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eth2.0-specs/specs/merge/beacon-chain.md

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Ethereum 2.0 The Merge

Warning: This document is currently based on Phase 0 and will be rebased on Altair.

Notice: This document is a work-in-progress for researchers and implementers.

Table of contents

Introduction

This patch adds transaction execution to the beacon chain as part of the Merge fork.

Custom types

Note: The Transaction type is a stub which is not final.

Name SSZ equivalent Description
OpaqueTransaction ByteList[MAX_BYTES_PER_OPAQUE_TRANSACTION] a typed transaction envelope structured as TransactionType || TransactionPayload
Transaction Union[OpaqueTransaction] a transaction

Constants

Execution

Name Value
MAX_BYTES_PER_OPAQUE_TRANSACTION uint64(2**20) (= 1,048,576)
MAX_TRANSACTIONS_PER_PAYLOAD uint64(2**14) (= 16,384)
BYTES_PER_LOGS_BLOOM uint64(2**8) (= 256)

Containers

Extended containers

BeaconBlockBody

class BeaconBlockBody(phase0.BeaconBlockBody):
    # Execution
    execution_payload: ExecutionPayload  # [New in Merge]

BeaconState

class BeaconState(phase0.BeaconState):
    # Execution
    latest_execution_payload_header: ExecutionPayloadHeader  # [New in Merge]

New containers

ExecutionPayload

class ExecutionPayload(Container):
    # Execution block header fields
    parent_hash: Hash32
    coinbase: Bytes20  # 'beneficiary' in the yellow paper
    state_root: Bytes32
    receipt_root: Bytes32  # 'receipts root' in the yellow paper
    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
    random: Bytes32  # 'difficulty' in the yellow paper
    block_number: uint64  # 'number' in the yellow paper
    gas_limit: uint64
    gas_used: uint64
    timestamp: uint64
    # Extra payload fields
    block_hash: Hash32  # Hash of execution block
    transactions: List[Transaction, MAX_TRANSACTIONS_PER_PAYLOAD]

ExecutionPayloadHeader

class ExecutionPayloadHeader(Container):
    # Execution block header fields
    parent_hash: Hash32
    coinbase: Bytes20
    state_root: Bytes32
    receipt_root: Bytes32
    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
    random: Bytes32
    block_number: uint64
    gas_limit: uint64
    gas_used: uint64
    timestamp: uint64
    # Extra payload fields
    block_hash: Hash32  # Hash of execution block
    transactions_root: Root

Helper functions

Predicates

is_merge_complete

def is_merge_complete(state: BeaconState) -> bool:
    return state.latest_execution_payload_header != ExecutionPayloadHeader()

is_merge_block

def is_merge_block(state: BeaconState, body: BeaconBlockBody) -> bool:
    return not is_merge_complete(state) and body.execution_payload != ExecutionPayload()

is_execution_enabled

def is_execution_enabled(state: BeaconState, body: BeaconBlockBody) -> bool:
    return is_merge_block(state, body) or is_merge_complete(state)

Misc

compute_timestamp_at_slot

Note: This function is unsafe with respect to overflows and underflows.

def compute_timestamp_at_slot(state: BeaconState, slot: Slot) -> uint64:
    slots_since_genesis = slot - GENESIS_SLOT
    return uint64(state.genesis_time + slots_since_genesis * SECONDS_PER_SLOT)

Beacon chain state transition function

Execution engine

The implementation-dependent ExecutionEngine protocol encapsulates the execution sub-system logic via:

  • a state object self.execution_state of type ExecutionState
  • a state transition function self.on_payload which mutates self.execution_state

on_payload

def on_payload(self: ExecutionEngine, execution_payload: ExecutionPayload) -> bool:
    """
    Returns ``True`` iff ``execution_payload`` is valid with respect to ``self.execution_state``.
    """
    ...

The above function is accessed through the EXECUTION_ENGINE module which instantiates the ExecutionEngine protocol.

Block processing

def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)
    if is_execution_enabled(state, block.body):
        process_execution_payload(state, block.body.execution_payload, EXECUTION_ENGINE)  # [New in Merge]

Execution payload processing

process_execution_payload

Note: This function depends on process_randao function call as it retrieves the most recent randao mix from the state. Implementations that are considering parallel processing of execution payload with respect to beacon chain state transition function should work around this dependency.

def process_execution_payload(state: BeaconState, payload: ExecutionPayload, execution_engine: ExecutionEngine) -> None:
    # Verify consistency of the parent hash, block number and random
    if is_merge_complete(state):
        assert payload.parent_hash == state.latest_execution_payload_header.block_hash
        assert payload.block_number == state.latest_execution_payload_header.block_number + uint64(1)
        assert payload.random == 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.on_payload(payload)
    # Cache execution payload
    state.latest_execution_payload_header = ExecutionPayloadHeader(
        parent_hash=payload.parent_hash,
        coinbase=payload.coinbase,
        state_root=payload.state_root,
        receipt_root=payload.receipt_root,
        logs_bloom=payload.logs_bloom,
        random=payload.random,
        block_number=payload.block_number,
        gas_limit=payload.gas_limit,
        gas_used=payload.gas_used,
        timestamp=payload.timestamp,
        block_hash=payload.block_hash,
        transactions_root=hash_tree_root(payload.transactions),
    )

Testing

Note: The function initialize_beacon_state_from_eth1 is modified for pure Merge testing only.

Note: The function initialize_beacon_state_from_eth1 is modified to use MERGE_FORK_VERSION and initialize latest_execution_payload_header.

def initialize_beacon_state_from_eth1(eth1_block_hash: Bytes32,
                                      eth1_timestamp: uint64,
                                      deposits: Sequence[Deposit]) -> BeaconState:
    fork = Fork(
        previous_version=GENESIS_FORK_VERSION,
        current_version=MERGE_FORK_VERSION,  # [Modified in Merge]
        epoch=GENESIS_EPOCH,
    )
    state = BeaconState(
        genesis_time=eth1_timestamp + GENESIS_DELAY,
        fork=fork,
        eth1_data=Eth1Data(block_hash=eth1_block_hash, deposit_count=uint64(len(deposits))),
        latest_block_header=BeaconBlockHeader(body_root=hash_tree_root(BeaconBlockBody())),
        randao_mixes=[eth1_block_hash] * EPOCHS_PER_HISTORICAL_VECTOR,  # Seed RANDAO with Eth1 entropy
    )

    # Process deposits
    leaves = list(map(lambda deposit: deposit.data, deposits))
    for index, deposit in enumerate(deposits):
        deposit_data_list = List[DepositData, 2**DEPOSIT_CONTRACT_TREE_DEPTH](*leaves[:index + 1])
        state.eth1_data.deposit_root = hash_tree_root(deposit_data_list)
        process_deposit(state, deposit)

    # Process activations
    for index, validator in enumerate(state.validators):
        balance = state.balances[index]
        validator.effective_balance = min(balance - balance % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)
        if validator.effective_balance == MAX_EFFECTIVE_BALANCE:
            validator.activation_eligibility_epoch = GENESIS_EPOCH
            validator.activation_epoch = GENESIS_EPOCH

    # Set genesis validators root for domain separation and chain versioning
    state.genesis_validators_root = hash_tree_root(state.validators)

    # [New in Merge] Initialize the execution payload header (with block number set to 0)
    state.latest_execution_payload_header.block_hash = eth1_block_hash
    state.latest_execution_payload_header.timestamp = eth1_timestamp
    state.latest_execution_payload_header.random = eth1_block_hash

    return state