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0 pointsderefr5y ago0 comments

You misinterpreted, I think. The point isn’t “snapshot isolation” in the MVCC sense (working with multiple snapshots-in-progress); it’s the ability to, in est, work with the database the way git works with commits: opening a transaction “on top of” a base commit, then “committing” that transaction to create a new commit object, with its own explicit ref, where you can later “check out” an arbitrary ref.

Except, unlike git, this database wouldn’t need to be able to create new commits off of anywhere but the HEAD; and also wouldn’t need to be able to have more than one in-progress write transaction open at a time. No need for MVCC at all; and no need for a DAG. The “refs” would always just be a dense linear sequence.

Also, unlike git (or a cryptographically-verified / append-only store), there’s no need to keep around “deleted” snapshots. It would actually be a huge benefit to be able to purge arbitrary snapshots from the database, without needing to do a mark-and-sweep liveness pass to write out a new copy of the database store.

The key constraint that differentiates this from e.g. a Kafka Streams-based CQRS/ES aggregate, is that you should be able to reopen and work with any historical database version instantly, with equal-amortized-cost lookups from any version, without needing to first do O(N) work to “replay” from the beginning of time to the snapshot, or to “rewind” from the HEAD to the snapshot. This system would need all snapshots to be equally “hot” / “online” for query access, not just the newest one.

In other words, such a database should work just like filesystem snapshots do in a copy-on-write filesystem.

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ec1096855y ago

It seems like coupling a database checkpoint process with file system’s snapshot process should be theoretically possible: 1) Database informed snapshot needed 2) Database finalizes any in progress writes and starts logging new writes to another file 3) Take file system snapshot 4) Inform database snapshot is fine

Between #3 and the file system snapshot, you should have a perfect and quick representation of the database at that point in time (when the database was informed it should stop logging).

derefrOP5y ago

File system snapshots are a system that have the analogous desired properties for files; but filesystem snapshots are actually quite heavyweight, because they deal with dirents, inodes, extents, etc. CoW filesystem snapshots are designed for ops-task-granularity usage, e.g. daily backups; not for per-transaction historical archiving. CoW filesystems tend to fall over once you get to 100K snapshots. (I tested!) A database that took a snapshot after every CQRS/ES transaction, could be expected to potentially have billions of snapshots.

A system that did its snapshots “inline” to itself, by e.g. managing a pool of pages with a free-list the way LMDB does — but where Txs ultimately add a new version to the root bucket as a snapshot, rather than replacing the root bucket page with themselves — would get a lot closer to allowing one to have at least tens-of-millions of snapshots online. At that point, to achieve a billion snapshots online, you “only” need to shard your timeline across a cluster of 100 nodes.

This is precisely one of the experiments I’m trying. :)

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