Struct ratelimit_meter::algorithms::gcra::GCRA
source · pub struct GCRA { /* private fields */ }
Expand description
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.
A note about batch decisions
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 = DirectRateLimiter::<GCRA>::per_second(nonzero!(20u32));
let now = Instant::now();
let ms = Duration::from_millis(1);
assert_eq!(Ok(()), limiter.check_at(now)); // the first cell is free
for i in 0..20 {
// Spam a lot:
assert!(limiter.check_at(now).is_ok(), "at {}", i);
}
// We have exceeded the bucket capacity:
assert!(limiter.check_at(now).is_err());
// After a sufficient time period, cells are allowed again:
assert_eq!(Ok(()), limiter.check_at(now + ms*50));
Trait Implementations
sourceimpl Algorithm for GCRA
impl Algorithm for GCRA
sourcefn test_and_update(
&self,
state: &Self::BucketState,
t0: Instant
) -> Result<(), Self::NegativeDecision>
fn test_and_update(
&self,
state: &Self::BucketState,
t0: Instant
) -> Result<(), Self::NegativeDecision>
Tests if a single cell can be accommodated by the rate-limiter. This is a threadsafe implementation of the method described directly in the GCRA algorithm.
sourcefn test_n_and_update(
&self,
state: &Self::BucketState,
n: u32,
t0: Instant
) -> Result<(), NegativeMultiDecision<Self::NegativeDecision>>
fn test_n_and_update(
&self,
state: &Self::BucketState,
n: u32,
t0: Instant
) -> Result<(), NegativeMultiDecision<Self::NegativeDecision>>
Tests if n
cells can be accommodated by the rate-limiter
and updates rate limiter state iff they can be.
As this method is an extension of GCRA (using multiplication), it is likely not as fast (and not as obviously “right”) as the single-cell variant.
type BucketState = State
type BucketState = State
type NegativeDecision = NotUntil
type NegativeDecision = NotUntil
NonConformance
, to ease
handling of how long to wait. Read moresourcefn construct(
capacity: NonZeroU32,
cell_weight: NonZeroU32,
per_time_unit: Duration
) -> Result<Self, InconsistentCapacity>
fn construct(
capacity: NonZeroU32,
cell_weight: NonZeroU32,
per_time_unit: Duration
) -> Result<Self, InconsistentCapacity>
capacity
is the number of cells to allow, weighing
cell_weight
, every per_time_unit
. Read more