavior.
Run and exercise the routes
Run the project using the command appropriate to its location and SDK setup (for example, from a project directory, dotnet run). Use the local HTTP address printed by the app as the base URL. Replace http://localhost:5000 below with that address:
curl -i http://localhost:5000/api/todoitems
curl -i -X POST http://localhost:5000/api/todoitems
-H 'Content-Type: application/json'
-d '{"name":"Write the API contract","isComplete":false}'
curl -i http://localhost:5000/api/todoitems/1
curl -i -X PUT http://localhost:5000/api/todoitems/1
-H 'Content-Type: application/json'
-d '{"name":"Review the API contract","isComplete":true}'
curl -i -X DELETE http://localhost:5000/api/todoitems/1Inspect both the status line and the body. A POST should return a success status and the created task; a request for a nonexistent ID should return not found. Microsoft’s tutorial also demonstrates testing endpoints with Endpoints Explorer and .http files, which keep repeatable requests beside the project: ASP.NET Core Minimal API tutorial.
Document and test behavior, not just the happy path
Tests should verify the contract from a client’s point of view. Use a .http file, an interactive OpenAPI/Swagger interface, Postman, or another HTTP client. OpenAPI documentation can make endpoints discoverable and provide a convenient way to issue requests, but it does not replace automated tests.
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- Send malformed JSON, missing required fields, wrong types, and values outside allowed limits.
- Request an unknown ID and verify the documented not-found behavior.
- Test missing credentials, invalid credentials, and users attempting actions they are not authorized to perform.
- Verify content types, response headers, and that errors do not disclose stack traces or secrets.
- Keep regression tests for bugs and edge cases so later changes do not silently break clients.
Postman outlines a general API testing workflow at Postman. SoapUI groups API testing practice into functional, load, security, automation, and mocking or virtualization tests: SoapUI API learning resources. Choose test types based on risk and expected usage rather than assuming a successful browser or Swagger request proves production readiness.
Secure the API before release
Security is part of the API design, not a final switch. Authentication answers who the caller is; authorization determines what that caller may do. Protect sensitive routes, validate every client-controlled value, and expose only fields the caller is permitted to change. Microsoft’s controller tutorial specifically calls out preventing over-posting—where a client submits fields it should not be allowed to set—and warns that enabling Swagger in production could expose potentially sensitive details about API structure and implementation: Microsoft Learn: controller-based Web API tutorial.
- Require HTTPS in deployed environments and avoid sending credentials in URLs.
- Validate body fields, query parameters, path identifiers, and headers; reject or safely handle invalid values.
- Use request models that expose only client-editable fields instead of binding arbitrary database models.
- Enforce authorization at the operation and resource level, not only at the user interface.
- Keep secrets in appropriate configuration or secret-management facilities, not source code or logs.
- Limit interactive API documentation to environments and users for whom the detail is appropriate.
Deploy, observe, and improve
Deployment depends on the language, framework, hosting platform, and data store, so there is no single command that fits every API. For ASP.NET Core, Microsoft documents publishing to Azure in its tutorial: Publish an ASP.NET Core app to Azure. Before release, configure the real database, production secrets, HTTPS, allowed origins if browser clients need cross-origin requests, and environment-specific documentation settings.
After deployment, monitor errors, latency, and usage, as Google Cloud recommends in its API design guidance: Google Cloud design-first API development. Track enough context to diagnose failed requests without logging passwords, tokens, or unnecessary personal data. A useful release check includes a health check, a test request against the deployed host, a rollback plan, and confirmation that the API’s documentation reflects the live contract.
Or skip the browser setup
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For example, this cURL request captures a page as WebP; see the ScreenshotNeo API documentation for request options and response details:
curl -G "https://api.screenshotneo.com/v1/shot"
-d access_key=YOUR_API_KEY
--data-urlencode url=https://stripe.com
-o shot.webpThere are 1,000 screenshots a month on the free plan with no card required; paid plans start at $5 for 3,000 screenshots. To try it, sign up for ScreenshotNeo.
Common API-building problems and fixes
The route returns 404
Check the method as well as the path: a registered GET route does not handle a POST. Confirm the local host and port printed by the app, the /api prefix, route spelling, and whether the ID constraint accepts the value.
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The request body is rejected or fields are empty
Send valid JSON and set Content-Type: application/json. Match the JSON field names and types expected by the model, and check required-field validation. Do not solve a binding problem by accepting arbitrary fields; define an input shape the API intends clients to send.
Writes appear to succeed but data disappears
The example stores tasks in process memory. Restarting the app clears them, and multiple app instances would not share that list. Use persistent storage for real data and plan migrations, backups, and concurrency handling.
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Swagger or interactive documentation is unavailable
Check whether the project has registered the OpenAPI/Swagger services and middleware and whether the current environment enables the UI. Keep the production exposure decision deliberate: documentation is useful, but it can reveal implementation details.
A client gets a server error instead of a useful response
Inspect server-side logs and reproduce the request with its method, path, headers, and body. Return a safe, consistent client error for invalid input; reserve server errors for unexpected failures and do not return stack traces or secrets in the response.
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Frequently asked questions
Do I need a database to build an API?
No. A small learning example can use in-memory data, but that data is temporary. An API whose data must survive restarts or be shared across instances needs persistent storage.
Is REST the only way to build an API?
No. This guide uses resource-oriented HTTP routes because they are a clear starting point for common web services. The right style depends on the clients, data needs, and contract your project requires.
Can I build an API without a frontend?
Yes. An API can be consumed by mobile apps, other services, command-line tools, or web frontends. HTTP clients and automated tests can exercise it without building a user interface.
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