ABCI interface / socket for queries from application to Tendermint

[cwgoes]

As discussed re: cosmos/cosmos-sdk#5340.

At present, it is not possible for the application to query (deterministic) Tendermint state such as past headers, commits, etc. without using the HTTP RPC interface, which is a bit of an anti-pattern (if used for consensus-critical state machine logic). If this kind of query would be useful for several different application use-cases, it seems worthwhile to consider adding an ABCI interface / socket and request-response pattern for queries from the application to Tendermint, which would need to be handled by Tendermint in a deterministic fashion.

Ref #3480
Ref https://github.com/tendermint/tendermint/issues/3956

[tomtau]

I'd be more in favour of extending https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-037-deliver-block.md to contain more information in the header that's currently only accessible via RPC

[melekes]

@tomtau what makes you prefer extending the ResponseBeginBlock/DeliverBlock approach?

[tomtau]

@melekes I think it's easier, more streamlined approach in the spirit of the existing ABCI design.
In our use case, we'd want to process SignedHeader (Header+Commit info) on each block as soon as it arrives -- so if that's just another piece of information to grab in ResponseBeginBlock, it's convenient.
If an application doesn't need that information, they can just ignore it.
If an application needs that information, they can process and store that information in whichever way makes sense for them.

[aaronc]

It would be great to see something like this for the Cosmos SDK (see cosmos/cosmos-sdk#8272).

Seems like there's a few different ideas here, but specifically it would be ideal for any SDK module to be able to at a minimum query 1) historical blocks by height and hash and 2) transactions by hash.

It would also be nice if modules could search for indexed tx's by tag, but this needs to be deterministic so there need to be strong guarantees about what is and isn't indexed on all nodes. Anyway this seems a fair bit harder...

Any updates on where the Tendermint team is possibly seeing this in the roadmap @marbar3778 ?