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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Design a RESTful web API by treating its public contract as a set of domain resources and representations, then defining stable URIs, standard HTTP method behavior, response formats, errors, collections, and evolution rules. The goal is not merely to put JSON behind URLs: clients should be able to understand what a request means and what its response says without knowing how the service is implemented.
REST is an architectural style; many APIs use REST-oriented HTTP conventions without satisfying every REST constraint. Use RFC 9110, HTTP Semantics (IETF, June 2022), as the authority for HTTP methods and response semantics. Microsoft Learn’s Azure Architecture Center and Google Cloud’s API design guide are useful implementation and design references, not substitutes for the HTTP standard.
1. Model the public domain contract
Start with the concepts clients need to work with, not the tables, classes, or service boundaries used internally. A resource is a target of a request, commonly identified by a URI; the API transfers a representation of that resource. The implementation may change while the public contract remains useful.
For a project-management service, clients might need projects, tasks, and comments. A relational schema could have many more tables, but that does not mean each table should become a public resource. Conversely, a client-facing concept can be a resource even if it is assembled from several internal records.
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Write down the client’s jobs
List the actions clients actually need: find projects, retrieve one project, create a task, change a task’s status, or list comments. For each, identify the relevant resource, the request representation, the response, and the expected outcome. This exposes missing concepts and prevents a public API from simply mirroring the database.
- Keep public names and identifiers stable enough for clients to rely on.
- Model meaningful relationships explicitly, such as a task belonging to a project.
- Keep implementation details private unless they are genuinely part of the client contract.
- Consider different client needs; a mobile client, for example, may have different payload constraints from an internal service.
2. Choose stable resource URIs
Use URIs to identify resources and collections. A common design uses a collection URI and an item URI, with a stable identifier for the individual resource:
/projectsidentifies a project collection./projects/42identifies one project./projects/42/tasksidentifies tasks associated with that project./tasks/913identifies one task, if tasks also need independent access.
Prefer resource-oriented names and let HTTP methods express the operation where that is appropriate. For example, retrieving /projects/42 uses GET; creating a project in /projects commonly uses POST. Do not create a verb path for every operation by default, such as /getProject or /createTask. Some domain actions do not map cleanly to ordinary resource manipulation, and an action-oriented endpoint can be a reasonable explicit choice. There is no single naming style required for every URI; consistency and client clarity matter.
Use predictable casing and collection naming throughout the API, and decide how identifiers and nested relationships work before clients depend on them. Avoid encoding transient implementation details in paths. A URI convention is practical design guidance; the semantics of the HTTP method are standardized separately.
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3. Define method behavior using HTTP semantics
Specify what each method does for each resource and honor its standard properties. RFC 9110 distinguishes safe methods, which are intended to be read-only, from idempotent methods, where repeating the same request has the same intended effect as making it once. Clients and intermediaries can rely on these properties when deciding whether to retry or cache requests.
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| Method | Typical API use | Design implication |
|---|---|---|
| GET | Retrieve a resource or collection representation. | Safe and idempotent; do not use it to perform a state-changing action. |
| HEAD | Retrieve the headers that a GET would return, without the response content. | Safe and idempotent; useful when a client needs metadata without the representation body. |
| POST | Create a subordinate resource or submit data for processing. | Not defined as safe or idempotent by default. Repeating a POST may create duplicate effects unless the API defines additional behavior. |
| PUT | Create or replace the state of the resource identified by the target URI, according to the API contract. | Idempotent; define whether the request is a complete replacement and document any creation behavior. |
| DELETE | Remove the association between the target URI and its current functionality, according to HTTP semantics. | Idempotent in intended effect; a repeated request need not return the same status as the first. |
Do not treat method names as decoration. If a client sends GET, it expects a safe retrieval rather than a hidden state change. If a client retries an idempotent operation after a network interruption, the repeated request should not multiply its intended effect. For POST operations that could be repeated by a client, document how duplicate submissions are handled rather than implying that POST is inherently retry-safe.
4. Specify representations, status codes, and errors
For every operation, document the accepted request media type, request fields, response media type, response fields, relevant headers, and possible status codes. JSON is common, but choosing JSON alone does not make an API RESTful. RFC 9110 describes HTTP’s uniform interface as interaction with a resource by sending messages that manipulate or transfer representations.
Make outcomes unambiguous
Use status codes to tell clients what happened, and include a response body when it gives them useful information. Typical choices include:
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- 200 OK: the request succeeded and the response provides a result.
- 201 Created: a request created a resource; identify the new resource, commonly with a Location header, and provide a representation when useful.
- 202 Accepted: processing has been accepted but is not complete. Explain how the client can discover progress or the eventual result.
- 204 No Content: the request succeeded and there is no response content to return.
- 400 Bad Request: the request cannot be processed as sent; give actionable validation details where appropriate.
- 404 Not Found: the target resource is not available at the requested URI.
- 409 Conflict: the request conflicts with the current state of the target resource.
- 500 Internal Server Error: an unexpected server-side failure occurred; do not expose internal stack traces as a client contract.
Choose the status that accurately represents the outcome rather than returning 200 for every case and burying success or failure in a custom JSON field. Define a consistent error representation with a stable machine-readable code and a human-readable explanation. Include field-level validation details when clients can use them to correct a request. Microsoft Learn’s Web API Implementation guidance likewise emphasizes correct status codes, headers, and parseable response bodies.
Example: create a task
A contract might accept a JSON representation at POST /projects/42/tasks, then return 201 Created, a Location header for the new task, and its representation. A validation failure should instead return an appropriate client-error status and identify the invalid input. Document both outcomes; the exact field names and error schema are decisions for the API, not properties supplied automatically by HTTP.
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5. Design collections, filtering, and pagination
Collection endpoints need explicit rules for narrowing and traversing results. Define supported filters, sort keys, default ordering, maximum page size, and what happens when a requested value is invalid. For example, document whether ?status=open filters tasks, whether clients can request a page size, and how they obtain the next page.
Offset-based pagination can be straightforward, while cursor-based pagination can be preferable when a collection changes frequently; choose based on the collection’s behavior and client needs. Whichever approach you use, describe whether results can shift between requests and how clients detect the end of the collection. Do not silently return an incomplete first page as if it were the whole collection.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesPartial responses can reduce payload size when clients need only selected fields. If supported, specify how a client requests fields and how the response behaves for unknown or inaccessible fields. Links to related resources or a next-page URI can help clients navigate when the API contract supports hypermedia, but they are not a substitute for documenting the available operations.
6. Design asynchronous and long-running operations
If work cannot reliably finish during one request, separate acceptance from completion. Return an appropriate accepted outcome, identify an operation-status resource or other documented means of checking progress, and define terminal success and failure responses. State whether clients should poll, whether the service can notify them, and how long the operation status remains available.
Do not return a success-shaped final resource before the work is complete unless that is explicitly what the representation means. Make it possible for a client to distinguish queued, running, completed, and failed states. For work that can take long enough to outlast a connection, this contract is more useful than asking clients to keep one HTTP request open indefinitely.
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7. Plan compatibility and versioning deliberately
Assume clients will depend on documented behavior. Adding an optional response field may be compatible with clients that ignore unknown fields; renaming a field, changing its meaning, or removing an operation can break them. Treat changes according to their effect on existing consumers, and publish a deprecation and migration path before removing a relied-upon contract.
Version only when needed to manage incompatible changes, and choose a strategy deliberately, such as a versioned path or a versioned media type. Each strategy has trade-offs for routing, caching, documentation, and client negotiation. Keep the number and support period of versions manageable. Most importantly, do not version the public API just because an internal database or service implementation changes.
8. Document and test the contract
Documentation should let a developer construct a valid request, understand the response, handle errors, and judge compatibility. Include authentication requirements, media types, examples, pagination rules, limits, and the behavior of every supported method. Google Cloud’s API design guide is a further reference for API design, including REST and RPC approaches and HTTP mapping.
Use a machine-readable contract where it fits your development workflow, and check that examples match the implementation. Test both expected success and failure behavior: method semantics, status codes, headers, validation errors, pagination boundaries, and compatibility-sensitive response fields. A contract test is especially useful when separate teams own the API implementation and its clients.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.9. Use the Richardson model as a teaching aid, not a score
Microsoft Learn summarizes a four-level model for discussing alignment with REST concepts:
- Level 0: one URI is used, often with POST, for multiple operations.
- Level 1: separate URIs identify resources.
- Level 2: HTTP methods are used to express operations on resources.
- Level 3: hypermedia controls add discoverable links and actions to representations.
The model can help teams explain a design’s progression, but it is not a complete quality measure. In a 2021 Delphi study, eight Web API experts considered a catalog of 82 design rules; the study reported rules associated with level 2 as critical and reaching level 3 as less important. That is a finding from those eight experts, not a universal consensus or proof that hypermedia is unimportant. Judge hypermedia by whether discovery and navigation benefit the actual clients.
10. Evaluate trade-offs against the client contract
When choosing between designs, compare them on the concerns that will shape client behavior:
- HTTP semantics: do methods, status codes, and headers accurately communicate standardized behavior?
- Domain clarity: do resources represent concepts clients understand, rather than internal storage structures?
- Discovery: can clients find related resources and collection continuations when they need to?
- Evolution: can the API add capabilities without unexpectedly breaking existing clients?
- Interaction fit: are payload size, filtering, and synchronous or asynchronous behavior appropriate to intended consumers?
- Operational behavior: can clients handle errors, retries, pagination, and long-running work predictably?
A consistent API is not necessarily a good one if it forces clients into oversized payloads or hides important outcomes. Conversely, adding elaborate navigation or abstraction that clients do not use can make a contract harder to maintain. Prefer the simplest design that expresses the domain accurately and preserves the HTTP semantics clients rely on.
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Frequently Asked Questions
Does an API need hypermedia links to be RESTful?
Hypermedia is one REST constraint, but a Richardson maturity level is not by itself proof that an API is good or bad. Decide whether embedded links and actions help the clients discover and navigate the resources they need.
Is a JSON API automatically a REST API?
No. JSON is a representation format. REST-oriented design also concerns resources, HTTP semantics, and the constraints of the architectural style; using JSON and CRUD-like URLs alone does not establish that an API implements them.
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