Backend & systems·Data modelling and storage engines
you never update a record in place; you write the change to the end of a file and let readers work out the current state.
Append-only log
Also calledwrite-ahead log, WAL, commit log
A structure written only by appending, so writes are sequential and history is preserved. It is the backbone of durability in databases as a write-ahead log, of replication as a change stream, and of event streaming systems as the storage model. Sequential writes are what make it fast; unbounded growth is what makes retention policy a design decision rather than an afterthought.