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# Overseer Protocol
This chapter contains message types sent to and from the overseer, and the underlying subsystem message types that are transmitted using these.
## Overseer Signal
Signals from the overseer to a subsystem to request change in execution that has to be obeyed by the subsystem.
```rust
enum OverseerSignal {
/// Signal to start work localized to the relay-parent hash.
StartWork(Hash),
/// Signal to stop (or phase down) work localized to the relay-parent hash.
StopWork(Hash),
}
```
All subsystems have their own message types; all of them need to be able to listen for overseer signals as well. There are currently two proposals for how to handle that with unified communication channels:
1. Retaining the `OverseerSignal` definition above, add `enum FromOverseer<T> {Signal(OverseerSignal), Message(T)}`.
1. Add a generic varint to `OverseerSignal`: `Message(T)`.
Either way, there will be some top-level type encapsulating messages from the overseer to each subsystem.
## All Messages
> TODO [now]
## Availability Distribution Message
Messages received by the availability distribution subsystem.
```rust
enum AvailabilityDistributionMessage {
/// An update on network state from the network bridge.
NetworkBridgeUpdate(NetworkBridgeEvent),
}
```
## Availability Store Message
Messages to and from the availability store.
```rust
enum AvailabilityStoreMessage {
/// Query the PoV of a candidate by hash.
QueryPoV(Hash, ResponseChannel<PoV>),
/// Query a specific availability chunk of the candidate's erasure-coding by validator index.
/// Returns the chunk and its inclusion proof against the candidate's erasure-root.
QueryChunk(Hash, ValidatorIndex, ResponseChannel<AvailabilityChunkAndProof>),
/// Store a specific chunk of the candidate's erasure-coding by validator index, with an
/// accompanying proof.
StoreChunk(Hash, ValidatorIndex, AvailabilityChunkAndProof),
}
```
## Bitfield Distribution Message
Messages received by the bitfield distribution subsystem.
```rust
enum BitfieldDistributionMessage {
/// Distribute a bitfield signed by a validator to other validators.
/// The bitfield distribution subsystem will assume this is indeed correctly signed.
DistributeBitfield(relay_parent, SignedAvailabilityBitfield),
/// Receive a network bridge update.
NetworkBridgeUpdate(NetworkBridgeEvent),
}
```
## Bitfield Signing Message
Currently, the bitfield signing subsystem receives no specific messages.
```rust
/// Non-instantiable message type
enum BitfieldSigningMessage { }
```
## Candidate Backing Message
```rust
enum CandidateBackingMessage {
/// Registers a stream listener for updates to the set of backable candidates that could be backed
/// in a child of the given relay-parent, referenced by its hash.
RegisterBackingWatcher(Hash, TODO),
/// Note that the Candidate Backing subsystem should second the given candidate in the context of the
/// given relay-parent (ref. by hash). This candidate must be validated.
Second(Hash, CandidateReceipt),
/// Note a peer validator's statement about a particular candidate. Disagreements about validity must be escalated
/// to a broader check by Misbehavior Arbitration. Agreements are simply tallied until a quorum is reached.
Statement(Statement),
}
```
## Candidate Selection Message
These messages are sent to the [Candidate Selection subsystem](../node/backing/candidate-selection.html) as a means of providing feedback on its outputs.
```rust
enum CandidateSelectionMessage {
/// We recommended a particular candidate to be seconded, but it was invalid; penalize the collator.
Invalid(CandidateReceipt),
}
```
## Network Bridge Message
Messages received by the network bridge. This subsystem is invoked by others to manipulate access
to the low-level networking code.
```rust
enum NetworkBridgeMessage {
/// Register an event producer with the network bridge. This should be done early and cannot
/// be de-registered.
RegisterEventProducer(ProtocolId, Fn(NetworkBridgeEvent) -> AllMessages),
/// Report a cost or benefit of a peer. Negative values are costs, positive are benefits.
ReportPeer(PeerId, cost_benefit: i32),
/// Send a message to one or more peers on the given protocol ID.
SendMessage([PeerId], ProtocolId, Bytes),
}
```
## Network Bridge Update
These updates are posted from the [Network Bridge Subsystem](../node/utility/network-bridge.html) to other subsystems based on registered listeners.
```rust
struct View(Vec<Hash>); // Up to `N` (5?) chain heads.
enum NetworkBridgeEvent {
/// A peer with given ID is now connected.
PeerConnected(PeerId, ObservedRole), // role is one of Full, Light, OurGuardedAuthority, OurSentry
/// A peer with given ID is now disconnected.
PeerDisconnected(PeerId),
/// We received a message from the given peer. Protocol ID should be apparent from context.
PeerMessage(PeerId, Bytes),
/// The given peer has updated its description of its view.
PeerViewChange(PeerId, View), // guaranteed to come after peer connected event.
/// We have posted the given view update to all connected peers.
OurViewChange(View),
}
```
## Misbehavior Arbitration Message
```rust
enum MisbehaviorReport {
/// These validator nodes disagree on this candidate's validity, please figure it out
///
/// Most likely, the list of statments all agree except for the final one. That's not
/// guaranteed, though; if somehow we become aware of lots of
/// statements disagreeing about the validity of a candidate before taking action,
/// this message should be dispatched with all of them, in arbitrary order.
///
/// This variant is also used when our own validity checks disagree with others'.
CandidateValidityDisagreement(CandidateReceipt, Vec<SignedStatement>),
/// I've noticed a peer contradicting itself about a particular candidate
SelfContradiction(CandidateReceipt, SignedStatement, SignedStatement),
/// This peer has seconded more than one parachain candidate for this relay parent head
DoubleVote(CandidateReceipt, SignedStatement, SignedStatement),
}
```
If this subsystem chooses to second a parachain block, it dispatches a `CandidateBackingSubsystemMessage`.
## PoV Distribution
Messages received by the PoV Distribution subsystem are unspecified and highly tied to gossip.
> TODO
## Provisioner Message
```rust
/// This data becomes intrinsics or extrinsics which should be included in a future relay chain block.
enum ProvisionableData {
/// This bitfield indicates the availability of various candidate blocks.
Bitfield(Hash, SignedAvailabilityBitfield),
/// The Candidate Backing subsystem believes that this candidate is valid, pending availability.
BackedCandidate(BackedCandidate),
/// Misbehavior reports are self-contained proofs of validator misbehavior.
MisbehaviorReport(Hash, MisbehaviorReport),
/// Disputes trigger a broad dispute resolution process.
Dispute(Hash, Signature),
}
/// Message to the Provisioner.
///
/// In all cases, the Hash is that of the relay parent.
enum ProvisionerMessage {
/// This message allows potential block authors to be kept updated with all new authorship data
/// as it becomes available.
RequestBlockAuthorshipData(Hash, Sender<ProvisionableData>),
/// This data should become part of a relay chain block
ProvisionableData(ProvisionableData),
}
```
## Runtime API Message
The Runtime API subsystem is responsible for providing an interface to the state of the chain's runtime.
Other subsystems query this data by sending these messages.
```rust
enum RuntimeApiRequest {
/// Get the current validator set.
Validators(ResponseChannel<Vec<ValidatorId>>),
/// Get a signing context for bitfields and statements.
SigningContext(ResponseChannel<SigningContext>),
/// Get the validation code for a specific para, assuming execution under given block number, and
/// an optional block number representing an intermediate parablock executed in the context of
/// that block.
ValidationCode(ParaId, BlockNumber, Option<BlockNumber>, ResponseChannel<ValidationCode>),
}
enum RuntimeApiMessage {
/// Make a request of the runtime API against the post-state of the given relay-parent.
Request(Hash, RuntimeApiRequest),
}
```
## Statement Distribution Message
The Statement Distribution subsystem distributes signed statements from validators to other validators.
It receives updates from the network bridge and signed statements to share with other validators.
```rust
enum StatementDistributionMessage {
/// An update from the network bridge.
NetworkBridgeUpdate(NetworkBridgeEvent),
/// We have validated a candidate and want to share our judgment with our peers.
/// The hash is the relay parent.
///
/// The statement distribution subsystem assumes that the statement should be correctly
/// signed.
Share(Hash, SignedStatement),
}
```
## Validation Request Type
Various modules request that the [Candidate Validation subsystem](../node/utility/candidate-validation.html) validate a block with this message
```rust
enum CandidateValidationMessage {
/// Validate a candidate with provided parameters. Returns `Err` if an only if an internal
/// error is encountered. A bad candidate will return `Ok(false)`, while a good one will
/// return `Ok(true)`.
Validate(ValidationCode, CandidateReceipt, PoV, ResponseChannel<Result<bool>>),
}
```