Snapshot range scan

[drmingdrmer]

Hi Spacejam,
I am a developer from https://github.com/datafuselabs/databend and we has been using sled to store meta-data of the databend-DB cluster. As an embedded high performance kv store, sled fits our needs very well.
Except that in some cases, we need to retrieve a list of key-value pairs, such as a bunch of users.

It would be very nice If sled is able to provide a way to create snapshot view for an application to repeatedly scan it and get the same response.

Is it possible to add such a feature to sled?
And we'd be glad to help on this:DDD

Use Case:

Meta data of a database cluster.
It stores database, table, schema.

Proposed Change:

Range scan on a snapshot view. E.g.:

let s = db.snapshot();

// Expect a snapshot always returns the same result no matter if other key-values are added or removed.
let prev_res = s.range(..);
db.insert(b"a", b"b");
db.remove(b"c", b"d");
assert_eq!(s.range(..), prev_res);

Who Benefits From The Change(s)?

The https://github.com/datafuselabs/databend team and some other guys :D

Alternative Approaches

Add a RWLock to contain the sled db, to prevent writes during db.range(..).

[tv42]

Related, it would be great to be able to atomically read a snapshot of the database, and start a subscription that sees changes from that snapshot onward.