#futures #rate-limit #semaphore #concurrency #rust


This library provides easy tools to help Rust applications guide critical resources or code paths from being overwhelmed

6 releases

✓ Uses Rust 2018 edition

0.1.5 Dec 16, 2019
0.1.4 Dec 12, 2019

#116 in Asynchronous

47 downloads per month

MIT license

550 lines


futures-rate on crates.io futures-rate on docs.rs

What is this library

This library provides easy tools to help Rust applications guide critical resources or code paths from being overwhelmed.

Depending on the configuration, the library will limit the amount of guarded futures being polled concurrently.

How to use


First of all, add the dependency to your Rust project:

$ cargo install futures-rate

Or in your project's Cargo.toml, add the following dependency:

futures-rate = "^0.1.0"

then run $ cargo install in your terminal from project's root directory.

Limit access rate

  • Create and manage a GateKeeper object in your main thread, which will set the access limit to a certain resource:
use futures_rate::GateKeeper;

/// then in main thread
fn main() {
    // ... other code

    // At most 10 futures can pass the gate at any given time  
    let gatekeeper = GateKeeper::new(10);

    // ... more code
  • Then register your future to the GateKeeper such that the resourceful future can be protected:
use futures_rate::{GateKeeper, Permit};

/// in the business logic which has access to the `gatekeeper` object
async fn work(gatekeeper: &GateKeeper) -> usize {
    // create the IO-heavy future
    let ioFut = async { 
        // do async work here 

    let permit = gatekeeper.register(async {
        // At most 10 IO work can be on-the-fly at any given time
        // the result of all questions is always 42

Naive Future Lock

If setting the


A classic use case is to place a GateKeeper over an a client socket pool, such that only a limited number of future visitors can be allowed to poll the resources and hence limit the amount of open connections.

use futures::{executor, future};
use futures_rate::{GateKeeper, Permit};
use std::future::Future;
use std::thread;
use std::time::Duration;

fn main() {
    let gatekeeper = GateKeeper::new(1);
    let fut_values = async {
        let fut_1 = build_fut(0, &gatekeeper);
        let fut_2 = build_fut(1, &gatekeeper);
        let fin = future::join(fut_1, fut_2);

    let values = executor::block_on(fut_values);

    println!("Values from fut_1={:?}", values.0);
    println!("Values from fut_2={:?}", values.1);

fn build_fut(
    offset: i32,
    gatekeeper: &GateKeeper,
) -> Permit<Vec<i32>, impl Future<Output = Vec<i32>>> {
    gatekeeper.register(async move {
        let mut values = Vec::with_capacity(100);
        (0..100).for_each(|v| {
            values.push(2 * v + offset);