Synchronous vs. Asynchronous Programming: What’s the Difference?

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Synchronous and asynchronous categorize two distinct programming styles, each with its own advantages and disadvantages.

You may not know what these terms mean just yet, but once you do, you’ll understand why knowing the difference between them is important for the success of your team.

Stay tuned to learn more about synchronous vs. asynchronous programming!

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What Is Asynchronous Programming?

Asynchronous programming relies on a non-blocking input and output (I/O) protocol. This signifies that an asynchronous program does not execute operations in a hierarchical or sequential order.

The resulting parallelization means an asynchronous program can take on multiple requests simultaneously and independently.

When one request fails, it has no effect on another request. And the program can move to another task before finishing the last.

In software development, an advantage of this kind is called loose coupling. Loosely coupled communication and/or programming allows for decentralized processes.

Fewer dependencies lead to higher fault tolerance and increased flexibility.

For businesses that prioritize getting things done, asynchronous programming can be a fundamental building block in meeting project requirements.

If you’re still struggling to fully grasp how asynchronous operations work, think outside of the box.

Imagine a non-technical example of how asynchronous communication would look like in a real-world scenario.

As an illustration, envision you’re at a restaurant. Your waiter has just taken multiple orders at your table and brings out your appetizer as soon as it’s ready, instead of waiting for your main entree to be prepared as well.

Not only is this simply efficient service, but it’s an exemplary demonstration of a non-blocking I/O or asynchronous architecture.

Pros of Asynchronous Programming

There is a common misconception that asynchronous programming means greater performance and speed. In reality, executing several tasks at once does not have a direct impact in these areas.

But there are quite a few benefits to asynchronous programming, which you can glimpse below.

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User Experience

In comparing asynchronous and synchronous programming, asynchronous programs improve user experience in a variety of ways.

First, all the scripts of a program or web page can load at one time, resulting in better responsiveness and decreased page load delays.

And the failure of a thread has no consequence on the ability of other threads to run or render. Therefore, there is more availability overall with asynchronous applications.


In practice, asynchronous programming requires writing callback functions in line with errors that may occur and disrupt a program.

These callbacks instruct the machine to move past the error and run the next task. This also gives programmers the opportunity to write a personalized error message.

As a result, errors do not mean that your program altogether fails to run. Users get a nice note while all the other features of the program work just fine.


Async/await is a well-known syntax structure of many programming languages that enables asynchronous functions to work with promises comfortably.

Promises are objects that encapsulate the desired behavior of an asynchronous operation.

Both of these concepts are integral to the notion of scalability. Scalability can take place in two ways — horizontally and vertically.

Synchronous and asynchronous programs can leverage horizontal scaling by adding additional servers to scale services.

On the other hand, vertical scaling is a simpler process that asynchronous programs can take advantage of to permit one server to augment the number of requests a server can handle.

By plugging in an async function using the async/await keyword, developers can simplify programs that would otherwise be synchronous using promise-based callback methods. 

Cons of Asynchronous Programming

Asynchronous programming may seem like the obvious solution to any bottlenecks that may appear in your software development projects.

But there are reasons why developers avoid using asynchronous programming. Check them out.


To successfully program asynchronous operations, developers must have an in-depth knowledge of callbacks and recursive functions.

And even if they do, programming this into an application can be a cumbersome task during development.

The software development lifecycle as a whole will be more complex and noticeably more sluggish.

For the same reason, the code itself can get complicated. Writing clean code becomes a more difficult endeavor and testing and debugging is similarly strenuous.


Though page refreshing is less of an issue with asynchronous programming vs. synchronous programming, rendering a page initially can take some time.

In addition, too many asynchronous requests can overload your server and your program may function at a slower pace, in spite of the concurrency that you gain in return.


C+ and JavaScript are the most distinguished programming languages that authorize asynchronous programming. 

In these languages, the async keyword is widely used and revered. But other languages do not make matters so easy.

While it’s certainly possible to program asynchronous programs in almost any language, it will be a time-consuming task if such an implementation is not pre-equipped with the language in question.

What Is Synchronous Programming?

Synchronous programs use blocking I/O operations in which each operation has to run before the next is executed. 

Generally, computers are fast, so synchronous communication isn’t necessarily disadvantageous.

However, in a real-life, non-technical application, this would be the equivalent of your waiter refusing to bring out any of the dishes you ordered until each and every dish were fully prepared.

At the very least your food would be cold. At worst, your waiter just wasted a pretty significant amount of time for what seems like no good reason at all.

In programming, synchronous programs allocate a single thread to handle a request or complete a task.

Since synchronous operations happen one at a time, a painstakingly long task like a database query will block all other threads in the meantime.

While asynchronous operations can run multiple tasks concurrently on a single thread, synchronous programs have a task queue where every other task remains idle while the first is completed.

By definition, synchronous means ‘connected’ or ‘dependent’. In essence, synchronous communication is tightly coupled.

For reactive systems, when programs work best when they react to environmental input, synchronous programs can be useful.

Pros of Synchronous Programming

Believe it or not, there are bonafide reasons as to why businesses and developers would turn to synchronous executions in turn for asynchronous programming. Here they are.

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Synchronous programming is well-supported amongst all programming languages.

Developers do not have to worry about whether it is possible to code asynchronous applications as this is the default.

Plus, writing synchronous programs is objectively easier than writing asynchronous programs.

In utilizing a blocking I/O architecture you can save time in the software development process and run tests with fewer complications.

Marketing Potential

Search engines find it easier to crawl for web pages that make use of traditional synchronous architecture. 

For marketers who depend on search engine optimization (SEO) to build their reputation and brand awareness, this is an appreciable advantage.

The more people who see your website via Google or Bing, the more visitors you’ll have on your webpage. Naturally, this will positively affect your return on investment (ROI).

Cons of Synchronous Programming

The cons of synchronous programming should be pretty obvious. They are the main disadvantages pushing many developers towards asynchronous programming.


Loading times can be slower with synchronous programming vs. asynchronous programming. This is to be expected given how synchronous programs handle multiple requests.

When a thread is locked, the other threads in line get blocked as well. To put it simply, synchronous programming is like going to Disney World without a VIP pass.

Resource Intensity

Enabling synchronous programming also requires a significant amount of resources.

While multiple asynchronous executions can function on one thread, this is not the case with synchronous functions.

You will need more threads to handle more requests and more often than not, this proves to be overwhelming.

When To Use Asynchronous Programming

The biggest contribution that asynchronous programming provides is enhanced throughput.

Throughput refers to the deliverables a product or company can produce within a specified time. 

That said, synchronous programming is not the end-all-be-all of programming methodologies.

There are certain scenarios where asynchronous programming fits, and others where asynchronous programming does not fit.

In order to know which is which, you need to identify the dependencies and processes in your system.

Parallel computing accommodates user-centric programming well, but any other type of program is not worth the complexity.

Limit your use of asynchronous programming to event-based task handling where it’s helpful to release valuable resources earlier.

I/O operations and database queries are common use cases for asynchronous programming.

When To Use Synchronous Programming

As established, computers are fast. Thus, synchronous programming is not as time-consuming as you might imagine.

If you merely wish to develop a front-end application or execute a basic central process unit (CPU) function, then asynchronous programming is way out of bounds.

Video rendering or mathematical computations, for instance, wield the CPU for peak functionality.

Employing asynchronous programming for these types of tasks would oversaturate the CPU, and do more bad than good.

Overengineering describes the act of designing solutions to a problem in a complicated manner. 

It goes without saying that you should avoid overengineering. Shoot for synchronous programming when you can make do.

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Synchronous vs. Asynchronous Programming: Which Is Better?

Between asynchronous and synchronous programming, no programming method is inherently better.

Rather, the key takeaway is to assess your programming needs and choose the most optimal solution for your software demands.

To get a better understanding of the specific needs of your upcoming software project, consider consulting with a professional developer.

Trio is composed of highly qualified and experienced developers who make it their personal mission to help businesses succeed in a technical capacity. 

Talk to Trio now to assess all of your development needs!

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With over 10 years of experience in software outsourcing, Alex has assisted in building high-performance teams before co-founding Trio with his partner Daniel. Today he enjoys helping people hire the best software developers from Latin America and writing great content on how to do that!
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