eth2.0-specs/specs/merge/fork-choice.md

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# The Merge -- Fork Choice
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**Notice**: This document is a work-in-progress for researchers and implementers.
## Table of contents
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- [Introduction](#introduction)
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- [Protocols](#protocols)
- [`ExecutionEngine`](#executionengine)
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- [`forkchoice_updated`](#forkchoice_updated)
- [Helpers](#helpers)
- [`PowBlock`](#powblock)
- [`get_pow_block`](#get_pow_block)
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- [`is_valid_terminal_pow_block`](#is_valid_terminal_pow_block)
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- [Updated fork-choice handlers](#updated-fork-choice-handlers)
- [`on_block`](#on_block)
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## Introduction
This is the modification of the fork choice according to the executable beacon chain proposal.
*Note*: It introduces the process of transition from the last PoW block to the first PoS block.
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## Protocols
### `ExecutionEngine`
*Note*: The `forkchoice_updated` function is added to the `ExecutionEngine` protocol to signal the fork choice updates.
The body of this function is implementation dependent.
The Engine API may be used to implement it with an external execution engine.
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#### `forkchoice_updated`
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This function performs two actions *atomically*:
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* Re-organizes the execution payload chain and corresponding state to make `head_block_hash` the head.
* Applies finality to the execution state: it irreversibly persists the chain of all execution payloads
and corresponding state, up to and including `finalized_block_hash`.
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```python
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def forkchoice_updated(self: ExecutionEngine, head_block_hash: Hash32, finalized_block_hash: Hash32) -> None:
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...
```
## Helpers
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### `PowBlock`
```python
@dataclass
class PowBlock(object):
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block_hash: Hash32
parent_hash: Hash32
total_difficulty: uint256
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difficulty: uint256
```
### `get_pow_block`
Let `get_pow_block(block_hash: Hash32) -> PowBlock` be the function that given the hash of the PoW block returns its data.
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*Note*: The `eth_getBlockByHash` JSON-RPC method may be used to pull this information from an execution client.
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### `is_valid_terminal_pow_block`
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Used by fork-choice handler, `on_block`.
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```python
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def is_valid_terminal_pow_block(block: PowBlock, parent: PowBlock) -> bool:
is_total_difficulty_reached = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
is_parent_total_difficulty_valid = parent.total_difficulty < TERMINAL_TOTAL_DIFFICULTY
return is_total_difficulty_reached and is_parent_total_difficulty_valid
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```
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## Updated fork-choice handlers
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### `on_block`
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*Note*: The only modification is the addition of the verification of transition block conditions.
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```python
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def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
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block = signed_block.message
# Parent block must be known
assert block.parent_root in store.block_states
# Make a copy of the state to avoid mutability issues
pre_state = copy(store.block_states[block.parent_root])
# Blocks cannot be in the future. If they are, their consideration must be delayed until the are in the past.
assert get_current_slot(store) >= block.slot
# Check that block is later than the finalized epoch slot (optimization to reduce calls to get_ancestor)
finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
assert block.slot > finalized_slot
# Check block is a descendant of the finalized block at the checkpoint finalized slot
assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root
# Check the block is valid and compute the post-state
state = pre_state.copy()
state_transition(state, signed_block, True)
# [New in Merge]
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if is_merge_block(pre_state, block.body):
pow_block = get_pow_block(block.body.execution_payload.parent_hash)
pow_parent = get_pow_block(pow_block.parent_hash)
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assert is_valid_terminal_pow_block(pow_block, pow_parent)
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# Add new block to the store
store.blocks[hash_tree_root(block)] = block
# Add new state for this block to the store
store.block_states[hash_tree_root(block)] = state
# Update justified checkpoint
if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
if state.current_justified_checkpoint.epoch > store.best_justified_checkpoint.epoch:
store.best_justified_checkpoint = state.current_justified_checkpoint
if should_update_justified_checkpoint(store, state.current_justified_checkpoint):
store.justified_checkpoint = state.current_justified_checkpoint
# Update finalized checkpoint
if state.finalized_checkpoint.epoch > store.finalized_checkpoint.epoch:
store.finalized_checkpoint = state.finalized_checkpoint
# Potentially update justified if different from store
if store.justified_checkpoint != state.current_justified_checkpoint:
# Update justified if new justified is later than store justified
if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
store.justified_checkpoint = state.current_justified_checkpoint
return
# Update justified if store justified is not in chain with finalized checkpoint
finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
ancestor_at_finalized_slot = get_ancestor(store, store.justified_checkpoint.root, finalized_slot)
if ancestor_at_finalized_slot != store.finalized_checkpoint.root:
store.justified_checkpoint = state.current_justified_checkpoint
```