Basic timelines support

mozilla / mentat

UNMAINTAINED A persistent, relational store inspired by Datomic and DataScript.

https://mozilla.github.io/mentat/

Apache License 2.0

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Basic timelines support #783

Closed grigoryk closed 6 years ago

grigoryk commented 6 years ago

This PR adds support for basic timelines to the transactor along with ability to move ranges of transactions off-of the main timeline.

Default timeline for all local transactions is the main timeline, 0.
A timeline of a transaction is indicated by an additional single field (same type as e).
Moving of transactions off of the main timeline is implemented as a "rewind": assertions are reversed and transacted in a descending order in order to achieve correct state of materialized views

grigoryk commented 6 years ago

Tests are failing only on 1.25.0 since I'm pattern matching a reference w/ a non-reference pattern.

grigoryk commented 6 years ago

One thing that might be a bit dicey is making sure transactions stack up well on the target timeline.

The way we're currently thinking of this - as, essentially, a one-way scratch pad for purposes of sync - this isn't necessary. In fact, it's probably not possible to "make sure transactions stack up well", since we're moving just partial slices of the transaction log. What does it mean for one slice to be compatible (i.e. non-conflicting) with another?

Perhaps a good follow-up would be to ensure we can't move transactions onto a non-empty timeline, since there's a good chance that would be quite meaningless. That change would firm up this a little.

I'll address this in the work that will start using timelines.

grigoryk commented 6 years ago

@ncalexan I've addressed your feedback in a few separate "review" commits; take a look when you get a chance.

grigoryk commented 6 years ago

Err, tests are passing, failure on rust beta seems to be a fluke of sorts.