Synchronous vs Asynchronous Programming: What It Is, Key Differences, & Which to Use

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Rafael Timbó
Rafael Timbó
Chief Technology Officer
Synchronous vs Asynchronous Programming: What It Is, Key Differences, & Which to Use

Table of Contents

Learn about synchronous and asynchronous programming models and which to choose for your team's next development project.
Published on
October 12, 2023
Updated on
October 12, 2023

In the ever-increasing complexity and interconnectedness of programming and software development, there are just two basic programming models — asynchronous and synchronous— to structure code. These models significantly impact the speed, performance, and efficiency of code execution.

Don’t let the “simplicity” of a binary choice fool you. Your development team must understand the subtle and not-so-subtle difference between an asynchronous vs. synchronous programming model to make the right choice for your next software development project.

This guide will explain the differences between asynchronous and synchronous programming, provide examples, and discuss which type is better for your project.

Synchronous vs. Asynchronous Programming

Say you're writing an application that needs to pull data from two sources. On the one hand, you have a server that continually generates data. On the other hand, you have an application programming interface (API) that supplies data from a third party. If you write the application using an asynchronous programming model, your code could move on to the following tasks while waiting for the server and API to supply data — and when received, they can be processed simultaneously without causing delays. If you use a synchronous programming model, your code would have to wait for the server and the API to provide data before it could continue. 

This is the key difference between asynchronous and synchronous programming — asynchronous programming allows components of a program to run independently, while synchronous programming requires components to run in sequence. The asynchronous programming model takes advantage of parallelism and distributed computing, while the synchronous programming model uses predictable, linear execution.

To help you understand asynchronous vs. synchronous programming better, here is a summary of the key differences:

Synchronous Programming

  • Synchronous programming code executes in a linear, predictable fashion.
  • Code execution must wait for an external process before it can continue.
  • All tasks are executed in a specific order.
  • Synchronous programming is not ideal for distributed computing.

Asynchronous Programming

  • Asynchronous programming code can execute tasks in parallel or independently of one another.
  • Code can continue executing without waiting for an external process.
  • Tasks execute in any order.
  • Asynchronous programming is ideal for distributed computing.


Now that you understand the difference between asynchronous and synchronous programming let's look at how you can use each model in different scenarios.

Asynchronous vs. Synchronous and Javascript

Javascript is a popular scripting language used to make websites interactive. Javascript is synchronous by default, meaning it runs each block of code in sequence. This allows code snippets to execute linearly and predictably, making it great for applications requiring more control.

Asynchronous programming is also possible in Javascript using asynchronous functions like setTimeout(), promises, and iteration methods like forEach(). These asynchronous features and the event loops in the architectural design allow Javascript to take advantage of parallelism and distributed computing without compromising the flow of code execution. A single thread can execute multiple asynchronous operations, making it great for distributed computing and web development. Javascript also supports asynchronous communication, meaning it can send and receive data from external sources without waiting for a response or blocking the main thread. 

If you want to execute two asynchronous tasks, you can use promises to ensure that the second task only runs after completing the first one. This asynchronous pattern allows Javascript to take full advantage of asynchronous programming without sacrificing the linear structure of synchronous code.

Asynchronous Function vs. Synchronous Function 

The ‘async’ function is a unique function that allows the asynchronous code to be written in a cleaner, more structured format. It makes asynchronous programming easier, and it does so by enabling the developer to write asynchronous code without explicitly configuring promise chains.

Sync functions execute all work on a single thread in a linear, predictable fashion. Although these functions can interact with other threads, the function stays on a single thread and executes its work sequentially. This makes synchronous functions great for applications that require precise, responsive control and predictability.

While asynchronous functions are great for distributed computing and communication, synchronous functions are great for applications requiring more control and precision.

Some uses of asynchronous functions include:

  • Web development
  • Distributed computing
  • Asynchronous communication, such as messaging

Some uses of synchronous functions include:

  • Data processing systems such as banking databases
  • Game development, where timing is important
  • Real-time applications, such as chat applications
  • Embedded systems
  • Event-driven programming

What Is an Asynchronous Operation?

An asynchronous operation allows multiple independent tasks to execute concurrently without blocking or delay in code execution. In short, an asynchronous operation is an independent, asynchronous task that can run in the background while other tasks are processing.

The Basics

When writing code, asynchronous operations allow you to run multiple independent tasks simultaneously, thus making the code more efficient and improving performance. It allows code to continue executing without waiting for an external process or operation to complete. 

The Pros

Advantages of using an asynchronous operation include:

  • Improved performance: Multiple tasks can process at once, thus improving the overall speed and efficiency of code execution.
  • Better scalability: Developers can easily scale up the system and add more tasks without worrying about blocking or delaying code executions.
  • Better error handling: Detecting and handling errors is easier, as the code is divided into multiple independent tasks that developers can debug individually.
  • Faster development: Developers can quickly add new features and tasks without worrying about code structure or performance.
  • Improved user experience: Asynchronous programs, such as web pages, respond faster by running multiple tasks simultaneously in the background and reducing wait times.

The Cons

Disadvantages of using an asynchronous operation include:

  • Complexity: Asynchronous programming can be more complex than synchronous code, requiring more intricate configuration and a better understanding of parallelism.
  • Higher memory usage: Operations can take up more memory as multiple tasks run in the background.
  • Increased risk of errors: Due to the complexity of asynchronous operations, errors and bugs increase as the code executes in different threads.

What Is a Synchronous Operation?

Synchronous operations require synchronous tasks to execute in sequence. This means the application must process the task queue from start to finish, and a task cannot run until the previous task is complete.

The Basics

When writing code, synchronous operations ensure that task executions are predictable and linear. Synchronous operations in programming require code to be written in a single task queue, with each task relying on the previous task to run before it can start.

The Pros

Advantages of using synchronous operations include:

  • Predictability: Synchronous operations allow you to control task executions, as each task must be executed in order before the next task can start.
  • Reliability: Synchronous operations are reliable, as the tasks will always execute in the same order, allowing for predictable and repeatable results.
  • Simpler debugging: Synchronous operations make it easier to debug code, as each task is completed linearly, and developers can identify errors quickly.
  • Less overhead: Synchronous operations require less overhead, as there is no need for complex configurations or asynchronous patterns.
  • Low memory usage: Synchronous operations use less memory, as only one task runs simultaneously.

The Cons

Advantages of using synchronous operations include:

  • Lower performance: Synchronous operations take more time to execute because tasks are completed in sequence and must wait for the previous task to finish. While this is acceptable for most applications, it can lead to delays in applications where speed is essential.
  • Cannot handle interrupts: Synchronous operations have difficulty handling interrupts, as they must wait for the current task to be completed before continuing.
  • Harder to scale: Synchronous operations can be more challenging to scale because code must be modified or rewritten each time a new task is added.

When to Use Synchronous or Asynchronous Programming

Now that you understand the differences between asynchronous vs. synchronous programming, you might be wondering when to use which model. That will depend on your project. Some instances where you may need to use asynchronous programming include:

  • Loading and downloading data: Using asynchronous programming, you can load or download information while an application executes — you can continue to use the application while data is being processed.
  • Running longer programs: If you need to run a program that will take longer than usual, asynchronous programming can help — it allows other tasks to execute while the longer task runs.
  • Visualizing the action: Asynchronous programming is good for creating applications with visuals. The application can load data while the user interacts with the application without delay.

On the other hand, some instances where you can consider using synchronous programming include:

  • Data processing: Synchronous programming processes data more sequentially, which is especially useful when dealing with large amounts of data.
  • Executing commands: If you need to execute instructions in order, synchronous programming is the best way.
  • Reacting to events: Synchronous programming is best for reacting to events — events and responses happen promptly and in the correct order.

While there are no hard and fast rules, some general guidelines exist to decide when asynchronous or synchronous programming is the right choice.

How Many Iterations Your Development Project Has

Iterations, or loops, are the number of processes a program must complete before it is done. Asynchronous programming might be the better choice if your project has many iterations and is constantly changing. Asynchronous programming allows tasks to run independently, making it great for applications requiring frequent feature updates. If your project has fewer iterations and is more predictable, synchronous programming might be better.

The Type of App You’re Creating

The type of application will help you choose the best programming model. For example, asynchronous programming might be the best choice for a web-based application where users must wait for server responses. Asynchronous programming makes it more efficient to handle these requests without blocking the main user interface thread. Furthermore, asynchronous programming is a better choice if your app heavily relies on asynchronous operations such as file downloads, web requests, and user input. 

Synchronous programming might be the way to go if you're developing a game. Games require precise timing and control, which makes synchronous operations a better choice, as they guarantee linear, predictable code execution.

How Many Operations You Need to Run Simultaneously

The number of operations you must run simultaneously is important in deciding between asynchronous vs. synchronous programming. Asynchronous programming might be more suitable if your operations are CPU intensive and you must run most of them simultaneously.

Synchronous programming might be better if your operations are more straightforward and can run sequentially. This approach allows you to simplify the code and make it easier to maintain.

Find Developers Who Sync With Your Operational Needs

Choosing between asynchronous vs. synchronous programming boils down to understanding your development project's needs. Not only do asynchronous and synchronous operations have different uses, but they also need different configurations and levels of understanding. Analyze each model’s pros and cons and decide which is best for your project. Ultimately, choosing the right model ensures that your code runs efficiently and your project succeeds.

Not sure which programming model your software development project needs? You can hire experienced developers to help you choose the right model. At Revelo, our talent marketplace has a pool of vetted remote software engineers from Latin America specializing in asynchronous and synchronous programming.

Contact Revelo today to get the best tech talent for your project.

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