Application Programming Interface (API)

For browser-side integrations, MDN’s Web API reference documents the interfaces available to website code. These differ from an affiliate platform’s own API, whose authentication, endpoints, and limits must be checked in that platform’s documentation.

What Is an API (Application Programming Interface)?

An application programming interface (API) is a defined contract between two software components that specifies how they communicate. One component makes a request in a structured format; the other returns a response with the requested data or performs the requested action.

The definition has three parts that matter:

1. Application — any piece of software with a distinct function, from a mobile weather app to a payment gateway to an affiliate tracking platform.
2. Programming — developers interact with the API through code, not through a graphical user interface. The API exposes specific functions, endpoints, or methods they can call.
3. Interface — the boundary layer. It shows only what the developer needs — a menu of available operations — and hides everything else: the database schema, the server infrastructure, the internal business logic.

Critically, an API is not a user interface. End users never see it. When a shopper clicks “Pay with PayPal” on an e-commerce site and sees a confirmation screen seconds later, the API handled the handshake between the store and PayPal’s servers — but the shopper only saw the result.

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What Does API Stand For?

API stands for Application Programming Interface. Breaking it down:

The term has been in use since at least the 1960s, but it entered mainstream software vocabulary with the rise of web APIs in the early 2000s. Today, when most people say “API” without qualification, they mean a web API — one that communicates over the internet using HTTP.

How Do APIs Work?

API communication follows a client-server request-response model. Here is the lifecycle of a single API call:

1. The client application initiates a request. A developer writes code that sends an HTTP request to a specific API endpoint — a URL that represents a resource or action. The request includes a method (GET, POST, PUT, DELETE), headers (metadata like authentication tokens), and optionally a body (the payload, often in JSON format).

2. The API receives and validates the request. The server checks that the request is well-formed, that the authentication credentials are valid, and that the client is authorized to perform the requested operation.

3. The server processes the request. This may involve querying a database, calling another internal service, transforming data, or triggering a business process. The API layer abstracts all of this away from the client.

4. The server returns a response. The response includes an HTTP status code (200 for success, 404 for not found, 500 for server error, etc.), response headers, and typically a body containing the requested data or a confirmation message — commonly formatted as JSON or XML.

5. The client application consumes the response. The calling code parses the data and uses it — updating a display, storing a record, triggering the next step in a workflow.

A concrete example: a weather app on your phone sends a request like GET /weather?city=Dallas to a weather service API. The API queries its data sources, constructs a JSON response with the temperature, humidity, and forecast, and returns it. The app parses that JSON and displays “82°F, partly cloudy.” The app never needs to know how the weather service collects or stores its data — it only needs to follow the API’s contract.

The API Call, Step by Step

• Endpoint: The URL where the API lives — e.g., https://api.weatherservice.com/v2/weather
• Method: The HTTP verb — GET (read), POST (create), PUT (update), DELETE (remove)
• Headers: Metadata — content type, authentication token, API key
• Parameters: Query-string values or path variables that refine the request — ?city=Dallas&units=imperial
• Body: The data payload sent with POST or PUT requests, typically JSON
• Response: Status code + headers + body (the data, an error message, or both)

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What Is an API Endpoint and Why Is It Important?

An API endpoint is a specific URL (or URI) within an API that corresponds to a particular resource or operation. While an API is the entire interface, an endpoint is one addressable touchpoint within it.

For example, a hypothetical affiliate marketing API might expose these endpoints:

Each endpoint represents a distinct capability. Well-designed APIs use consistent, predictable endpoint naming so developers can reason about the interface without constantly consulting documentation. This pattern — treating URLs as resources and using HTTP methods to describe actions — is the foundation of REST API design.

Endpoints matter because they define the granularity of what you can ask the system to do. An API with well-designed endpoints lets developers compose complex workflows from simple, reusable calls. An API with poorly designed endpoints forces developers into workarounds — making multiple calls to assemble data that should have been available in one, or parsing oversized responses to find the one field they need.

What Are the Different Types of APIs?

APIs can be categorized two ways: by access level (who can use them) and by technical layer (where they operate in the software stack).

Types of APIs by Access Level

Public APIs — sometimes called open APIs — are the most visible category. Companies like Google (Maps), Stripe (payments), and OpenAI (ChatGPT) expose public APIs that any developer can integrate after registering for credentials. The business model varies: some are free with rate limits, others charge per call, and still others drive adoption of a platform.

Partner APIs sit between public and private. They are exposed to specific business partners under negotiated terms. An affiliate network, for example, might expose a partner API that lets approved advertisers query commission data programmatically — but the same API is not available to the general public.

Private APIs — also called internal APIs — are used exclusively within an organization. A company’s mobile app might talk to its backend through a private API that a public user could never discover. These APIs prioritize internal developer productivity and security over general-purpose usability.

Composite APIs address a performance problem: when a single user action requires data from five different services, making five separate API calls adds latency. A composite API bundles those requests into one call and returns a unified response. They are common in microservice architectures and API gateway configurations.

Types of APIs by Technical Layer

• Hardware and firmware APIs let software communicate with physical components — processors, sensors, graphics cards. They are the lowest-level interface.
• Operating system APIs expose OS services — file management, process scheduling, memory allocation, network stacks. Windows API, POSIX, and Cocoa are examples.
• Library and framework APIs let developers call prewritten functions within the same runtime. A Python developer importing requests and calling requests.get() is using a library API. No network is involved.
• Database APIs standardize communication with database management systems. ODBC, JDBC, and native drivers like psycopg2 for PostgreSQL fall into this category.
• Web APIs enable communication between separate applications over a network, typically using HTTP. This is what most people mean by “API” today, and it is the focus of the rest of this article.

Web API Protocols and Styles

Not all web APIs are built the same way. The protocol — sometimes called the architectural style — defines how requests and responses are structured, what formats they use, and what constraints they follow.

REST APIs (Representational State Transfer)

• Client-server separation: The client handles the user interface; the server handles data storage. They evolve independently.
• Statelessness: Each request from the client must contain all the information the server needs to fulfill it. The server stores no client context between requests.
• Cacheability: Responses must explicitly declare whether they can be cached, improving performance.
• Uniform interface: Resources are identified by URLs, manipulated through representations (JSON, XML), and accessed through standard HTTP methods.
• Layered system: The client cannot tell whether it is connected directly to the end server or to an intermediary — a load balancer, a cache, or an API gateway.

Benefits of REST APIs:

• Language- and platform-agnostic — any HTTP client can call them
• Scalable, thanks to statelessness and caching
• Simple to understand and test — you can explore a REST API with nothing more than a browser or curl
• Rich ecosystem of tools: OpenAPI/Swagger for documentation, Postman for testing, API gateways for management

Limitations of REST:

• Over-fetching: an endpoint might return 20 fields when you only need two
• Under-fetching: you might need to call multiple endpoints to assemble the data for one screen (the N+1 query problem)
• No built-in support for real-time updates

SOAP APIs (Simple Object Access Protocol)

SOAP is an older, XML-based protocol that predates REST. It is more rigid: messages must conform to a strict XML schema, and the protocol defines its own security and transaction rules through WS-* standards.

SOAP is still used in enterprise environments — financial services, payment processing, and government systems — where its built-in security, transaction integrity, and formal contracts (WSDL files) matter more than REST’s simplicity. SOAP is legacy infrastructure that persists, not a technology teams choose for greenfield projects.

GraphQL

Instead of the server defining fixed endpoints with fixed response shapes, the client specifies exactly which fields it wants in a single query:

query {
  affiliate(id: "4821") {
    name
    totalEarnings
    recentCommissions(limit: 5) {
      amount
      date
      campaign { name }
    }
  }
}

RPC and WebSocket APIs

• Remote Procedure Call (RPC) APIs treat server-side operations as function calls. The client calls createInvoice(customer, amount) and the server returns the result. gRPC, developed by Google, is the modern RPC framework — it uses Protocol Buffers for efficient binary serialization and is popular in microservice architectures where performance matters more than human readability.

• WebSocket APIs maintain a persistent two-way connection. Unlike REST’s request-response model, WebSocket lets the server push data to the client without waiting for a request. This is essential for real-time features: chat applications, live dashboards, collaborative editing, and streaming data feeds.

What Are API Integrations?

An API integration is the process of connecting two or more software systems through their APIs so they can exchange data and trigger actions automatically. The integration defines:

• Which endpoints are called
• What triggers the call (a scheduled job, a user action, a webhook event)
• How data is mapped between the two systems’ formats
• How errors and retries are handled

For example, an API integration between an e-commerce platform and an affiliate tracking system might work like this: when a customer completes a purchase, the e-commerce platform sends a POST request to the tracking system’s /conversions endpoint with the order details, affiliate ID, and commission amount. The tracking system records the conversion and returns a confirmation. No human copied data between spreadsheets; no CSV files were emailed.

The average mid-market company uses over 100 SaaS applications, and the ones that work together seamlessly — through documented, stable APIs — are the ones that win.

How APIs Power Affiliate Marketing

Affiliate marketing runs on data: clicks, conversions, commission rates, payout schedules, performance metrics. Every piece of that data must move between systems reliably and in near real time. APIs make that possible.

For Advertisers (Merchants)

Advertisers use APIs to:

• Automate conversion tracking. When a sale happens, the e-commerce platform calls the affiliate platform’s API to register the conversion, attribute it to the correct affiliate, and calculate the commission — all within milliseconds.
• Sync product catalogs. Product feeds, pricing changes, and inventory updates flow from the advertiser’s system to the affiliate platform through API calls, keeping publisher listings accurate.
• Pull performance reports. Rather than logging into a dashboard and exporting CSVs, advertisers can use the API to pipe real-time performance data into their own business intelligence tools — Tableau, Power BI, or a custom analytics stack.
• Manage affiliates programmatically. Approve applications, adjust commission tiers, and send messages to affiliates through API calls instead of manual admin panel clicks.

For Publishers (Affiliates)

Publishers use APIs to:

• Retrieve affiliate links and creative assets dynamically. Instead of copying links from a dashboard, a publisher can use the API to fetch the right tracking link for any product in real time, ensuring links never break and always point to the current offer.
• Check commission status. Publishers can query the API for pending, approved, and paid commissions — and build their own dashboards or alerts on top of that data.
• Automate content updates. A content site reviewing products can use the API to pull current pricing and availability, keeping reviews accurate without manual edits.

For Affiliate Program Managers

Program managers sit between advertisers and publishers. APIs let them:

• Monitor program health. Aggregate data across all affiliates and campaigns, detect anomalies (a publisher whose conversion rate suddenly drops to zero), and trigger alerts.
• Automate payouts. Integrate the affiliate platform’s API with accounting software or a payment gateway to batch-process commission payments on schedule.
• Enforce compliance. Query the API for suspicious patterns — click flooding, unusually high chargeback rates — and take action programmatically.

Limitations to Understand

APIs in affiliate marketing are powerful but not magic:

• Rate limits. Most platforms cap the number of requests per minute or per day. A publisher pulling live pricing for 10,000 products every hour may hit those limits and need to cache responses locally.
• Latency. An API call that takes 500ms is fast for a human but slow for a page-load-critical request. Always make API calls server-side or asynchronously, never inline in a page render.
• Versioning. APIs change. The tracking endpoint that works today may be deprecated in six months. Subscribe to changelogs, pin your integrations to specific API versions, and test against sandbox environments before deployment.
• Authentication management. API keys and tokens are credentials. Rotating them, storing them securely (never in client-side code or public repositories), and managing scoped permissions is an ongoing operational responsibility — not a one-time setup.

What Is an API Gateway?

An API gateway is a server that sits between clients and a collection of backend services, acting as a single entry point for all API requests. Instead of clients calling each microservice directly, they call the gateway, which routes requests, enforces policies, and aggregates responses.

An API gateway typically handles:

• Routing: Directing requests to the correct backend service based on the URL path, headers, or parameters
• Authentication and authorization: Validating API keys, JWT tokens, or OAuth credentials before the request reaches the backend
• Rate limiting: Enforcing per-client request quotas to protect backend services from overload
• Request and response transformation: Converting between protocols (REST to gRPC), adding or removing headers, or reshaping payloads
• Caching: Returning cached responses for identical requests to reduce backend load
• Logging and monitoring: Capturing metrics on request volume, latency, and error rates

In the context of an affiliate platform, an API gateway might handle authentication for thousands of publishers, rate-limit individual API keys to prevent abuse, and route different request types — tracking calls, reporting queries, administrative commands — to different backend services — all behind a single, consistent domain like api.yourplatform.com.

They are not optional for any platform handling production API traffic at scale.

API Security

APIs expose business logic and data to the network, which makes them a high-value target. Securing an API involves multiple layers:

• Authentication. Every API request must prove who is making it. Common mechanisms: API keys (simple but easily leaked), OAuth 2.0 tokens (industry standard for delegated access), and mutual TLS (certificate-based, common in B2B and financial APIs).

• Authorization. Authentication proves identity; authorization determines what that identity is allowed to do. A publisher’s API key should be able to query their own commissions — not anyone else’s. Implement scoped permissions and validate them on every request.

• Transport security. Always use HTTPS (TLS 1.2 or higher). Never expose an API endpoint over plain HTTP in production. This encrypts data in transit and prevents man-in-the-middle attacks.

• Input validation. Treat every API request as potentially malicious. Validate data types, string lengths, numerical ranges, and enum values before processing. Reject anything unexpected — do not try to “fix” malformed input.

• Rate limiting. Cap the number of requests a client can make in a given window. This prevents both accidental overload and deliberate denial-of-service attacks. Return HTTP 429 (Too Many Requests) with a Retry-After header when limits are exceeded.

• Logging and monitoring. Log every API request — timestamp, client ID, endpoint, method, status code, and latency. Monitor logs for anomalies: sudden spikes, unusual error patterns, requests from new IP ranges. Security incidents are first visible in the logs, not in user reports.

• API gateway as security layer. An API gateway centralizes authentication, rate limiting, and request validation, reducing the attack surface of individual backend services. It is the single choke point where security policies are enforced consistently.

Broken object-level authorization (accessing another user’s data by changing an ID in the URL) consistently ranks as the most common API vulnerability.

How to Create an API

1. Define the Contract First

Before writing code, define what the API will do. Use the OpenAPI Specification (formerly Swagger) to describe every endpoint, its parameters, its response format, and its error conditions in a machine-readable YAML or JSON document. A well-written OpenAPI spec serves as:

• A design tool that forces clarity before implementation
• A contract that frontend and backend teams can work against in parallel
• A source for auto-generated documentation, client SDKs, and mock servers

2. Choose an Architectural Style

For most new APIs, REST with JSON is the default choice — it is widely understood, tooling-rich, and sufficient for most use cases. Choose GraphQL if clients need flexible, client-driven queries and under-fetching is a known problem. Choose gRPC if you need high-throughput, low-latency internal service-to-service communication.

3. Implement the Backend

Build the server-side logic that fulfills the API contract. This involves:

• Setting up HTTP routes that match the defined endpoints
• Writing handlers that validate input, call business logic, and format responses
• Connecting to databases, caches, and other backend services
• Implementing authentication and authorization middleware

4. Document the API

Documentation is not an afterthought. It is the first thing a developer integrating with your API will see, and it determines whether they succeed or give up. Good API documentation includes:

• An overview of authentication (how to get a key, how to include it in requests)
• A reference of every endpoint with example requests and responses
• Error codes and their meanings
• Rate limits and pagination conventions
• A changelog or version history

Tools like Swagger UI, Redoc, and Postman can generate interactive documentation from an OpenAPI spec, letting developers test endpoints directly in the browser.

5. Test the API

API testing verifies that the implementation matches the contract — and holds up under load. Testing categories include:

• Functional testing: Does each endpoint return the correct response for valid input and the correct error for invalid input?
• Integration testing: Does the API work correctly when connected to real databases, authentication services, and downstream systems?
• Load testing: How does the API perform under concurrent requests? At what point does latency spike or errors appear?
• Security testing: Can an unauthenticated request access a protected endpoint? Does input validation block SQL injection and cross-site scripting?

6. Deploy Behind an API Gateway

Route production traffic through an API gateway that handles TLS termination, authentication, rate limiting, and monitoring. Deploy the API in a containerized or serverless environment that can scale horizontally as traffic grows. Monitor latency, error rates, and throughput continuously — and set alerts for deviations from baseline.

What Is API Testing?

API testing is the practice of verifying that an API behaves as specified — returning the right data for valid requests, the right errors for invalid ones, and maintaining performance under expected load. It is distinct from UI testing: there is no browser, no page rendering, no visual layout to validate. The test sends an HTTP request and checks the response.

API testing covers:

• Contract testing: Does the API response match the schema defined in the OpenAPI spec? Are all required fields present? Are data types correct?
• Functional testing: Does GET /commissions/123 return commission 123’s data — and only commission 123’s data? Does POST /commissions with valid input create a new record and return a 201 status?
• Error handling: Does a request with a missing required field return a 400 (Bad Request)? Does an expired token return a 401 (Unauthorized)? Does a request for a nonexistent resource return a 404 (Not Found)?
• Performance testing: How many concurrent requests can the API handle before latency exceeds the service-level objective (SLO)? Where is the breaking point?
• Security testing: Does the API reject requests without authentication? Does it prevent access to resources the requester does not own?

Automated API tests — run in CI/CD pipelines on every commit — catch regressions before they reach production. Tools like Postman, REST Assured, and Supertest make it practical to build and maintain test suites that cover every endpoint.