eth2.0-specs/specs/merge/validator.md

Ethereum 2.0 The Merge

Warning: This document is currently based on Phase 0 but will be rebased to Altair once the latter is shipped.

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

Table of contents

• Introduction
• Prerequisites
• Protocols
  • ExecutionEngine
    • assemble_block
• Beacon chain responsibilities
  • Block proposal
    • Constructing the BeaconBlockBody
      • Execution Payload
        • get_pow_chain_head

Introduction

This document represents the changes to be made in the code of an "honest validator" to implement executable beacon chain proposal.

Prerequisites

This document is an extension of the Phase 0 -- Validator. All behaviors and definitions defined in the Phase 0 doc carry over unless explicitly noted or overridden.

All terminology, constants, functions, and protocol mechanics defined in the updated Beacon Chain doc of The Merge are requisite for this document and used throughout. Please see related Beacon Chain doc before continuing and use them as a reference throughout.

Protocols

ExecutionEngine

The following methods are added to the ExecutionEngine protocol for use as a validator:

assemble_block

Produces a new instance of an execution payload, with the specified timestamp, on top of the execution payload chain tip identified by block_hash.

The body of this function is implementation dependent. The Consensus API may be used to implement this with an external execution engine.

def assemble_block(self: ExecutionEngine, block_hash: Hash32, timestamp: uint64, random: Bytes32) -> ExecutionPayload:
    ...

Beacon chain responsibilities

All validator responsibilities remain unchanged other than those noted below. Namely, the transition block handling and the addition of ExecutionPayload.

Block proposal

Constructing the BeaconBlockBody

Execution Payload

get_pow_chain_head

Let get_pow_chain_head() -> PowBlock be the function that returns the head of the PoW chain. The body of the function is implementation specific.

• Set block.body.execution_payload = get_execution_payload(state, transition_store, execution_engine) where:

def compute_randao_mix(state: BeaconState, randao_reveal: BLSSignature) -> Bytes32:
    epoch = get_current_epoch(state)
    return xor(get_randao_mix(state, epoch), hash(randao_reveal))


def produce_execution_payload(state: BeaconState,
                              parent_hash: Hash32,
                              randao_reveal: BLSSignature,
                              execution_engine: ExecutionEngine) -> ExecutionPayload:
    timestamp = compute_timestamp_at_slot(state, state.slot)
    randao_mix = compute_randao_mix(state, randao_reveal)
    return execution_engine.assemble_block(parent_hash, timestamp, randao_mix)


def get_execution_payload(state: BeaconState,
                          transition_store: TransitionStore,
                          randao_reveal: BLSSignature,
                          execution_engine: ExecutionEngine) -> ExecutionPayload:
    if not is_merge_complete(state):
        pow_block = get_pow_chain_head()
        if not is_valid_terminal_pow_block(transition_store, pow_block):
            # Pre-merge, empty payload
            return ExecutionPayload()
        else:
            # Signify merge via producing on top of the last PoW block
            return produce_execution_payload(state, pow_block.block_hash, randao_reveal, execution_engine)

    # Post-merge, normal payload
    parent_hash = state.latest_execution_payload_header.block_hash
    return produce_execution_payload(state, parent_hash, randao_reveal, execution_engine)