Backend & systems·Testing and test doubles
you write a working in-memory version of the repository, with real behaviour, just not one backed by a database.
Fake
A double with a real, working implementation that is unsuitable for production — an in-memory store, an embedded database, a local queue. It lets tests exercise genuine behaviour without the real dependency's cost. The risk is divergence: a fake that behaves differently from the real thing gives you green tests and a broken system.