# Capella -- The Beacon Chain ## Table of contents ## 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` to `ETH1_ADDRESS_WITHDRAWAL_PREFIX` versioned withdrawal credentials to enable withdrawals for a validator ## Custom types ## Constants We define the following Python custom types for type hinting and readability: | Name | SSZ equivalent | Description | | - | - | - | | `WithdrawalIndex` | `uint64` | an index of a `Withdrawal`| ## Preset ### State list lengths | Name | Value | Unit | Duration | | - | - | :-: | :-: | | `WITHDRAWALS_QUEUE_LIMIT` | `uint64(2**40)` (= 1,099,511,627,776) | withdrawals enqueued in state| ### Execution | Name | Value | Description | | - | - | - | | `MAX_WITHDRAWALS_PER_PAYLOAD` | `uint64(2**4)` (= 16) | Maximum amount of withdrawals allowed in each payload | ## Configuration ## Containers ### Extended Containers #### `Validator` ```python 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 withdrawn_epoch: Epoch # [New in Capella] ``` #### `BeaconState` ```python 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] ``` #### `ExecutionPayload` ```python 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` ```python 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] ``` ### New containers #### `Withdrawal` ```python class Withdrawal(Container): index: WithdrawalIndex address: ExecutionAddress amount: Gwei ``` ## Helpers ### Beacon state mutators #### `withdraw` ```python def withdraw(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` ```python def is_fully_withdrawable_validator(validator: Validator, epoch: Epoch) -> bool: """ Check if ``validator`` is fully withdrawable. """ is_eth1_withdrawal_prefix = validator.withdrawal_credentials[0:1] == ETH1_ADDRESS_WITHDRAWAL_PREFIX return is_eth1_withdrawal_prefix and validator.withdrawable_epoch <= epoch < validator.withdrawn_epoch ``` ## Beacon chain state transition function ### Epoch processing ```python 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. ```python 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(state, ValidatorIndex(index), state.balances[index]) validator.withdrawn_epoch = current_epoch ``` ### Block processing ```python 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` ```python 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_receipt, withdrawal_transaction in zip(dequeued_withdrawals, payload.withdrawals): assert dequeued_receipt == withdrawal_transaction # Ensure no withdrawal type transactions in the normal payload transactions # assert no_withdrawal_type_transactions_in(payload.transactions) # Remove dequeued receipts 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. ```python 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), ) ```