Synchronous vs Asynchronous API Calls: Which One Is Faster and why?

 

When building applications that interact with third-party APIs, we often have to decide whether to make synchronous (sync) or asynchronous (async) API calls. At first glance, it might seem that one approach will inherently be faster than the other. However, the real answer is a bit more nuanced.

In this post, we’ll explore how both approaches work, when to use them, and which one can give you better performance under different conditions.

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Understanding Sync and Async Calls

Synchronous API Call

A synchronous call is a blocking call. When you make a sync API call, your program waits until the third-party API responds before moving to the next line of code. This means your application thread is held up, doing nothing, while waiting for the external service to return data.

public string MakeSyncCallToApi()
{
    var client = new HttpClient();
    var response = client.GetStringAsync("https://thirdpartyapi.com/data").Result;
    return response;
}

In this example, the thread will stop and wait for the third-party API to respond. This is a blocking operation.

Asynchronous API Call

An asynchronous call, on the other hand, is non-blocking. When an async API call is made, your program doesn’t wait for the response. Instead, the thread is released to perform other tasks while the API call is processed in the background. When the response comes back, a callback or continuation is triggered to resume processing.

public async Task<string> MakeAsyncCallToApi()
{
    var client = new HttpClient();
    var response = await client.GetStringAsync("https://thirdpartyapi.com/data");
    return response;
}

Here, the thread is not held up waiting for the third-party API to respond. It can be used for other tasks until the API returns a result, at which point the code picks up where it left off.

Is Asynchronous Always Faster?

At first, it might seem like async calls should always be faster because they free up the thread, but that’s not entirely accurate. Let’s break down the differences.

1. Actual Response Time from the API

The actual time it takes for the third-party API to process the request and return a response is the same whether you make a synchronous or asynchronous call. This is because the third-party service operates independently of how your application is handling the request.

In other words, the third-party API doesn’t care if you’re waiting for the response or doing something else while it processes the request—it will still take the same amount of time to return the data.

2. Application-Level Performance

Where the difference comes in is how your application handles the wait time for that response. This is where async has a significant advantage, especially when dealing with I/O-bound operations like API calls.

Synchronous Call:

With a sync call, your application thread is blocked, meaning it can’t do anything else until the API responds. This might not seem like a big deal for a single call, but in a high-traffic application, this can cause major performance bottlenecks.

Let’s say your application has to make multiple API calls. With synchronous calls, each call blocks the thread, and if enough threads get blocked, your application could experience slowdowns or even exhaust its available threads, leading to failure.

Asynchronous Call:

With async calls, on the other hand, the thread is released back into the pool while waiting for the API’s response. This allows the system to handle other requests or operations while waiting. The result is better scalability and improved responsiveness, especially under high loads.

Even though the response time from the third-party API is the same, async calls can result in better performance overall because they make more efficient use of system resources. This is particularly true in web applications where multiple requests can be handled concurrently without each one waiting for the previous one to complete.

When Does Async Win?

Let’s look at a few scenarios where async clearly shines:

1. Multiple Concurrent API Calls

When your application needs to make multiple API calls (either to the same or different services), synchronous calls will cause each request to wait for the previous one to finish. This leads to long wait times, inefficient use of resources, and a poor user experience.

With async calls, your application can fire off all the API requests at once without blocking. This allows your system to perform other tasks while waiting for the API responses.

Example:

Let’s say you’re calling an external payment processing API multiple times to check the status of several transactions. Here’s how it would look:

Synchronous Version:

public List<string> CheckTransactionsSync(List<string> transactionIds)
{
    var client = new HttpClient();
    List<string> responses = new List<string>();

    foreach (var id in transactionIds)
    {
        var response = client.GetStringAsync($"https://api.paymentgateway.com/status/{id}").Result;
        responses.Add(response);
    }
    
    return responses;
}

Each transaction status call waits for the previous one to finish. This is not ideal when you have many transactions to check.

Asynchronous Version:

public async Task<List<string>> CheckTransactionsAsync(List<string> transactionIds)
{
    var client = new HttpClient();
    List<Task<string>> tasks = new List<Task<string>>();

    foreach (var id in transactionIds)
    {
        tasks.Add(client.GetStringAsync($"https://api.paymentgateway.com/status/{id}"));
    }

    return await Task.WhenAll(tasks);
}

Here, all the transaction status requests are sent off at the same time, and your app continues processing while waiting for the responses.

2. Resource Optimization

Even if you’re making only a single API call, async still has its advantages when it comes to resource optimization. By not blocking a thread during the wait, your application can handle other work, resulting in a more efficient use of system resources. This is particularly important in high-traffic web applications where the number of concurrent requests can be high.

When Sync Might Be Enough

In some cases, a synchronous call might be just fine:

1. Simple, Low-Traffic Applications: If your application only needs to make a few API calls occasionally and isn’t serving hundreds or thousands of concurrent users, synchronous calls are easier to implement and understand.

2. Quick API Responses: If the third-party API responds almost instantly, the benefits of async might be negligible for your specific case.

However, as your application grows and traffic increases, the limitations of sync calls become more apparent.

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Conclusion: Which is Faster?

• For a single API call, async and sync are roughly the same in terms of total time spent waiting for a response from the third-party API. The actual time taken by the third-party API to respond will not change whether you call it synchronously or asynchronously.

• In a high-concurrency environment, async calls will result in better performance because they free up threads to handle other tasks while waiting for the API response. This makes async calls more resource-efficient and allows your application to scale more effectively.

In short:

• Async is not necessarily “faster” in terms of response time, but it is more efficient and scalable, especially when handling multiple requests or high traffic.
• Synchronous calls are easier to work with but can quickly lead to bottlenecks in more complex applications.

When in doubt, use async to future-proof your application, improve scalability, and optimize performance under load.