State Sync

    Channels

    State sync has four distinct channels. The channel identifiers are listed below.

    Name Number
    SnapshotChannel 96
    ChunkChannel 97
    LightBlockChannel 98
    ParamsChannel 99

    Message Types

    SnapshotRequest

    When a new node begin state syncing, it will ask all peers it encounters if it has any available snapshots:

    | Name | Type | Description | Field Number | |———-|——–|————-|————–|

    SnapShotResponse

    The receiver will query the local ABCI application via ListSnapshots, and send a message containing snapshot metadata (limited to 4 MB) for each of the 10 most recent snapshots: and stored at the application layer. When a peer is starting it will request snapshots.

    Name Type Description Field Number
    height uint64 Height at which the snapshot was taken 1
    format uint32 Format of the snapshot. 2
    chunks uint32 How many chunks make up the snapshot 3
    hash bytes Arbitrary snapshot hash 4
    metadata bytes Arbitrary application data. May be non-deterministic. 5

    ChunkRequest

    The node running state sync will offer these snapshots to the local ABCI application via OfferSnapshot ABCI calls, and keep track of which peers contain which snapshots. Once a snapshot is accepted, the state syncer will request snapshot chunks from appropriate peers:

    Name Type Description Field Number
    height uint64 Height at which the chunk was created 1
    format uint32 Format chosen for the chunk. May be non-deterministic. 2
    index uint32 Index of the chunk within the snapshot. 3

    ChunkResponse

    The receiver will load the requested chunk from its local application via LoadSnapshotChunk, and respond with it (limited to 16 MB):

    Name Type Description Field Number
    height uint64 Height at which the chunk was created 1
    format uint32 Format chosen for the chunk. May be non-deterministic. 2
    index uint32 Index of the chunk within the snapshot. 3
    hash bytes Arbitrary snapshot hash 4
    missing bool Arbitrary application data. May be non-deterministic. 5

    Here, Missing is used to signify that the chunk was not found on the peer, since an empty chunk is a valid (although unlikely) response.

    The returned chunk is given to the ABCI application via ApplySnapshotChunk until the snapshot is restored. If a chunk response is not returned within some time, it will be re-requested, possibly from a different peer.

    The ABCI application is able to request peer bans and chunk refetching as part of the ABCI protocol.

    LightBlockRequest

    To verify state and to provide state relevant information for consensus, the node will ask peers for light blocks at specified heights.

    Name Type Description Field Number
    height uint64 Height of the light block 1

    LightBlockResponse

    The receiver will retrieve and construct the light block from both the block and state stores. The receiver will verify the data by comparing the hashes and store the header, commit and validator set if necessary. The light block at the height of the snapshot will be used to verify the AppHash.

    Name Type Description Field Number
    light_block LightBlock Light block at the height requested 1

    State sync will use light client verification to verify the light blocks.

    If no state sync is in progress (i.e. during normal operation), any unsolicited response messages are discarded.

    ParamsRequest

    In order to build the state, the state provider will request the params at the height of the snapshot and use the header to verify it.

    Name Type Description Field Number
    height uint64 Height of the consensus params 1

    ParamsResponse

    A receiver to the request will use the state store to fetch the consensus params at that height and return it to the sender.

    Name Type Description Field Number
    height uint64 Height of the consensus params 1
    consensus_params ConsensusParams Consensus params at the height requested 2

    Message

    Message is a oneof protobuf type. The oneof consists of eight messages.

    Name Type Description Field Number
    snapshots_request SnapshotRequest Request a recent snapshot from a peer 1
    snapshots_response SnapshotResponse Respond with the most recent snapshot stored 2
    chunk_request ChunkRequest Request chunks of the snapshot. 3
    chunk_response ChunkRequest Response of chunks used to recreate state. 4
    light_block_request LightBlockRequest Request a light block. 5
    light_block_response LightBlockResponse Respond with a light block 6
    params_request ParamsRequest Request the consensus params at a height. 7
    params_response ParamsResponse Respond with the consensus params 8
    Decorative Orb