Sequential vs parallel: when order actually matters

Not every Promise.all is a win.

Sequential is for work where step B depends on step A’s result, or where shared mutable state must be updated in order. Example: adjusting a balance then recording a ledger line — if you parallelize naively you race yourself.

Parallel is for independent units: generating chapters that do not touch each other, fan-out HTTP calls to different services, fetching static assets.

I used to reach for parallel first because it felt “modern.” Now I ask: if these finish out of order, is anything wrong? If yes, sequential (or explicit locking / transactions).

When parallel is safe, it is often a big speedup. When it is not, it is a subtle bug factory.