Interface development: The ultimate guide

What an Interface Is and Why It Matters

Short: An interface — also called an API (Application Programming Interface) — is a formally defined agreement between software components.

An interface — also called an API (Application Programming Interface) — is a formally defined agreement between software components. It acts as an intermediary: it receives requests, forwards them to the underlying system, and returns standardized responses.

The requesting component does not need to understand how the backend works internally. It only needs to know the interface contract.

Quality Criteria for Well-Designed APIs

Short: API quality determines how well systems integrate and how maintainable the integration remains over time.

API quality determines how well systems integrate and how maintainable the integration remains over time. The four key criteria:

• Reliability — Consistent, predictable behavior. Failures in one API can cascade through dependent systems.
• Security — Protection against unauthorized access to data and functionality.
• Performance — Speed and efficiency of data transmission directly affects user experience and system scalability.
• Scalability — The API must handle growing request volumes without degrading performance.

A well-designed API is more than a technical connection. It is a contract that must be honored over years.

Types of Interfaces and API Architectures

REST (Representational State Transfer)

REST has become the most widely used API style for web services. It uses standard HTTP methods (GET, POST, PUT, DELETE) and returns data in JSON or XML format.

REST APIs are stateless: each request contains all the information needed to process it. This makes them scalable and easy to cache.

Strengths: Simple to implement, broad tool support, easy to test, widely understood.

Best for: Public APIs, web and mobile application backends, service-to-service communication.

GraphQL

GraphQL is a query language for APIs developed by Meta. Instead of fixed endpoints returning fixed data structures, GraphQL lets the client specify exactly what data it needs.

This eliminates over-fetching (receiving more data than needed) and under-fetching (requiring multiple requests to get all needed data).

Strengths: Flexible data retrieval, reduces network payload, strong tooling.

Best for: Complex frontend applications with diverse data requirements, mobile apps where bandwidth matters.

gRPC

gRPC uses Protocol Buffers (Protobuf) for binary serialization. It is faster than REST for high-throughput, low-latency communication. gRPC supports streaming — both server-to-client and bidirectional.

Strengths: High performance, strongly typed contracts, excellent for microservice-to-microservice communication.

Best for: Internal service communication, real-time data streaming, systems where performance is critical.

SOAP (Simple Object Access Protocol)

SOAP is an older, XML-based protocol. It is formally defined by a WSDL (Web Services Description Language) contract. SOAP offers built-in standards for security (WS-Security) and transactions.

Strengths: Strong formal contracts, extensive security standards, mature tooling for enterprise environments.

Best for: Legacy enterprise integrations, financial and insurance systems where formal contracts are required.

Webhooks

Webhooks are event-driven callbacks. Instead of a client polling an API repeatedly to check for changes, the server sends a notification to a defined URL when an event occurs.

Strengths: Real-time event delivery, reduces unnecessary API calls, efficient for event-driven architectures.

Best for: Payment notifications, CI/CD pipelines, integration with third-party services (Stripe, GitHub, Shopify).

Architecture Decision Factors

Short: Choosing the right API architecture depends on:

Choosing the right API architecture depends on:

• Who consumes the API — external developers, internal services, or end-user applications?
• Performance requirements — is low latency critical, or is occasional delay acceptable?
• Data complexity — do consumers need flexible data queries, or fixed data structures?
• Existing technology landscape — what do connected systems already support?

There is no universal best choice. The right architecture serves the specific integration requirements.

What Good API Documentation Includes

Short: An API without documentation is nearly unusable for external consumers.

An API without documentation is nearly unusable for external consumers. Comprehensive documentation covers:

• Authentication and authorization method
• All endpoints with request parameters and response formats
• Error codes and their meaning
• Rate limiting rules
• Versioning strategy and deprecation policy
• Working code examples in relevant languages

Tools like OpenAPI/Swagger generate interactive documentation directly from the API specification.

"A well-designed API is the invisible bridge between systems — and often the biggest lever for efficiency." — Björn Groenewold, Managing Director, Groenewold IT Solutions