feat(Cosmos): eqx query, Category.TryHydrateTip

[bartelink]

• Add --index-unfolds / -U option to eqx init to index unfolds
  • "/u/[]/c/?" (To be able to filter based on the name of the Unfold)
  • "/u/[]/d/*" (To be able to search based on values in an uncompressed unfolds via feat(Cosmos): compressUnfolds -> shouldCompress #436)
• Add eqx query facility to validate costs / correctness of the indexes
• Add CosmosStoreCategory.TryHydrateTip to generate a memento based on u (and optionally the _etag if it's for Transact purposes)
• Add TryLoad shortcut (based on TryHydrateTip, but solely returns the state)
• Reinstate Load = LoadOption.FromMemento from feat!: Remove ISyncContext.CreateMemento #413 (but not ISyncContext.CreateMemento)

Future/unanswered questions:

• Should CosmosStoreContainer expose a Querying mechanism (and should that let you express filters via IQueryable<UnfoldBody>)

[nordfjord]

Would be good to see an example of how to use it in actual applications as well

[bartelink]

Learnings/decisions relative to the original overview at the outset

• Main difference/learning/variance is that providing a function to generate a memento means no TransactMany/QueryMany API variant.
• Searching based on u/[]/c means no need to filter based on id = "-1"
• eqx query shows 3 modes (inc retrieving the p values); How to use the p value is an external concern

Original proposal:

We can:

• pay more on writes to maintain 2 more indexed things (affects write ops, but there's only one per stream and the first path should remain stable)
  • no, have not assessed the damage in terms of write costs
  • note that the /d/* means only data in uncompressed folds, so should be relatively low
• add uncompressed unfolds (so we can query against u.d.<FieldName>)
• identify streams to read or transact against by category name + an unfold name + arbitrary criteria targeting uncompressed unfolds for 2.8RU
• QueryMany: pull back u (and, optionally p) and provide a paged result based on hydrating the State from the (potentially compressed) Snapshot and rendering that (can convey Version and State)
  • p lets you identify the stream, but that's optional if it's state is sufficient
• TransactMany: pull back u, p, _etag and then (potentially concurrently) do a Transact read and/or write roundtrip based on only that input
  • could also do a null update to freshen the Cache
• for streams that are AccessStrategy.Unoptimized (events only), it would also work
  • would have to get e in lieu of u
  • as there is no u, there's no ability to filter based on uncompressed folds
  • would also need to filter by id = "-1" (to rule out other event batches), or retrieve those too and sort by c.i (if you know you'll hydrate the bulk of them)