Crate ratelimit_meter

Leaky Bucket Rate-Limiting (as a meter) in Rust

This crate implements the generic cell rate algorithm (GCRA) for rate-limiting and scheduling in Rust.

Interface

There is currently one rate limiter implementation in this crate, the Generic Cell Rate Algorithm. Use it by creating a builder from the GCRA struct:

use std::time::Duration;
use ratelimit_meter::{Decider, GCRA, Decision};

let mut lim = GCRA::for_capacity(50).unwrap() // Allow 50 units of work
    .per(Duration::from_secs(1)) // We calculate per-second (this is the default).
    .cell_weight(1).unwrap() // Each cell is one unit of work "heavy".
    .build(); // Construct a non-threadsafe GCRA decider.
assert_eq!(Decision::Yes, lim.check().unwrap());

The rate-limiter interface is intentionally geared towards only providing callers with the information they need to make decisions about what to do with each cell. Whenever possible, additional information about why a cell should be denied - the GCRA implementation will return a time::Instant alongside the decision to allow callers to e.g. provide better error messages to users.

Due to this, the ratelimit_meter crate does not provide any facility to wait until a cell would be allowed - if you require this, you should use the Instant returned with negative decisions and wait in your own, e.g. event loop.

Design and implementation of GCRA

Unlike token bucket algorithms, the GCRA one assumes that all units of work are of the same "weight", and so allows some optimizations which result in much more consise and fast code (it does not even use multiplication or division in the "hot" path).

See the documentation of the GCRA type for more details on its implementation and on trade-offs that apply to it.

Thread-safe operation

The default GCRA implementation can not be used across threads. However, there is a wrapper struct Threadsafe, that wraps the hot path in an atomically reference-counted mutex. It still manages to be pretty fast (see the benchmarks above), but the lock comes with an overhead even in single-threaded operation.

Example:

use std::time::Duration;
use ratelimit_meter::{Decider, GCRA, Decision};

let mut lim = GCRA::for_capacity(50).unwrap() // Allow 50 units of work
    .per(Duration::from_secs(1)) // We calculate per-second (this is the default).
    .cell_weight(1).unwrap() // Each cell is one unit of work "heavy".
    .build_sync(); // Construct a threadsafe GCRA decider.
assert_eq!(Decision::Yes, lim.check().unwrap());

Reexports

pub use errors::*;

pub use self::algorithms::*;

Modules

algorithms
errors
example_algorithms

Enums

Decision

A decision on a single cell from the metered rate-limiter.

Traits

Decider	The main decision trait. It allows checking a single cell against the rate-limiter, either at the current time instant, or at a given instant in time, both destructively.
MultiDecider
TypedDecider	A prerequisite for implementing any Decider trait. It provides the associated type for Decision's additional information for negative decisions.