Asynchronous vs Synchronous APIs

Fri Dec 24 2021

2 min read

APIs let you perform complex business operations by working with the client and the server. They help applications communicate with services on the backend servers. Different communication mechanisms are used for this purpose, broadly classified into asynchronous and synchronous.

So, there are two kinds of APIs: Asynchronous and Synchronous. Let’s take a look at them.

Synchronous APIs

They use traditional protocols such as REST, SOAP, etc. These APIs handle requests synchronously, and users have to wait for the response from the API before they can continue in the application. They can not process two requests at the same time. Therefore, they can bottleneck performance if a request takes too long.

Asynchronous APIs

Asynchronous APIs can process multiple requests at the same time. Due to this mechanism, the APIs allow relatively time-consuming requests to be processed in the background while more minor requests are serviced right away.

In JavaScript, if you add an async keyword to a script, JavaScript processes it in the background without stopping the execution of the following scripts. These APIs work in a similar fashion.

These APIs are implemented using different protocols like WebHooks, Websockets, GraphQL Subscriptions, and Server-Sent Events. Like the OpenAPI specification, AsyncAPI provides a specification for Asynchronous APIs that allows developers, architects, and product managers to define asynchronous API interfaces.

Advantages of Asynchronous APIs

Let’s look at some of the advantages of using Asynchronous APIs over Synchronous ones.

Multiple Responses

Asynchronous APIs can send more than one response for a single request. This design enables them to provide a stream of events to the user end instead of sending data periodically. This is why they are also called event-driven APIs.

Performance at Scale

These APIs ensure a smooth flow of requests by allowing multiple requests to process simultaneously. They can shift a resource-intensive request to a separate thread that is processed in the background and does not bottleneck other requests.


Async APIs are more efficient in terms of bandwidth usage because they utilize remote back communication channels between client and server, avoiding additional requests.

Drawbacks of Asynchronous APIs

Since the concept of Asynchronous APIs is inherently different from traditional alternatives like REST, the management and design of these APIs can be complex. They require other mechanisms to implement rate limiting, analytics, and throttling. Securing these APIs can also be a challenge because of their dedicated backchannel communication.


Asynchronous APIs provide a reliable way of communication, particularly when complex computations, multiple processes, and scalable performance are required.