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Comment by jpgvm

3 years ago

Logical/vector clocks etc can be a real pain with ephemeral clients like web browsers, it can be hard to work out when it's safe to trim the now dormant nodes.

Though to address the GP a bit, the problem of concurrent writers without a sequencer (like a database) is less common than you might think. It definitely still comes up and there are things like CRDTs to help you address these cases (which do generally rely on either logical clocks or hybrid logical clocks). However most cases of event streaming to the browser you have each write round-tripping through the DB anyway and you use a feed like this to push a CDC stream or similar down into the browser to get "instant" feedback of a change that occurred after initial load.

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jpgvm

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brasetvik  3 years ago

> Though to address the GP a bit, the problem of concurrent writers without a sequencer (like a database) is less common than you might think.

My point was that even if you have a single non-scaleout database with a single time source, a sequence or a timestamp or a combination of both isn't as reliable a sequencer as you might think, unless you have at most one writer.

Thus, I think a "standard" should encourage a cursor concept that can use something that may reliably provide _all_ changes. If you have a single writer, you have a pretty easy job implementing that, as a plain sequence would work. (A timestamp could still break on clock adjustments, though)

This pertains to the "data replication" part of the listed goals, where getting everything is more important than in e.g. a social media news feed style thing where chronological order may be tenable - or less consequential if an item is missed.

  • toomim  3 years ago

    > Thus, I think a "standard" should encourage a cursor concept that can use something that may reliably provide _all_ changes.

    I think the Braid standard might support what you want: https://datatracker.ietf.org/doc/html/draft-toomim-httpbis-b...

    We don't call it a "cursor", but the idea is to encode whatever clock or versioning information you want as an arbitrary string, with the only general requirement that the string is unique to that version.

    You can use this with Lamport clocks, Vector Clocks, Version Vectors, Wall Clocks, (Agentid+Sequence) pairs, SHA hashes, or even blockchain transactions.