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Revert "document state sync ABCI interface and P2P protocol (#90)" (#92)

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This reverts commit 9842b4b0fb.
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Tess Rinearson
2020-05-05 14:53:49 +02:00
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spec/reactors/state_sync/reactor.md
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# 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.