update valset-changes.md
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Ethan Buchman
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# Slashing
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# Validator Set Changes
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A validator bond is an economic commitment made by a validator signing key to both the safety and liveness of
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the consensus. Validator keys must not sign invalid messages which could
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violate consensus safety, and their signed precommit messages must be regularly included in
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block commits.
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The validator set may be updated by state transitions that run at the beginning and
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end of every block. This can happen one of three ways:
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The incentivization of these two goals are treated separately.
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- voting power of a validator changes due to bonding and unbonding
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- voting power of validator is "slashed" due to conflicting signed messages
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- validator is automatically unbonded due to inactivity
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## Safety
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## Voting Power Changes
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Messges which may compromise the safety of the underlying consensus protocol ("equivocations")
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result in some amount of the offending validator's shares being removed ("slashed").
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At the end of every block, we run the following:
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Currently, such messages include only the following:
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- prevotes by the same validator for more than one BlockID at the same
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Height and Round
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- precommits by the same validator for more than one BlockID at the same
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Height and Round
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We call any such pair of conflicting votes `Evidence`. Full nodes in the network prioritize the
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detection and gossipping of `Evidence` so that it may be rapidly included in blocks and the offending
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validators punished.
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For some `evidence` to be valid, it must satisfy:
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`evidence.Height >= block.Height - MAX_EVIDENCE_AGE`
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If valid evidence is included in a block, the offending validator loses
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a constant `SLASH_PROPORTION` of their current stake:
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```
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oldShares = validator.shares
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validator.shares = oldShares * (1 - SLASH_PROPORTION)
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```
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This ensures that offending validators are punished the same amount whether they
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act as a single validator with X stake or as N validators with collectively X
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stake.
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## Liveness
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Every block includes a set of precommits by the validators for the previous block,
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known as the LastCommit. A LastCommit is valid so long as it contains precommits from +2/3 of voting power.
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Proposers are incentivized to include precommits from all
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validators in the LastCommit by receiving additional fees
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proportional to the difference between the voting power included in the
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LastCommit and +2/3 (see [TODO](https://github.com/cosmos/cosmos-sdk/issues/967)).
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Validators are penalized for failing to be included in the LastCommit for some
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number of blocks by being automatically unbonded.
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TODO: do we do this by trying to track absence directly in the state, using
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something like the below, or do we let users notify the app when a validator has
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been absent using the
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[TxLivenessCheck](https://github.com/cosmos/cosmos-sdk/blob/develop/docs/spec/staking/spec-technical.md#txlivelinesscheck).
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A list, `ValidatorAbsenceInfos`, is stored in the state and used to track how often
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validators were included in a LastCommit.
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```go
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// Ordered by ValidatorAddress.
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// One entry for each validator.
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type ValidatorAbsenceInfos []ValidatorAbsenceInfo
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type ValidatorAbsenceInfo struct {
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ValidatorAddress []byte // address of the validator
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FirstHeight int64 // first height the validator was absent
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Count int64 // number of heights validator was absent since (and including) first
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}
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```
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### BeginBlock Handling
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### EndBlock Handling
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This is where we inflate the Atoms and deal with validator set changes.
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(TODO remove inflation from here)
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```golang
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tick(ctx Context):
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@ -172,3 +100,91 @@ unbondedToBondedPool(candidate Candidate):
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return transfer(address of the unbonded pool, address of the bonded pool, removedTokens)
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```
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## Slashing
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Messges which may compromise the safety of the underlying consensus protocol ("equivocations")
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result in some amount of the offending validator's shares being removed ("slashed").
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Currently, such messages include only the following:
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- prevotes by the same validator for more than one BlockID at the same
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Height and Round
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- precommits by the same validator for more than one BlockID at the same
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Height and Round
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We call any such pair of conflicting votes `Evidence`. Full nodes in the network prioritize the
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detection and gossipping of `Evidence` so that it may be rapidly included in blocks and the offending
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validators punished.
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For some `evidence` to be valid, it must satisfy:
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`evidence.Timestamp >= block.Timestamp - MAX_EVIDENCE_AGE`
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where `evidence.Timestamp` is the timestamp in the block at height
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`evidence.Height` and `block.Timestamp` is the current block timestamp.
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If valid evidence is included in a block, the offending validator loses
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a constant `SLASH_PROPORTION` of their current stake at the beginning of the block:
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```
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oldShares = validator.shares
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validator.shares = oldShares * (1 - SLASH_PROPORTION)
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```
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This ensures that offending validators are punished the same amount whether they
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act as a single validator with X stake or as N validators with collectively X
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stake.
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## Automatic Unbonding
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Every block includes a set of precommits by the validators for the previous block,
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known as the LastCommit. A LastCommit is valid so long as it contains precommits from +2/3 of voting power.
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Proposers are incentivized to include precommits from all
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validators in the LastCommit by receiving additional fees
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proportional to the difference between the voting power included in the
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LastCommit and +2/3 (see [TODO](https://github.com/cosmos/cosmos-sdk/issues/967)).
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Validators are penalized for failing to be included in the LastCommit for some
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number of blocks by being automatically unbonded.
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The following information is stored with each validator candidate, and is only non-zero if the candidate becomes an active validator:
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```go
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type ValidatorSigningInfo struct {
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StartHeight int64
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SignedBlocksBitArray BitArray
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}
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```
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Where:
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* `StartHeight` is set to the height that the candidate became an active validator (with non-zero voting power).
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* `SignedBlocksBitArray` is a bit-array of size `SIGNED_BLOCKS_WINDOW` that records, for each of the last `SIGNED_BLOCKS_WINDOW` blocks,
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whether or not this validator was included in the LastCommit. It uses a `0` if the validator was included, and a `1` if it was not.
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Note it is initialized with all 0s.
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At the beginning of each block, we update the signing info for each validator and check if they should be automatically unbonded:
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```
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h = block.Height
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index = h % SIGNED_BLOCKS_WINDOW
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for val in block.Validators:
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signInfo = val.SignInfo
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if val in block.LastCommit:
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signInfo.SignedBlocksBitArray.Set(index, 0)
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else
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signInfo.SignedBlocksBitArray.Set(index, 1)
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// validator must be active for at least SIGNED_BLOCKS_WINDOW
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// before they can be automatically unbonded for failing to be
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// included in 50% of the recent LastCommits
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minHeight = signInfo.StartHeight + SIGNED_BLOCKS_WINDOW
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minSigned = SIGNED_BLOCKS_WINDOW / 2
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blocksSigned = signInfo.SignedBlocksBitArray.Sum()
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if h > minHeight AND blocksSigned < minSigned:
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unbond the validator
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```
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