[slifin]

If your database is mutable then you have a third party network resource, essentially you can't test it because you have no stable basis in time and no means of dealing in values

All you have is a shared place, mocking is essentially putting up a fence around it to prevent testing into that boundary

If your database is immutable and is indexed for time travel, you can rewind or fast-forward your database into your desired state, if your database supports speculative writes you can even build up a non-committable state

I'm sure Rich Hickey will have a talk on this in relation to databases