eth2.0-specs/specs/core/0_fork-choice.md

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2019-04-22 13:20:48 +00:00
# Ethereum 2.0 Phase 0 -- Beacon Chain Fork Choice
**NOTICE**: This document is a work in progress for researchers and implementers.
## Table of contents
<!-- TOC -->
- [Ethereum 2.0 Phase 0 -- Beacon Chain Fork Choice](#ethereum-20-phase-0----beacon-chain-fork-choice)
- [Table of contents](#table-of-contents)
- [Introduction](#introduction)
- [Constants](#constants)
- [Time parameters](#time-parameters)
- [Beacon chain processing](#beacon-chain-processing)
- [Beacon chain fork choice rule](#beacon-chain-fork-choice-rule)
<!-- /TOC -->
## Introduction
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This document represents is the specification for the beacon chain fork choice rule, part of Ethereum 2.0 phase 0.
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## Constants
### Time parameters
| Name | Value | Unit | Duration |
| - | - | :-: | :-: |
| `SECONDS_PER_SLOT` | `6` | seconds | 6 seconds |
## Beacon chain processing
Processing the beacon chain is similar to processing the Ethereum 1.0 chain. Clients download and process blocks and maintain a view of what is the current "canonical chain", terminating at the current "head". For a beacon block, `block`, to be processed by a node, the following conditions must be met:
* The parent block with root `block.previous_block_root` has been processed and accepted.
* An Ethereum 1.0 block pointed to by the `state.latest_eth1_data.block_hash` has been processed and accepted.
* The node's Unix time is greater than or equal to `state.genesis_time + block.slot * SECONDS_PER_SLOT`.
Note: Leap seconds mean that slots will occasionally last `SECONDS_PER_SLOT + 1` or `SECONDS_PER_SLOT - 1` seconds, possibly several times a year.
Note: Nodes needs to have a clock that is roughly (i.e. within `SECONDS_PER_SLOT` seconds) synchronized with the other nodes.
### Beacon chain fork choice rule
The beacon chain fork choice rule is a hybrid that combines justification and finality with Latest Message Driven (LMD) Greediest Heaviest Observed SubTree (GHOST). At any point in time a [validator](#dfn-validator) `v` subjectively calculates the beacon chain head as follows.
* Abstractly define `Store` as the type of storage object for the chain data and `store` be the set of attestations and blocks that the [validator](#dfn-validator) `v` has observed and verified (in particular, block ancestors must be recursively verified). Attestations not yet included in any chain are still included in `store`.
* Let `finalized_head` be the finalized block with the highest epoch. (A block `B` is finalized if there is a descendant of `B` in `store` the processing of which sets `B` as finalized.)
* Let `justified_head` be the descendant of `finalized_head` with the highest epoch that has been justified for at least 1 epoch. (A block `B` is justified if there is a descendant of `B` in `store` the processing of which sets `B` as justified.) If no such descendant exists set `justified_head` to `finalized_head`.
* Let `get_ancestor(store: Store, block: BeaconBlock, slot: Slot) -> BeaconBlock` be the ancestor of `block` with slot number `slot`. The `get_ancestor` function can be defined recursively as:
```python
def get_ancestor(store: Store, block: BeaconBlock, slot: Slot) -> BeaconBlock:
"""
Get the ancestor of ``block`` with slot number ``slot``; return ``None`` if not found.
"""
if block.slot == slot:
return block
elif block.slot < slot:
return None
else:
return get_ancestor(store, store.get_parent(block), slot)
```
* Let `get_latest_attestation(store: Store, index: ValidatorIndex) -> Attestation` be the attestation with the highest slot number in `store` from the validator with the given `index`. If several such attestations exist, use the one the [validator](#dfn-validator) `v` observed first.
* Let `get_latest_attestation_target(store: Store, index: ValidatorIndex) -> BeaconBlock` be the target block in the attestation `get_latest_attestation(store, index)`.
* Let `get_children(store: Store, block: BeaconBlock) -> List[BeaconBlock]` returns the child blocks of the given `block`.
* Let `justified_head_state` be the resulting `BeaconState` object from processing the chain up to the `justified_head`.
* The `head` is `lmd_ghost(store, justified_head_state, justified_head)` where the function `lmd_ghost` is defined below. Note that the implementation below is suboptimal; there are implementations that compute the head in time logarithmic in slot count.
```python
def lmd_ghost(store: Store, start_state: BeaconState, start_block: BeaconBlock) -> BeaconBlock:
"""
Execute the LMD-GHOST algorithm to find the head ``BeaconBlock``.
"""
validators = start_state.validator_registry
active_validator_indices = get_active_validator_indices(validators, slot_to_epoch(start_state.slot))
attestation_targets = [(i, get_latest_attestation_target(store, i)) for i in active_validator_indices]
# Use the rounded-balance-with-hysteresis supplied by the protocol for fork
# choice voting. This reduces the number of recomputations that need to be
# made for optimized implementations that precompute and save data
def get_vote_count(block: BeaconBlock) -> int:
return sum(
start_state.validator_registry[validator_index].high_balance
for validator_index, target in attestation_targets
if get_ancestor(store, target, block.slot) == block
)
head = start_block
while 1:
children = get_children(store, head)
if len(children) == 0:
return head
# Ties broken by favoring block with lexicographically higher root
head = max(children, key=lambda x: (get_vote_count(x), hash_tree_root(x)))
```