An online schema change tool that works by adding triggers, on a table an outbox relay polls.
the tool: copies the table, keeps the copy current
with INSERT/UPDATE/DELETE triggers, then swaps.
our table: has an outbox relay polling it every 200ms,
and a trigger on the original that writes to a second
table.
two triggers on one table is permitted and the
interaction is not something I wanted to reason about
during a four-hour operation on production.
rejected in favour of expand-and-contract, which is
four releases and no triggers.The tool is well understood and the interaction with an existing trigger is not, which is a judgement about my own confidence rather than about the tool. Four releases over six weeks is the more expensive option and it is composed entirely of operations we do every week.