db.get() doesn't read from a snapshot
#796
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vweevers
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Sep 25, 2022
At the moment, this is a documentation-only PR, acting as an RFC. In particular I'd like review of my choice to use a token-based approach (as first suggested by Rod Vagg in Level/community#45) as opposed to a dedicated snapshot API surface (as suggested by Julian Gruber in Level/community#47). Main reasons for not choosing the latter: - It would be a third API surface. We have the main database API, the sublevel API which is equal to that except for implementation- specific additional methods, and would now add another API which only has read methods (get, iterator, etc, but not put and batch). If the API was reusable, meaning you could pass around a snapshot as if it was a regular database (like you can with sublevels) then I'd be cool with it. But for that to happen, we'd have to implement transactions as well, which I consider to be out of scope although transactions are in fact a use case of snapshots. - It would have a higher complexity once you factor in sublevels. I.e. to make `db.sublevel().snapshot().get(key)` read from the snapshot but also prefix the given key. By instead doing `db.sublevel().get(key, { snapshot })`, the sublevel can just forward that snapshot option to its parent database. - Furthermore, with the token approach, you can pass a snapshot to multiple sublevels, which cleanly solves the main use case of retrieving data from an index (I included an example in the docs). I renamed the existing snapshot mechanism to "implicit snapshots" and attempted to clarify the behavior of those as well. If accepted, some related issues can be closed, because this PR: - Includes (a more complete write-up than) Level/classic-level#40. - Mentions Level/leveldown#796 (which should be moved rather than closed). - Answers Level/classic-level#28. - Solves the main use case as described in Level/leveldown#486. - Effectively completes Level/awesome#19.
This was referenced Sep 25, 2022
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Tracked in Level/community#118. It won't be fixed in |
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We don't set the LevelDB snapshot option for
database_->Get(), so rather than reading from a snapshot created at the time thatdb.get()was called, it will read from an implicit snapshot (i.e. latest state) at the time the async worker runs:leveldown/binding.cc
Line 1123 in bd33f6f
For comparison, see
db.getMany()where we do set the snapshot option:leveldown/binding.cc
Line 1173 in bd33f6f
This may have been the case for a long time and I've personally not found this to be a problem in real-world apps. Putting this in the backlog until there are upvotes.
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