An API is a collection of commands and functions that allow software components to communicate with each other. APIs power many essential features within applications and serve both to retrieve and modify information, as clients request content and perform database operations daily.
For example, third-party Reddit apps leverage the Reddit API to grab content from Reddit's servers/databases and serve it to users. Since Reddit owns this data, the API helps bridge the gap between Reddit and those outside services. The same is true for transit applications that use APIs (like the Chicago CTA's Train Tracker APIs) to display arrival times, to name another example.
While some APIs are publicly available (and open source), others are private and reserved largely for internal use. API developers and organizations often charge for access, but this isn't always the case.
APIs connect services with services, apps with other apps, and ultimately offer clients access to features or data to support their workflows. APIs also allow the automation of sophisticated processes such as deploying, scaling, and backing up web applications.
While the term "interface" may seem somewhat misleading to modern readers, the idea of a programming interface predates the graphical user interfaces (GUIs) many users might initially envision.
How do APIs (application programming interfaces) work?
When a client does something like logging into an online portal, the sign-in process authenticates and authorizes them before displaying personalized data. This is often tied to one's digital identity and therefore their access permissions.
Typically, an API works in the following way:
A client visits a webpage such as YouTube and clicks on a video thumbnail to open it.
The browser initiates an API call (or request) to the backend server while outlining which information it wants to retrieve or modify. This often, but not exclusively, happens via the HTTP protocol.
The server receives this request and processes it, while hunting for that specific data.
The server forms a response in a specified format and the API delivers it directly to the client.
APIs rely on Uniform Resource Identifiers (URIs) to pass crucial client IP address information, verb typing (such as GET, POST, PUT, PATCH, and DELETE), supplemental headers, and an optional request body to the server. This effectively fingerprints each client. It also enables other functions such as load balancing, blocking, allowlisting, denylisting, rate limiting, and more. API developers govern access to their APIs through keys (which verify the app itself) and tokens (which support user verification).
API calls happen in the background and users don't necessarily "see" this communication actively happening following their request and an API's response. Not only does this happen in mere moments, but an API provides a level of abstraction to separate users from sensitive backend processes.
This is where mechanisms like status codes (such as 404 page not found) can contextually tell us when things are or aren't working as expected. However, developers must closely monitor these status codes to keep sensitive business logic hidden. A response that reveals too much about an API's configuration can pose a security risk in the wrong hands.
What types of APIs are there?
While APIs are essential tools in software development, not all APIs are created equally. Just like numerous internet protocols exist—powering communication across the web—underlying protocols also determine how APIs function. APIs are thus categorized in the following ways:
REST APIs – The most common type of API, Representative State Transfer (REST) APIs are stateless (no data is saved between requests) and flexible, with defined functions such as GET or POST. Requests and responses are sent as data.
SOAP APIs – Powered by the Simple Object Access Protocol, SOAP APIs leverage XML for requests and responses. These APIs aren't as popular currently since they're more rigid.
WebSocket APIs – These APIs support two-way communication between clients and servers via JSON. Servers can leverage callback messages with clients to optimize the request-response pathway.
RPC APIs – Remote Procedure Call (RPC) APIs enable clients to complete certain functions on the server, and the server produces an associated output.
How an API is designed can influence its performance, scalability, interoperability, and maintenance demands. Organizations are (and should be) building APIs that best fit their use cases and infrastructure needs. And due to the popularity of API-powered apps, businesses are embracing solutions such as API gateways and rate limiting to help manage and secure everything.
How does HAProxy support APIs?
Not only is HAProxy the world's fastest software load balancer, but it's also a powerful API gateway. Our products help organizations scale, secure, simplify, accelerate, and manage their APIs. To learn more, check out our API Gateway solution page and our introductory API gateway blog post.
