API Architecture - Synchronous, Asynchronous, and Parallel calls

Abdul Rafee Wahab
4 min readMay 6, 2023

A crash course with infographics on what these mean, and when to use which one.

Photo by Volodymyr Dedyshyn on Unsplash

Background

One critical aspect of REST API development is the way in which clients make requests and receive responses.

In this segment, I will cover the 3 main methods of making requests in REST APIs - synchronous, asynchronous, and parallel calls.

I will outline the pros and cons of each method and when to use which for maximum performance gains.

Related Reading

I’ve written extensively about API design, performance, security, versioning, and diagramming best practices. Check out my series linked below to learn more! 👇

APIs & Technical Architecture Best Practices

6 stories

Synchronous Calls

Synchronous calls are the most common method of making requests in a REST API.

In synchronous calls, the client application sends a request to the server and waits for a response before proceeding to the next step. This means that the client application is blocked until it receives a response from the server.

Pros:

  • Synchronous calls are easy to implement and understand.
  • They are useful for small applications with low traffic volumes.

Cons:

  • Synchronous calls can be slow and inefficient, particularly for large applications with high traffic volumes.
  • They can cause performance issues if the server takes too long to respond.

When to use:

  • Use synchronous calls when the application has low traffic volumes or is relatively small.
  • They are also useful for applications where the client needs to wait for a response before proceeding to the next step.

Pictorial view of Synchronous calls

Source: Abdul R. Wahab

Asynchronous Calls

Asynchronous calls are a method of making requests in which the client application sends a request to the server and continues to perform other tasks while waiting for a response.

This means that the client application is not blocked while waiting for a response from the server.

Pros:

  • Asynchronous calls are faster and more efficient than synchronous calls.
  • They are useful for large applications with high traffic volumes.

Cons:

  • Asynchronous calls are more complex to implement than synchronous calls.
  • They can be challenging to debug if there are errors in the request or response.

When to use:

  • Use asynchronous calls when the application has high traffic volumes or is relatively large.
  • They are also useful for applications where the client does not need to wait for a response before proceeding to the next step.

Pictorial view of Asynchronous calls

Source: Abdul R. Wahab

Parallel Calls

Parallel calls are a method of making multiple requests simultaneously.

In parallel calls, the client application sends multiple requests to the server at the same time, and the server responds to each request individually.

Pros:

  • Parallel calls are faster and more efficient than sequential calls.
  • They are useful for large applications with high traffic volumes.

Cons:

  • Parallel calls can cause performance issues if the server is not designed to handle multiple requests simultaneously.
  • They can also cause issues if there are dependencies between requests.

When to use:

  • Use parallel calls when the application has high traffic volumes or is relatively large.
  • They are also useful for applications where the client needs to make multiple requests simultaneously.

Pictorial view of Parallel Calls

Source: Abdul R. Wahab

Closing thoughts 👏

So, pretty much, the choice of making synchronous, asynchronous, or parallel calls in an API depends on the specific needs of the application.

While synchronous calls are easy to implement, they may not be suitable for large-scale applications that require fast and efficient communication between the client and server. Asynchronous and parallel calls offer better performance and scalability, but they require more advanced implementation and may have their own set of challenges.

It is essential to weigh the pros and cons of each method and consider the traffic volume, the size of the application, and the type of requests to be made before choosing a particular approach.

By choosing the appropriate method, engineering teams can ensure that their API provides optimal performance, scalability, and responsiveness for the end-users.

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Abdul Rafee Wahab

Tech guy. I like building cool software, & also leading others in building cool things. All views shared are my own.