sync/

README

State sync

Overview

Normally, a node joins the network through bootstrapping: First it fetches all blocks from genesis to the chain's last accepted block from peers, then it applies the state transition specified in each block to reach the state necessary to join consensus.

State sync is an alternative in which a node downloads the state of the chain from its peers at a specific syncable block height. Then, the node processes the rest of the chain's blocks (from syncable block to tip) via normal bootstrapping. Blocks at heights divisible by defaultSyncableInterval (= 16,384 or 2**14) are considered syncable. Note: defaultSyncableInterval must be divisible by CommitInterval (= 4096). This is so the state corresponding to syncable blocks is available on nodes with pruning enabled.

State sync is faster than bootstrapping and uses less bandwidth and computation:

• Nodes joining the network do not process all the state transitions.
• The amount of data sent over the network is porportionate to the amount of state not the chain's length

Note: nodes joining the network thorugh state sync will not have historical state prior to the syncable block.

What is the chain state?

The node needs the following data from its peers to continue processing blocks from a syncable block:

• Accounts trie & storage tries for all accounts (at the state root corresponding to the syncable block),
• Contract code referenced in the account trie,
• 256 parents of the syncable block (required for the BLOCKHASH opcode)

Code structure

State sync code is structured as follows:

• sync/handlers: Nodes that have joined the network are expected to respond to valid requests for the chain state:
  • LeafsRequestHandler: handles requests for trie data (leafs)
  • CodeRequestHandler: handles requests for contract code
  • BlockRequestHandler: handles requests for blocks
  • Note: There are response size and time limits in place so peers joining the network do not overload peers providing data. Additionally, the engine tracks the CPU usage of each peer for such messsages and throttles inbound requests accordingly.
• sync/client: Validates reponses from peers and provides support for syncing tries.
• sync/statesync: Uses sync/client to sync EVM related state: Accounts, storage tries, and contract code.
• plugin/evm/: The engine expects the VM to implement StateSyncableVM interface,
  • StateSyncServer: Contains methods executed on nodes serving state sync requests.
  • StateSyncClient: Contains methods executed on nodes joining the network via state sync, and orchestrates the top level steps of the sync.
• peer: Contains abstractions used by sync/statesync to send requests to peers (AppRequest) and receive responses from peers (AppResponse).
• message: Contains structs that are serialized and sent over the network during state sync.

Sync summaries & engine involvement

When a new node wants to join the network via state sync, it will need a few pieces of information as a starting point so it can make valid requests to its peers:

• Number (height) and hash of the latest available syncable block,
• Root of the account trie,

The above information is called a state summary, and each syncable block corresponds to one such summary (see message.SyncSummary). The engine and VM interact as follows to find a syncable state summary:

1. The engine calls StateSyncEnabled. The VM returns true to initiate state sync, or false to start bootstrapping. In subnet-evm, this is controlled by the state-sync-enabled flag.
2. The engine calls GetOngoingSyncStateSummary. If the VM has a previously interrupted sync to resume it returns that summary. Otherwise, it returns ErrNotFound. By default, subnet-evm will resume an interrupted sync.
3. The engine samples peers for their latest available summaries, then verifies the correctness and availablility of each sampled summary with validators. The messaging flow is documented here.
4. The engine calls Accept on the chosen summary. The VM may return false to skip syncing to this summary (subnet-evm skips state sync for less than defaultStateSyncMinBlocks = 300_000 blocks). If the VM decides to perform the sync, it must return true without blocking and fetch the state from its peers asynchronously.
5. The VM sends common.StateSyncDone on the toEngine channel on completion.
6. The engine calls VM.SetState(Bootstrapping). Then, blocks after the syncable block are processed one by one.

Syncing state

The following steps are executed by the VM to sync its state from peers (see stateSyncClient.StateSync):

1. Wipe snapshot data
2. Sync 256 parents of the syncable block (see BlockRequest),
3. Sync the EVM state: account trie, code, and storage tries,
4. Update in-memory and on-disk pointers.

Steps 3 and 4 involve syncing tries. To sync trie data, the VM will send a series of LeafRequests to its peers. Each request specifies:

• Root of the trie to sync,
• Start and End specify a range of keys.

Peers responding to these requests send back trie leafs (key/value pairs) beginning at Start and up to End (or a maximum number of leafs). The response must also contain include a merkle proof for the range of leafs it contains. Nodes serving state sync data are responsible for constructing these proofs (see sync/handlers/leafs_request.go)

client.GetLeafs handles sending a single request and validating the response. This method will retry the request from a different peer up to maxRetryAttempts (= 32) times if the peer's response is:

• malformed,
• does not contain a valid merkle proof,
• or is not received in time.

If there are more leafs in a trie than can be returned in a single response, the client will make successive requests to continue fetching data (with Start set to the last key received) until the trie is complete. CallbackLeafSyncer manages this process and does a callback on each batch of received leafs.

EVM state: Account trie, code, and storage tries

sync/statesync.stateSyncer uses CallbackLeafSyncer to sync the account trie. When the leaf callback is invoked, each leaf represents an account:

• If the account has contract code, it is requested from peers using client.GetCode
• If the account has a storage root, it is added to the list of trie roots returned from the callback. CallbackLeafSyncer has defaultNumThreads (= 4) goroutines to fetch these tries concurrently. If the account trie encounters a new storage trie task and there are already 4 in-progress trie tasks (1 for the account trie and 3 for in-progress storage trie tasks), then the account trie worker will block until one of the storage trie tasks finishes and it can create a new task.

When an account leaf is received, it is converted to SlimRLP format and written to the snapshot. To reconstruct the trie, stateSyncer inserts leafs as they arrive in a StackTrie. Since leafs arrive sorted by increasing key order, the StackTrie can create intermediary trie nodes as soon as all possible children for a given path are known (by hashing the children). This allows the sync process to recreate the trie locally, without the need to transmit non-leaf nodes over the network. When the trie is complete, an OnFinish callback is called and we hash any remaining nodes (resulting in the trie root).

When a storage trie leaf is received, it is stored in the account's storage snapshot. A StackTrie is used here to reconstruct intermediary trie nodes & root as well.

Updating in-memory and on-disk pointers

plugin/evm.stateSyncClient.StateSyncSetLastSummaryBlock is the last step in state sync. Once the tries have been synced, this method:

• Verifies the block the engine has received matches the expected block hash and block number in the summary,
• Adds a checkpoint to the core.ChainIndexer (to avoid indexing missing blocks)
• Resets in-memory and on disk pointers on the core.BlockChain struct.
• Updates VM's last accepted block.

Resuming a partial sync operation

While state sync is faster than normal bootstrapping, the process may take several hours to complete. In case the node is shut down in the middle of a state sync, progress on syncing the account trie and storage tries is preserved:

• When starting a sync, stateSyncClient persists the state summary to disk. This is so if the node is shut down while the sync is ongoing, this summary can be found and returned to the engine from GetOngoingSyncStateSummary upon node restart.
• If enough validators indicate this summary is available and valid, the engine will prefer it and stateSyncClient will use this summary for the rest of the sync.
• sync/statesync.stateSyncer maintains a set of in-progress tries (see sync/statesync/state_syncer_progress.go), and will resume syncing these tries
• For each in-progress trie, leafs are restored by iterating keys from the snapshot (account or storage) to the StackTrie, and syncing continues from the next key.
• When the sync is complete, the ongoing state summary is removed from disk.

Configuration flags

flag	type	description	default
state-sync-enabled	bool	set to true to enable state sync	false
state-sync-skip-resume	bool	set to true to avoid resuming an ongoing sync	false
state-sync-min-blocks	uint64	Minimum number of blocks the chain must be ahead of local state to prefer state sync over bootstrapping	300,000
state-sync-server-trie-cache	int	Size of trie cache to serve state sync data in MB. Should be set to multiples of 64.	64
state-sync-ids	string	a comma seperated list of NodeID- prefixed node IDs to sync data from. If not provided, peers are randomly selected.