How do I obtain key-value storage proof for a runtime call?

Question

I want to run an experiment to see how big are PoVs for my parachain blocks.

I figured I can leverage the same API that is used during syncing execute_block for the blocks I am interested in and collect the key-value pairs along with the merkle-proof similar to what a collator does during parachain block creation.

Is that possible to do in Substrate?

Answer 1

It is possible to collect a witness for having an runtime API handle at hand. The key methods are record_proof and extract_proof.

Assuming you have a Client instance at hand, you can do the following:

let mut runtime_api = client.runtime_api();
runtime_api.record_proof(); 
runtime_api.execute_block(at, block);
let witness = runtime_api
    .extract_proof()
    .unwrap("proof is always present if called after `record_proof`");

Currently, Cumulus' PoV consists of the witness and the block part. The block part can be obtained by just SCALE encoding the block, i.e.

block.encode()