Long gone are the days when computers could only process one thread of data at a speed that left much to be desired. Today, even an average smartphone has more computing power than PCs used to send astronauts to the moon. With modern devices, we can watch videos on YouTube, while one app makes complex computations in the background, and the other monitors multiple email accounts.

Multi-threading has enabled wide possibilities and, among other things, brought to life two essential development models. Asynchronous and synchronous programming are two terms that may sound confusing, but understanding their differences helps to spend effort more efficiently and build applications providing outstanding user experience. Today, we’ll learn the major pros and cons of both approaches.

The More Threads, The Better. The Basics of Asynchronous Programming

Let’s deal with asynchronous programming first. When a development team uses this approach, they write a code that tells an application not to wait until a specific operation ends before starting executing the next one. If an application performs many functions simultaneously (and most of them work this way) it doesn’t line them up in a queue. Instead, it receives multiple parallel requests and processes them immediately. In this case, the app relies on a non-blocking input and output protocol. If a specific request fails, the application doesn’t stop working and simply proceeds to the next one.

When you need to fill a complex online form, for example, asynchronous programming helps to check specific fields independently without blocking other requests or the entire application. It may be pretty helpful to check if a password is complex enough, the email address has a valid format, and a username is not taken all at once even before clicking the submit button.

Asynchronous programming has some obvious benefits. For example:

  • Good user experience is guaranteed by simultaneously loaded scripts and lack of lags in the web application work. End-users don’t have to reload the page every time they need new data to appear on the screen. Of course, it largely depends on the skills of the development team. For experienced programmers who have a couple of complex projects behind their backs, writing some issues-free asynchronous code won’t be an unbearable task;
  • Increased responsibility can be achieved thanks to parallel execution of multiple functions. Say if a request to the online database takes too long for some reason, the application built using the asynchronous programming model won’t freeze, and all other functions will continue to work;
  • Better scalability in the case of asynchronous programming can be achieved by increasing the number of concurrent requests as long as a server can handle them. This, besides adding new servers, enables relatively easy horizontal and vertical scalability;
  • More customizable approach to error handling in case of asynchronous programming is caused by the nature of callback functions. When development teams write the code, they take care of possible errors that may or may not occur while the program is running. The application, in case of such errors, will handle them according to the instructions and move to the next function. From the user standpoint, a personalized error message will be the only reminder that something went wrong. The web page won’t freeze and the end-user’s day won’t be ruined.

Unfortunately, asynchronous programming has some disadvantages all development teams have to face. Most likely you have already guessed what the first item on the list of disadvantages will be:

  • Asynchronous code is more complex to write and maintain. If you deal with multiple concurrently running functions, be prepared to face some challenges;
  • You may face decreased performance if you don’t have enough experience in asynchronous applications development. You can get carried away by the possibilities that multi-thread approach provinces and overload the app with too many requests, most of which are not necessary for the app to work properly;
  • Not all languages are equally suitable for asynchronous programming. There won’t be any problems if you choose JavaScript, but if you prefer to use something more exotic for your web development projects, you better ensure that your language provides all the possibilities you may need.

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Dealing With Tasks One at a Time. Synchronous Programming in a Nutshell

Now, it’ll be pretty easy to understand what synchronous programming is. Just remember what we said about the asynchronous approach and imagine its opposite. Here, applications use blocking operations, which means no subsequent functions will be called until the previous ones complete their work. At a first sight, it may seem that such software will work as slow as a sloth. However, modern computers and smartphones are pretty fast, and therefore the synchronous model is not necessarily a disadvantage.

Synchronous programming has some features one should remember. There are some cases where, for example, an online database is not reachable and a function whose purpose is to load some data from it will block the execution of all subsequent tasks. In this scenario, all functions line up and are processed by a single thread. It may sound like a synchronous model is something you should never use, but nonetheless, there are applications that must process the user’s actions to proceed. Without a new portion of data, such software can’t move forward. Reacting to environmental inputs is their primary driver and synchronous programming suits such tasks pretty well.

Among the advantages of the synchronous programming model, we can mention the following ones:

  • Simplicity is probably the most important thing about this methodology. Making things this way is pretty intuitive. Any beginner in web development will understand the basic principles of synchronous programming. You call a function, get a result, and pass it to another function until the goal is reached;
  • More straightforward SEO strategy will be especially important for small development teams. Web pages built following the synchronous programming have a less complex structure compared to those that work asynchronously, which simplifies the job for search engines;
  • Good choice for apps using simple requests.

The weak sides of synchronous programming you should remember are:

  • Lower speed of processing requests may become a pretty serious problem if you decide to build something complex. Such applications will load in no time, but tasks requiring large numbers of database requests will become a real headache. They’ll block the execution of all other functions, which won’t make end-users happy;
  • The app may become demanding on resources if the number of requests rises over time.

Choosing the Right Model

The choice seems pretty obvious. If you want the tasks to start independently, choose asynchronous programming. If you want them to run one after another, synchronous programming is what you need. In most cases, you will not be limited in the choice of programming languages. Major server-side technologies such as Python, Java, or PHP support asynchronous calls, so you can decide whether you want to implement a multi-thread or single-thread model. Node.js is a pretty popular server-side framework that allows using the JavaScript programming language and also has no issues with an asynchronous approach. For the client-side of your software product, you’ll most probably use JavaScript. Luckily, it also has no problems with the asynchronous code. Thus, in terms of technologies, there are no boundaries and you can implement any model you want following the purpose of your software system.

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Because of the features of asynchronous programming, it’ll suit those projects where the development team has to implement multiple independent tasks. For instance, an ecommerce system with responsive UI design that must react to the users’ actions instantly, offer products that are often ordered under the interests of the buyer, change the appearance when users pull up their orders, etc. Also, if a software system works with tons of data records, it’ll take too long to wait until all database requests are completed one by one. Asynchronous programming can run them in parallel, which will save a lot of time.

If you plan to build a software system that uses all the computing capabilities that a processor offers, synchronous programming will suit you better. For example, video rendering is a pretty complex task that will load the CPU to the maximum in a blink of an eye. Processing a video stream with a single-thread won’t overload the processor and will enable lag-free experience for the end-users.


Choosing between synchronous and asynchronous programming models is not a matter of the habit, style, or preference of the development team members. When you build a UI for your web project, you can prefer to use React or Vue.js for plenty of reasons and most probably end-users won’t notice any difference. If you follow the asynchronous programming approach without considering the purpose of the system you’re going to build, you risk facing the consequences of so-called overengineering. On the other hand, if you don’t have enough web development experience to write asynchronous code, your system risks being stuck in case a certain request cannot be fulfilled.

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