Asynchronous vs. Synchronous Programming: Key Similarities and Differences

In computer programming, synchronous and asynchronous models are essential. The terms offer a clue about what each programming model does and the differences between them. Synchronous tasks happen in order — you must finish task one before moving on to the next. Asynchronous tasks can be executed in any order, or even simultaneously. How can this be understood in terms of programming?

What Is the Difference Between Asynchronous and Synchronous Programming?

Synchronous, sometimes referred to as “sync,” and asynchronous, also known as “async,” are two types of programming models. Understanding how these two models differ is critical in building application programming interfaces (APIs), creating event-based architectures, and deciding how to handle long-running tasks. In choosing which method to use and when, it’s important to know a few key things about synchronous programming and asynchronous programming.

Mendix relies on an asynchronous programming approach for its speed and agility

Asynchronous Programming

Asynchronous programming, conversely, is a multithreaded model that’s most applicable to networking and communications. Asynchronous is a non-blocking architecture, which means it doesn’t block further execution while one or more operations are in progress.

With asynchronous programming, multiple related operations can run concurrently without waiting for other tasks to complete. During asynchronous communication, parties receive and process messages when it’s convenient or possible to do so, rather than responding immediately upon receipt.

Texting is an asynchronous communication method. One person can send a text message and the recipient can respond at their leisure. In the meantime, the sender may do other things while waiting for a response.

Synchronous Programming

Synchronous is known as a blocking architecture and is ideal for programming reactive systems. As a single-thread model, it follows a strict set of sequences, which means that operations are performed one at a time, in perfect order. While one operation is being performed, other operations’ instructions are blocked. The completion of the first task triggers the next, and so on.

To illustrate how synchronous programming works, think of a telephone. During a phone call, while one person speaks, the other listens. When the first person finishes, the second tends to respond immediately.


In the conversation about synchronous and asynchronous programming, there is the curveball called JavaScript. A common scripting language that’s used to make websites interactive, JavaScript is single-threaded, like sync, but also non-blocking, like async. JavaScript is an asynchronous and concurrent programming language that offers a lot of flexibility but is also simultaneously single-threaded and non-blocking.

Although it’s synchronous by nature, JavaScript can benefit from asynchronous code. Long-running JavaScript functions can make the user interface (UI) or server unresponsive until the function has returned, resulting in a less-than-stellar user experience. However, there are some instances where users can benefit from blocking programming; for example, when making an online payment. The beauty of JavaScript is that it offers the best of both worlds: single-thread and multi-thread, blocking and non-blocking. With this flexibility, programmers can write code in a single programming language instead of two—one for synchronous operations and another for asynchronous operations.

Asynchronous vs. Synchronous Programming

Ultimately, the choice comes down to operational dependencies. Do you want the start of an operation to be dependent on the completion of another operation, or do you want it to run independently?

Asynchronous is a non-blocking architecture, so the execution of one task isn’t dependent on another. Tasks can run simultaneously. Synchronous is a blocking architecture, so the execution of each operation is dependent on the completion of the one before it. Each task requires an answer before moving on to the next iteration.

The differences between asynchronous and synchronous include:

  • Async is multi-thread, which means operations or programs can run in parallel. Sync is single-thread, so only one operation or program will run at a time.
  • Async is non-blocking, which means it will send multiple requests to a server. Sync is blocking — it will only send the server one request at a time and will wait for that request to be answered by the server.
  • Async increases throughput because multiple operations can run at the same time. Sync is slower and more methodical.

Differences aside, asynchronous and synchronous methods both offer advantages, but for different stakeholders: async for users, sync for developers.

Asynchronous programming enhances a user’s experience by decreasing the lag time between when a function is called and when the value of that function is returned. In the real world, this translates to a faster, more seamless flow. For example, users want their apps to run fast, but it takes time to fetch data from an application programming interface (API). In these cases, asynchronous programming helps the app screens load faster, improving the user experience.

Asynchronous and synchronous methods offer advantages for different stakeholders; async for users, sync for developers.

Synchronous programming, on the other hand, is advantageous for developers. Quite simply, synchronous programming is much easier to code. It’s well supported among all programming languages, and as the default programming method, developers don’t have to spend time learning something new that could open the door to bugs.

Use cases

Programming makes our digital world run, but without the right pairing of programs and operations, chaos and poor user experiences would ensue. If operations are inappropriately relying on asynchronous programming, our digital world could spin into a mad, hyperactive frenzy. And if operations are inappropriately relying on synchronous programming, our digital world could come to a screeching halt. It’s imperative to understand when to use each type of programming.

When to Use Async

Asynchronous programming should only be used in programming independent tasks, where it plays a critical role. For instance, asynchronous programs are ideal for development projects with a large number of iterations. Because steps don’t have to follow a fixed sequence, asynchronous programming keeps development moving forward.

Responsive UI is a great use case for asynchronous planning. Take, for example, a shopping app. When a user pulls up their order, the font size should increase. Instead of first waiting to load the history and update the font size, asynchronous programming can make both actions happen simultaneously.

When to Use Sync

Asynchronous programming is relatively complex. It can overcomplicate things and make code difficult to read. Synchronous programming, on the other hand, is fairly straightforward; its code is easier to write and doesn’t require tracking and measuring process flows (as async does). Because tasks are dependent on each other, there’s a need to know if they could run independently without interrupting each other.

Synchronous programming could be appropriate for a shopping app, for example. When checking out online, a user wants to buy all of their items together, not individually. Instead of completing an order every time the user adds something to their cart, synchronous programming ensures that the payment method and shipping destination for all items are selected at the same time.

How to Choose Between Asynchronous and Synchronous Programming

When deciding which approach to take, it may be helpful to think of asynchronous programming as adaptable, and synchronous programming as strict. Asynchronous programming is the multitasker, moving from one task to the other and alerting the system when each one is complete. Synchronous programming functions as a one-track mind, checking off one task at a time in a rigid sequence.

Asynchronous programming allows more things to be done at the same time and is typically used to enhance the user experience by providing an effortless, quick-loading flow.

Synchronous programming is best utilized in reactive systems. While it is simpler for developers to code and is recognized by every programming language, sync is resource-intensive and can slow things down.

If you’re interested in more information, do read Asynchronous vs. Synchronous Programming: Key Similarities and Differences blog from Mendix.

Posts by David Bevans

Leave a Reply

Your email address will not be published. Required fields are marked *

Fill out this field
Fill out this field
Please enter a valid email address.