Lesson 03 · Removing a backend - Build a Load Balancer

Removing a backend

Pools change - a server is decommissioned and must leave the rotation for good. Today you remove a backend by id while preserving the order of the rest.

The goal

Remove a backend from the pool by id, leaving the remaining backends in their original order.

Start here - the target

TO DO

Scenario: A backend can be removed by id

Givena pool holding backends A, B, C in that order

WhenRemove("B") is called

ThenLen reports 2, Get("B") returns nil, and the remaining order is A then C

AndRemove("Z") for an id not in the pool leaves the pool unchanged with Len still 2

Background

Removing a backend is how a server permanently leaves the pool - decommissioned, scaled down, or replaced. This is different from marking it down: a down backend is still in the pool and can come back, while a removed one is gone. Later you will also learn to drain a backend, which is different again. Keep the three ideas separate; today is only the permanent removal.

The one thing to get right is order preservation. Round-robin and weighted selection walk the pool in order, so removing the middle backend must leave the others in their original relative order, not swap the last one into the gap. A filter that copies survivors in place does exactly that, and removing an id that is not present simply keeps every backend.

Make it work

func (p *Pool) Remove(id string) {
  out := p.backends[:0]           // reuse the backing array
  for _, b := range p.backends {
    if b.ID != id { out = append(out, b) }
  }
  p.backends = out
}
// Order is preserved because you append survivors in their original order.

CheckpointDONE

You can remove a backend by id without disturbing the order of the others. Commit and stop here.