Concurrency Limiter
The Concurrency Limiter middleware controls the number of concurrent requests that can be processed simultaneously. For detailed API information, please check the documentation.
Use Cases
The concurrency limiter is particularly useful in the following scenarios:
- High-traffic website protection: Limiting the number of requests processed simultaneously during traffic surges to prevent server overload
- Resource-intensive operation control: Restricting concurrent resource-intensive operations such as file uploads and database writes
- Third-party API call limitation: Controlling concurrent requests when calling external services with rate limits
Key Benefits
- Server overload prevention: Maintaining server stability during peak traffic by limiting concurrent requests
- Improved system reliability: Avoiding system crashes caused by resource exhaustion
- Optimized resource allocation: Ensuring critical services have sufficient resources to process requests
Usage
Simply add the max_concurrency(n) middleware to your routes, where n is the maximum number of concurrent requests allowed. Requests exceeding this limit will receive a 503 Service Unavailable response.
Example Code
concurrency-limiter/src/main.rs
use std::fs::create_dir_all;
use std::path::Path;
use salvo::prelude::*;
// Handler for serving the index page with upload forms
#[handler]
async fn index(res: &mut Response) {
res.render(Text::Html(INDEX_HTML));
}
// Handler for processing file uploads with a simulated delay
#[handler]
async fn upload(req: &mut Request, res: &mut Response) {
// Extract file from the multipart form data
let file = req.file("file").await;
// Simulate a long-running operation (10 seconds)
tokio::time::sleep(tokio::time::Duration::from_secs(10)).await;
if let Some(file) = file {
// Generate destination path for the uploaded file
let dest = format!("temp/{}", file.name().unwrap_or("file"));
tracing::debug!(dest = %dest, "upload file");
// Copy file to destination
if let Err(e) = std::fs::copy(file.path(), Path::new(&dest)) {
res.status_code(StatusCode::INTERNAL_SERVER_ERROR);
res.render(Text::Plain(format!("file not found in request: {e}")));
} else {
res.render(Text::Plain(format!("File uploaded to {dest}")));
}
} else {
res.status_code(StatusCode::BAD_REQUEST);
res.render(Text::Plain("file not found in request"));
}
}
#[tokio::main]
async fn main() {
// Initialize logging system
tracing_subscriber::fmt().init();
// Create temporary directory for file uploads
create_dir_all("temp").unwrap();
// Configure router with two upload endpoints:
// - /limited: Only allows one concurrent upload (with concurrency limiter)
// - /unlimit: Allows unlimited concurrent uploads
let router = Router::new()
.get(index)
.push(
Router::new()
.hoop(max_concurrency(1)) // Limit concurrent requests to 1
.path("limited")
.post(upload),
)
.push(Router::with_path("unlimit").post(upload));
// Bind server to port 5800 and start serving
let acceptor = TcpListener::new("0.0.0.0:5800").bind().await;
Server::new(acceptor).serve(router).await;
}
// HTML template for the upload forms page
static INDEX_HTML: &str = r#"<!DOCTYPE html>
<html>
<head>
<title>Upload file</title>
</head>
<body>
<h1>Upload file</h1>
<form action="/unlimit" method="post" enctype="multipart/form-data">
<h3>Unlimit</h3>
<input type="file" name="file" />
<input type="submit" value="upload" />
</form>
<form action="/limited" method="post" enctype="multipart/form-data">
<h3>Limited</h3>
<input type="file" name="file" />
<input type="submit" value="upload" />
</form>
</body>
</html>
"#;
