Lesson 02 · The backend pool - Build a Load Balancer

The backend pool

A balancer needs a collection of backends to choose among. Today you build the pool - an ordered set you can add backends to, count, and look up by id.

The goal

Build a pool that holds backends, reports its size, and finds a backend by id.

Start here - the target

TO DO

Scenario: A pool holds and finds backends

Givena new empty pool, then Add(A) and Add(B) where A and B are backends with ids "A" and "B"

Whenthe pool is queried

ThenLen reports 2, Get("A") returns backend A, and Get("B") returns backend B

AndGet("Z") returns nil for an id that was never added

Background

The pool is the set of backends the balancer distributes requests over. Keep it as an ordered collection, not an unordered set: round-robin, weighted round-robin, and the deterministic tie-breaks you will build all depend on a stable order, so preserving insertion order matters. A slice is exactly right.

Two operations cover today: append a backend, and find one by its id. Get returning nil for an unknown id is the honest answer and keeps callers simple - they can check for nil rather than juggle a second “found” boolean. Everything the selection algorithms do starts from walking this ordered list.

Make it work

type Pool struct { backends []*Backend }
func NewPool() *Pool { return &Pool{} }
func (p *Pool) Add(b *Backend) { p.backends = append(p.backends, b) }
func (p *Pool) Len() int { return len(p.backends) }
func (p *Pool) Get(id string) *Backend {
  for _, b := range p.backends {
    if b.ID == id { return b }
  }
  return nil
}

CheckpointDONE

You have a pool that stores backends in order and looks them up by id. Commit and stop here.