Schema drift as a slow leak, not a single bang

Crews remember the night a job died with a hard error. They forget the week a source renamed cust_id and the warehouse kept loading, minus a join that nobody watched because row counts still matched.

That is drift as a leak. The feed is “green” in the only sense anyone checked. The handoff downstream starts dropping keys into a pile that grows slowly enough to look like weather.

On a health review we look for shape changes inside the observation window: new columns, missing columns, dates that became text, codes that gained a prefix. We mark the leak on the handoff, not only on the source, because the source may have announced the change in a letter nobody in the night room received.

If your firm ships source changes on Thursdays, tell us. We will put those Thursdays on the lag sheet even if run times look ordinary. Drift is not dramatic. It is how a loyal extract becomes a stranger without ever failing loudly.