Lesson 09 · Random selection - Build a Load Balancer

Random selection

Random selection spreads load with no per-request memory - just pick a healthy backend at random. To keep it exactly testable, the randomness is injected, so today you build a selector driven by a source you control.

The goal

Select a healthy backend using an injected index source, so the sequence is deterministic under test.

Start here - the target

TO DO

Scenario: Random selection follows the injected source

Givena random selector over A, B, C all up, driven by an index source that returns 2, 0, 1, 2 on successive calls

WhenSelect is called four times

Thenit returns C, A, B, C - each pick is Available()[source(len)]

Andthe source is always called with the current number of available backends, so it only ever returns a valid index

Background

Random selection is appealing because it needs no shared cursor - handy when many workers balance independently and you do not want them all marching in lockstep. The catch for a spec-first project is that “random” is not a value you can assert. The fix is dependency injection: the selector takes an IndexSource, a function that returns an index into the available backends. In production it wraps a real random-number generator; in a test you hand it a fixed script and the output becomes exactly predictable.

This injection pattern is worth internalizing because the next algorithm, power-of-two-choices, uses the very same source to sample two backends. Always call the source with len(Available()) so it returns a valid index into the current healthy set, never a position that might be down or out of range.

Make it work

// inject randomness so the test is deterministic and the spec is language-neutral
type IndexSource func(n int) int   // returns an index in [0, n)
type Random struct { pool *Pool; src IndexSource }
func (r *Random) Select() (*Backend, error) {
  avail := r.pool.Available()
  return avail[r.src(len(avail))], nil
}
// in production src wraps a real RNG; in tests it replays a fixed script

CheckpointDONE

Random selection picks a healthy backend through an injected, testable source. Commit and stop here.