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# Ethereum 2.0 Phase 1 -- The Beacon Chain with Shards
**Notice**: This document is a work-in-progress for researchers and implementers.
## Table of contents
<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
- [Introduction](#introduction)
- [Custom types](#custom-types)
- [Configuration](#configuration)
- [Misc](#misc)
- [Shard block configs](#shard-block-configs)
- [Precomputed size verification points](#precomputed-size-verification-points)
- [Gwei values](#gwei-values)
- [Time parameters](#time-parameters)
- [Domain types](#domain-types)
- [Updated containers](#updated-containers)
- [`AttestationData`](#attestationdata)
- [`BeaconBlock`](#beaconblock)
- [`BeaconState`](#beaconstate)
- [New containers](#new-containers)
- [`DataCommitment`](#datacommitment)
- [`ShardHeader`](#shardheader)
- [`PendingShardHeader`](#pendingshardheader)
- [Helper functions](#helper-functions)
- [Misc](#misc-1)
- [`compute_previous_slot`](#compute_previous_slot)
- [`compute_shard_from_committee_index`](#compute_shard_from_committee_index)
- [`compute_updated_gasprice`](#compute_updated_gasprice)
- [`compute_committee_source_epoch`](#compute_committee_source_epoch)
- [Beacon state accessors](#beacon-state-accessors)
- [Updated `get_committee_count_per_slot`](#updated-get_committee_count_per_slot)
- [`get_active_shard_count`](#get_active_shard_count)
- [`get_shard_committee`](#get_shard_committee)
- [`get_shard_proposer_index`](#get_shard_proposer_index)
- [`get_start_shard`](#get_start_shard)
- [Predicates](#predicates)
- [Block processing](#block-processing)
- [Operations](#operations)
- [New Attestation processing](#new-attestation-processing)
- [Updated `process_attestation`](#updated-process_attestation)
- [`update_pending_votes`](#update_pending_votes)
- [`process_shard_header`](#process_shard_header)
- [Shard transition processing](#shard-transition-processing)
- [Epoch transition](#epoch-transition)
- [Pending headers](#pending-headers)
- [Phase 1 final updates](#phase-1-final-updates)
- [Custody game updates](#custody-game-updates)
<!-- END doctoc generated TOC please keep comment here to allow auto update -->
## Introduction
This document describes the extensions made to the Phase 0 design of The Beacon Chain to support data sharding, based on the ideas [here](https://hackmd.io/@HWeNw8hNRimMm2m2GH56Cw/r1XzqYIOv) and more broadly [here](https://arxiv.org/abs/1809.09044), using Kate commitments to commit to data to remove any need for fraud proofs (and hence, safety-critical synchrony assumptions) in the design.
## Custom types
We define the following Python custom types for type hinting and readability:
| Name | SSZ equivalent | Description |
| - | - | - |
| `Shard` | `uint64` | A shard number |
| `BLSCommitment` | `bytes48` | A G1 curve point |
## Configuration
### Misc
| Name | Value | Notes |
| - | - | - |
| `MAX_SHARDS` | `uint64(2**10)` (= 1024) | Theoretical max shard count (used to determine data structure sizes) |
| `INITIAL_ACTIVE_SHARDS` | `uint64(2**6)` (= 64) | Initial shard count |
| `GASPRICE_ADJUSTMENT_COEFFICIENT` | `uint64(2**3)` (= 2) | Gasprice may decrease/increase by at most exp(1 / this value) *per epoch* |
| `MAX_SHARD_HEADERS` | `MAX_SHARDS * 4` | |
### Shard block configs
| Name | Value | Notes |
| - | - | - |
| `POINTS_PER_SAMPLE` | `uint64(2**3)` (= 8) | 31 * 8 = 248 bytes |
| `MAX_SAMPLES_PER_BLOCK` | `uint64(2**11)` (= 2,048) | 248 * 2,048 = 507,904 bytes |
| `TARGET_SAMPLES_PER_BLOCK` | `uint64(2**10)` (= 1,024) | 248 * 1,024 = 253,952 bytes |
### Precomputed size verification points
| Name | Value |
| - | - |
| `G2_ONE` | The G2 generator |
| `SIZE_CHECK_POINTS` | Type `List[G2, MAX_SAMPLES_PER_BLOCK + 1]`; TO BE COMPUTED |
These points are the G2-side Kate commitments to `product[a in i...MAX_SAMPLES_PER_BLOCK] (X - w ** revbit(a))` for each `i` in `[0...MAX_SAMPLES_PER_BLOCK]`, where `w` is the root of unity and `revbit` is the reverse-bit-order function. They are used to verify block size proofs. They can be computed with a one-time O(N^2/log(N)) calculation using fast-linear-combinations in G2.
### Gwei values
| Name | Value | Unit | Description |
| - | - | - | - |
| `MAX_GASPRICE` | `Gwei(2**24)` (= 16,777,216) | Gwei | Max gasprice charged for an TARGET-sized shard block |
| `MIN_GASPRICE` | `Gwei(2**3)` (= 8) | Gwei | Min gasprice charged for an TARGET-sized shard block |
### Time parameters
| Name | Value | Unit | Duration |
| - | - | :-: | :-: |
| `SHARD_COMMITTEE_PERIOD` | `Epoch(2**8)` (= 256) | epochs | ~27 hours |
### Domain types
| Name | Value |
| - | - |
| `DOMAIN_SHARD_HEADER` | `DomainType('0x80000000')` |
## Updated containers
The following containers have updated definitions in Phase 1.
### `AttestationData`
```python
class AttestationData(Container):
slot: Slot
index: CommitteeIndex
# LMD GHOST vote
beacon_block_root: Root
# FFG vote
source: Checkpoint
target: Checkpoint
# Shard header root
shard_header_root: Root
```
### `BeaconBlock`
```python
class BeaconBlock(phase0.BeaconBlock):
shard_headers: List[Signed[ShardHeader], MAX_SHARD_HEADERS]
```
### `BeaconState`
```python
class BeaconState(phase0.BeaconState):
current_epoch_pending_headers: List[PendingHeader, MAX_PENDING_HEADERS * SLOTS_PER_EPOCH]
previous_epoch_pending_headers: List[PendingHeader, MAX_PENDING_HEADERS * SLOTS_PER_EPOCH]
most_recent_confirmed_commitments: Vector[Vector[DataCommitment, SLOTS_PER_EPOCH], MAX_SHARDS]
shard_gasprice: uint64
current_epoch_start_shard: Shard
```
## New containers
The following containers are new in Phase 1.
### `DataCommitment`
```python
class DataCommitment(Container):
# Kate commitment to the data
point: BLSCommitment
# Length of the data in samples
length: length
```
### `ShardHeader`
```python
class ShardHeader(Container):
# Slot and shard that this header is intended for
slot: Slot
shard: Shard
# The actual data commitment
commitment: DataCommitment
# Proof of the length (more precisely, proof that values at
# positions >= the length all equal zero)
length_proof: BLSCommitment
```
### `PendingShardHeader`
```python
class PendingShardHeader(Container):
# Slot and shard that this header is intended for
slot: uint64
shard: Shard
# Kate commitment to the data
commitment: BLSCommitment
# hash_tree_root of the ShardHeader (stored so that attestations
# can be checked against it)
root: Hash
# Length of the data in samples
length: uint64
# Who voted for the header
votes: Bitlist[MAX_COMMITTEE_SIZE]
# Has this header been confirmed?
confirmed: bool
```
## Helper functions
### Misc
#### `compute_previous_slot`
```python
def compute_previous_slot(slot: Slot) -> Slot:
if slot > 0:
return Slot(slot - 1)
else:
return Slot(0)
```
#### `compute_shard_from_committee_index`
```python
def compute_shard_from_committee_index(state: BeaconState, index: CommitteeIndex, slot: Slot) -> Shard:
active_shards = get_active_shard_count(state)
return Shard((index + get_start_shard(state, slot)) % active_shards)
```
#### `compute_updated_gasprice`
```python
def compute_updated_gasprice(prev_gasprice: Gwei, shard_block_length: uint64, adjustment_quotient: uint64) -> Gwei:
if shard_block_length > TARGET_SAMPLES_PER_BLOCK:
delta = (prev_gasprice * (shard_block_length - TARGET_SAMPLES_PER_BLOCK)
// TARGET_SAMPLES_PER_BLOCK // adjustment_quotient)
return min(prev_gasprice + delta, MAX_GASPRICE)
else:
delta = (prev_gasprice * (TARGET_SAMPLES_PER_BLOCK - shard_block_length)
// TARGET_SAMPLES_PER_BLOCK // adjustment_quotient)
return max(prev_gasprice, MIN_GASPRICE + delta) - delta
```
#### `compute_committee_source_epoch`
```python
def compute_committee_source_epoch(epoch: Epoch, period: uint64) -> Epoch:
"""
Return the source epoch for computing the committee.
"""
source_epoch = Epoch(epoch - epoch % period)
if source_epoch >= period:
source_epoch -= period # `period` epochs lookahead
return source_epoch
```
### Beacon state accessors
#### Updated `get_committee_count_per_slot`
```python
def get_committee_count_per_slot(state: BeaconState, epoch: Epoch) -> uint64:
"""
Return the number of committees in each slot for the given ``epoch``.
"""
return max(uint64(1), min(
get_active_shard_count(state, epoch),
uint64(len(get_active_validator_indices(state, epoch))) // SLOTS_PER_EPOCH // TARGET_COMMITTEE_SIZE,
))
```
#### `get_active_shard_count`
```python
def get_active_shard_count(state: BeaconState, epoch: Epoch) -> uint64:
"""
Return the number of active shards.
Note that this puts an upper bound on the number of committees per slot.
"""
return INITIAL_ACTIVE_SHARDS
```
#### `get_shard_committee`
```python
def get_shard_committee(beacon_state: BeaconState, epoch: Epoch, shard: Shard) -> Sequence[ValidatorIndex]:
"""
Return the shard committee of the given ``epoch`` of the given ``shard``.
"""
source_epoch = compute_committee_source_epoch(epoch, SHARD_COMMITTEE_PERIOD)
active_validator_indices = get_active_validator_indices(beacon_state, source_epoch)
seed = get_seed(beacon_state, source_epoch, DOMAIN_SHARD_COMMITTEE)
return compute_committee(
indices=active_validator_indices,
seed=seed,
index=shard,
count=get_active_shard_count(beacon_state, epoch),
)
```
#### `get_shard_proposer_index`
```python
def get_shard_proposer_index(beacon_state: BeaconState, slot: Slot, shard: Shard) -> ValidatorIndex:
"""
Return the proposer's index of shard block at ``slot``.
"""
epoch = compute_epoch_at_slot(slot)
committee = get_shard_committee(beacon_state, epoch, shard)
seed = hash(get_seed(beacon_state, epoch, DOMAIN_SHARD_COMMITTEE) + uint_to_bytes(slot))
return compute_proposer_index(state, committee, seed)
```
#### `get_start_shard`
```python
def get_start_shard(state: BeaconState, slot: Slot) -> Shard:
"""
Return the start shard at ``slot``.
"""
current_epoch_start_slot = compute_start_slot_at_epoch(get_current_epoch(state))
shard = state.current_epoch_start_shard
if slot > current_epoch_start_slot:
# Current epoch or the next epoch lookahead
for _slot in range(current_epoch_start_slot, slot):
committee_count = get_committee_count_per_slot(state, compute_epoch_at_slot(Slot(_slot)))
active_shard_count = get_active_shard_count(state, compute_epoch_at_slot(Slot(_slot)))
shard = (shard + committee_count) % active_shard_count
return Shard(shard)
elif slot < current_epoch_start_slot:
# Previous epoch
for _slot in list(range(slot, current_epoch_start_slot))[::-1]:
committee_count = get_committee_count_per_slot(state, compute_epoch_at_slot(Slot(_slot)))
active_shard_count = get_active_shard_count(state, compute_epoch_at_slot(Slot(_slot)))
# Ensure positive
shard = (shard + active_shard_count - committee_count) % active_shard_count
return Shard(shard)
```
### Predicates
### Block processing
```python
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_light_client_aggregate(state, block.body)
process_operations(state, block.body)
```
#### Operations
```python
def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
# Verify that outstanding deposits are processed up to the maximum number of deposits
assert len(body.deposits) == min(MAX_DEPOSITS, state.eth1_data.deposit_count - state.eth1_deposit_index)
def for_ops(operations: Sequence[Any], fn: Callable[[BeaconState, Any], None]) -> None:
for operation in operations:
fn(state, operation)
for_ops(body.proposer_slashings, process_proposer_slashing)
for_ops(body.attester_slashings, process_attester_slashing)
# New attestation processing
for_ops(body.attestations, process_attestation)
for_ops(body.deposits, process_deposit)
for_ops(body.voluntary_exits, process_voluntary_exit)
# Limit is dynamic based on active shard count
assert len(body.shard_headers) <= 4 * get_active_shard_count(state, get_current_epoch(state))
for_ops(body.shard_headers, process_shard_header)
# See custody game spec.
process_custody_game_operations(state, body)
process_shard_transitions(state, body.shard_transitions, body.attestations)
# TODO process_operations(body.shard_receipt_proofs, process_shard_receipt_proofs)
```
### New Attestation processing
#### Updated `process_attestation`
```python
def process_attestation(state: BeaconState, attestation: Attestation) -> None:
phase0.process_attestation(state, attestation)
update_pending_votes(
state=state,
attestation: Attestation,
root=,
aggregation_bits=attestation.aggregation_bits
)
```
#### `update_pending_votes`
```python
def update_pending_votes(state: BeaconState,
attestation: Attestation) -> None:
if compute_epoch_at_slot(slot) == get_current_epoch(state):
pending_headers = state.current_epoch_pending_headers
else:
pending_headers = state.previous_epoch_pending_headers
# Create or update the PendingShardHeader object
pending_header = None
for header in pending_headers:
if header.root == attestation.data.shard_header_root:
pending_header = header
assert pending_header is not None
assert pending_header.slot == attestation.data.slot + 1
assert pending_header.shard == compute_shard_from_committee_index(
state,
attestation.data.index,
attestation.data.slot
)
pending_header.votes = bitwise_or(
pending_header.votes,
attestation.aggregation_bits
)
# Check if the PendingShardHeader is eligible for expedited confirmation
# Requirement 1: nothing else confirmed
all_candidates = [
c for c in pending_headers if
(c.slot, c.shard) == (pending_header.slot, pending_header.shard)
]
if True not in [c.confirmed for c in all_candidates]:
# Requirement 2: >= 2/3 of balance attesting
participants = get_attesting_indices(state, data, pending_commitment.votes)
participants_balance = get_total_balance(state, participants)
full_committee = get_beacon_committee(state, data.slot, data.shard)
full_committee_balance = get_total_balance(state, full_committee)
if participants_balance * 2 > full_committee_balance:
pending_header.confirmed = True
```
#### `process_shard_header`
```python
def process_shard_header(state: BeaconState,
signed_header: Signed[ShardDataHeader]) -> None:
header = signed_header.message
header_root = hash_tree_root(header)
# Verify signature
signer_index = get_shard_proposer_index(state, header.slot, header.shard)
assert bls.Verify(
state.validators[signer_index].pubkey,
compute_signing_root(header, get_domain(state, DOMAIN_SHARD_HEADER)),
signed_header.signature
)
# Verify length of the header
assert (
bls.Pairing(header.length_proof, SIZE_CHECK_POINTS[header.commitment.length]) ==
bls.Pairing(header.commitment.point, G2_ONE)
)
# Get the correct pending header list
if compute_epoch_at_slot(header.slot) == get_current_epoch(state):
pending_headers = state.current_epoch_pending_headers
else:
pending_headers = state.previous_epoch_pending_headers
# Check that this header is not yet in the pending list
for pending_header in pending_headers:
assert header_root != pending_header.root
# Include it in the pending list
committee_length = len(get_beacon_committee(state, header.slot, header.shard))
pending_headers.append(PendingShardHeader(
slot=header.slot,
shard=header.shard,
commitment=header.commitment,
root=header_root,
votes=Bitlist[MAX_COMMITTEE_SIZE]([0] * committee_length),
confirmed=False
))
```
### Shard transition processing
### Epoch transition
This epoch transition overrides the phase0 epoch transition:
```python
def process_epoch(state: BeaconState) -> None:
process_justification_and_finalization(state)
process_rewards_and_penalties(state)
process_registry_updates(state)
# Proof of custody
process_reveal_deadlines(state)
process_challenge_deadlines(state)
process_slashings(state)
# Sharding
process_pending_headers(state)
charge_confirmed_header_fees(state)
reset_pending_headers(state)
# Final updates
# Phase 0
process_eth1_data_reset(state)
process_effective_balance_updates(state)
process_slashings_reset(state)
process_randao_mixes_reset(state)
process_historical_roots_update(state)
process_participation_record_updates(state)
# Proof of custody
process_custody_final_updates(state)
# Update current_epoch_start_shard
state.current_epoch_start_shard = get_start_shard(state, Slot(state.slot + 1))
```
#### Pending headers
```python
def process_pending_headers(state: BeaconState):
for slot in range(SLOTS_PER_EPOCH):
for shard in range(SHARD_COUNT):
# Pending headers for this (slot, shard) combo
candidates = [
c for c in state.previous_epoch_pending_headers if
(c.slot, c.shard) == (slot, shard)
]
if True not in [c.confirmed for c in candidates]:
# The entire committee (and its balance)
full_committee = get_beacon_committee(state, slot, shard)
full_committee_balance = get_total_balance(state, full_committee)
# The set of voters who voted for each header
# (and their total balances)
voting_sets = [
[v for i, v in enumerate(full_committee) if c.votes[i]]
for c in candidates
]
voting_balances = [
get_total_balance(state, voters)
for voters in voting_sets
]
# Get the index with the most total balance voting for them.
# NOTE: if two choices get exactly the same voting balance,
# the candidate earlier in the list wins
if max(voting_balances) > 0:
winning_index = voting_balances.index(max(voting_balances))
else:
# If no votes, zero wins
winning_index = [c.root for c in candidates].index(Root())
candidates[winning_index].confirmed = True
for slot in range(SLOTS_PER_EPOCH):
for shard in range(SHARD_COUNT):
state.most_recent_confirmed_commitments[shard][slot] = DataCommitment()
for c in state.previous_epoch_pending_headers:
if c.confirmed:
state.most_recent_confirmed_commitments[c.shard][c.slot % SLOTS_PER_EPOCH] = c.commitment
```
```python
def charge_confirmed_header_fees(state: BeaconState) -> None:
new_gasprice = state.shard_gasprice
adjustment_quotient = get_active_shard_count(state) * SLOTS_PER_EPOCH * GASPRICE_ADJUSTMENT_COEFFICIENT
for slot in range(SLOTS_PER_EPOCH):
for shard in range(SHARD_COUNT):
confirmed_candidates = [
c for c in state.previous_epoch_pending_headers if
(c.slot, c.shard, c.confirmed) == (slot, shard, True)
]
if confirmed_candidates:
candidate = confirmed_candidates[0]
# Charge EIP 1559 fee
proposer = get_shard_proposer(state, slot, shard)
fee = (
(state.shard_gasprice * candidates[i].commitment.length) //
TARGET_SAMPLES_PER_BLOCK
)
decrease_balance(state, proposer, fee)
new_gasprice = compute_updated_gasprice(
new_gasprice,
candidates[i].commitment.length,
adjustment_quotient
)
state.shard_gasprice = new_gasprice
```
```python
def reset_pending_headers(state: BeaconState):
state.previous_epoch_pending_headers = state.current_epoch_pending_headers
shards = [
compute_shard_from_committee_index(state, index, slot)
for i in range()
state,
attestation.data.index,
attestation.data.slot
)
state.current_epoch_pending_headers = []
# Add dummy "empty" PendingAttestations
# (default to vote for if no shard header available)
for slot in range(SLOTS_IN_EPOCH):
for index in range(get_committee_count_per_slot(get_current_epoch(state))):
shard = compute_shard_from_committee_index(state, index, slot)
committee_length = len(get_beacon_committee(
state,
header.slot,
header.shard
))
state.current_epoch_pending_headers.append(PendingShardHeader(
slot=slot,
shard=shard,
commitment=DataCommitment(),
root=Root(),
votes=Bitlist[MAX_COMMITTEE_SIZE]([0] * committee_length),
confirmed=False
))
```
#### Custody game updates
`process_reveal_deadlines`, `process_challenge_deadlines` and `process_custody_final_updates` are defined in [the Custody Game spec](./custody-game.md).
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