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sync: add Semaphore example for limiting the number of outgoing reque…
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…sts (#6419)
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czzrr authored Mar 22, 2024
1 parent f9d78fb commit baad270
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53 changes: 53 additions & 0 deletions tokio/src/sync/semaphore.rs
Original file line number Diff line number Diff line change
Expand Up @@ -76,6 +76,59 @@ use std::sync::Arc;
/// }
/// ```
///
/// ## Limit the number of outgoing requests being sent at the same time
///
/// In some scenarios, it might be required to limit the number of outgoing
/// requests being sent in parallel. This could be due to limits of a consumed
/// API or the network resources of the system the application is running on.
///
/// This example uses an `Arc<Semaphore>` with 10 permits. Each task spawned is
/// given a reference to the semaphore by cloning the `Arc<Semaphore>`. Before
/// a task sends a request, it must acquire a permit from the semaphore by
/// calling [`Semaphore::acquire`]. This ensures that at most 10 requests are
/// sent in parallel at any given time. After a task has sent a request, it
/// drops the permit to allow other tasks to send requests.
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::Semaphore;
///
/// #[tokio::main]
/// async fn main() {
/// // Define maximum number of parallel requests.
/// let semaphore = Arc::new(Semaphore::new(10));
/// // Spawn many tasks that will send requests.
/// let mut jhs = Vec::new();
/// for task_id in 0..100 {
/// let semaphore = semaphore.clone();
/// let jh = tokio::spawn(async move {
/// // Acquire permit before sending request.
/// let _permit = semaphore.acquire().await.unwrap();
/// // Send the request.
/// let response = send_request(task_id).await;
/// // Drop the permit after the request has been sent.
/// drop(_permit);
/// // Handle response.
/// // ...
///
/// response
/// });
/// jhs.push(jh);
/// }
/// // Collect responses from tasks.
/// let mut responses = Vec::new();
/// for jh in jhs {
/// let response = jh.await.unwrap();
/// responses.push(response);
/// }
/// // Process responses.
/// // ...
/// }
/// # async fn send_request(task_id: usize) {
/// # // Send request.
/// # }
/// ```
///
/// ## Limit the number of incoming requests being handled at the same time
///
/// Similar to limiting the number of simultaneously opened files, network handles
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