Investigate on how to decide if it is a backable candidate in candidate backing subsystem.

[axaysagathiya]

Issue summary

• in host implementors guide it is mentioned that 'The Candidate Backing subsystem ensures every parablock considered for relay block inclusion has been seconded by at least one validator, and approved by a quorum'
• I didn't come across any code related to quorum. So, we need to check how it is implemented in Rust and how we can implement it.

Other information and links

Backable Candidate: If a candidate receives a majority of supporting Statements from the Parachain Validators currently assigned, then that candidate is considered backable. Backed Candidate: A Backable Candidate noted in a relay-chain block

Acceptance criteria's or Outcomes

• We should know which subsystem implements a functionality to decide if it is a backable candidate.
• We should know how to implement this functionality.

[axaysagathiya]

I found this documentation about accepting and rejecting the candidate based on the approval of a quorum.

[axaysagathiya]

it seems we don't need majority validity votes in the legacy candidate backing(before the support of async backing).

There is a predefined number of minimum backing votes required for legacy backing.

• in legacy backing, we need a minimum of 2 valid votes to consider the candidate backable.
• in async backing, we need to call the runtime method to get the minimum number of backing votes required.

[axaysagathiya]

We store all the validity votes in the statement table in the candidate backing subsystem. The Provisioner subsystem sends a message to the candidate backing subsystem to get backable candidates.

Conclusion:

• need to implement the provisioner subsystem.
• We already have an implementation to get the backable candidate in the candidate backing subsystem. It is just that the message name is GetBackedCandidates instead of GetbackableCandidates. We can rename it to GetBackableCandidate.