evidence: batch vs individual processing of evidence

[cmwaters]

Summary

Currently all evidence is submitted individually i.e. one validator per evidence, however we detect evidence in batches when a light client detects two conflicting headers, i.e. all validators that misbehaved in these two headers have evidence created against them.

The introduction of amnesia evidence meant that there was no longer irrevocable proof of malicious behavior and instead accused validators had to prove their innocence. When this was introduced, naturally, it was thought to batch the suspects together along with the proofs.

It is possible to have amnesia evidence individually however this now begs the question of what is better - individual processing of evidence or batch processing of evidence

Pros of each option

Individual

• 😃 Easy to match offences to validators, this stops malicious validators from spamming the system with extra evidence as only the earliest evidence is considered.
• 😃 At the moment, evidence is made from conflicting headers by light client but some evidence detection could be built into the consensus package - this would most likely be individually
• 😃 Already implemented so not much extra work has to be done

Batch

• 😃 Quicker to validate
• 😃 Easier to prioritize evidence as we know the total voting power of the attacks
• 😃 Evidence can still be individual.

NOTE: the sending of extra messages is not a problem because they are sent and received in batches already

Please add more if you have ideas

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[cmwaters]

Amnesia Evidence will be individually processed. Verdict is reflected in the changes made to the Amnesia ADR in this PR #4994

[cmwaters]

Reopening as there are still elements to this issue that are outstanding as outlined here:

tendermint/evidence/reactor.go

Lines 154 to 158 in 15c1936

	// TODO: Refactor the Evidence type to not contain a list since we only ever
	// send and receive one piece of evidence at a time. Or potentially consider
	// batching evidence.
	//
	// see: https://github.com/tendermint/tendermint/issues/4729