open API (public API)

What is open API (public API)?

An open API, also called public API, is an application programming interface made publicly available to software developers. Open APIs are published on the internet and shared freely, allowing the owner of a network-accessible service to give a universal access to consumers.

While some may consider open and public APIs an analogous term, some may site a difference in that open APIs are shared freely, and that public APIs are typically more restrictive in terms of sharing assets. In this case, because the proprietary organization is sharing its own API and back-end data publicly and wants to maintain a sense of security and management over their application. Or, a company might want to publish a series of APIs to encourage third-party developers in vertical industries discover and share new ways to use the company's software product.

Open APIs can be designed in a variety of different ways, but the main priority of any open API is to be easily consumed and accessed by as many different clients as possible.

As a result, using proprietary protocols or custom data formats to create open APIs is discouraged, while using Open Source technology and community-driven standards makes the most sense.


The most common open API architectures fall into two categories: REST APIs and SOAP APIs.

SOAP and REST offer different methods to invoke a web service. SOAP-based APIs typically use XML as a data exchange format, while RESTful APIs typically use JSON back and forth. Both approaches have supporters and opponents.

The current trend in the industry is largely toward REST APIs and away from SOAP-based APIs. Many older open APIs provide both a SOAP and REST base to support older clients, but newer implementations typically only provide REST-based access.

Open API management

Once an API is made public, it is difficult for an organization to control who uses the API and how they use it. As a result, API management must be taken seriously; otherwise, issues can arise in terms of customer satisfaction.

For example, organizations must be careful when they decommission older APIs, change the syntax of a RESTful method call, alter the structure of an XML or JSON payload, or retire a given piece of functionality, as the control that might exist within an organization over how an API is used and who is using it simply does not apply to the public at large.

When changes to an API are not managed properly, end users become disaffected, and that can hurt the reputation of the organization providing the public API.

Open API security

While an open API might be universally accessible, it may also use various means to restrict access, encrypt data transmission and leverage API security measures.

For example, Transport Layer Security (TLS) can encrypt data sent across a network. For authenticating the identity of the caller, SSL certificates prove useful, and any number of back-end authentication mechanisms can map users to the private data that is associated with a given user principle.

Open vs. closed APIs

Even though there are some well-known and popular open APIs such as Jenkins Remote Access API and GitLab API, many APIs are closed, private APIs. This privacy is different from the form of security found in open APIs. An open API has access restrictions because they are openly accessible to the public and can be invoked from anywhere on the open internet.

On the other hand, a closed API, also known as a private API, is not accessible openly on the internet. Accessing a closed API normally requires making calls through highly restrictive firewalls or a VPN service, if any external access is allowed at all.

Since closed APIs typically reside in highly secure environments, they often do not employ any form of user authentication at all, as it is assumed that every resource making calls to them originates from within a secured and trusted realm.

Also, closed APIs are often used for internal services and processes, such as microservices and container orchestration tools, to communicate with each other, whereas open APIs tend to focus on delivering more user-driven services and capabilities.

Benefits of open APIs

Benefits of open APIs include benefits for both the API publisher and the developers who want to use the publishers API.

For the publisher, the benefits of an open API include:

  • The ability to expand a user base without the cost of niche industry software development;
  • A chance to create revenue streams by licensing new programs; and
  • The option for an organization to retain its proprietary source code.

For the consuming developers, the benefits using an open API include:

  • Reduced dependencies between development teams and certain application components;
  • Less time spent fixing mistakes made when writing code; and
  • The ability to still use preferred development tools alongside an open API.

Drawbacks of open APIs

An open API should be treated like any other customer-facing product because the reputation of the company can improve or suffer damage depending upon how the open API is received by the developers.

Organizations should ensure any of the possible downsides an open API may have can be avoided. Just like any application an organization might release to the public, it is important to ensure an open API does not:

  • Contain software bugs;
  • Cause delays due to poor performance;
  • Contain significant security flaws; or
  • Leak any private company data.

Furthermore, open APIs can be problematic for developers because the company publishing the API has all the power. If a startup ever decides to change the terms of use for its API, for example, or decides to charge a fee for licensing the API, a third-party developer has no choice but to accept it.

Examples of open APIs

There are many uses of open APIs. One of the most common areas for both the availability and consumption of open APIs is in social media.

For example, Facebook provides an open API that allows third-party tools to create photo albums or post to a user's news feed. Facebook has a built-in function that allows the items a user to add publicly to their news feed to also be posted to their Twitter feed. In that case, the Facebook application is using the open API from Twitter to make that interaction happen.

Many software products also come with an open API, allowing programmatic access to the data they contain. For example, Microsoft's SharePoint collaboration platform provides a REST API that allows access to the FAST Search service. When queried using the open API, authenticated users can query the SharePoint platform for published documents based on date, title, ID and a variety of other attributes.

In a world where systems are increasingly interconnected, organizations are realizing that one way to gain widespread adoption of the services they provide is to make an open API available so the public can access those services in new and creative ways.

An open API case study

The rise of Twitter illustrates the benefits of open APIs, along with the potential drawbacks.

When Twitter started, it provided a text-only format and a rudimentary web-based interface. However, Twitter provided an open API that allowed programmatic access to its systems.

An ecosystem of tools and feature enhancements were quickly developed around this API. TwitPic, which added the ability to share photos through Twitter, became popular. Customizable desktop interfaces, such as TweetDeck and Falcon Pro, earned favor from users who were not impressed by Twitter's own interface. A large part of the Twitter success story was due to the additional features and software applications that were developed using its open API.

However, some of the Twitter clients rejected Twitter advertisements or injected their own ads, which had a negative effect on Twitter's business model. Through acquisitions, API restrictions and trademark challenges, Twitter pushed back against some companies that used its open API.

Some members of the software development community felt betrayed by Twitter. They believed the community that developed apps using Twitter's open API played a large part in why Twitter became successful. Shutting down those APIs and filing trademark infringements against supporting services showed disregard for the many developers who played a role in Twitter's success, according to some in the developer community. However, Twitter executives said their actions sought to protect the company's trademarks and intellectual property, as well as the platform's security.

This was last updated in October 2021

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