14 KiB
14 KiB
Capella -- The Beacon Chain
Table of contents
- Introduction
- Custom types
- Constants
- Preset
- Configuration
- Containers
- Helpers
- Beacon chain state transition function
Introduction
Capella is a consensus-layer upgrade containing a number of features related to validator withdrawals. Including:
- Automatic withdrawals of
withdrawable
validators - Partial withdrawals during block proposal
- Operation to change from
BLS_WITHDRAWAL_PREFIX
toETH1_ADDRESS_WITHDRAWAL_PREFIX
versioned withdrawal credentials to enable withdrawals for a validator
Custom types
We define the following Python custom types for type hinting and readability:
Name | SSZ equivalent | Description |
---|---|---|
WithdrawalIndex |
uint64 |
an index of a Withdrawal |
Constants
Domain types
Name | Value |
---|---|
DOMAIN_BLS_TO_EXECUTION_CHANGE |
DomainType('0x0A000000') |
Preset
State list lengths
Name | Value | Unit | Duration |
---|---|---|---|
WITHDRAWALS_QUEUE_LIMIT |
uint64(2**40) (= 1,099,511,627,776) |
withdrawals enqueued in state |
Max operations per block
Name | Value |
---|---|
MAX_BLS_TO_EXECUTION_CHANGES |
2**4 (= 16) |
Execution
Name | Value | Description |
---|---|---|
MAX_WITHDRAWALS_PER_PAYLOAD |
uint64(2**4) (= 16) |
Maximum amount of withdrawals allowed in each payload |
Configuration
Containers
New containers
Withdrawal
class Withdrawal(Container):
index: WithdrawalIndex
address: ExecutionAddress
amount: Gwei
BLSToExecutionChange
class BLSToExecutionChange(Container):
validator_index: ValidatorIndex
from_bls_pubkey: BLSPubkey
to_execution_address: ExecutionAddress
SignedBLSToExecutionChange
class SignedBLSToExecutionChange(Container):
message: BLSToExecutionChange
signature: BLSSignature
Extended Containers
ExecutionPayload
class ExecutionPayload(Container):
# Execution block header fields
parent_hash: Hash32
fee_recipient: ExecutionAddress # 'beneficiary' in the yellow paper
state_root: Bytes32
receipts_root: Bytes32
logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
prev_randao: Bytes32 # 'difficulty' in the yellow paper
block_number: uint64 # 'number' in the yellow paper
gas_limit: uint64
gas_used: uint64
timestamp: uint64
extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
base_fee_per_gas: uint256
# Extra payload fields
block_hash: Hash32 # Hash of execution block
transactions: List[Transaction, MAX_TRANSACTIONS_PER_PAYLOAD]
withdrawals: List[Withdrawal, MAX_WITHDRAWALS_PER_PAYLOAD] # [New in Capella]
ExecutionPayloadHeader
class ExecutionPayloadHeader(Container):
# Execution block header fields
parent_hash: Hash32
fee_recipient: ExecutionAddress
state_root: Bytes32
receipts_root: Bytes32
logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
prev_randao: Bytes32
block_number: uint64
gas_limit: uint64
gas_used: uint64
timestamp: uint64
extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
base_fee_per_gas: uint256
# Extra payload fields
block_hash: Hash32 # Hash of execution block
transactions_root: Root
withdrawals_root: Root # [New in Capella]
Validator
class Validator(Container):
pubkey: BLSPubkey
withdrawal_credentials: Bytes32 # Commitment to pubkey for withdrawals
effective_balance: Gwei # Balance at stake
slashed: boolean
# Status epochs
activation_eligibility_epoch: Epoch # When criteria for activation were met
activation_epoch: Epoch
exit_epoch: Epoch
withdrawable_epoch: Epoch # When validator can withdraw funds
fully_withdrawn_epoch: Epoch # [New in Capella]
BeaconBlockBody
class BeaconBlockBody(Container):
randao_reveal: BLSSignature
eth1_data: Eth1Data # Eth1 data vote
graffiti: Bytes32 # Arbitrary data
# Operations
proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS]
attestations: List[Attestation, MAX_ATTESTATIONS]
deposits: List[Deposit, MAX_DEPOSITS]
voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
sync_aggregate: SyncAggregate
# Execution
execution_payload: ExecutionPayload
# Capella operations
bls_to_execution_changes: List[SignedBLSToExecutionChange, MAX_BLS_TO_EXECUTION_CHANGES] # [New in Capella]
BeaconState
class BeaconState(Container):
# Versioning
genesis_time: uint64
genesis_validators_root: Root
slot: Slot
fork: Fork
# History
latest_block_header: BeaconBlockHeader
block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
historical_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
# Eth1
eth1_data: Eth1Data
eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
eth1_deposit_index: uint64
# Registry
validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
# Randomness
randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR]
# Slashings
slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR] # Per-epoch sums of slashed effective balances
# Participation
previous_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
current_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
# Finality
justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH] # Bit set for every recent justified epoch
previous_justified_checkpoint: Checkpoint
current_justified_checkpoint: Checkpoint
finalized_checkpoint: Checkpoint
# Inactivity
inactivity_scores: List[uint64, VALIDATOR_REGISTRY_LIMIT]
# Sync
current_sync_committee: SyncCommittee
next_sync_committee: SyncCommittee
# Execution
latest_execution_payload_header: ExecutionPayloadHeader
# Withdrawals
withdrawal_index: WithdrawalIndex
withdrawals_queue: List[Withdrawal, WITHDRAWALS_QUEUE_LIMIT] # [New in Capella]
Helpers
Beacon state mutators
withdraw
def withdraw_balance(state: BeaconState, index: ValidatorIndex, amount: Gwei) -> None:
# Decrease the validator's balance
decrease_balance(state, index, amount)
# Create a corresponding withdrawal receipt
withdrawal = Withdrawal(
index=state.withdrawal_index,
address=state.validators[index].withdrawal_credentials[12:],
amount=amount,
)
state.withdrawal_index = WithdrawalIndex(state.withdrawal_index + 1)
state.withdrawals_queue.append(withdrawal)
Predicates
is_fully_withdrawable_validator
def is_fully_withdrawable_validator(validator: Validator, epoch: Epoch) -> bool:
"""
Check if ``validator`` is fully withdrawable.
"""
is_eth1_withdrawal_prefix = validator.withdrawal_credentials[:1] == ETH1_ADDRESS_WITHDRAWAL_PREFIX
return is_eth1_withdrawal_prefix and validator.withdrawable_epoch <= epoch < validator.fully_withdrawn_epoch
Beacon chain state transition function
Epoch processing
def process_epoch(state: BeaconState) -> None:
process_justification_and_finalization(state)
process_inactivity_updates(state)
process_rewards_and_penalties(state)
process_registry_updates(state)
process_slashings(state)
process_eth1_data_reset(state)
process_effective_balance_updates(state)
process_slashings_reset(state)
process_randao_mixes_reset(state)
process_historical_roots_update(state)
process_participation_flag_updates(state)
process_sync_committee_updates(state)
process_full_withdrawals(state) # [New in Capella]
Withdrawals
Note: The function process_full_withdrawals
is new.
def process_full_withdrawals(state: BeaconState) -> None:
current_epoch = get_current_epoch(state)
for index, validator in enumerate(state.validators):
if is_fully_withdrawable_validator(validator, current_epoch):
# TODO, consider the zero-balance case
withdraw_balance(state, ValidatorIndex(index), state.balances[index])
validator.fully_withdrawn_epoch = current_epoch
Block processing
def process_block(state: BeaconState, block: BeaconBlock) -> None:
process_block_header(state, block)
if is_execution_enabled(state, block.body):
process_withdrawals(state, block.body.execution_payload) # [New in Capella]
process_execution_payload(state, block.body.execution_payload, EXECUTION_ENGINE) # [Modified in Capella]
process_randao(state, block.body)
process_eth1_data(state, block.body)
process_operations(state, block.body)
process_sync_aggregate(state, block.body.sync_aggregate)
New process_withdrawals
def process_withdrawals(state: BeaconState, payload: ExecutionPayload) -> None:
num_withdrawals = min(MAX_WITHDRAWALS_PER_PAYLOAD, len(state.withdrawals_queue))
dequeued_withdrawals = state.withdrawals_queue[:num_withdrawals]
assert len(dequeued_withdrawals) == len(payload.withdrawals)
for dequeued_withdrawal, withdrawal in zip(dequeued_withdrawals, payload.withdrawals):
assert dequeued_withdrawal == withdrawal
# Remove dequeued withdrawals from state
state.withdrawals_queue = state.withdrawals_queue[num_withdrawals:]
Modified process_execution_payload
Note: The function process_execution_payload
is modified to use the new ExecutionPayloadHeader
type.
def process_execution_payload(state: BeaconState, payload: ExecutionPayload, execution_engine: ExecutionEngine) -> None:
# Verify consistency of the parent hash with respect to the previous execution payload header
if is_merge_transition_complete(state):
assert payload.parent_hash == state.latest_execution_payload_header.block_hash
# Verify prev_randao
assert payload.prev_randao == get_randao_mix(state, get_current_epoch(state))
# Verify timestamp
assert payload.timestamp == compute_timestamp_at_slot(state, state.slot)
# Verify the execution payload is valid
assert execution_engine.notify_new_payload(payload)
# Cache execution payload header
state.latest_execution_payload_header = ExecutionPayloadHeader(
parent_hash=payload.parent_hash,
fee_recipient=payload.fee_recipient,
state_root=payload.state_root,
receipts_root=payload.receipts_root,
logs_bloom=payload.logs_bloom,
prev_randao=payload.prev_randao,
block_number=payload.block_number,
gas_limit=payload.gas_limit,
gas_used=payload.gas_used,
timestamp=payload.timestamp,
extra_data=payload.extra_data,
base_fee_per_gas=payload.base_fee_per_gas,
block_hash=payload.block_hash,
transactions_root=hash_tree_root(payload.transactions),
withdrawals_root=hash_tree_root(payload.withdrawals), # [New in Capella]
)
Modified process_operations
Note: The function process_operations
is modified to process BLSToExecutionChange
operations included in the block.
def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
# Verify that outstanding deposits are processed up to the maximum number of deposits
assert len(body.deposits) == min(MAX_DEPOSITS, state.eth1_data.deposit_count - state.eth1_deposit_index)
def for_ops(operations: Sequence[Any], fn: Callable[[BeaconState, Any], None]) -> None:
for operation in operations:
fn(state, operation)
for_ops(body.proposer_slashings, process_proposer_slashing)
for_ops(body.attester_slashings, process_attester_slashing)
for_ops(body.attestations, process_attestation)
for_ops(body.deposits, process_deposit)
for_ops(body.voluntary_exits, process_voluntary_exit)
for_ops(body.bls_to_execution_changes, process_bls_to_execution_change) # [New in Capella]
New process_bls_to_execution_change
def process_bls_to_execution_change(state: BeaconState,
signed_address_change: SignedBLSToExecutionChange) -> None:
address_change = signed_address_change.message
assert address_change.validator_index < len(state.validators)
validator = state.validators[address_change.validator_index]
assert validator.withdrawal_credentials[:1] == BLS_WITHDRAWAL_PREFIX
assert validator.withdrawal_credentials[1:] == hash(address_change.from_bls_pubkey)[1:]
domain = get_domain(state, DOMAIN_BLS_TO_EXECUTION_CHANGE)
signing_root = compute_signing_root(address_change, domain)
assert bls.Verify(address_change.from_bls_pubkey, signing_root, signed_address_change.signature)
validator.withdrawal_credentials = (
ETH1_ADDRESS_WITHDRAWAL_PREFIX
+ (0x00).to_bytes(11, 'little')
+ address_change.to_execution_address
)