Struct ratelimit_meter::algorithms::gcra::GCRA

pub struct GCRA { /* fields omitted */ }

Implements the virtual scheduling description of the Generic Cell Rate Algorithm, attributed to ITU-T in recommendation I.371 Traffic control and congestion control in B-ISDN; from Wikipedia.

While algorithms like leaky-bucket rate limiters allow cells to be distributed across time in any way, GCRA is a rate-limiting and traffic-shaping algorithm. It mandates that a minimum amount of time passes between cells being measured. For example, if your API mandates that only 20 requests can be made per second, GCRA will ensure that each request is at least 50ms apart from the previous request. This makes GCRA suitable for shaping traffic in networking and telecom equipment (it was initially made for asynchronous transfer mode networks), or for outgoing workloads on consumers of attention, e.g. distributing outgoing emails across a day.

In a blatant side-stepping of the above traffic-shaping criteria, this implementation of GCRA comes with an extension that allows measuring multiple cells at once, assuming that if a pause of n*(the minimum time between cells) has passed, we can allow a single big batch of n cells through. This assumption may not be correct for your application, but if you depend on GCRA's traffic-shaping properties, it's better to not use the _n suffixed check functions.

Example

In this example, we construct a rate-limiter with the GCR algorithm that can accommodate 20 cells per second. This translates to the GCRA parameters τ=1s, T=50ms (that's 1s / 20 cells).

let mut limiter = GCRA::for_capacity(20).unwrap().cell_weight(1).unwrap().build();
let now = Instant::now();
let ms = Duration::from_millis(1);
assert_eq!(Decision::Yes, limiter.check_at(now).unwrap()); // the first cell is free
for i in 0..20 {
    // Spam a lot:
    assert_eq!(Decision::Yes, limiter.check_at(now).unwrap(), "at {}", i);
}
// We have exceeded the bucket capacity:
assert!(!limiter.check_at(now).unwrap().is_compliant());

// After a sufficient time period, cells are allowed again:
assert_eq!(Decision::Yes, limiter.check_at(now + ms*50).unwrap());

Methods

impl GCRA

fn for_capacity(capacity: u32) -> Result<Builder>

Constructs a builder object for a GCRA rate-limiter with the given capacity per second, at cell weight=1. See Builder for options.

fn with_parameters(t: Duration, tau: Duration, tat: Option<Instant>) -> GCRA

Constructs a GCRA rate-limiter with the parameters T (the minimum amount of time that single cells are spaced apart), tau (τ, the number of cells that fit into this buffer), and an optional t_at (the earliest instant that the rate-limiter would accept another cell).

Trait Implementations

impl Decider for GCRA

fn check(&mut self) -> Result<Decision<Self::T>>

Tests if a single cell can be accommodated at Instant::now(). See check_at.

fn check_at(&mut self, at: Instant) -> Result<Decision<Self::T>>

Tests is a single cell can be accommodated at the given time stamp.

impl MultiDecider for GCRA

This crate's GCRA implementation also allows checking multiple cells at once, assuming that (counter the traffic-shaping properties of GCRA) if a sufficiently long pause (n*t) has occurred between cells, the algorithm can accommodate n cells.

As this assumption does not necessarily hold in all circumstances, users of this trait on GCRA limiters should ensure that this is ok.

fn check_n_at(&mut self, n: u32, at: Instant) -> Result<Decision<Self::T>>

Tests if n cells can be accommodated at the given time stamp. An error ErrorKind::InsufficientCapacity is returned if n exceeds the bucket capacity.

fn check_n(&mut self, n: u32) -> Result<Decision<Self::T>>

Tests if n cells can be accommodated at the current time (Instant::now()). An error ErrorKind::InsufficientCapacity is returned if n exceeds the bucket capacity.

impl TypedDecider for GCRA

type T = Instant

In GCRA, negative decisions come with the time at which the next cell was expected to arrive; client code of GCRA can use this to decide what to do with the non-conforming cell.

impl Debug for GCRA

fn fmt(&self, __arg_0: &mut Formatter) -> Result

Formats the value using the given formatter.

impl Clone for GCRA

fn clone(&self) -> GCRA

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl From<Builder> for GCRA

Allows converting from a GCRA builder directly into a GCRA decider. Same as the borrowed implementation, except for owned Builders.

Example:

use ratelimit_meter::{GCRA, Decider, Decision};
let mut gcra: GCRA = GCRA::for_capacity(50).unwrap().into();
assert_eq!(Decision::Yes, gcra.check().unwrap());

fn from(b: Builder) -> Self

Performs the conversion.

impl<'a> From<&'a mut Builder> for GCRA

Allows converting a GCRA builder directly into a GCRA decider.

Example:

use ratelimit_meter::{GCRA, Decider, Decision};
let mut gcra: GCRA = GCRA::for_capacity(50).unwrap().cell_weight(2).unwrap().into();
assert_eq!(Decision::Yes, gcra.check().unwrap());

fn from(b: &'a mut Builder) -> Self

Performs the conversion.

impl<'a> Into<(Duration, Duration, Option<Instant>)> for &'a GCRA

Allows converting a GCRA into a tuple containing its T (the minimum amount of time that single cells are spaced apart), tau (τ, the number of cells that fit into this buffer), and an optional t_at (the earliest instant that the rate-limiter would accept another cell).

These parameters can be used with .with_parameters to persist and construct a copy of the GCRA rate-limiter.

fn into(self) -> (Duration, Duration, Option<Instant>)

Performs the conversion.

impl From<(Duration, Duration, Option<Instant>)> for GCRA

Allows converting the parameters returned from Into<(Duration, Duration, Option<Instant>)> back into a GCRA.

fn from(params: (Duration, Duration, Option<Instant>)) -> GCRA

Performs the conversion.