Refactor schema for the durable storage

[ieQu1]

Fixes https://emqx.atlassian.net/browse/EMQX-11862

Release version: v/e5.?

Summary

• Reduce configuration nesting
• Allow to change storage layout
• Add dynamic dispatching of the DS configuration handler

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix|breaking)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update

[keynslug]

I wonder if we could turn that option into a time interval instead. Work on the consistent replication has so far shown that one of the most practical ways to deal with ordering guarantees is to just use microsecond-presicion timestamps for message keys, but changing precision will suddenly make this option backward-incompatible. One significant downside is that we won't be able to respect specified value exactly and need to tell the user what's the actual epoch size somehow.

[ieQu1]

I think I mentioned in the other PR that using microsecond-precision timestamp for serialization is fine, but it must be implemented in a way that does NOT affect neither EMQX application, NOR the storage layer. Otherwise we leak abstractions from the replication layer.

EMQX application owns the #message record, and assumes that the timestamp is in milliseconds. DS must store and restore message as is (at least type- and unit-wise), so changing replication layer in the future doesn't result in breakage.

There is no reason why the storage layer must be desined otherwise. I guess proper way to address this is to pass unique message id (which could be derived from the microsecond TS), from replication layer to the storage layer, instead of generating it in the storage layer.

[ieQu1]

...Made it hidden for now.

[keynslug]

Thanks!

EMQX application owns the #message record, and assumes that the timestamp is in milliseconds.

That's why I mentioned message key and not emqx-level message timestamp.

NOR the storage layer.

Arguable. I do not honestly see why storage layer could not accommodate replication layer needs and handle timestamps with variable precision for the sake of overall simplicity and compactness.

[ieQu1]

Arguable. I do not honestly see why storage layer could not accommodate replication layer needs and handle timestamps with variable precision for the sake of overall simplicity and compactness.

Well, I would like to avoid it if possible, since it will, to some degree, break the layered model of the DS. It should be possible to replace the replication layer (if we come up with a new & faster replication protocol, for example), and reuse the storage layer modules. Storage layer may in theory implement some clever way of serializing messages that depends on the timestamp's fixed size.
It's all hypothetical, of course, but it illustrates why we should strive to keep the layers untangled.

I don't think it will lead to any complixity, to be honest. bitfield_lts module makes it quite trivial to map the serialization key next to the timestamp, so, in effect, one gets a microsecond timestamp in the RocksDB key. It's simply a matter of the API.

Sure, treating microseconds as a separate entity will make it impossible to configure bitfield_lts module to seek to an epoch with a microsecond precision, but I don't see it as a problem, because the top-level API (emqx_ds:make_iterator) won't be able to make use of it anyway.