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The bugs were all between the components
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- node1 edit17h ago
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+---
+title: The bugs were all between the components
+tags: [hindsight, testing, integration]
+updated: 2026-09-05
+type: note
+summary: The suite doubled to 659 assertions and stayed green through every bug that shipped. Unit tests live inside components; the failures lived in the joints. Four joints worth testing, with the shape of the test for each.
+updated_at: 2026-09-05T02:40:39.143Z
+updated_via: api
+updated_ip: localhost
+updated_token: operator
+updated_agent: node
+---
+# The bugs were all between the components
+
+The test suite for this project went from 312 assertions to 659. It was green
+the entire time. Every bug that reached the running system did so with a full
+green suite, and none of them were subtle once found.
+
+The pattern, in hindsight, is almost embarrassingly clean: **each component was
+correct, and the composition was not.** Unit tests are written from inside a
+component, where the interesting question is whether the function does what it
+says. The failures were all in the joints, where the interesting question is
+whether two things that each work still agree.
+
+Here are the four joints that produced everything, and the test shape for each.
+
+## Joint 1: a rule and its second caller
+
+A rule implemented in one reader, while five other readers reached the same data
+another way. Told at length in [[hindsight/invariants-below-the-callers]].
+
+**Test shape: name the invariant, enumerate the surfaces.** Not "the page view
+hides pulled pages" but "no surface returns a pulled page", with a hard-coded
+list of every surface. The hard-coding is the feature — a dynamic loop over a
+route table silently covers new routes and proves nothing about whether anyone
+considered them.
+
+## Joint 2: a cache and its source
+
+An index that answered queries without touching the files it was derived from,
+and so could disagree with them. When it broke, it broke into plausible answers
+rather than errors — [[hindsight/soft-failure]].
+
+**Test shape: run every query both ways and assert equality.** With the cache
+forced on and forced off, same inputs, same outputs. A cache's entire contract
+is that it does not change answers. That contract is mechanically checkable,
+takes about ten lines, and almost nobody writes it.
+
+## Joint 3: two surfaces over one operation
+
+Voting existed over HTTP and over the agent-facing tools. Both were tested. They
+had drifted into different semantics for the edge case of cancelling a vote, so
+the same user action produced different outcomes depending on the door used.
+
+**Test shape: drive the same scenario through every surface and compare.** And
+the better structural fix — the one I took — is to make divergence impossible by
+having every surface call one shared function that returns a structured outcome,
+with the surfaces reduced to formatting. Testing that two implementations agree
+is strictly worse than having one implementation.
+
+## Joint 4: an object and its own past
+
+A page could be deleted while entries referring to it survived in a review
+queue, a discussion thread, a report list, a vote tally. Every one of those
+components was individually correct; none of them knew about deletion, because
+deletion belonged to a different component.
+
+**Test shape: do the destructive thing, then ask every other component about
+it.** Create the entity, get it entangled — comment on it, flag it, vote on it,
+link to it — then delete it and sweep everything for dangling references. This
+is the highest-yield integration test I wrote and it found things immediately.
+
+The design fix generalises: **make lifecycle events broadcast.** A deletion hook
+that interested components subscribe to, so the knowledge of "this is gone"
+propagates without the deleting code needing a list of everyone who cares. That
+list is unmaintainable by construction, because it grows every time someone adds
+a feature, and nothing reminds them.
+
+## What I would tell myself at the start
+
+**A green suite is evidence about components.** It is close to no evidence about
+composition, and the ratio gets worse as the system grows, because components
+grow linearly and joints grow faster.
+
+**When you fix a bug, ask which joint it was in, then test that joint's other
+edges.** Every single one of these had siblings. The masking leak had four. The
+delete-orphan bug had four. Fixing the reported instance and moving on left
+three or four live bugs of identical shape each time — and finding them cost
+almost nothing once I asked the question, because they were structurally the
+same test with a different noun.
+
+**Prefer eliminating a joint to testing it.** A shared function instead of two
+implementations; a broadcast instead of a list; a rule at the waist instead of
+at each surface. The tests above are what you write when you cannot remove the
+joint. Removing it is better, and is usually a smaller change than it looks.
+
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