Modularize beacon node backend

[eserilev]

Issue Addressed

#4669

Proposed Changes

Modularize the beacon node backend to make it easier to add new database implementations

Additional Info

The existing KeyValueStore trait already does some abstraction, however the codebases assumes the KV store is always LevelDB.

I created a BeaconNodeBackend type that implements the KeyValueStore and ItemStore traits. I then replaced all references of LevelDb to the new BeaconNodeBackend type. Within the BeaconNodeBackend type I used the cfg macro which should allow us to switch between different database implementations via config changes.

[eserilev]

I'm currently working on adding Redb to the beacon node backend. Here are some notes I've taken so far on my work

Redb

Theres a few difference's between LevelDB and Redb that I'll describe below

fsync

Info about redb transaction durability can be found here
https://docs.rs/redb/latest/redb/enum.Durability.html#

For now I translate WriteOptions.sync = true to be Durability::Immediate and WriteOptions.sync = false to be Durability::Eventual

We could decide to use Paranoid instead of Immediate, but I think the additional guarantees of Paranoid are probably unecessary for our use case.

Compaction

LevelDB allows for compacting over a range of values. Redb only allows for compacting the full db. It seems like our LevelDB implementation compacts across all keys in the DB. If thats the case we should be all good here.

Tables

Redb introduces the concept of tables. We can either use one table for everything, or spin up tables by column name. I'm currently going down the path of spinning up tables by column name. This causes issues with do_atomically. We may need to make changes to this function to also accept a col: &str as an argument.

[eserilev]

the last compaction run took 7 min to complete

I think we were running an older version of Redb that had introduced a regression to compaction. I'm really hoping the new version of Redb, which fixed the regression and introduced some optimizations, will improve those times. We should see compaction run on gown on sep 23rd.. will update here once that runs.

If compaction speeds still suck, one option is to have two separate databases for redb. The first contains the tables we dont want to run compaction for, the second contains tables we do want to run compaction for. That way we can run compaction against these three tables:

  DBColumn::BeaconState,
  DBColumn::BeaconStateSummary,
  DBColumn::BeaconBlock,

And not against the full set of tables (mimicking leveldb functionality).

[michaelsproul]

It would be ideal to have a test that sets this to redb, as the flag was actually completely defunct and this test still passed. But I think this would require compiling with beacon-node-redb as a default feature. Hmm 🤔