What is an API gateway? APIs, web services and integration in modern architectures

For many years, two fundamental concepts have consistently appeared alongside the evolution of flexibility, integration capabilities and distributed architectures in the software world: APIs and API management. In recent years, these concepts have moved beyond purely technical teams and become part of the agenda for product and business stakeholders as well. The main reason for this shift is that systems are no longer isolated and closed structures; instead, they have evolved into living ecosystems shaped by numerous internal and external integrations.

In this article, we will explore what an API is, where the distinction between APIs and web services truly begins, and why components such as API gateways have become essential today. Rather than focusing solely on theoretical definitions, we will approach these questions through architectural and operational realities.

What is an API?

An API (Application Programming Interface), in its simplest form, is an interface that defines how two software components communicate with each other. It specifies which capabilities an application exposes to another application and under what rules.

Although today the term API is often associated with services accessed over the internet, this represents only a small part of the concept. APIs existed long before the web. Communication between two applications running on the same machine — or even between modules within a single application — also happens through APIs. This interaction does not always require HTTP, REST or even a network connection.

Functions provided by operating systems, methods exposed by software libraries and custom protocols that transfer raw data over TCP are all examples of APIs. Therefore, limiting the concept of an API to “web services” alone means overlooking its architectural depth.

What is a web service?

A web service is a narrower and more specific subset of the API concept. It refers to structures that enable two systems to communicate over a network, most commonly using HTTP or HTTPS.

A web service listens for incoming requests on a specific endpoint and produces a response accordingly. That response may consist of structured data such as JSON or XML, but it can also be HTML, images or other types of binary content. The key distinguishing factor here is that web services are inherently network-dependent.

In other words, when we talk about a web service, communication over a network is always involved.

Where does the difference between APIs and web services begin?

Although these two concepts are often used interchangeably in daily conversation, they do not mean the same thing from a technical perspective. Every web service is an API because it exposes an interface to the outside world. However, not every API is a web service, since APIs do not require web protocols or a network connection to function.

Web services are constrained by specific communication models such as SOAP, REST or XML-RPC, whereas APIs have no such obligation. This distinction becomes particularly critical when designing architectures or defining integration strategies.

Why did API gateways emerge?

APIs have been part of our systems for a long time, but in the past, most of them were used between a limited number of systems located within the same data centre. Integration scenarios were relatively few, and management was comparatively straightforward.

With the widespread adoption of cloud technologies, this picture changed fundamentally. Services became geographically distributed, the number of systems increased, and both internal and external integrations diversified significantly. This diversification led to the emergence of an uncontrolled and rapidly growing API ecosystem.

Backend services that were initially meant to contain only business logic gradually started to take on additional responsibilities in order to meet new integration requirements, such as:

Authentication and authorisation
Data validation
Logging and error handling
Rate limiting and traffic control

As a result, business logic and integration-related code became tightly coupled, producing fragile systems that were difficult to change and maintain. The need to remove this burden from backend services and manage common integration concerns from a central point gave rise to the concept of the API gateway.

What does an API gateway do?

An API gateway is a centralised layer positioned between clients and backend services. Its primary purpose is to separate common integration concerns from backend services and manage them consistently from a single point.

With this approach, backend services become simpler, more autonomous components that focus solely on their core business responsibilities.

Core capabilities provided by an API gateway

The value of a modern API gateway does not come solely from routing traffic, but from centralising the integration-related responsibilities that accumulate around services over time. These capabilities elevate the gateway from a simple proxy to a strategic architectural component.

A modern API gateway typically provides value across six main areas.

1. Authentication & authorisation

Authentication and authorisation are common challenges in almost every integration scenario. Determining who can call a service and at what level of access is often independent of the service’s business logic. Historically, however, these controls were embedded directly into backend code, causing services to become increasingly complex as integration scenarios multiplied.

With an API gateway approach, authentication and authorisation can be fully managed at the gateway layer. Supported mechanisms typically include:

OAuth 2.0
API key
Basic authentication
LDAP / directory services

For example, a service protected by LDAP at the backend can be exposed to external consumers via OAuth. These two different security models are bridged at the gateway level, completely isolating the backend service from this complexity. This approach not only enhances security but also significantly improves service reusability.

2. Data transformation & validation

In real-world integrations, it is unrealistic to expect all systems to communicate using the same data format. A SOAP-based service that has been used internally for years may need to be consumed by a newly developed mobile or web application over REST.

API gateways prevent backend complexity by handling transformations such as SOAP-to-REST or XML-to-JSON at the gateway level. In addition, the gateway can perform:

Request schema validation
Type checks
Regex-based validations

As a result, malformed, incomplete or unexpected requests are blocked before they reach backend systems. This approach protects backend resources and directly improves overall system stability.

3. Logging & monitoring

It is impossible to manage a service effectively without understanding how it is being used. API gateways make all traffic passing through them centrally observable.

Through the gateway, it is possible to collect:

Request and response logs
Performance metrics
Error and audit records

These data can be stored within the gateway itself or forwarded to external systems such as log analytics platforms, SIEM tools or similar platforms. This visibility allows service owners to clearly see who is using their services, how often they are called and under which scenarios issues occur.

4. QoS (Quality of Service)

Traffic patterns for internet-facing services are rarely predictable. Unexpected spikes or malicious requests can easily cause system-wide outages.

API gateways use mechanisms such as:

Rate limiting
Throttling
Resource reservation

to ensure that critical services remain available even under heavy load. Instead of a total system failure, this approach enables controlled degradation.

5. API catalogue & documentation

As the number of services grows, the question “Which service does what?” quickly becomes a serious problem. The centralised API catalogue provided by API gateways allows services to be listed in a single place, with up-to-date documentation readily accessible.

This structure:

Simplifies service discovery
Improves cross-team collaboration
Reduces duplicate code and services

and contributes to the development of a shared API culture across the organisation.

6. Security

API gateways serve as the first line of defence for services exposed to the outside world. Attacks covered by the OWASP Top 10 — such as SQL injection, XSS and DDoS — can be blocked at the gateway level before they ever reach backend systems.

Enterprise-grade and licensed solutions provide regular security updates, allowing rapid adaptation to newly discovered attack vectors. This significantly reduces operational risk, especially for organisations with a high number of external integrations.

Conclusion

Using an API gateway provides organisations with not only technical but also operational and strategic advantages. Reducing integration complexity, centralising security and simplifying backend services are among the most important of these benefits.

In short, an API gateway is not just a technical tool. When designed correctly, it becomes a critical architectural component that makes an organisation’s API ecosystem sustainable, manageable and scalable.

Frequently asked questions

What is the difference between an API gateway and a load balancer?

An API gateway provides application-level capabilities such as authentication, authorisation, data transformation, rate limiting and logging. A load balancer, on the other hand, primarily focuses on distributing incoming traffic across backend services. An API gateway operates at a higher level of abstraction and addresses integration and security concerns beyond traffic distribution.

Is an API gateway mandatory in every microservices architecture?

An API gateway is not mandatory in microservices architectures, but it is strongly recommended. As the number of services grows, managing security, traffic control and observability from a central point provides significant operational benefits.

Does an API gateway cause performance issues?

A properly configured API gateway does not create performance issues; instead, it can improve overall system performance by protecting backend services. Features such as rate limiting, caching and request validation prevent unnecessary load from reaching backend systems.

What is the difference between an API gateway and a service mesh?

An API gateway is typically positioned as the entry point for external traffic and manages north-south communication. A service mesh focuses on service-to-service (east-west) communication. They are not alternatives but complementary components in modern architectures.

Does an API gateway fully handle backend security?

An API gateway plays a critical role as the first line of defence, but it does not replace backend security entirely. A defence-in-depth approach requires backend services to maintain their own security controls as well.

Is API management the same as an API gateway?

An API gateway is the technical core of API management solutions. API management extends gateway capabilities with lifecycle management, developer portals, versioning and analytics.