Lesson 05 · The healthy view of the pool

The healthy view of the pool

Selection must only ever consider backends that can take work. Today you build Available - the filtered, order-preserving view of the pool that every selection algorithm will choose from.

The goal

Return the backends that are up, in pool order, skipping any that are down.

Start here - the target

TO DO

Scenario: Available lists only the up backends

Givena pool holding backends A, B, C all up

WhenAvailable is queried

Thenit returns [A, B, C] in that order

Andafter B is marked down, Available returns [A, C] with the order preserved

Background

Every selection algorithm in the next chapter answers the same question - “which backend gets this request?” - and none of them should ever answer with a backend that is down. Rather than teach each algorithm to check health itself, build the filter once: Available returns the subset of the pool that is currently up, keeping the original order so round-robin and weighted selection stay deterministic.

This one method is the seam between health and selection. Later, when a backend is Draining as well as Down, Available is the single place that decides it no longer takes new work - so a draining backend disappears from selection for free, without touching any algorithm. Selecting from an empty Available (every backend down) is the all-down case you will handle explicitly near the end.

Make it work

func (p *Pool) Available() []*Backend {
  var out []*Backend
  for _, b := range p.backends {
    if b.IsUp() { out = append(out, b) }
  }
  return out
}

CheckpointDONE

The pool can hand back just its healthy backends, in order. Commit and stop here.