159 lines
6.3 KiB
Markdown
159 lines
6.3 KiB
Markdown
# Beacon Block Flow
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This is a WIP document to explain the beacon block flows.
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## Validation & Verification flow
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Important distinction:
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- We distinguish block `validation` which is defined in the P2P specs:
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https://github.com/ethereum/consensus-specs/blob/v1.4.0/specs/phase0/p2p-interface.md#beacon_block.
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A validated block can be forwarded on gossipsub.
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- and we distinguish `verification` which is defined in consensus specs:
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https://github.com/ethereum/consensus-specs/blob/v1.5.0-alpha.5/specs/phase0/beacon-chain.md#block-processing
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A block needs to be verified to enter fork choice, the DAG and the BeaconChainDB
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In particular in terms of costly checks validating a block only requires checking:
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- the **block proposer signature**
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while verifying a block requires checking:
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- the state transition
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- the block proposer signature
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- and also the signatures within a block:
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- Randao Reveal
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- Proposer slashings
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- Attester slashings
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- Attestations
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- VoluntaryExits
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### Inputs
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Blocks can be received from the following sources:
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- Gossipsub
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- via `topicBeaconBlocks` (`&"/eth2/{$forkDigest}/{topicBeaconBlocksSuffix}"`). They are then validated by `blockValidator` (in `eth2_processor.nim`) which calls `validateBeaconBlock`
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- ETH2 RPC
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- via SyncManager (when Nimbus syncs)
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- via RequestManager (when Nimbus needs ancestor blocks)
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- the NBC database
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- a local validator block proposal
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- Devtools: test suite, ncli, fuzzing
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The related base types are:
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- BeaconBlockBody
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- BeaconBlock
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- BeaconBlockBody
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- + metadata (slot, blockchain state before/after, proposer)
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- BeaconBlockHeader
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- metadata (slot, blockchain state before/after, proposer)
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- Merkle hash of the BeaconBlockBody
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- SignedBeaconBlock
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- BeaconBlock
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- + BLS signature
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The base types are defined in the Eth2 specs.
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On top, Nimbus builds new types to represent the level of trust and validation we have with regards to each BeaconBlock.
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Those types allow the Nim compiler to help us ensure proper usage at compile-time and zero runtime cost.
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#### BeaconBlocks
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Those are spec-defined types.
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On deserialization the SSZ code guarantees that BeaconBlock are correctly max-sized
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according to:
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- MAX_PROPOSER_SLASHINGS
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- MAX_ATTESTER_SLASHINGS
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- MAX_ATTESTATIONS
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- MAX_DEPOSITS
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- MAX_VOLUNTARY_EXITS
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#### TrustedBeaconBlocks
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A block that has been fully checked to be sound
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both in regards to the blockchain protocol and its cryptographic signatures is known as a `TrustedBeaconBlock` or `TrustedSignedBeaconBlock`.
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This allows skipping expensive signature checks.
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Blocks are considered trusted if they come from:
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- the NBC database
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- produced by a local validator
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#### SigVerifiedBeaconBlocks
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A block with a valid cryptographic signature is considered SigVerified.
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This is a weaker guarantee than Trusted as the block might still be invalid according to the state transition function.
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Such a block is produced if incoming gossip blocks' signatures are batched together for batch verification **before** being passed to state transition.
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#### TransitionVerifiedBeaconBlocks
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A block that passes the state transition checks and can be successfully applied to the beacon chain is considered `TransitionVerified`.
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Such a block can be produced if incoming blocks' signatures are batched together for batch verification **after** successfully passing state transition.
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_This is not used in Nimbus at the moment_
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## Block processing architecture
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How the various modules interact with block is described in a diagram:
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It is important to note that 3 data structures are sharing the same `AsyncQueue[BlockEntry]`:
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- Eth2Processor.blockQueue
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- SyncManager.outQueue
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- RequestManager.outQueue
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### Gossip flow in
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Blocks are listened to via the gossipsub topic `/eth2/{$forkDigest}/beacon_block/ssz_snappy` (`topicBeaconBlocks` variable)
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They are then:
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- validated by `blockValidator()` in the Eth2Processor by `validateBeaconBlock()` according to spec https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.4/specs/phase0/p2p-interface.md#beacon_block
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- Important: P2P validation is not full verification (state transition and internal cryptographic signatures were not checked)
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- enqueued in the shared block queue `AsyncQueue[BlockEntry]` in case of success
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- dropped in case of error
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Logs:
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- debug: "Dropping already-seen gossip block"
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- debug: "Block received"
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- trace: "Block validated"
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### Gossip flow out
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- After validation in `blockValidator()` in the Eth2Processor by `validateBeaconBlock()` according to spec https://github.com/ethereum/consensus-specs/blob/v1.4.0/specs/phase0/p2p-interface.md#beacon_block
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- Important: P2P validation is not full verification (state transition and internal cryptographic signatures were not checked)
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- We jump into libp2p/protocols/pubsub/pubsub.nim in the method `validate(PubSub, message)`
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- which was called by `rpcHandler(GossipSub, PubSubPeer, RPCMsg)`
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### Block syncing
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See [Block syncing](../beacon_chain/sync/README.md)
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for flow explanation and performance characteristics.
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### Consuming the shared block queue
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The SharedBlockQueue is consumed by the Eth2Processor via `runQueueProcessingLoop(Eth2Processor)`
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To mitigate blocking networking and timeshare between Io and compute, blocks are processed 1 by 1 by `processBlock(Eth2Processor, BlockEntry)`
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This in turn calls:
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- `storeBlock(Eth2Processor, SignedBeaconBlock, Slot)`
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- `addHeadBlock(ChainDAGRef, var BatchVerifier, SignedBeaconBlock, forkChoiceCallback)`
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- trigger sending attestation if relevant
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### Steady state (synced to head)
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#### Bottlenecks when synced
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The SyncManager is deactivated
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RequestManager and Gossip are activated.
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##### Backpressure
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There is no backpressure handling at the RequestManager and Gossip level with regards to the SharedBlockQueue
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There is backpressure handling at the Quarantine level:
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- Blocks in the SharedBlockQueue that are missing parents
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are put in quarantine, only 16 can be stored and new candidates are dropped as long as the older ones are unresolved.
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##### Latency & Throughput sensitiveness
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When synced, blocks are a small part of the whole processing compared to attestations, there is no more throughput constraint however a single block should be processed ASAP as it blocks attestation flow.
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Furthermore to ensure the stability of the gossip mesh, blocks should be validated and rebroadcasted ASAP as well.
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