API development: the foundation of a connected software landscape

An API is the agreed interface through which software systems communicate. You pay online, your CRM fills automatically from your web shop, your dashboards show real-time data — in every case, an API is handling the connection. API development is therefore not a technical detail. It is a strategic decision that determines how flexible and scalable your software landscape can become.

What is an API?

API stands for Application Programming Interface: a defined set of rules that specifies how two software components can communicate. One side makes a request (the client), the other delivers a response (the server) — through an agreed protocol.

The most useful analogy is a restaurant. You (the customer) order from a waiter (the API). The waiter goes to the kitchen (the system) and returns with what you ordered. You never see how the dish is prepared, and you don't need to. The interface — the menu and the ordering process — is everything you require.

APIs are everywhere. When you open a weather app, it fetches current conditions from an external service via an API. When you pay online, your browser sends an API call to the payment provider. When your CRM updates automatically after someone fills in a form, that is an API integration running silently in the background.

According to Postman's State of the API Report 2024, more than 91% of developers use APIs multiple times per week — and over 70% of software initiatives have API integrations as a critical component. APIs are not optional. They are the infrastructure on which modern software runs.

REST vs. GraphQL

The two dominant styles for building web APIs are REST and GraphQL. Both are mature, widely supported, and battle-tested in production. The right choice depends on your use case.

REST (Representational State Transfer) works with URL-based endpoints that represent resources. The endpoint `/customers/123` returns customer 123. HTTP methods determine the action: GET to retrieve, POST to create, PUT to update, DELETE to remove. REST is stateless — every call contains all information needed to process the request.

GraphQL is a query language developed by Meta. Rather than multiple endpoints for different resources, GraphQL has a single endpoint. The client specifies exactly which fields it needs and receives precisely that — nothing more, nothing less. A mobile app that only needs a customer's name and email requests only those fields.

REST is the right choice for most public APIs, straightforward CRUD operations, and situations where HTTP caching provides value. GraphQL fits better with complex frontend applications that have varied data needs, mobile apps conserving bandwidth, and product teams who want to iterate quickly without changing the backend each time.

There is also gRPC — a protocol developed by Google for fast machine-to-machine communication. It is binary (not human-readable JSON), fast, and efficient, but less suited as a public API. Use gRPC for internal services that require high throughput or low latency.

API-first development

API-first development is an approach where the API definition comes first — before any implementation begins. You write the contract, then you build the code behind it.

It sounds slower. It is not. Teams can work in parallel: frontend developers use the API specification to build mocks while backend developers write the implementation. When the implementation is ready, everything fits seamlessly — because the interface was agreed upfront.

The OpenAPI standard (formerly Swagger) is the most widely used format for API specifications. It describes endpoints, request and response structures, authentication methods, and error codes in a machine-readable YAML or JSON file. From that specification you can automatically generate documentation, mocks, and sometimes even client code in multiple programming languages.

1. Write the OpenAPI specification. Define every endpoint, parameters, response structures, and error codes.
2. Review the contract with all stakeholders — frontend, backend, integration partners. This is the moment to correct mistakes, not after the build.
3. Generate mocks from the specification. Frontend can build and test immediately.
4. Implement the backend according to the specification. Automated validation ensures the implementation respects the contract.
5. Generate and publish documentation automatically. Documentation that must be maintained by hand always goes stale.

API-first development fits naturally into a microservices architecture, but the benefits apply to monolithic applications too. It forces you to think about system boundaries before writing code — and those are the most expensive decisions to reverse later.

Security and authentication

Poorly secured APIs are one of the most common attack vectors on web applications. The OWASP API Security Top 10 — the reference standard for API security — lists broken authentication, excessive data exposure, and insufficient rate limiting as the top risks. In practice, these problems appear regularly at companies that build APIs as a byproduct rather than as a designed interface.

• HTTPS always. An unencrypted API endpoint is not an endpoint — it is a liability. All communication must be encrypted, including internal traffic.
• OAuth 2.0 for delegated authorization. This is the protocol behind 'Sign in with Google' — and the standard for machine-to-machine authentication in B2B scenarios.
• JWT (JSON Web Tokens) for stateless authentication. A JWT contains signed claims about who the caller is. The server does not need to maintain a session.
• API keys for simple integrations. Rotate them regularly and never store them in code or version control repositories.
• Rate limiting to prevent abuse. Limit the number of calls per time window per client. Without rate limiting, every public API is vulnerable to volumetric abuse.
• Input validation on every endpoint. Never trust input from the client — even when that client is your own frontend.

Security is not a feature you add afterwards. It is a design dimension that must be part of every decision. An API that starts as internal and later becomes public pays a high price if security was not part of the original design.

“Security is not a product, but a process.”— Bruce Schneier, Secrets and Lies (2000)

API strategy for your business

For many businesses, an API strategy starts not with external integrations but with cleaning up internal system communication. Legacy environments are built with point-to-point connections: system A talks directly to systems B, C, and D. Each new system adds more connections. After five years, it is a tangle that no one fully understands.

An API gateway solves this by acting as the single entry point for all API calls. External clients talk to the gateway, which routes calls to the appropriate internal services. Authentication, logging, rate limiting, and routing are managed centrally — not implemented separately in each service.

Companies that work API-first launch new integrations 3.4 times faster than companies that build point-to-point connections, according to MuleSoft's Connectivity Benchmark Report 2023. The difference is not better code — it is better architecture.

1. Map which systems communicate with each other and through what mechanism. Understand the current state before you build anything new.
2. Decide which integrations are strategic (supporting core processes) and which are ad hoc (built for a temporary need). Strategic integrations deserve a proper API. Ad hoc connections deserve reconsideration.
3. Design an API layer that decouples your systems. An API gateway acts as the single entry point for external calls and centralises cross-cutting concerns.
4. Document the APIs. Undocumented APIs either go unused or get used incorrectly. Both cost time.
5. Introduce versioning. Changes to an API must not break existing integrations. Use semantic versioning and keep multiple versions active during migration windows.

An API strategy is also an organisational choice. Who owns which API? Who decides on breaking changes? How are APIs deprecated? Without answers to those questions, an API strategy becomes a technical layer without governance — and that is a new form of technical debt.

Conclusion.

API development is the backbone of a flexible software landscape. Not as a trend, but as a technical reality: every system you want to integrate, every process you want to automate, every partner you want to share data with — it all runs through an API.

The question is not whether you will deal with APIs. The question is whether those APIs are well designed. Good APIs are consistent, documented, secured, and built for change. Bad APIs are technical debt that stays hidden until an integration breaks at the worst possible moment.