AsyncAPI vs. OpenAPI: Which Specification Is Right for Your App?

Depending on how your application is designed and what it needs to accomplish, you probably want to consider choosing one type of communication protocol for your API over another—namely, synchronous or asynchronous. And which protocol you use determines which specification you need to follow for your API—OpenAPI or AsyncAPI.

Let’s take a brief look at the differences between synchronous and asynchronous APIs, and then we’ll talk about whether OpenAPI or AsyncAPI is best for communicating the ins and outs of your API to your users.

Synchronous vs. Asynchronous APIs #

Synchronous and asynchronous APIs differ mainly in how they process information.

Synchronous APIs respond to each client request with only minimal elapsed time, and clients must wait for the response before code execution can continue. This is the type of API that supports most of the internet, and they typically use the REST protocol.

Asynchronous APIs may provide a callback or notification when requested resources are ready, allowing clients to continue functioning without tying up resources waiting for a response from the API. These APIs use pub-sub messaging queues like MQTT and RabbitMQ, data streams like Kafka, or bidirectional communication protocols like WebSockets. IoT devices also commonly use asynchronous APIs.

Choosing Between OpenAPI and AsyncAPI #

Documentation of your APIs is critical for communicating between backend and frontend developers. And good documentation can go beyond just improving the development experience - it can even increase adoption of your API.

Industry-standard specifications like OpenAPI and AsyncAPI ensure that you’re effectively communicating to your users how they can interact with your API through an API contract. Which spec you use to define and document your API depends on your use case, available resources, engineering team and community support, and chosen protocol to transmit data.

In general, synchronous APIs can be defined using the OpenAPI standard, and asynchronous APIs can be defined using the AsyncAPI standard.

Protocols #

OpenAPI is typically used for RESTful APIs, which leverage HTTP or HTTPS as a transport protocol for data. The client makes a request to the server, which responds quickly (ideally within milliseconds or seconds). HTTP is mostly used as a unidirectional protocol, meaning the connection is closed after a single request and response. Each request must create a new connection to the server.

To receive updates on any server changes, the client must rely on polling. Since the application does not know when a task will be completed, it must continue calling the API until the update is detected.

AsyncAPI is based on event driven architectures (EDA). In this context, the sender and recipient don’t have to wait for a response from the other to do their next task or request.

In event driven architectures, as you might have guess, it all revolves around events. An event is defined as a change of state. In an EDA, when an change of state occurs, the producer captures that event and sends it to all consumers who have subscribed to this event. There can be a broker in the middle in some cases. This way of architecturing APIs is refered to as Asynchronous.

Asynchronous APIs can use bidirectional protocols, where a connection is maintained until it’s terminated by either the client or the server. Communication protocols like WebSockets allow the application to request data from the server but continue other processing while it waits for the response. Once resources are available to generate the response, the API produces a response, and the WebSocket connection streams the information back to the application.

Applications can also subscribe to a channel on a message broker like MQTT, Kafka, or RabbitMQ where they can receive updates or notifications as events occur. When updates are no longer needed, the subscription can be terminated.

A single API can make use of multiple protocols. We’ll talk more about that in a bit, but for now, suffice it to say that an API can combine synchronous and asynchronous methodologies, as well as use multiple asynchronous protocols. GraphQL APIs, for example, send queries over HTTP (a synchronous protocol), but also provide asynchronous messaging with WebSockets.

Simplicity of Implementation #

Since OpenAPI and AsyncAPI support different communication protocols, they require different skills to implement and different amounts of work as well.

To implement an API designed according to OpenAPI, developers can use web application frameworks to set up the actual API. Authentication is also built into the standard, so everything needed to create your synchronous API is present once your OpenAPI document is created. Since synchronous APIs send messages directly to the endpoint, there’s no additional architecture required to build the API.

On the other hand, implementing an asynchronous API is more complicated. Developers can still use web application frameworks, but they do need more architecture support than that. If your API uses a message broker as a middleman between application and API, that needs to be designed and built. If you’re using WebSockets to push notifications, both the server and the application need listener functions that keep the connection open.

Community Support #

AsyncAPI and OpenAPI are both open-source standards and part of the Linux Foundation. Both are stable, well-maintained specifications and should be supported well into the future. Still, there are a handful of differences when it comes to their ecosystems.

OpenAPI is a more established standard, since it was developed for synchronous REST APIs. The OpenAPI specification has many contributors from the Linux Foundation and the community. Many more have contributed to the standard through discussion and examples. The governance model of the OpenAPI Initiative allows both community and industry contributions in an effort to keep the specification vendor-neutral. To contribute, you must meet requirements for membership in the initiative.

AsyncAPI is a newer standard based on OpenAPI. While it has fewer code contributors than OpenAPI, its popularity is quickly growing. AsyncAPI attributes its rising success to its community support and all the hosted tools related to AsyncAPI. In late 2020, AsyncAPI also formed an open governance model to assure the community that no single company has control over AsyncAPI and that this initiative is, in fact, community driven. They even are a Linux Foundation Project since 2021.

Use Cases #

Now, as we’ve already mentioned, whether you would use OpenAPI or AsyncAPI specification depends on what protocols your API uses. Which protocol you use depends on your specific use case and what backend resources are available for support. In general, OpenAPI is used to define APIs that provide immediate feedback (synchronous). The AsyncAPI spec defines APIs that don’t need to respond immediately to a request (asynchronous).

But let’s see how that works out in some everyday use cases. When does one implementation have an advantage over the other?

RESTful APIs #

REST is a software architecture style commonly used to support APIs used on the web, and as mentioned earlier, synchronous APIs carry the day here. These are simple, uniform interfaces that are often used to support cloud applications and cloud services because they’re stateless, consistently available, and widely accessible. With these traits, a RESTful API can provide information on virtually any topic easily and reliably.

Internet of Things #

Internet of Things (IoT) devices need to handle inconsistent communications between the device and the API. For example, an IoT device may go out of internet range periodically, but will still need to send data to the API for processing. This means that data may be sent in bursts to the API when a connection is available. IoT devices may also need to notify the API when a significant event has occurred, as when a smoke alarm goes off or a camera detects motion.

As a result of event processing, an IoT device may need to receive asynchronous notifications from the server. While the device itself holds raw data, servers collect and perform calculations of the data over time. The server may need to send notifications to the device when an event has occurred, like an IoT device losing power in your home.

Combining Synchronous and Asynchronous APIs #

Many SaaS API offerings exist to set up shopping carts for users and store inventory management for owners. These solutions tend to include a mixture of synchronous and asynchronous APIs.

Inventory management applications, for example, use asynchronous API resources that allow users and owners to retrieve information about available inventory, user accounts, and pricing. Business owners can update their inventory, and it’s automatically available to users when the API pushes data to the message broker. However, remember that synchronous APIs support most of the internet. An online retail shop may very well want the reliability of a RESTful API as part of its system to facilitate communication with a wide array of other organizations and developers.

Documenting in Any Case #

Regardless of which specification you choose to develop your API with, users need to be aware of new features and changes. Maintaining your API’s documentation and ensuring that it keeps to the standards of your spec can be a time-hungry task.

Bump.sh transforms your OpenAPI and AsyncAPI documents into beautiful documentation. It lets your developers focus on building products rather than maintaining docs. Bump.sh provides automatic change detection by integrating with CI tools, pushing notifications of the change to stakeholders, and enabling validation via its simple-to-use interface. Take its free trial for a spin to see how Bump.sh can make documenting any API - OpenAPI or AsyncAPI - simple. You can also try any AsyncAPI or OpenAPI file in the preview at the bottom of this page.

So… Which one to pick? #

To summarize, the API specification you should use depends entirely on what type of API you want to build. Long story short, synchronous APIs use simple, direct endpoint communication to link a client to a server. When data consists of simple requests that can be completed by the server quickly, a synchronous, RESTful API that follows the OpenAPI specification is what you want.

Asynchronous APIs use message brokers or pub-sub to communicate between server and application as data is made available. They’re useful for real-time applications that require frequent updates. They can be more complex to implement since they require more architecture than synchronous APIs, but you can stay organized thanks to the AsyncAPI specification.

Both OpenAPI and AsyncAPI are open-source specifications and provide a standard method for designing and implementing APIs. A key asset of both specs is the ability to create comprehensive, industry-standard documentation for your API. By connecting your documents to Bump.sh, you can rest assured that your documentation is automatically updated for each API release.