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Rail signals

A rail can return items and still not mean what it looks like it means, and a rail can return nothing for two very different reasons. Member Preview marks both cases so you read each rail correctly instead of trusting the tiles at face value.

Cold start · fallback​

A personalized rail is built to rank items against a member's own history. When the member has no interaction history feeding the engine, there is nothing to personalize from, so the engine falls back to a generic result — popular items, a default shelf — and serves that instead.

Those fallback tiles look exactly like personalized ones, which is the trap. So a personalized rail serving fallback carries a Cold start · fallback badge. The badge tells you: this member is cold, and what you are seeing is the fallback, not a personalized ranking. Without it, a product manager could mistake a generic shelf for evidence that personalization is working for a brand-new member.

The badge appears only where both things are true — the rail is personalized and the member is cold. A rail that ranks by popularity for everyone is never personalized, so it never carries the badge; its generic result is simply what it always does.

Unavailable​

A rail is unavailable when its placement is not serving at all — the placement errored, or is not deployed on this project's engine. The rail shows a distinct unavailable state and names the reason, so a broken placement never reads as an ordinary result.

This is deliberately different from a rail that ran fine and simply returned no items:

  • Unavailable — the placement did not run. Something is wrong with the placement itself, and the state says what. This is a signal to go fix the placement.
  • Empty — the placement ran correctly and returned nothing for this member. Nothing is broken; this member just has no items for this shelf right now.

Keeping the two apart matters because they call for opposite responses. An empty rail is a fact about one member; an unavailable rail is a fact about the placement, and it needs attention no matter which member you are viewing.