Woman looking at asynchronous APIs

If you have worked mainly with REST APIs, there’s a chance you might not be acquainted with asynchronous APIs and their protocols, such as WebSockets, Kafka, and AMQP. Asynchronous APIs enable you to manage multiple requests simultaneously to communicate between services effectively.

Many similar tasks can be performed using both these APIs; however, some use cases can be managed better with a particular API. Let’s delve deeper into the difference between synchronous and asynchronous APIs.

The Difference Between Synchronous and Asynchronous APIs

Both synchronous and asynchronous application programming interfaces function by returning data requests. The significant difference is that synchronous APIs provide requests for data and services immediately. In contrast, their asynchronous counterparts give a callback to the requester as soon as the request is ready.

However, in the case of synchronous APIs, the requests happen sequentially, and with every interaction, the process gets blocked until it is complete. It implies that the response time to the client using the application is a sum of the response times of each microservice provided by the app.  Another drawback of synchronous APIs is that if a user wants an update, the API needs to be invoked continuously to look for data modifications.  

Making APIs event-driven or asynchronous can solve these bottlenecks and enhance the real-time experience.  Moreover, employing an event-driven architecture has significant benefits, including a great user experience and increased system efficiency.

Asynchronous APIs work by returning the API calls instantaneously with a polling URL while the asynchronous request continues to be processed in the background. It ensures that the functionality of an application is maintained, thereby keeping the app resources free to process newer requests. 

Moreover, synchronous APIs work best in places with higher connectivity and low latency. If the connectivity is lacking or oversaturated, it is best to use asynchronous APIs. Also, when some of your API requests have a longer execution time, it is good to consider expensive operations outside the request. So, the responses to these requests can be provided faster.  

For instance, a REST API operation sometimes takes a lot of time to process. Rather than keeping the client waiting for the operation’s completion, it is best to return an instantaneous response and process the request later.

Using AsyncAPI to Define Event-Driven APIs

An AsyncAPI is an open-source tool that enables you to describe event-driven architectures and document asynchronous applications in a format that is machine-readable.

The elements of AsyncAPI are similar to an OpenAPI, but it has some additional features regarding the accommodation of an event. It supports many messaging protocols and transports, including WebSockets, Kafka, STOMP, JMA, AMQP, MQTT, and HTTP, along with event schema format. 

So, when you define an event-driven architecture with AsyncAPI, it includes the event payload definition, name of the channel, protocols, application headers, and other eventing semantics.

What Are Asynchronous Messaging Protocols for APIs?

  • Webhooks 

A webhook can be considered a type of API that operates on events. This service enables data transfer from one program to another when an event occurs. So, there is no specification regarding the request from your server other than the confirmation of receipt. 

These are also known as reverse APIs, as the app that sends data is initiating the communication rather than receiving the data. With the increased interconnectedness of web services, webhooks are being used as a more straightforward solution to enable data updates and push notifications to one or more endpoints in real time, and that too without using a robust API solution. 

  • WebSockets 

This protocol ensures that the server and the client can have two-way communication. It implies that both entities can share information as and when required. WebSockets enable you to send messages to a server and get event-driven responses without polling the server for a response.  

WebSockets do not have a point-to-point connection or any pub-sub support. 

  • GraphQL subscriptions

GraphQL is a query language used for APIs. You can carry out asynchronous communications from a GraphQL server through live queries and subscriptions. If you are looking forward to adding real-time functionalities to an app powered by GraphQL, then subscriptions are the key. For instance, the live comments and streaming reactions feature on Facebook were made possible because of GraphQL subscriptions developed internally by the company. 

Live queries were created later and are not as widely used. Also, they are not yet a part of the GraphQL subscription. The main difference between a subscription and a query is that subscriptions are used to observe events, whereas live queries are employed for observing data. 

The GraphQL subscriptions are inbuilt in GraphQL and enable asynchronous and synchronous interactions through a single API experience. So, a single API can support both request-response commands and queries as well as asynchronous event-based notifications. 

  •  Server-sent events

Standard request-response interactions send HTTP requests to the server as soon as the client opens the connection. It then gets an HTTP response, and the connection is closed upon completion of the interaction. So, this scenario always takes place when a client requests data.   On the other hand, Server-Sent Events (SSE) enable the data to be pushed by the server asynchronously as soon as the client opens the client-server connection. As soon as the client establishes a connection, the server provides the data and transfers it to the client whenever any new data event occurs. So, this process has acquired the name Server-Sent-Events.

What Are Some Practical Use Cases of Asynchronous APIs?

Whether it is social media or other sectors such as banking or gaming, asynchronous APIs are being used widely to boost application performance and improve responsiveness.


Billions of people worldwide use the messaging feature of apps such as Facebook, Instagram, and WhatsApp to stay connected. Therefore, there are a large number of user interactions that these apps have to process.

These apps use asynchronous APIs to process the numerous parallel requests in the background without requiring users to wait for a response. Many of these async requests do not need to be processed simultaneously—for instance, a post from a friend or a notification regarding a message.

Additionally, some of these may need to be processed faster. Facebook uses Async to manage these types of asynchronous requests, which handles billions of requests efficiently.


Asynchronous transactions are common in banking, where credit transactions are made frequently. These transactions are managed by a processor that functions as a mediator between the merchant and the bank.

Customers make payments through the website hosted by the processor. The merchant initiates the transaction, and the server returns the response later. If the customer abandons a transaction, the server does not send a response.

Asynchronous APIs in credit transactions act as a single point of entry for different processors requiring the customer to provide payment details on the payment site hosted by the bank. As soon as the transaction is initiated, the merchant provides a bank identifier for the bank that will process the transaction.


Asynchronous multiplayer games have been a huge success recently. These allow you to play against friends and other people without real-time data exchange.

Asynchronous multiplayer games are widely used on mobile platforms, and they employ asynchronous APIs since the game is frequently interrupted because players are busy with other activities.

Final Thoughts

Using asynchronous APIs has many benefits, such as enhanced responsiveness and improved communication.

Moreover, in the wake of increasing interconnectedness and complexities in applications, asynchronous APIs are here to stay. However, it is essential to know when to use them and when they cannot replace their synchronous counterparts. Professional developers understand that both these APIs have unique benefits, and they may even use a hybrid architecture to improve communication within the app.

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