What are the three types of APIs?

What are the three types of APIs: Public, Private, Partner

Selecting the correct what are the three types of APIs protects organizational data while expanding digital reach. Different access levels prevent unauthorized usage and ensure developers implement the right tools for integration. Understanding these categories avoids security risks and improves system efficiency. Explore the roles of each interface to optimize your strategy.

Understanding the Three Types of APIs by Accessibility

Application Programming Interfaces (APIs) are the invisible connective tissue of the modern internet - and they are everywhere. Whether you are checking the weather on your phone or paying for a coffee with an app, an API is facilitating that data exchange. However, when we ask about what are the three types of APIs, we usually arent talking about how they are built, but rather who can see them. There is one counterintuitive factor that 90% of beginners overlook regarding which type of API is actually the most common - I will reveal that specific insight in the section on Private APIs below.

Broadly speaking, APIs are categorized by their level of accessibility and the relationship between the provider and the consumer. In recent reports, a high majority of organizations (often cited around 80-90% in various surveys) integrate APIs deeply into their operations and digital strategies, but they do not treat every endpoint the same way. The three main types of APIs you need to know are Public, Private, and Partner APIs. ^[1]

1. Public APIs: The Open Ecosystems

Public APIs, also known as Open APIs or External APIs, are designed to be accessed by any developer who wants to use them. These are the APIs that power the developer ecosystems for giants like Google, X, or Spotify. They are often the most visible because they are built to encourage innovation and widespread adoption.

Public APIs usually fall into two sub-categories: open and commercial. Open APIs are fully available to the public without restrictions, while commercial APIs might require a subscription fee after a certain usage threshold. Interestingly, while these get the most press, they represent a relatively small fraction of the total API landscape.

Ill be honest - when I first started coding, I thought Public APIs were the only ones that existed. I spent weeks trying to find a public endpoint for a feature that turned out to be locked behind a private company firewall. It was a frustrating lesson in digital boundaries.

The goal of a Public API is typically to increase brand reach or monetize data. For instance, a payment processor might offer a Public API so that thousands of e-commerce sites can integrate their checkout system easily. This creates a win-win: the developer gets a powerful tool, and the provider gets more transaction volume.

2. Private APIs: The Internal Workhorses

Here is the critical factor I mentioned earlier: Private APIs (or internal vs external APIs) are actually the most common type of API in existence, accounting for a majority share (often reported around 50-60%) of API usage and traffic within large enterprises. ^[2] Unlike their public counterparts, these are hidden from the outside world. They are used exclusively within an organization to connect different internal systems, such as linking a HR database to a payroll processing tool.

Why hide them? Security and efficiency. By keeping these APIs internal, companies can streamline their microservices architecture without exposing sensitive data structures to the public internet. It reduces the surface area for cyberattacks while allowing different teams within the same company to share data seamlessly.

In my experience building backend systems, Internal APIs are where the real complexity lives. They dont need the polished documentation of a Public API, so they often grow into sprawling, tangled webs if not managed carefully. Seldom does a developer realize how much a company relies on these until one goes down - and suddenly the entire internal dashboard turns into a wall of error messages. It is a messy, high-stakes environment.

3. Partner APIs: The B2B Bridge

Partner APIs sit in the middle ground between public and private. They are not open to everyone, but they are also not restricted to internal staff. Instead, they are shared with specific business partners who have a signed agreement or a formal relationship with the provider. These are common in business-to-business (B2B) integrations.

For example, an airline might provide a Partner API to a specific travel agency website. The agency gets real-time access to flight data and booking capabilities, but a random developer off the street cannot just sign up and start calling those same endpoints. This controlled access ensures that only trusted parties are interacting with high-value data.

Security is the top priority here. Developers working on what is a partner API frequently cite authentication and data privacy as primary concerns.^[3] This is because a breach in a Partner API does not just affect one company; it can compromise the entire supply chain or partnership network. You want a secure bridge? This is it.

Protocols vs. Types: Clearing the Confusion

A common mistake beginners make is confusing different types of APIs by access with API protocols (How they are built). You might hear people talk about REST, SOAP, or GraphQL. These are the technical architectures or languages used to create the API. Any of these protocols can be used to build any of the three types of APIs. A Public API can be REST-based, just as a Private API can use SOAP.

Think of it like this: the API type is the guest list for a party, while the protocol is the language spoken at that party. You could have a private party (Internal API) where everyone speaks English (REST), or a public festival (Public API) where everyone speaks English too. Understanding this distinction is vital for anyone moving from a hobbyist to a professional developer role. I spent way too much time early in my career arguing that an API couldnt be Public because it was SOAP - I was dead wrong, and my senior architect let me know it.

API Types Comparison: Accessibility and Usage

Public API

Lower barrier to entry, but relies heavily on API keys and rate limiting

Innovation, ecosystem growth, and public data sharing

Open to any external developer or business

Private API

Highest; often sits behind internal firewalls and VPNs

Internal system integration and microservices efficiency

Restricted to internal employees and systems

Partner API

High; requires custom authentication and partnership agreements

B2B integration and shared business functionality

Exclusive to authorized business partners

The API Evolution of SwiftShip Logistics

SwiftShip, a logistics startup in Austin, initially built one massive Internal API to handle everything from tracking to billing. As they grew to 50 employees, this single API became a bottleneck, causing 400ms lag in their warehouse management app.

They decided to split the system into microservices. First attempt: they tried to make every endpoint public to simplify remote work. Result: They suffered a minor data leak when a developer accidentally exposed customer addresses without an API key.

The team realized they needed a tiered approach. They locked down sensitive data behind Private APIs and created a specialized Partner API for their delivery contractors. It took 3 weeks of re-coding the auth layer, which was incredibly tedious.

By Q3 2026, SwiftShip reported a 60% improvement in internal system speed. Their new Public API for tracking also led to a 25% increase in small business sign-ups within the first 60 days.