What is an API? What is an SDK? (And What’s the Difference?)


All about auth

Julianna Lamb

January 25, 2022

APIs and SDKs make it easier for developers to integrate different features and functionalities into their applications. Instead of development teams having to build all of their solutions in-house, APIs and SDKs enable them to connect to other apps and platforms and leverage existing services and technologies, providing them with the tools and resources they need to get functions up and running quickly.

In this post, we’ll cover what APIs and SDKs are, what they do, how they’re different, and why a development team might choose to use one over the other for a specific project.

What is an API?

API stands for application programming interface. The key word here is “interface,” as APIs essentially work as mediators or messengers that allow different applications to communicate. More specifically, APIs are responsible for relaying an app user’s request to a separate system and, once the information is processed, relaying the system’s response back to the app and user.

APIs are often explained through IRL customer service analogies. For instance, if you’re working with a travel agent to plan a trip, they’re tasked with taking your information (travel dates, traveler data, etc.), reaching out to airlines, and booking you a suitable flight. The travel agent and, say, Delta Airlines, are two separate entities. The agent isn’t supplying or flying the plane itself, but you trust them to interpret your needs and book you the flight you want through the Delta system.

In this case, the travel agent is the API. They’re saving you the time and maybe even the higher cost of researching and making a reservation yourself by engaging and coordinating with an outside service.

API examples and use cases

In our growing world of “headless” CMS development—where back-end content is increasingly decoupled from a front-end user experience that changes from channel to channel—APIs are what connect the two while maintaining a streamlined user experience.

Most modern apps use some form of API to allow users to accomplish specific tasks without ever leaving their platform. For example, APIs are working behind the scenes whenever you:

  • Reserve a flight through a booking website: To expand on the analogy from above, online booking sites like Expedia and Travelocity use airline companies’ APIs to aggregate flight data (like schedules and pricing) and present you with all available options.
  • Make an online payment: When you make a purchase online, you’re prompted to select a method of payment at checkout, like a Visa card or PayPal account. Whichever you choose, the e-commerce app uses that financial service’s APIs to exchange your order and payment information and complete the transaction.
  • Log in through a third party: Many websites will let you log in using your credentials from Google, Facebook, Twitter, or another social media account, authenticating you through these platforms’ APIs.
  • Add a newly scheduled appointment to your calendar: If you’ve just booked an appointment with your dentist online, their website might use an API to let you add that session to your iCal or Google Calendar.
  • Find the nearest storefront: Many retail sites use APIs to connect to platforms like Google Maps and embed locational services into their apps, so users can identify and get directions to the store branch nearest them.

Benefits of APIs

By facilitating interactions between systems, APIs make it easier and faster for developers to integrate and innovate around essential services and for users to access them. 

In the above examples, for instance, the online transactions are optimized for all involved:

  • App developers can provide advanced services and solutions without having to spend extra time and effort on programming.
  • Users gain access to services within the apps they’re already using—and without being bounced around to third-party platforms, which would introduce extra steps and friction.
  • API providers are able to tap into new use cases and sources of revenue for their products and services.

It’s win-win-win, and it means everyone enjoys a more seamless digital experience.

APIs and security

Because APIs play such a significant role in digital transformation—and because they transmit sensitive user data like payment and account information between apps—they’re becoming a key target for common cyber attacks, which are growing more sophisticated by the day.

When integrating APIs, it’s important for developers to make security a priority and keep up with best practices. This includes encrypting traffic to prevent against man-in-the-middle (MitM) attacks, using strategies like multi-factor authentication (MFA) to guard against tactics like credential stuffing, and using just-in-time (JiT) methods to monitor what data users can access and when.

What is an SDK?

SDK stands for software development kit. As this name suggests, an SDK works as a “kit” or package that gives developers a full set of tools and programs to build, simplifying the implementation of an API.

To keep with our IRL analogies, if an API is like a travel agent—an intermediary that provides and interfaces with various other providers for you, but whom you need to give a set of instructions to—then an SDK is a cruise where everything is handled for you and the itinerary is set, you just need to show up. It can be a valuable tool for people who want to spend minimal time on planning and development.

As part of the “kit,” an SDK will typically include a compiler for translating code, a debugger to test and locate errors within the program, and the API itself. But it could also include elements like:

  • Coding samples and/or libraries to teach developers how to create basic programs
  • Documentation on how to use and integrate the API
  • Guides and/or tutorials to walk them through the process
  • Licensing by the provider that regulates the use of the SDK material

There are different terminologies developers use to refer to SDKs working on the front and back end. Some people, for instance, refer to client libraries (for back-end code) as “admin” SDKs, and some call front-end SDKs “browser-side” or “client-side” SDKs to clarify their function.

SDK examples and use cases

There are many ways app developers might use pre-built, ready-to-go SDKs, including:

  • Platform-specific development for apps meant to be used on a particular platform, one prominent example being the iOS SDK developers use to build apps for iPhone and iPads
  • Language-specific development for facilitating and standardizing development around particular programming languages like Java, Python, Ruby, and others
  • Mobile advertising whereby apps connect to publishers and ad networks via SDKs to run ads on their platform and generate revenue
  • Analytics tools like the Google Analytics SDK that can be integrated with an app to collect data around users, usage, and behavior patterns

APIs vs. SDKs

Comparing APIs and SDKs is not as straightforward as it may seem; the two cover similar ground and overlap in significant ways.

However, there are times when a developer may choose one over the other. While both APIs and SDKs can be used to build essential features into an app, APIs are a bit more complex than out-of-the-box SDKs and require a bit more time, knowledge, and effort to integrate correctly and effectively. However, they can also provide greater opportunity for creativity and customization.

Going back to our travel analogy, creating your own itinerary (like an API) means you can choose the stops you want to make—but it leaves greater room for error and requires more work on your part. With a cruise, you’ll have some choices to make but the bulk of the planning is out of your hands.

Keep in mind, SDKs aren’t just for beginners—and they definitely don’t limit what you can build. Maybe you just need to set one program or service in motion quickly and effortlessly, so you can focus on the core, innovative product at the heart of your app.

Learn more about Stytch’s APIs and SDKs

When it comes to passwordless authentication, Stytch offers both easy-to-integrate APIs and fully customizable SDKs that work with your brand, at your level.

Jump into our docs for a full tour of our solutions, or sign up for a free account to try them out.


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