Lesson 13 · The balancer and a connection lease

The balancer and a connection lease

Now the pieces come together into a Balancer that owns a pool and a selector. Its core move is a lease - pick a backend and mark it busy, handing back a release you call when the work is done.

The goal

Build a Balancer whose Begin selects a backend, raises its active count, and returns a release that lowers it exactly once.

Start here - the target

TO DO

Scenario: Begin leases a backend and release frees it

Givena Balancer over a round-robin selector for pool A, B, C all up

WhenBegin is called, then Begin is called again

Thenthe first returns A with A.Active() now 1, the second returns B with B.Active() now 1

Andcalling A's release once drops A.Active() to 0, and calling that same release a second time leaves it at 0 (release is idempotent)

Background

The Balancer is the object a caller actually holds. It owns the pool and a chosen Selector, and its job is to turn “give me a backend for this request” into a backend plus the bookkeeping that keeps the active counts honest. The clean way to express that is a lease: Begin selects a backend, raises its active count so least-connections and friends see it as busy, and returns a release closure. The caller runs the request, then calls release to lower the count.

The one correctness rule is that release must decrement exactly once, no matter how many times it is called. A double release would push the active count below the real number of in-flight requests and corrupt every load-aware decision. Guard it with a done flag captured in the closure. This lease shape is what lets the next lessons run a request through a transport and, crucially, hold several requests in flight at once to prove least-connections reacts to live load.

Make it work

type Balancer struct { pool *Pool; sel Selector }
func (bal *Balancer) Begin() (*Backend, func(), error) {
  b, err := bal.sel.Select()
  if err != nil { return nil, nil, err }
  b.Incr()
  done := false
  release := func() { if !done { done = true; b.Decr() } } // decrement once
  return b, release, nil
}

CheckpointDONE

The Balancer leases a backend, tracking it as busy until released. Commit and stop here.