The signal to split

A story is the unit of work the Operator hands the Author, and INVEST (A2.03) is the gate read before it is handed over. Of the six letters, two name a story that is too big. Small asks whether the story fits one rung — one turn of the loop, small enough to specify, build, and demo in a single pass. Estimable asks whether its cost is a guess anyone can make. A No on either is not a defect in the wording; it is a statement about size, and the repair for size is a split.

Reading the signal early is the whole point. A story over the one-rung budget cannot be specified once, so it cannot be accepted once: handed to the Author as written, it produces a sprawl no one can demonstrate against a single criterion. Splitting it first is cheaper than unwinding a rung later. The trigger is read off the gate, not guessed: when Small or Estimable answers No, the story is split before any code is generated.

The hero scores the signal; this view shows its cause. A too-big story is almost always one verb — “manage”, “handle”, “support” — standing in for several distinct behaviours. Each hidden behaviour is a story in its own right, and the count of them is the count of slices the split will produce. That is why the story is unestimable: its cost is the sum of several guesses, and a sum of guesses is still a guess.

Reveal the behaviours bundled inside one Portal story, one row at a time, and watch the count cross the one-rung budget. The moment more than one behaviour is in the story, Small fails — and the number of rows is the number of rungs the split yields. The failing letter does not only flag the problem; it sizes the solution.

On the Portal — the learning platform this course builds — “manage the whole catalogue” fails Small and Estimable. Read the verb: “manage” hides four behaviours — browse, add, edit, and remove a course. The double failure (S and E) and the four hidden behaviours are the same fact seen twice: the story is unestimable precisely because its cost is the sum of four guesses. Contrast “enrol in a course”, which carries one behaviour, passes both letters, and ships as one rung.

The signal is never “the identifiers are missing.” The ids the rungs rest on are already minted — Portal.ID.generate/1 hands back a typed id, and Portal.ID.decode/1 reads its .type back — so nothing blocks a small story from shipping. What blocks a too-big one is its size, read off Small and Estimable before a line is generated. The how of the cut is the next dive; the signal is here.

# Read INVEST before handing the story to the Author.
# "enrol in a course"           → Small: pass · Estimable: pass → 1 rung, hand over
# "manage the whole catalogue"  → Small: FAIL · Estimable: FAIL → the signal to split

# "manage" hides four behaviours — the failing letter sizes the split:
#   browse → add → edit → remove a course      (split into 4)

id = Portal.ID.generate(:course)        # ids are already minted — size, not identity, is the blocker
assert Portal.ID.decode(id).type == :course

A split is a repair triggered by a failing letter: a No on Small or Estimable says the story is too big for one rung. The two move together because size is their common cause, and that size shows up as a bundle of behaviours hidden behind one verb. The count of those behaviours is the count of rungs the split will yield, so the failing letter both flags the problem and sizes the solution. On the Portal, “manage the whole catalogue” fails both letters and hides four behaviours; “enrol” passes and ships. With the signal read, the next dive makes the cut.

The sources behind the too-big signal and the readiness gate it sits in, and where it connects in the course.