What is an API?

An API, or Application Programming Interface, is a set of rules and protocols that allows different software applications to communicate and interact with each other. APIs define the methods and data formats that applications can use to request and exchange information, effectively serving as a bridge between different software systems.

In the context of server-to-server (S2S) tracking, APIs are a powerful way to pass data directly between servers without relying on a user’s browser. This enables faster, more secure, and more reliable data sharing—crucial for tracking user actions like purchases or sign-ups across devices and platforms.

How do APIs work?

APIs work by setting up a way for two systems to send requests and receive responses. When an app needs data or a service from another system, it sends a request to a specific URL, known as an API endpoint. This endpoint processes the request and sends back a response with the needed data or action confirmation, usually in a standard format like JSON or XML.

Most APIs rely on HTTP protocols to manage these interactions, ensuring they are secure and efficient. For example, a weather app can use a weather API to request forecast data for a specific city from a central database. The API processes the request and sends back the forecast, which the app then displays to the user.

Types of APIs:

There are several types of APIs, each suited to specific tasks and systems:

• Open APIs (Public APIs): These APIs are publicly available and often free to use, though some may require registration. Open APIs enable external developers to interact with a company’s data or services.
• Partner APIs: Unlike open APIs, partner APIs require specific rights or licenses to access and are typically used between business partners. They allow companies to share data and services selectively to support mutual goals.
• Internal APIs (Private APIs): Internal APIs are used within a company to connect systems, departments, and teams. They streamline internal processes and make data sharing efficient.
• Composite APIs: Composite APIs combine multiple API requests into a single call. This is useful when an application needs data from several sources simultaneously, saving time and reducing the number of requests required.
• Event APIS: Event APIs are a type of server-to-server (S2S) API that transmits real-time, event-based data—such as purchases or clicks—as they happen. They are designed for high-speed, reliable tracking and data syncing between platforms, making them ideal for analytics, ad tech, and personalization engines.

How to measure APIs:

APIs can be measured by several key performance indicators (KPIs) to ensure they’re functioning optimally. These KPIs include:

• Latency: how quickly an API responds to a request
• Uptime: percentage of time an API is available
• Throughput: the amount of data processed
• Error rate: the frequency of errors encountered

Monitoring tools provide insights into these metrics, helping companies optimize their APIs for speed, reliability, and user satisfaction.

What is the difference between an API and a postback?

An API is a more flexible and dynamic connection between systems. You ask for something (request), and you get an answer (response). With API, there’s two-way communication, meaning the system can send and receive data, and can also retry if something goes wrong.

A postback is like a receipt—it just confirms something has happened but doesn’t request or send any further information. It’s a one-way notification and does not provide an option to retry if an error occurs.

Why are APIs important to marketers?

APIs allow marketing teams to access customer data, track campaign performance, and update content across platforms in real-time, enhancing personalization efforts and customer experiences.

For instance, when launching a post-transaction advertising campaign, the most effective way to ensure accurate tracking and avoid data fragmentation is by using an event API. This helps align data across advertisers, supply-side partners, and merchants, driving higher performance.

APIs are also essential in software development. They allow developers to access predefined functions and data from other software applications, reducing the time needed to build features from scratch.

Who needs to know about APIs?

While developers primarily use APIs, understanding them can benefit a broader audience, including product managers, marketers, data analysts, and business executives. Having basic knowledge of APIs helps these roles make informed decisions about software integrations, data sharing, and collaboration with tech teams.

Use API in a sentence:

“By leveraging Fluent’s event API for audience attribution, advertisers can optimize post-purchase campaigns across various factors like audience segments, device types, basket sizes, and even time of day.”