Preperation for ValidatorSet

[colibrishin]

cc. pbft @greymistcube @limebell, pos @riemannulus @OnedgeLee

Context

Currently, we are using static validator lists for simplicity (Policy.GetValidatorSet()) in PBFT. If we are moving onto the ValidatorSet that changes, some changes seem to be required. This addresses from an abstract perspective. These are not sealed and stamped or "must have".

ValidatorSet

A ValidatorSet is a set of Validator's PublicKeys that have joined into consensus. There should be a place to store and track changes (e.g., State). The content of ValidatorSet can be changed. See also Transaction.

Based on #2558 and #2541 use cases, the previous and current ValidatorSets are needed (In this example, Block # ${2}$ is a current block, and in appending.) :

• A previous ValidatorSet for validating the LastCommit.
• A current ValidatorSet for validating the current Block's Commit.

The validation of BlockCommit¹² (LastCommit and Commit) is about checking whether Votes in BlockCommit are from the given ValidatorSet.

Implementation status

Currently, we have two ValidatorSet definitions in libplanet/pbft and libplanet/pos. A ValidatorSet for validating the current and previous BlockCommit is from Policy.GetValidatorSet(), which returns static Validators. It seems a baseline needs to be defined.

Previous ValidatorSet for LastCommit

A LastCommit is a set of commits of the previous Block. For validating, a LastCommit needs the previous ValidatorSet. There are multiple ways to retrieve the previous ValidatorSet with simplicity (e.g., State, from the previous Block, store the previous ValidatorSet temporarily.)

Implementation status

We don't track the previous ValidatorSet. This work looks dependent on LastCommit and storing ValidatorSet. It seems to be in PBFT scope.

Transaction (Partially PoS)

A ValidatorSet can be changed by a node or validator's decision, which means a Transaction of "mutating" ValidatorSet is needed.³ A Transaction is created by the node itself and it holds whether a node registers/unregisters as a validator. The detailed specification and implementation need to be discussed.

Implementation status

The implementation exists in libplanet/pos. This seems partially in PBFT scope.

Block

A Block holds the Txs of the node's decision and the ValidatorSet of the current Block. The current ValidatorSet is used for checking whether the known node's ValidatorSet (e.g., in State) is identical to Block's ValidatorSet (Added for checking integrity.)

Implementation status

We don't have the current ValidatorSet field in Block. Above Tx is defined in libplanet/pos. It seems to be decided in PBFT.

See also

• Tendermint
• Ethereum

Footnotes

1. In consensus, If +2/3 PreCommit messages are collected for a height ( $\simeq$ index), and a locked Block, then the Block is committed (appended) into BlockChain. ↩

2. Having +2/3 commits in BlockCommit is checked in the constructor. ↩

3. A Validator node will join into or remove itself from the Validator "pool" by sending Tx. A Validator node participates in consensus, then votes and proposes a next Block. ↩