# State Sync Reactor

State sync allows new nodes to rapidly bootstrap and join the network by discovering, fetching,
and restoring state machine snapshots. For more information, see the [state sync ABCI section](../../abci/apps.md#state-sync).

The state sync reactor has two main responsibilites:

* Serving state machine snapshots taken by the local ABCI application to new nodes joining the
  network.

* Discovering existing snapshots and fetching snapshot chunks for an empty local application
  being bootstrapped.

The state sync process for bootstrapping a new node is described in detail in the section linked
above. While technically part of the reactor (see `statesync/syncer.go` and related components),
this document will only cover the P2P reactor component.

For details on the ABCI methods and data types, see the [ABCI documentation](../../abci/abci.md).

## State Sync P2P Protocol

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

```go
type snapshotsRequestMessage struct{}
```

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:

```go
type snapshotsResponseMessage struct {
 Height   uint64
 Format   uint32
 Chunks   uint32
 Hash     []byte
 Metadata []byte
}
```

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:

```go
type chunkRequestMessage struct {
 Height uint64
 Format uint32
 Index  uint32
}
```

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

```go
type chunkResponseMessage struct {
 Height  uint64
 Format  uint32
 Index   uint32
 Chunk   []byte
 Missing bool
}
```

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.

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