APIs unlock critical communication between our apps and services, and developers tirelessly optimize these communication mechanisms. Traditionally, performance limitations and fledgling app designs dictated that APIs make requests synchronously. This meant that users taking action within an application — like pulling specific info from a database — had to await a backend response before continuing onward.

Naturally, this could hamper workflows to varying degrees. Users hoping to harness multiple API-dependent features in quick succession face bottlenecks. Synchronous API requests can be lightning-fast or take multiple minutes to execute. The latter case is especially concerning. Developers needed a better solution.

Introducing Asynchronous

Now, consider that individualized experience and how the problem might compound across an entire user base. Modern, containerized applications are designed to scale quickly with user activity. More users means more requests, and a litany of competing priorities from an application perspective. Asynchronous APIs help tackle this challenge. These APIs allow relatively time-consuming requests to process in the background while other requests are made.

This approach keeps requests flowing more smoothly. It’s essential for robust apps — and especially enterprise software — that can log hundreds of thousands of user sessions per day. Each user can generate over a dozen API calls per session, making asynchronicity essential.

The practice mirrors frontend web development with respect to JavaScript. Many web pages require JS to summon dynamic content, render effectively, or provide some degree of interactivity. Because these scripts can take so long to load (within the DOM) and execute, adding an async attribute conveniently lets other scripts run simultaneously.

Overall, it’s argued that asynchronous APIs delegate communication between internal and external services more intelligently. They’re also known as event-driven APIs, or even reactive APIs. Asynchronous technologies leverage remote back channels of communication between client and server, saving bandwidth for additional requests. Asynchronous technologies include the following:

• WebSockets
• WebHooks
• GraphQL subscriptions
• Server-sent events

It’s assumed that an asynchronous API will experience some sort of delay since it’ll often tap into resources not immediately available. When the resource is ready, the API gives a callback to the requester. This approach is also preferable when service availability is low. Crucially, asynchronous APIs have risen to support demand-saturated applications across multiple platforms. These APIs are nothing new (boasting a nearly 20-year history) yet have become more prominent lately.

Benefits of Asynchronous APIs

We know that asynchronous APIs are great in instances where user activity is heavy. These API requests excel at executing background tasks without getting in each other’s way. This is essential in a microservices environment where features are smaller, more integrated, and APIs are more plentiful.

Today’s applications are now more interactive than ever from a UI standpoint. Many elements are dynamic, and a smooth user experience means eliminating delays characteristic of synchronous APIs. This is why async is often synonymous with “reactive.” When the user provides an input, it’s the API’s job to translate backend changes into frontend updates. Though far from an exhaustive list, this is useful for apps that provide event-driven notifications, messaging, content feeds, or even real-time collaboration.

These APIs are also receiving widespread support from the developer community. The OpenAPI Initiative has been pivotal in getting RESTful architectures off the ground in recent years. Similarly, developers have banded together to create the AsyncAPI Initiative. This community effort aims to build open-source tools for building event-driven applications. Backed by prominent companies and enthusiasts alike, the technology has plenty of buy-in. There’s a push towards forming a definitive async standard to make development consistent and approachable.

Because asynchronous API communication is standardized to a notable extent, it’s much easier to define how multiple APIs can interact. In the past, each individual procedure or “protocol” had to be crafted on a case-by-case basis. That’s not only time-consuming, but it’s costly. Additionally, these asynchronous message formats have defined structures. Asynchronous design tends to be unified, schema-friendly, and protocol agnostic.

Asynchronous APIs are ideal for long-running computations on the complexity side of the spectrum. This is one way to boost efficiency across your entire infrastructure, which is almost sure to be more complex than the monoliths of old. They’re best used for multi-threaded applications where complex operations exist in a loop. We see this a lot when combining frontend JS with backend Node — because asynchronous loops are important.

Due to the nature of async-friendly conditions, it’s both common and beneficial to leverage these APIs in client-server environments. These utilize busy network-and-web servers. The subsequent rise of Node.js, Twisted, and other related libraries can be attributed to this relationship.

If performance and user feedback are critical within a complex environment, asynchronous APIs are definitely worth a second look. Developers dependent on parallel programming will gain plenty of performance without incurring unneeded overhead.

Popular Examples in the Wild

As you might imagine, many small and large companies leverage asynchronous APIs within their applications. One of the world’s largest payment API providers, Stripe, leverages asynchronous APIs for pull and push requests. These are key in several payment protocols using Stripe’s Sources API. The company’s integrations with partners like Plaid also have asynchronous elements tied to WebHooks.

Earlier this year, eBay adopted the evolving AsyncAPI standard for asynchronous API contracts. The company has found this approach extremely favorable:

“[AsyncAPI is] extremely helpful because of the ease of use that results from the delineation of channels, protocols, and bindings, separation of concerns between publish and subscribe, and separation of protocol-specific and application-specific headers.”

Thanks to the presence of reusable schemas, AsyncAPI is also compatible with the OpenAPI specification. This helped cement eBay’s decision, and certainly those of others across the industry.

Lastly, IBM’s own CICS Asynchronous Interface makes it easy to hit multiple external services with requests in an efficient fashion. The request process immensely reduces wait times; it allows “multiple child transactions” to process simultaneously. CICS uses specialized channels and containers to funnel data back to the requester once it’s available.

In any case, asynchronous APIs aren’t simply another passing fad in the tech space — they’re here to stay. With applications becoming more complex and interdependent, fostering more efficient communication is essential. It’s worth noting that asynchronous styles aren’t designed exclusively to replace synchronous alternatives. Conversely, both API architectures have their own use cases while complimenting each other in unique ways.