Lesson 08 · Weighted round-robin - Build a Load Balancer

Weighted round-robin

Backends are not equal - a bigger box should get more traffic. Weighted round-robin gives each backend a share proportional to its weight. Today you build it by expanding weights into a repeating schedule.

The goal

Distribute selection in proportion to weight, cycling A,A,A,B for weights 3 and 1.

Start here - the target

TO DO

Scenario: Weighted round-robin cycles in proportion to weight

Givena weighted round-robin selector over backend A with weight 3 and backend B with weight 1, both up

WhenSelect is called five times

Thenit returns A, A, A, B, A - one full cycle of A,A,A,B and then the start of the next

Andacross each full cycle of length 4, A appears exactly 3 times and B exactly once

Background

Weighted round-robin lets an operator send more traffic to a more capable backend. The simplest correct way to do it is schedule expansion: repeat each backend in a list as many times as its weight, then run ordinary round-robin over that expanded list. Weights 3 and 1 expand to [A, A, A, B], so four consecutive picks give three As and one B, and the fifth pick wraps to A again.

The order within the cycle here is the blunt grouped one - all of A, then B - which is easy to reason about and exact to test. Production balancers often use a smooth weighted round-robin instead, which interleaves the picks (A, A, B, A) so the same 3-to-1 ratio arrives more evenly spread over time; that is a natural extension noted in the caveats. Build the schedule from Available() so a down backend contributes no slots, and rebuild it when weights or membership change.

Make it work

// build a schedule by repeating each backend weight times, in pool order:
// A(3), B(1) -> [A, A, A, B]; then round-robin over that schedule
type WeightedRR struct { pool *Pool; sched []*Backend; n int }
func (w *WeightedRR) build() {
  w.sched = nil
  for _, b := range w.pool.Available() {
    for i := 0; i < b.Weight(); i++ { w.sched = append(w.sched, b) }
  }
}
// Select: build once (or when the pool changes), then sched[n % len]; n++

CheckpointDONE

Weighted round-robin gives each backend a share of traffic equal to its weight. Commit and stop here.