eth2.0-specs/specs/phase0/fork-choice.md

# Phase 0 -- Beacon Chain Fork Choice

## Table of contents
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- [Introduction](#introduction)
- [Fork choice](#fork-choice)
  - [Constant](#constant)
  - [Configuration](#configuration)
  - [Helpers](#helpers)
    - [`LatestMessage`](#latestmessage)
    - [`Store`](#store)
    - [`is_previous_epoch_justified`](#is_previous_epoch_justified)
    - [`get_forkchoice_store`](#get_forkchoice_store)
    - [`get_slots_since_genesis`](#get_slots_since_genesis)
    - [`get_current_slot`](#get_current_slot)
    - [`compute_slots_since_epoch_start`](#compute_slots_since_epoch_start)
    - [`get_ancestor`](#get_ancestor)
    - [`get_checkpoint_block`](#get_checkpoint_block)
    - [`get_weight`](#get_weight)
    - [`get_voting_source`](#get_voting_source)
    - [`filter_block_tree`](#filter_block_tree)
    - [`get_filtered_block_tree`](#get_filtered_block_tree)
    - [`get_head`](#get_head)
    - [`update_checkpoints`](#update_checkpoints)
    - [`update_unrealized_checkpoints`](#update_unrealized_checkpoints)
    - [Pull-up tip helpers](#pull-up-tip-helpers)
      - [`compute_pulled_up_tip`](#compute_pulled_up_tip)
    - [`on_tick` helpers](#on_tick-helpers)
      - [`on_tick_per_slot`](#on_tick_per_slot)
    - [`on_attestation` helpers](#on_attestation-helpers)
      - [`validate_target_epoch_against_current_time`](#validate_target_epoch_against_current_time)
      - [`validate_on_attestation`](#validate_on_attestation)
      - [`store_target_checkpoint_state`](#store_target_checkpoint_state)
      - [`update_latest_messages`](#update_latest_messages)
  - [Handlers](#handlers)
    - [`on_tick`](#on_tick)
    - [`on_block`](#on_block)
    - [`on_attestation`](#on_attestation)
    - [`on_attester_slashing`](#on_attester_slashing)

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## Introduction

This document is the beacon chain fork choice spec, part of Phase 0. It assumes the [beacon chain state transition function spec](./beacon-chain.md).

## Fork choice

The head block root associated with a `store` is defined as `get_head(store)`. At genesis, let `store = get_forkchoice_store(genesis_state, genesis_block)` and update `store` by running:

- `on_tick(store, time)` whenever `time > store.time` where `time` is the current Unix time
- `on_block(store, block)` whenever a block `block: SignedBeaconBlock` is received
- `on_attestation(store, attestation)` whenever an attestation `attestation` is received
- `on_attester_slashing(store, attester_slashing)` whenever an attester slashing `attester_slashing` is received

Any of the above handlers that trigger an unhandled exception (e.g. a failed assert or an out-of-range list access) are considered invalid. Invalid calls to handlers must not modify `store`.

*Notes*:

1) **Leap seconds**: Slots will last `SECONDS_PER_SLOT + 1` or `SECONDS_PER_SLOT - 1` seconds around leap seconds. This is automatically handled by [UNIX time](https://en.wikipedia.org/wiki/Unix_time).
2) **Honest clocks**: Honest nodes are assumed to have clocks synchronized within `SECONDS_PER_SLOT` seconds of each other.
3) **Eth1 data**: The large `ETH1_FOLLOW_DISTANCE` specified in the [honest validator document](./validator.md) should ensure that `state.latest_eth1_data` of the canonical beacon chain remains consistent with the canonical Ethereum proof-of-work chain. If not, emergency manual intervention will be required.
4) **Manual forks**: Manual forks may arbitrarily change the fork choice rule but are expected to be enacted at epoch transitions, with the fork details reflected in `state.fork`.
5) **Implementation**: The implementation found in this specification is constructed for ease of understanding rather than for optimization in computation, space, or any other resource. A number of optimized alternatives can be found [here](https://github.com/protolambda/lmd-ghost).


### Constant

| Name                 | Value       |
| -------------------- | ----------- |
| `INTERVALS_PER_SLOT` | `uint64(3)` |

### Configuration

| Name                   | Value        |
| ---------------------- | ------------ |
| `PROPOSER_SCORE_BOOST` | `uint64(40)` |

- The proposer score boost is worth `PROPOSER_SCORE_BOOST` percentage of the committee's weight, i.e., for slot with committee weight `committee_weight` the boost weight is equal to `(committee_weight * PROPOSER_SCORE_BOOST) // 100`.

### Helpers

#### `LatestMessage`

```python
@dataclass(eq=True, frozen=True)
class LatestMessage(object):
    epoch: Epoch
    root: Root
```

#### `Store`

The `Store` is responsible for tracking information required for the fork choice algorithm. The important fields being tracked are described below:

- `justified_checkpoint`: the justified checkpoint used as the starting point for the LMD GHOST fork choice algorithm.
- `finalized_checkpoint`: the highest known finalized checkpoint. The fork choice only considers blocks that are not conflicting with this checkpoint.
- `unrealized_justified_checkpoint` & `unrealized_finalized_checkpoint`: these track the highest justified & finalized checkpoints resp., without regard to whether on-chain ***realization*** has occurred, i.e. FFG processing of new attestations within the state transition function. This is an important distinction from `justified_checkpoint` & `finalized_checkpoint`, because they will only track the checkpoints that are realized on-chain. Note that on-chain processing of FFG information only happens at epoch boundaries.
- `unrealized_justifications`: stores a map of block root to the unrealized justified checkpoint observed in that block.

```python
@dataclass
class Store(object):
    time: uint64
    genesis_time: uint64
    justified_checkpoint: Checkpoint
    finalized_checkpoint: Checkpoint
    unrealized_justified_checkpoint: Checkpoint
    unrealized_finalized_checkpoint: Checkpoint
    proposer_boost_root: Root
    equivocating_indices: Set[ValidatorIndex]
    blocks: Dict[Root, BeaconBlock] = field(default_factory=dict)
    block_states: Dict[Root, BeaconState] = field(default_factory=dict)
    checkpoint_states: Dict[Checkpoint, BeaconState] = field(default_factory=dict)
    latest_messages: Dict[ValidatorIndex, LatestMessage] = field(default_factory=dict)
    unrealized_justifications: Dict[Root, Checkpoint] = field(default_factory=dict)
```

#### `is_previous_epoch_justified`

```python
def is_previous_epoch_justified(store: Store) -> bool:
    current_slot = get_current_slot(store)
    current_epoch = compute_epoch_at_slot(current_slot)
    return store.justified_checkpoint.epoch + 1 == current_epoch
```

#### `get_forkchoice_store`

The provided anchor-state will be regarded as a trusted state, to not roll back beyond.
This should be the genesis state for a full client.

*Note* With regards to fork choice, block headers are interchangeable with blocks. The spec is likely to move to headers for reduced overhead in test vectors and better encapsulation. Full implementations store blocks as part of their database and will often use full blocks when dealing with production fork choice.

```python
def get_forkchoice_store(anchor_state: BeaconState, anchor_block: BeaconBlock) -> Store:
    assert anchor_block.state_root == hash_tree_root(anchor_state)
    anchor_root = hash_tree_root(anchor_block)
    anchor_epoch = get_current_epoch(anchor_state)
    justified_checkpoint = Checkpoint(epoch=anchor_epoch, root=anchor_root)
    finalized_checkpoint = Checkpoint(epoch=anchor_epoch, root=anchor_root)
    proposer_boost_root = Root()
    return Store(
        time=uint64(anchor_state.genesis_time + SECONDS_PER_SLOT * anchor_state.slot),
        genesis_time=anchor_state.genesis_time,
        justified_checkpoint=justified_checkpoint,
        finalized_checkpoint=finalized_checkpoint,
        unrealized_justified_checkpoint=justified_checkpoint,
        unrealized_finalized_checkpoint=finalized_checkpoint,
        proposer_boost_root=proposer_boost_root,
        equivocating_indices=set(),
        blocks={anchor_root: copy(anchor_block)},
        block_states={anchor_root: copy(anchor_state)},
        checkpoint_states={justified_checkpoint: copy(anchor_state)},
        unrealized_justifications={anchor_root: justified_checkpoint}
    )
```

#### `get_slots_since_genesis`

```python
def get_slots_since_genesis(store: Store) -> int:
    return (store.time - store.genesis_time) // SECONDS_PER_SLOT
```

#### `get_current_slot`

```python
def get_current_slot(store: Store) -> Slot:
    return Slot(GENESIS_SLOT + get_slots_since_genesis(store))
```

#### `compute_slots_since_epoch_start`

```python
def compute_slots_since_epoch_start(slot: Slot) -> int:
    return slot - compute_start_slot_at_epoch(compute_epoch_at_slot(slot))
```

#### `get_ancestor`

```python
def get_ancestor(store: Store, root: Root, slot: Slot) -> Root:
    block = store.blocks[root]
    if block.slot > slot:
        return get_ancestor(store, block.parent_root, slot)
    return root
```

#### `get_checkpoint_block`

```python
def get_checkpoint_block(store: Store, root: Root, epoch: Epoch) -> Root:
    """
    Compute the checkpoint block for epoch ``epoch`` in the chain of block ``root``
    """
    epoch_first_slot = compute_start_slot_at_epoch(epoch)
    return get_ancestor(store, root, epoch_first_slot)
```

#### `get_weight`

```python
def get_weight(store: Store, root: Root) -> Gwei:
    state = store.checkpoint_states[store.justified_checkpoint]
    unslashed_and_active_indices = [
        i for i in get_active_validator_indices(state, get_current_epoch(state))
        if not state.validators[i].slashed
    ]
    attestation_score = Gwei(sum(
        state.validators[i].effective_balance for i in unslashed_and_active_indices
        if (i in store.latest_messages
            and i not in store.equivocating_indices
            and get_ancestor(store, store.latest_messages[i].root, store.blocks[root].slot) == root)
    ))
    if store.proposer_boost_root == Root():
        # Return only attestation score if ``proposer_boost_root`` is not set
        return attestation_score

    # Calculate proposer score if ``proposer_boost_root`` is set
    proposer_score = Gwei(0)
    # Boost is applied if ``root`` is an ancestor of ``proposer_boost_root``
    if get_ancestor(store, store.proposer_boost_root, store.blocks[root].slot) == root:
        committee_weight = get_total_active_balance(state) // SLOTS_PER_EPOCH
        proposer_score = (committee_weight * PROPOSER_SCORE_BOOST) // 100
    return attestation_score + proposer_score
```

#### `get_voting_source`

```python
def get_voting_source(store: Store, block_root: Root) -> Checkpoint:
    """
    Compute the voting source checkpoint in event that block with root ``block_root`` is the head block
    """
    block = store.blocks[block_root]
    current_epoch = compute_epoch_at_slot(get_current_slot(store))
    block_epoch = compute_epoch_at_slot(block.slot)
    if current_epoch > block_epoch:
        # The block is from a prior epoch, the voting source will be pulled-up
        return store.unrealized_justifications[block_root]
    else:
        # The block is not from a prior epoch, therefore the voting source is not pulled up
        head_state = store.block_states[block_root]
        return head_state.current_justified_checkpoint

```

#### `filter_block_tree`

*Note*: External calls to `filter_block_tree` (i.e., any calls that are not made by the recursive logic in this function) MUST set `block_root` to `store.justified_checkpoint`.

```python
def filter_block_tree(store: Store, block_root: Root, blocks: Dict[Root, BeaconBlock]) -> bool:
    block = store.blocks[block_root]
    children = [
        root for root in store.blocks.keys()
        if store.blocks[root].parent_root == block_root
    ]

    # If any children branches contain expected finalized/justified checkpoints,
    # add to filtered block-tree and signal viability to parent.
    if any(children):
        filter_block_tree_result = [filter_block_tree(store, child, blocks) for child in children]
        if any(filter_block_tree_result):
            blocks[block_root] = block
            return True
        return False

    current_epoch = compute_epoch_at_slot(get_current_slot(store))
    voting_source = get_voting_source(store, block_root)

    # The voting source should be at the same height as the store's justified checkpoint
    correct_justified = (
        store.justified_checkpoint.epoch == GENESIS_EPOCH
        or voting_source.epoch == store.justified_checkpoint.epoch
    )

    # If the previous epoch is justified, the block should be pulled-up. In this case, check that unrealized
    # justification is higher than the store and that the voting source is not more than two epochs ago
    if not correct_justified and is_previous_epoch_justified(store):
        correct_justified = (
            store.unrealized_justifications[block_root].epoch >= store.justified_checkpoint.epoch and
            voting_source.epoch + 2 >= current_epoch
        )

    finalized_checkpoint_block = get_checkpoint_block(
        store,
        block_root,
        store.finalized_checkpoint.epoch,
    )

    correct_finalized = (
        store.finalized_checkpoint.epoch == GENESIS_EPOCH
        or store.finalized_checkpoint.root == finalized_checkpoint_block
    )

    # If expected finalized/justified, add to viable block-tree and signal viability to parent.
    if correct_justified and correct_finalized:
        blocks[block_root] = block
        return True

    # Otherwise, branch not viable
    return False
```

#### `get_filtered_block_tree`

```python
def get_filtered_block_tree(store: Store) -> Dict[Root, BeaconBlock]:
    """
    Retrieve a filtered block tree from ``store``, only returning branches
    whose leaf state's justified/finalized info agrees with that in ``store``.
    """
    base = store.justified_checkpoint.root
    blocks: Dict[Root, BeaconBlock] = {}
    filter_block_tree(store, base, blocks)
    return blocks
```

#### `get_head`

```python
def get_head(store: Store) -> Root:
    # Get filtered block tree that only includes viable branches
    blocks = get_filtered_block_tree(store)
    # Execute the LMD-GHOST fork choice
    head = store.justified_checkpoint.root
    while True:
        children = [
            root for root in blocks.keys()
            if blocks[root].parent_root == head
        ]
        if len(children) == 0:
            return head
        # Sort by latest attesting balance with ties broken lexicographically
        # Ties broken by favoring block with lexicographically higher root
        head = max(children, key=lambda root: (get_weight(store, root), root))
```

#### `update_checkpoints`

```python
def update_checkpoints(store: Store, justified_checkpoint: Checkpoint, finalized_checkpoint: Checkpoint) -> None:
    """
    Update checkpoints in store if necessary
    """
    # Update justified checkpoint
    if justified_checkpoint.epoch > store.justified_checkpoint.epoch:
        store.justified_checkpoint = justified_checkpoint

    # Update finalized checkpoint
    if finalized_checkpoint.epoch > store.finalized_checkpoint.epoch:
        store.finalized_checkpoint = finalized_checkpoint
```

#### `update_unrealized_checkpoints`

```python
def update_unrealized_checkpoints(store: Store, unrealized_justified_checkpoint: Checkpoint,
                                  unrealized_finalized_checkpoint: Checkpoint) -> None:
    """
    Update unrealized checkpoints in store if necessary
    """
    # Update unrealized justified checkpoint
    if unrealized_justified_checkpoint.epoch > store.unrealized_justified_checkpoint.epoch:
        store.unrealized_justified_checkpoint = unrealized_justified_checkpoint

    # Update unrealized finalized checkpoint
    if unrealized_finalized_checkpoint.epoch > store.unrealized_finalized_checkpoint.epoch:
        store.unrealized_finalized_checkpoint = unrealized_finalized_checkpoint
```


#### Pull-up tip helpers

##### `compute_pulled_up_tip`

```python
def compute_pulled_up_tip(store: Store, block_root: Root) -> None:
    state = store.block_states[block_root].copy()
    # Pull up the post-state of the block to the next epoch boundary
    process_justification_and_finalization(state)

    store.unrealized_justifications[block_root] = state.current_justified_checkpoint
    update_unrealized_checkpoints(store, state.current_justified_checkpoint, state.finalized_checkpoint)

    # If the block is from a prior epoch, apply the realized values
    block_epoch = compute_epoch_at_slot(store.blocks[block_root].slot)
    current_epoch = compute_epoch_at_slot(get_current_slot(store))
    if block_epoch < current_epoch:
        update_checkpoints(store, state.current_justified_checkpoint, state.finalized_checkpoint)
```

#### `on_tick` helpers

##### `on_tick_per_slot`

```python
def on_tick_per_slot(store: Store, time: uint64) -> None:
    previous_slot = get_current_slot(store)

    # Update store time
    store.time = time

    current_slot = get_current_slot(store)

    # If this is a new slot, reset store.proposer_boost_root
    if current_slot > previous_slot:
        store.proposer_boost_root = Root()

    # If a new epoch, pull-up justification and finalization from previous epoch
    if current_slot > previous_slot and compute_slots_since_epoch_start(current_slot) == 0:
        update_checkpoints(store, store.unrealized_justified_checkpoint, store.unrealized_finalized_checkpoint)
```

#### `on_attestation` helpers


##### `validate_target_epoch_against_current_time`

```python
def validate_target_epoch_against_current_time(store: Store, attestation: Attestation) -> None:
    target = attestation.data.target

    # Attestations must be from the current or previous epoch
    current_epoch = compute_epoch_at_slot(get_current_slot(store))
    # Use GENESIS_EPOCH for previous when genesis to avoid underflow
    previous_epoch = current_epoch - 1 if current_epoch > GENESIS_EPOCH else GENESIS_EPOCH
    # If attestation target is from a future epoch, delay consideration until the epoch arrives
    assert target.epoch in [current_epoch, previous_epoch]
```

##### `validate_on_attestation`

```python
def validate_on_attestation(store: Store, attestation: Attestation, is_from_block: bool) -> None:
    target = attestation.data.target

    # If the given attestation is not from a beacon block message, we have to check the target epoch scope.
    if not is_from_block:
        validate_target_epoch_against_current_time(store, attestation)

    # Check that the epoch number and slot number are matching
    assert target.epoch == compute_epoch_at_slot(attestation.data.slot)

    # Attestation target must be for a known block. If target block is unknown, delay consideration until block is found
    assert target.root in store.blocks

    # Attestations must be for a known block. If block is unknown, delay consideration until the block is found
    assert attestation.data.beacon_block_root in store.blocks
    # Attestations must not be for blocks in the future. If not, the attestation should not be considered
    assert store.blocks[attestation.data.beacon_block_root].slot <= attestation.data.slot

    # LMD vote must be consistent with FFG vote target
    assert target.root == get_checkpoint_block(store, attestation.data.beacon_block_root, target.epoch)

    # Attestations can only affect the fork choice of subsequent slots.
    # Delay consideration in the fork choice until their slot is in the past.
    assert get_current_slot(store) >= attestation.data.slot + 1
```

##### `store_target_checkpoint_state`

```python
def store_target_checkpoint_state(store: Store, target: Checkpoint) -> None:
    # Store target checkpoint state if not yet seen
    if target not in store.checkpoint_states:
        base_state = copy(store.block_states[target.root])
        if base_state.slot < compute_start_slot_at_epoch(target.epoch):
            process_slots(base_state, compute_start_slot_at_epoch(target.epoch))
        store.checkpoint_states[target] = base_state
```

##### `update_latest_messages`

```python
def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIndex], attestation: Attestation) -> None:
    target = attestation.data.target
    beacon_block_root = attestation.data.beacon_block_root
    non_equivocating_attesting_indices = [i for i in attesting_indices if i not in store.equivocating_indices]
    for i in non_equivocating_attesting_indices:
        if i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch:
            store.latest_messages[i] = LatestMessage(epoch=target.epoch, root=beacon_block_root)
```


### Handlers

#### `on_tick`

```python
def on_tick(store: Store, time: uint64) -> None:
    # If the ``store.time`` falls behind, while loop catches up slot by slot
    # to ensure that every previous slot is processed with ``on_tick_per_slot``
    tick_slot = (time - store.genesis_time) // SECONDS_PER_SLOT
    while get_current_slot(store) < tick_slot:
        previous_time = store.genesis_time + (get_current_slot(store) + 1) * SECONDS_PER_SLOT
        on_tick_per_slot(store, previous_time)
    on_tick_per_slot(store, time)
```

#### `on_block`

```python
def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
    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 they 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
    finalized_checkpoint_block = get_checkpoint_block(
        store,
        block.parent_root,
        store.finalized_checkpoint.epoch,
    )
    assert store.finalized_checkpoint.root == finalized_checkpoint_block    

    # Check the block is valid and compute the post-state
    state = pre_state.copy()
    block_root = hash_tree_root(block)
    state_transition(state, signed_block, True)
    # Add new block to the store
    store.blocks[block_root] = block
    # Add new state for this block to the store
    store.block_states[block_root] = state

    # Add proposer score boost if the block is timely
    time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT
    is_before_attesting_interval = time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT
    if get_current_slot(store) == block.slot and is_before_attesting_interval:
        store.proposer_boost_root = hash_tree_root(block)

    # Update checkpoints in store if necessary
    update_checkpoints(store, state.current_justified_checkpoint, state.finalized_checkpoint)

    # Eagerly compute unrealized justification and finality
    compute_pulled_up_tip(store, block_root)
```

#### `on_attestation`

```python
def on_attestation(store: Store, attestation: Attestation, is_from_block: bool=False) -> None:
    """
    Run ``on_attestation`` upon receiving a new ``attestation`` from either within a block or directly on the wire.

    An ``attestation`` that is asserted as invalid may be valid at a later time,
    consider scheduling it for later processing in such case.
    """
    validate_on_attestation(store, attestation, is_from_block)

    store_target_checkpoint_state(store, attestation.data.target)

    # Get state at the `target` to fully validate attestation
    target_state = store.checkpoint_states[attestation.data.target]
    indexed_attestation = get_indexed_attestation(target_state, attestation)
    assert is_valid_indexed_attestation(target_state, indexed_attestation)

    # Update latest messages for attesting indices
    update_latest_messages(store, indexed_attestation.attesting_indices, attestation)
```

#### `on_attester_slashing`

*Note*: `on_attester_slashing` should be called while syncing and a client MUST maintain the equivocation set of `AttesterSlashing`s from at least the latest finalized checkpoint.

```python
def on_attester_slashing(store: Store, attester_slashing: AttesterSlashing) -> None:
    """
    Run ``on_attester_slashing`` immediately upon receiving a new ``AttesterSlashing``
    from either within a block or directly on the wire.
    """
    attestation_1 = attester_slashing.attestation_1
    attestation_2 = attester_slashing.attestation_2
    assert is_slashable_attestation_data(attestation_1.data, attestation_2.data)
    state = store.block_states[store.justified_checkpoint.root]
    assert is_valid_indexed_attestation(state, attestation_1)
    assert is_valid_indexed_attestation(state, attestation_2)

    indices = set(attestation_1.attesting_indices).intersection(attestation_2.attesting_indices)
    for index in indices:
        store.equivocating_indices.add(index)
```