What are the differences between Reqwest's blocking and non-blocking clients?

Reqwest is a popular HTTP client for Rust. It offers both synchronous (blocking) and asynchronous (non-blocking) APIs for making HTTP requests. The choice between using the blocking and non-blocking clients typically depends on the context of your application and your performance requirements. Here's an overview of the differences between the two:

Blocking Client

The blocking client is synchronous, which means that when you make an HTTP request, your thread of execution is blocked until the server responds. This is similar to how traditional web requests work in many other programming languages. The blocking client is easier to use and understand, especially for those who are new to Rust or do not require asynchronous features.

Here's a simple example of using Reqwest's blocking client to make a GET request:

use reqwest;

fn main() -> Result<(), reqwest::Error> {
    let response = reqwest::blocking::get("https://httpbin.org/get")?
        .text()?;
    println!("{}", response);
    Ok(())
}

The blocking client is suitable for:

• CLI applications where simplicity and ease of use are more important than performance.
• Small scripts where concurrency is not a concern.
• Applications where you're already running in a multi-threaded environment and you can afford to block threads.

Non-Blocking (Async) Client

The non-blocking client is asynchronous and uses Rust's async/await features. When you make an HTTP request using the non-blocking client, the execution is not blocked. Instead, the future representing the HTTP request is polled, and the task is suspended until the response is ready. This allows you to handle many requests concurrently without creating new threads for each request, which can lead to better performance and resource utilization on I/O-bound tasks.

Here's a simple example of using Reqwest's non-blocking client to make a GET request:

use reqwest;
use tokio;

#[tokio::main]
async fn main() -> Result<(), reqwest::Error> {
    let response = reqwest::get("https://httpbin.org/get").await?
        .text().await?;
    println!("{}", response);
    Ok(())
}

The non-blocking client is suitable for:

• Web servers and other I/O-bound applications where concurrency is crucial.
• Applications that require handling a large number of simultaneous connections without using many threads.
• Situations where you want to perform other tasks while waiting for an HTTP response.

Summary of Differences

• Concurrency: The non-blocking client allows for concurrent request handling, while the blocking client handles requests sequentially.
• Performance: For I/O-bound applications, the non-blocking client can provide better performance and resource utilization.
• Complexity: The blocking client is simpler to use, while the non-blocking client requires understanding of Rust's asynchronous programming model.
• Runtime: The non-blocking client requires an asynchronous runtime like Tokio or async-std, while the blocking client does not.

In summary, choose the blocking client if your application's simplicity and ease of development are at the forefront. Opt for the non-blocking client if you're building a performance-sensitive application where concurrency and efficient resource usage are key factors.