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Replace committee exponential backoff with max progress 

Removes the mechanism that only rotates committees if blocks have been finalized and every shard has been crosslinked or at exponentially decreasing intervals, and replaces it with a rule that shard committees can only progress a maximum of 64 epochs at a time to preserve the invariant that maximum possible work required per epoch for a validator is O(1).
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vbuterin 2019-03-19 09:32:06 -05:00 committed by GitHub
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specs/core/0_beacon-chain.md
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@ -185,8 +185,10 @@ Code snippets appearing in `this style` are to be interpreted as Python code.
| `MAX_INDICES_PER_SLASHABLE_VOTE` | `2**12` (= 4,096) |
| `MAX_EXIT_DEQUEUES_PER_EPOCH` | `2**2` (= 4) |
| `SHUFFLE_ROUND_COUNT` | 90 |
| `MAX_CROSSLINK_EPOCHS` | `2**6` (= 64) |
* For the safety of crosslinks `TARGET_COMMITTEE_SIZE` exceeds [the recommended minimum committee size of 111](https://vitalik.ca/files/Ithaca201807_Sharding.pdf); with sufficient active validators (at least `SLOTS_PER_EPOCH * TARGET_COMMITTEE_SIZE`), the shuffling algorithm ensures committee sizes of at least `TARGET_COMMITTEE_SIZE`. (Unbiasable randomness with a Verifiable Delay Function (VDF) will improve committee robustness and lower the safe minimum committee size.)
* `MAX_CROSSLINK_EPOCHS` should be a small constant times `SHARD_COUNT // EPOCH_LENGTH`
### Deposit contract
@ -598,12 +600,7 @@ The types are defined topologically to aid in facilitating an executable version
# Randomness and committees
'latest_randao_mixes': ['bytes32', LATEST_RANDAO_MIXES_LENGTH],
'previous_shuffling_start_shard': 'uint64',
'current_shuffling_start_shard': 'uint64',
'previous_shuffling_epoch': 'uint64',
'current_shuffling_epoch': 'uint64',
'previous_shuffling_seed': 'bytes32',
'current_shuffling_seed': 'bytes32',
# Finality
'previous_epoch_attestations': [PendingAttestation],
@ -849,7 +846,7 @@ def get_current_epoch_committee_count(state: BeaconState) -> int:
"""
current_active_validators = get_active_validator_indices(
state.validator_registry,
state.current_shuffling_epoch,
get_current_epoch(state),
)
return get_epoch_committee_count(len(current_active_validators))
```
@ -886,40 +883,30 @@ def get_crosslink_committees_at_slot(state: BeaconState,
next_epoch = current_epoch + 1
assert previous_epoch <= epoch <= next_epoch
committees_per_epoch = get_epoch_committee_count(get_active_validator_indices(
state.validator_registry,
epoch,
))
if epoch == current_epoch:
committees_per_epoch = get_current_epoch_committee_count(state)
seed = state.current_shuffling_seed
shuffling_epoch = state.current_shuffling_epoch
shuffling_start_shard = state.current_shuffling_start_shard
elif epoch == previous_epoch:
committees_per_epoch = get_previous_epoch_committee_count(state)
seed = state.previous_shuffling_seed
shuffling_epoch = state.previous_shuffling_epoch
shuffling_start_shard = state.previous_shuffling_start_shard
shuffling_start_shard = (
state.current_shuffling_start_shard - EPOCH_LENGTH * committees_per_epoch
) % SHARD_COUNT
elif epoch == next_epoch:
epochs_since_last_registry_update = current_epoch - state.validator_registry_update_epoch
if registry_change:
committees_per_epoch = get_next_epoch_committee_count(state)
seed = generate_seed(state, next_epoch)
shuffling_epoch = next_epoch
current_committees_per_epoch = get_current_epoch_committee_count(state)
shuffling_start_shard = (state.current_shuffling_start_shard + current_committees_per_epoch) % SHARD_COUNT
elif epochs_since_last_registry_update > 1 and is_power_of_two(epochs_since_last_registry_update):
committees_per_epoch = get_next_epoch_committee_count(state)
seed = generate_seed(state, next_epoch)
shuffling_epoch = next_epoch
shuffling_start_shard = state.current_shuffling_start_shard
else:
committees_per_epoch = get_current_epoch_committee_count(state)
seed = state.current_shuffling_seed
shuffling_epoch = state.current_shuffling_epoch
shuffling_start_shard = state.current_shuffling_start_shard
current_epoch_committees = get_epoch_committee_count(get_active_validator_indices(
state.validator_registry,
current_epoch,
))
shuffling_start_shard = (
state.current_shuffling_start_shard + EPOCH_LENGTH * current_epoch_committees
) % SHARD_COUNT
shuffling = get_shuffling(
seed,
generate_seed(state, epoch),
state.validator_registry,
shuffling_epoch,
epoch,
)
offset = slot % SLOTS_PER_EPOCH
committees_per_slot = committees_per_epoch // SLOTS_PER_EPOCH
@ -1529,12 +1516,7 @@ def get_genesis_beacon_state(genesis_validator_deposits: List[Deposit],
# Randomness and committees
latest_randao_mixes=Vector([ZERO_HASH for _ in range(LATEST_RANDAO_MIXES_LENGTH)]),
previous_shuffling_start_shard=GENESIS_START_SHARD,
current_shuffling_start_shard=GENESIS_START_SHARD,
previous_shuffling_epoch=GENESIS_EPOCH,
current_shuffling_epoch=GENESIS_EPOCH,
previous_shuffling_seed=ZERO_HASH,
current_shuffling_seed=ZERO_HASH,
# Finality
previous_epoch_attestations=[],
@ -1574,7 +1556,6 @@ def get_genesis_beacon_state(genesis_validator_deposits: List[Deposit],
genesis_active_index_root = hash_tree_root(get_active_validator_indices(state.validator_registry, GENESIS_EPOCH))
for index in range(LATEST_ACTIVE_INDEX_ROOTS_LENGTH):
state.latest_active_index_roots[index] = genesis_active_index_root
state.current_shuffling_seed = generate_seed(state, GENESIS_EPOCH)
return state
```
@ -1855,7 +1836,7 @@ def process_crosslinks(state: BeaconState) -> None:
total_balance = get_total_balance(state, crosslink_committee)
if 3 * participating_balance >= 2 * total_balance:
state.latest_crosslinks[shard] = Crosslink(
epoch=slot_to_epoch(slot),
epoch=min(slot_to_epoch(slot), state.latest_crosslinks[shard].epoch + MAX_CROSSLINK_EPOCHS),
crosslink_data_root=winning_root
)
```
@ -2060,14 +2041,6 @@ def should_update_validator_registry(state: BeaconState) -> bool:
# Must have finalized a new block
if state.finalized_epoch <= state.validator_registry_update_epoch:
return False
# Must have processed new crosslinks on all shards of the current epoch
shards_to_check = [
(state.current_shuffling_start_shard + i) % SHARD_COUNT
for i in range(get_current_epoch_committee_count(state))
]
for shard in shards_to_check:
if state.latest_crosslinks[shard].epoch <= state.validator_registry_update_epoch:
return False
return True
```
@ -2119,30 +2092,17 @@ def update_validator_registry(state: BeaconState) -> None:
Run the following function:
```python
def update_registry_and_shuffling_data(state: BeaconState) -> None:
# First set previous shuffling data to current shuffling data
state.previous_shuffling_epoch = state.current_shuffling_epoch
state.previous_shuffling_start_shard = state.current_shuffling_start_shard
state.previous_shuffling_seed = state.current_shuffling_seed
def update_registry(state: BeaconState) -> None:
current_epoch = get_current_epoch(state)
next_epoch = current_epoch + 1
# Check if we should update, and if so, update
if should_update_validator_registry(state):
update_validator_registry(state)
# If we update the registry, update the shuffling data and shards as well
state.current_shuffling_epoch = next_epoch
state.current_shuffling_start_shard = (
state.current_shuffling_start_shard +
get_current_epoch_committee_count(state) % SHARD_COUNT
)
state.current_shuffling_seed = generate_seed(state, state.current_shuffling_epoch)
else:
# If processing at least one crosslink keeps failing, then reshuffle every power of two,
# but don't update the current_shuffling_start_shard
epochs_since_last_registry_update = current_epoch - state.validator_registry_update_epoch
if epochs_since_last_registry_update > 1 and is_power_of_two(epochs_since_last_registry_update):
state.current_shuffling_epoch = next_epoch
state.current_shuffling_seed = generate_seed(state, state.current_shuffling_epoch)
# If we update the registry, update the shuffling data 2/3 or and shards as well
state.current_shuffling_start_shard = (
state.current_shuffling_start_shard +
get_current_epoch_committee_count(state) % SHARD_COUNT
)
```
**Invariant**: the active index root that is hashed into the shuffling seed actually is the `hash_tree_root` of the validator set that is used for that epoch.
@ -2397,7 +2357,8 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
# the attestation is trying to create
Crosslink(
crosslink_data_root=attestation.data.crosslink_data_root,
epoch=slot_to_epoch(attestation.data.slot)
epoch=min(slot_to_epoch(attestation.data.slot),
attestation.data.previous_crosslink.epoch + MAX_CROSSLINK_EPOCHS)
)
}
assert state.latest_crosslinks[attestation.data.shard] in acceptable_crosslink_data