Package dev.relism.flash.ext.limiter.strategy

Class SlidingWindowStrategy

java.lang.Object

dev.relism.flash.ext.limiter.strategy.SlidingWindowStrategy

All Implemented Interfaces:

RateLimitStrategy

public final class SlidingWindowStrategy
extends Object
implements RateLimitStrategy

Sliding-window counter rate limit: approximates a true sliding window by interpolating between the previous fixed window's count and the current window's count.

   estimate = prevCount × (1 − elapsed / windowMs) + currentCount
 

This is the same approximation used by Redis. It eliminates the boundary burst problem of FixedWindowStrategy while staying O(1) memory and lock-free. The error is bounded: in the worst case the true rate at the boundary can exceed the limit by at most limit × (1 − elapsed/windowMs) — typically a few percent.

Slot layout

• Bucket.slot0 — packed (reducedEpoch << 32 | currentCount)
• Bucket.slot1 — count from the immediately preceding epoch (0 = none)

On a window transition the thread that wins the slot0 CAS also writes slot1. A concurrent thread that reads slot0 after the transition but before slot1 is written sees a slightly stale previous count — acceptable for an approximation algorithm.

Constructor Summary

Constructors

Constructor	Description
SlidingWindowStrategy()

Method Summary

Modifier and Type	Method	Description
boolean	check(Bucket bucket, LimitConfig cfg, long[] out)	Checks whether this request is within the limit and updates the bucket atomically.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

SlidingWindowStrategy

public SlidingWindowStrategy()

Method Details

check

public boolean check(Bucket bucket,
 LimitConfig cfg,
 long[] out)

Description copied from interface: RateLimitStrategy

Checks whether this request is within the limit and updates the bucket atomically.

On return, out contains:

• out[0] — remaining allowed requests in the current window (≥ 0).
• out[1] — Unix epoch seconds at which the quota resets (for X-RateLimit-Reset and Retry-After headers).

Specified by:

check in interface RateLimitStrategy

Parameters:

bucket - per-key state carrier (pre-allocated, never null)

cfg - immutable rule configuration

out - caller-supplied two-element array; values are overwritten on every call

Returns:

true if the request is within the limit and should proceed