Scala algorithm: Token Bucket Rate Limiter

Published

Algorithm goal

While similar to LeakyBucketRateLimiter, it works a bit differently, and is specifically useful as a network layer algorithm in order rate-limit packets:

Each packet consumes a number of tokens; tokens are added to the bucket at a particular rate, up to a limit and any overflow is discarded.If there are not enough tokens for a packet to consume, then the packet is discarded; otherwise it is passed on.

Design an algorithm to model this, in pure-functional immutable Scala.

Test cases in Scala

assert(
  sampleRefillRate.checkPossibleFills(java.time.Duration.ofSeconds(4)) ==
    None
)
assert(
  sampleRefillRate.checkPossibleFills(java.time.Duration.ofSeconds(5)) ==
    Some(5)
)
assert(
  sampleRefillRate.checkPossibleFills(java.time.Duration.ofSeconds(10)) ==
    Some(10)
)
assert(
  sampleRefillRate.checkPossibleFills(java.time.Duration.ofSeconds(11)) ==
    Some(10)
)
assert(
  sample.requestTokens(1).isEmpty,
  "Initially there is no capacity, so we cannot request anything"
)
assert(
  sample
    .maybeRefill(sampleStartTime.plusSeconds(4))
    .flatMap(_.requestTokens(1))
    .isEmpty,
  "After 4 seconds, we have not refilled, so we cannot request tokens"
)
assert(
  sample
    .maybeRefill(sampleStartTime.plusSeconds(5))
    .flatMap(_.requestTokens(1))
    .nonEmpty,
  "After 5 seconds, we have refilled, so we can request tokens"
)
assert(
  sample
    .maybeRefill(sampleStartTime.plusSeconds(5))
    .flatMap(_.requestTokens(6))
    .isEmpty,
  "After 5 seconds, we have only 5 tokens, so cannot request 6"
)
assert(
  sample
    .maybeRefill(sampleStartTime.plusSeconds(10))
    .flatMap(_.requestTokens(6))
    .nonEmpty,
  "After 10 seconds, we have 10 tokens, so can request 6"
)
assert(
  sample
    .maybeRefill(sampleStartTime.plusSeconds(10))
    .flatMap(_.requestTokens(6))
    .flatMap(_.requestTokens(4))
    .nonEmpty,
  "After 10 seconds, we have 10 tokens, so can request 6, and then 4"
)
assert(
  sample
    .maybeRefill(sampleStartTime.plusSeconds(10))
    .flatMap(_.requestTokens(6))
    .flatMap(_.requestTokens(4))
    .flatMap(_.requestTokens(1))
    .isEmpty,
  "After 10 seconds, we have 10 tokens, so can request 6, and then 4, but not 1 more"
)

Algorithm in Scala

41 lines of Scala (compatible versions 2.13 & 3.0).

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Explanation

This solution was developed through test-driven development (TDD).

The token bucket is fully described by a combination of the rate, capacity and the time there was a last refill. (this is © from www.scala-algorithms.com)

Using immutable programming, it is even possible to have the rate and capacities vary across time very easily.

Scala concepts & Hints

  1. Drop, Take, dropRight, takeRight

    Scala's `drop` and `take` methods typically remove or select `n` items from a collection.

    assert(List(1, 2, 3).drop(2) == List(3))

assert(List(1, 2, 3).take(2) == List(1, 2))

assert(List(1, 2, 3).dropRight(2) == List(1))

assert(List(1, 2, 3).takeRight(2) == List(2, 3))

assert((1 to 5).take(2) == (1 to 2))

  2. Option Type

    The 'Option' type is used to describe a computation that either has a result or does not. In Scala, you can 'chain' Option processing, combine with lists and other data structures. For example, you can also turn a pattern-match into a function that return an Option, and vice-versa!

    assert(Option(1).flatMap(x => Option(x + 2)) == Option(3))

assert(Option(1).flatMap(x => None) == None)

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