Archive storage

[Scooletz]

An archive node is an instance of an Ethereum client configured to build an archive of all historical states. There are two queries that it needs to answer:

1. the account information at the given block
2. the storage information at the given block for the given account.

This means that both queries could be transformed into a query that:

1. starts with the account = @account equality,
2. then is followed by storage = @storage (for storage)
3. then is followed by block <= @block as the change might have been applied in the past

It should be possible then to use the same db structure where the key is encoded as a NibblePath by concatenating account + storage + block and mapping some payload to it. This concatenation would allow Paprika's prefixing behavior to potentially make the dataset smaller. Potentially a separate page type with a different flushing behavior and a different fan out could be applied.

From the consumption point of view, archive processing could be a sidecar that processes storage & state slots that are flushed by the Blockchain.FlusherTask method. Then, each block that is applied could be inspected and applied to a separate archive root. This could be done by extending RootPage by only one address that would point to the separate Archive tree.

Remarks:

1. The application of the data could be done in a relaxed manner, where the flush to disk happens only now and then. There are options for this already.
2. Storing or calculating hashes, could be made optional.
3. Querying the state should take into consideration the transient in memory state and the archive.
4. The other option is to have the Archive totally separated. That could be considered, but there's something into having it embedded into the block processing, where the data are fresh, in-memory and ready to be stored by this archiving sidecar.

[LukaszRozmej]

There should be a (slow) way to support deep reorganistations with archive storage.
I expect archive storage to be indexed by block (or each transactions within blocks), it would be good to have an option to prune archive storage, keeping some arbitrary number of blocks there (for example from only last year or month).