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Synchronous API vs asynchronous API: What is the difference?

Synchronous API vs asynchronous API: What is the difference?

Erin Lansdown

Dec 18, 2023

Discover the key differences between synchronous and asynchronous APIs, and learn which suits your application’s needs for optimal performance and efficiency.

Application programming interfaces (APIs) are crucial for seamless software communication. With Nylas’ state of developer experience report revealing that 86% of developers anticipate increased API usage in the coming years, understanding API functionality is more relevant than ever. Today, we focus on a key aspect: the distinction between synchronous and asynchronous APIs.

To contextualize the difference, let’s compare each API type to a different communication method. Synchronous APIs are like two-way radios, demanding immediate responses and operating on a request-reply basis. This immediacy is a significant advantage in many applications, from financial transactions to user authentication, where delays can be costly or disruptive. Conversely, asynchronous APIs resemble a voicemail, providing an initial ‘status’ update to confirm receipt of a request and allowing users to proceed with other tasks. Later, a complete ‘response’ is given, efficiently managing application resources without demanding immediate attention.

Grasping the nuances between synchronous and asynchronous APIs is critical for building an effective digital strategy or deciding which API provider to use for your application. This blog post explores these differences in depth, providing insights into the best contexts and applications for each API type and helping you make informed decisions for your business needs.

What is a synchronous API?

Let’s take a closer look at the official definition of a synchronous API. A synchronous API is an interface that processes requests sequentially, requiring each operation to be completed before the next one can begin. In this model, the client initiating the request must wait for the server to respond before proceeding. Scenarios requiring immediate feedback or real-time data processing often utilize synchronous APIs.

Pros of synchronous APIs

Synchronous APIs are advantageous, particularly for immediate response and real-time interaction. For example, an airline may want to use a synchronous API for its booking system. When a user selects a flight, the API immediately checks the availability for those specific dates and times, ensuring the user receives up-to-date information on availability and price. When they decide to book the flight, the synchronous API processes the transaction, confirming the booking instantly, preventing double bookings, and ensuring a smooth user experience.

Additional advantages of a synchronous API include:

Immediate feedback: They operate on a real-time request-and-reply basis, which is beneficial when making quick decisions(e.g., financial transactions, real-time data validation, or user authentication processes).
Simple design: With a linear execution model, synchronous APIs are generally easier to implement and debug than their asynchronous counterparts.
Predictability: Each request is processed and completed before moving on to the next, providing a clear and consistent flow of operations.
Easy to understand: The request-wait-response pattern closely mimics everyday human interactions, making the conceptual model familiar and easier to grasp.
Direct data management: Data management can be more straightforward since the response is received immediately after the request. This can simplify data handling, particularly in apps that require direct and immediate processing of input/output operations.

Limitations of synchronous APIs

While they offer many pros, synchronous APIs also have limitations that might impact their suitability. Some of these challenges include:

Scalability: Synchronous APIs may face scalability issues due to their sequential handling of requests. However, this can be mitigated by optimizing server resources and implementing efficient load-balancing strategies.
Increased server load: When opting for synchronous APIs, it’s important to use modern server technologies and architectures that are designed to handle numerous simultaneous connections efficiently. Also, leveraging caching mechanisms and optimizing API endpoints can reduce the load significantly.
Risk of blocking operations: A synchronous API waiting for a response can block other operations, especially in single-threaded environments. That’s why it’s important to use non-blocking I/O operations or multi-threading techniques to ensure the application remains responsive and efficient.
Dependency on external systems: If synchronous APIs depend on external systems or services, timeout mechanisms and fallback strategies must be in place to mitigate this risk.

What is an asynchronous API?

An asynchronous API is an interface that lets the client request and proceed with other tasks, not waiting for an immediate response. The server works on the request in the background and responds after completion. Ideal when immediate feedback isn’t crucial, asynchronous APIs are good to use in background data processing, batch operations, or services requiring extended response times.

Pros of asynchronous APIs

Asynchronous APIs offer several advantages, especially in complex or resource-intensive environments:

Scalability: These APIs can handle multiple requests simultaneously, making them well-suited for high-traffic systems. They do not require each request to be completed before starting the next one, leading to improved resource utilization.
Efficiency: By allowing other processes to continue while waiting for a response, asynchronous APIs can improve the overall efficiency of an application. This is especially beneficial in applications that perform extensive data processing or need to interact with slow external services.
Non-blocking operations: Asynchronous APIs reduce the risk of blocking operations, as the system doesn’t need to wait idly for a response.
Flexibility in long processes: These APIs are ideal for tasks that require significant processing time, as they avoid the issues of timeouts and unresponsiveness associated with long-running synchronous operations.

Limitations of asynchronous APIs

However, asynchronous APIs also come with their own challenges:

Complex implementation: They can be more complex to implement and understand, particularly in handling the callbacks or events associated with the completion of tasks.
Difficulty debugging: Tracing and debugging asynchronous operations can be challenging due to their non-linear nature and potential for concurrent processes.
Dependency on callbacks or event handling: These APIs rely heavily on callbacks or event handling mechanisms, which can complicate the code structure and flow, especially in large-scale applications.

Synchronous API vs asynchronous API: Top differences

When comparing synchronous and asynchronous APIs, several key differences stand out. Here’s a chart to give a quick summary:

	Synchronous API	Asynchronous API
Response time and interaction models	Operate in real-time on a request-and-reply basis, ideal for immediate feedback tasks like user authentication.	Handle requests in the background, suitable for long-running operations where immediate response isn’t necessary.
Resource utilization and scalability	Less efficient in resource usage, a potential limit in high-traffic scenarios.	Enhance scalability by allowing simultaneous processes, ideal for extensive background processing.
Implementation and maintenance	Easier to implement and maintain.	More complex due to callbacks and event loops, beneficial for advanced applications requiring non-blocking operations and scalability.

Choosing between synchronous and asynchronous APIs

Deciding whether to use a synchronous or asynchronous API in your application hinges on understanding the specific needs of your project and the nature of the tasks at hand.

When to opt for synchronous APIs:

You need real-time interaction
You need a straightforward, easy-to-follow flow in your application
Direct and immediate information processing is crucial, such as transaction processing

When asynchronous APIs are the better choice:

You’re dealing with extensive data processing or sending bulk communications
You anticipate high traffic or need to handle numerous operations simultaneously
Your operations take significant time, like processing large datasets

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