# Subsystems and Jobs
In this section we define the notions of Subsystems and Jobs. These are guidelines for how we will employ an architecture of hierarchical state machines. We'll have a top-level state machine which oversees the next level of state machines which oversee another layer of state machines and so on. The next sections will lay out these guidelines for what we've called subsystems and jobs, since this model applies to many of the tasks that the Node-side behavior needs to encompass, but these are only guidelines and some Subsystems may have deeper hierarchies internally.
Subsystems are long-lived worker tasks that are in charge of performing some particular kind of work. All subsystems can communicate with each other via a well-defined protocol. Subsystems can't generally communicate directly, but must coordinate communication through an [Overseer](overseer.md), which is responsible for relaying messages, handling subsystem failures, and dispatching work signals.
Most work that happens on the Node-side is related to building on top of a specific relay-chain block, which is contextually known as the "relay parent". We call it the relay parent to explicitly denote that it is a block in the relay chain and not on a parachain. We refer to the parent because when we are in the process of building a new block, we don't know what that new block is going to be. The parent block is our only stable point of reference, even though it is usually only useful when it is not yet a parent but in fact a leaf of the block-DAG expected to soon become a parent (because validators are authoring on top of it). Furthermore, we are assuming a forkful blockchain-extension protocol, which means that there may be multiple possible children of the relay-parent. Even if the relay parent has multiple children blocks, the parent of those children is the same, and the context in which those children is authored should be the same. The parent block is the best and most stable reference to use for defining the scope of work items and messages, and is typically referred to by its cryptographic hash.
Since this goal of determining when to start and conclude work relative to a specific relay-parent is common to most, if not all subsystems, it is logically the job of the Overseer to distribute those signals as opposed to each subsystem duplicating that effort, potentially being out of synchronization with each other. Subsystem A should be able to expect that subsystem B is working on the same relay-parents as it is. One of the Overseer's tasks is to provide this heartbeat, or synchronized rhythm, to the system.
The work that subsystems spawn to be done on a specific relay-parent is known as a job. Subsystems should set up and tear down jobs according to the signals received from the overseer. Subsystems may share or cache state between jobs.
Subsystems must be robust to spurious exits. The outputs of the set of subsystems as a whole comprises of signed messages and data committed to disk. Care must be taken to avoid issuing messages that are not substantiated. Since subsystems need to be safe under spurious exits, it is the expected behavior that an `OverseerSignal::Conclude` can just lead to breaking the loop and exiting directly as opposed to waiting for everything to shut down gracefully.
## Subsystem Message Traffic
Which subsystems send messages to which other subsystems.
**Note**: This diagram omits the overseer for simplicity. In fact, all messages are relayed via the overseer.
**Note**: Messages with a filled diamond arrowhead ("♦") include a `oneshot::Sender` which communicates a response from the recipient.
Messages with an open triangle arrowhead ("Δ") do not include a return sender.
```dot process
digraph {
rankdir=LR;
node [shape = oval];
concentrate = true;
av_store [label = "Availability Store"]
avail_dist [label = "Availability Distribution"]
avail_rcov [label = "Availability Recovery"]
bitf_dist [label = "Bitfield Distribution"]
bitf_sign [label = "Bitfield Signing"]
cand_back [label = "Candidate Backing"]
cand_sel [label = "Candidate Selection"]
cand_val [label = "Candidate Validation"]
chn_api [label = "Chain API"]
coll_gen [label = "Collation Generation"]
coll_prot [label = "Collator Protocol"]
net_brdg [label = "Network Bridge"]
pov_dist [label = "PoV Distribution"]
provisioner [label = "Provisioner"]
runt_api [label = "Runtime API"]
stmt_dist [label = "Statement Distribution"]
av_store -> runt_api [arrowhead = "diamond", label = "Request::CandidateEvents"]
av_store -> chn_api [arrowhead = "diamond", label = "BlockNumber"]
av_store -> chn_api [arrowhead = "diamond", label = "BlockHeader"]
av_store -> runt_api [arrowhead = "diamond", label = "Request::Validators"]
av_store -> chn_api [arrowhead = "diamond", label = "FinalizedBlockHash"]
avail_dist -> net_brdg [arrowhead = "onormal", label = "Request::SendValidationMessages"]
avail_dist -> runt_api [arrowhead = "diamond", label = "Request::AvailabilityCores"]
avail_dist -> net_brdg [arrowhead = "onormal", label = "ReportPeer"]
avail_dist -> av_store [arrowhead = "diamond", label = "QueryDataAvailability"]
avail_dist -> av_store [arrowhead = "diamond", label = "QueryChunk"]
avail_dist -> av_store [arrowhead = "diamond", label = "StoreChunk"]
avail_dist -> runt_api [arrowhead = "diamond", label = "Request::Validators"]
avail_dist -> chn_api [arrowhead = "diamond", label = "Ancestors"]
avail_dist -> runt_api [arrowhead = "diamond", label = "Request::SessionIndexForChild"]
avail_rcov -> net_brdg [arrowhead = "onormal", label = "ReportPeer"]
avail_rcov -> av_store [arrowhead = "diamond", label = "QueryChunk"]
avail_rcov -> net_brdg [arrowhead = "diamond", label = "ConnectToValidators"]
avail_rcov -> net_brdg [arrowhead = "onormal", label = "SendValidationMessage::Chunk"]
avail_rcov -> net_brdg [arrowhead = "onormal", label = "SendValidationMessage::RequestChunk"]
bitf_dist -> net_brdg [arrowhead = "onormal", label = "ReportPeer"]
bitf_dist -> provisioner [arrowhead = "onormal", label = "ProvisionableData::Bitfield"]
bitf_dist -> net_brdg [arrowhead = "onormal", label = "SendValidationMessage"]
bitf_dist -> net_brdg [arrowhead = "onormal", label = "SendValidationMessage"]
bitf_dist -> runt_api [arrowhead = "diamond", label = "Request::Validatiors"]
bitf_dist -> runt_api [arrowhead = "diamond", label = "Request::SessionIndexForChild"]
bitf_sign -> av_store [arrowhead = "diamond", label = "QueryChunkAvailability"]
bitf_sign -> runt_api [arrowhead = "diamond", label = "Request::AvailabilityCores"]
bitf_sign -> bitf_dist [arrowhead = "onormal", label = "DistributeBitfield"]
cand_back -> av_store [arrowhead = "diamond", label = "StoreAvailableData"]
cand_back -> pov_dist [arrowhead = "diamond", label = "FetchPoV"]
cand_back -> cand_val [arrowhead = "diamond", label = "ValidateFromChainState"]
cand_back -> cand_sel [arrowhead = "onormal", label = "Invalid"]
cand_back -> provisioner [arrowhead = "onormal", label = "ProvisionableData::MisbehaviorReport"]
cand_back -> provisioner [arrowhead = "onormal", label = "ProvisionableData::BackedCandidate"]
cand_back -> pov_dist [arrowhead = "onormal", label = "DistributePoV"]
cand_back -> stmt_dist [arrowhead = "onormal", label = "Share"]
cand_sel -> coll_prot [arrowhead = "diamond", label = "FetchCollation"]
cand_sel -> cand_back [arrowhead = "onormal", label = "Second"]
cand_sel -> coll_prot [arrowhead = "onormal", label = "ReportCollator"]
cand_val -> runt_api [arrowhead = "diamond", label = "Request::PersistedValidationData"]
cand_val -> runt_api [arrowhead = "diamond", label = "Request::ValidationCode"]
cand_val -> runt_api [arrowhead = "diamond", label = "Request::CheckValidationOutputs"]
coll_gen -> coll_prot [arrowhead = "onormal", label = "DistributeCollation"]
coll_prot -> net_brdg [arrowhead = "onormal", label = "ReportPeer"]
coll_prot -> net_brdg [arrowhead = "onormal", label = "Declare"]
coll_prot -> net_brdg [arrowhead = "onormal", label = "AdvertiseCollation"]
coll_prot -> net_brdg [arrowhead = "onormal", label = "Collation"]
coll_prot -> net_brdg [arrowhead = "onormal", label = "RequestCollation"]
coll_prot -> cand_sel [arrowhead = "onormal", label = "Collation"]
net_brdg -> avail_dist [arrowhead = "onormal", label = "NetworkBridgeUpdateV1"]
net_brdg -> bitf_dist [arrowhead = "onormal", label = "NetworkBridgeUpdateV1"]
net_brdg -> pov_dist [arrowhead = "onormal", label = "NetworkBridgeUpdateV1"]
net_brdg -> stmt_dist [arrowhead = "onormal", label = "NetworkBridgeUpdateV1"]
net_brdg -> coll_prot [arrowhead = "onormal", label = "NetworkBridgeUpdateV1"]
pov_dist -> net_brdg [arrowhead = "onormal", label = "SendValidationMessage"]
pov_dist -> net_brdg [arrowhead = "onormal", label = "ReportPeer"]
provisioner -> cand_back [arrowhead = "diamond", label = "GetBackedCandidates"]
provisioner -> chn_api [arrowhead = "diamond", label = "BlockNumber"]
stmt_dist -> net_brdg [arrowhead = "onormal", label = "SendValidationMessage"]
stmt_dist -> net_brdg [arrowhead = "onormal", label = "ReportPeer"]
stmt_dist -> cand_back [arrowhead = "onormal", label = "Statement"]
stmt_dist -> runt_api [arrowhead = "onormal", label = "Request::Validators"]
stmt_dist -> runt_api [arrowhead = "onormal", label = "Request::SessionIndexForChild"]
}
```
## The Path to Inclusion (Node Side)
Let's contextualize that diagram a bit by following a parachain block from its creation through finalization.
Parachains can use completely arbitrary processes to generate blocks. The relay chain doesn't know or care about
the details; each parachain just needs to provide a [collator](collators/collation-generation.md).
**Note**: Inter-subsystem communications are relayed via the overseer, but that step is omitted here for brevity.
**Note**: Dashed lines indicate a request/response cycle, where the response is communicated asynchronously via
a oneshot channel. Adjacent dashed lines may be processed in parallel.
```mermaid
sequenceDiagram
participant Overseer
participant CollationGeneration
participant RuntimeApi
participant CollatorProtocol
Overseer ->> CollationGeneration: ActiveLeavesUpdate
loop for each activated head
CollationGeneration -->> RuntimeApi: Request availability cores
CollationGeneration -->> RuntimeApi: Request validators
Note over CollationGeneration: Determine an appropriate ScheduledCore
and OccupiedCoreAssumption
CollationGeneration -->> RuntimeApi: Request full validation data
Note over CollationGeneration: Build the collation
CollationGeneration ->> CollatorProtocol: DistributeCollation
end
```
The `DistributeCollation` messages that `CollationGeneration` sends to the `CollatorProtocol` contains
two items: a `CandidateReceipt` and `PoV`. The `CollatorProtocol` is then responsible for distributing
that collation to interested validators. However, not all potential collations are of interest. The
`CandidateSelection` subsystem is responsible for determining which collations are interesting, before
`CollatorProtocol` actually fetches the collation.
```mermaid
sequenceDiagram
participant CollationGeneration
participant CS as CollatorProtocol::CollatorSide
participant NB as NetworkBridge
participant VS as CollatorProtocol::ValidatorSide
participant CandidateSelection
CollationGeneration ->> CS: DistributeCollation
CS -->> NB: ConnectToValidators
Note over CS,NB: This connects to multiple validators.
CS ->> NB: Declare
NB ->> VS: Declare
Note over CS: Ensure that the connected validator is among
the para's validator set. Otherwise, skip it.
CS ->> NB: AdvertiseCollation
NB ->> VS: AdvertiseCollation
VS ->> CandidateSelection: Collation
Note over CandidateSelection: Lots of other machinery in play here,
but there are only three outcomes from the
perspective of the `CollatorProtocol`:
alt happy path
CandidateSelection -->> VS: FetchCollation
Activate VS
VS ->> NB: RequestCollation
NB ->> CS: RequestCollation
CS ->> NB: Collation
NB ->> VS: Collation
Deactivate VS
else collation invalid or unexpected
CandidateSelection ->> VS: ReportCollator
VS ->> NB: ReportPeer
else CandidateSelection already selected a different candidate
Note over CandidateSelection: silently drop
end
```
Assuming we hit the happy path, flow continues with `CandidateSelection` receiving a `(candidate_receipt, pov)` as
the return value from its
`FetchCollation` request. The only time `CandidateSelection` actively requests a collation is when
it hasn't yet seconded one for some `relay_parent`, and is ready to second.
```mermaid
sequenceDiagram
participant CS as CandidateSelection
participant CB as CandidateBacking
participant CV as CandidateValidation
participant PV as Provisioner
participant SD as StatementDistribution
participant PD as PoVDistribution
CS ->> CB: Second
% fn validate_and_make_available
CB -->> CV: ValidateFromChainState
Note over CB,CV: There's some complication in the source, as
candidates are actually validated in a separate task.
alt valid
Note over CB: This is where we transform the CandidateReceipt into a CommittedCandidateReceipt
% CandidateBackingJob::sign_import_and_distribute_statement
% CandidateBackingJob::import_statement
CB ->> PV: ProvisionableData::BackedCandidate
% CandidateBackingJob::issue_new_misbehaviors
opt if there is misbehavior to report
CB ->> PV: ProvisionableData::MisbehaviorReport
end
% CandidateBackingJob::distribute_signed_statement
CB ->> SD: Share
% CandidateBackingJob::distribute_pov
CB ->> PD: DistributePoV
else invalid
CB ->> CS: Invalid
end
```
At this point, you'll see that control flows in two directions: to `StatementDistribution` to distribute
the `SignedStatement`, and to `PoVDistribution` to distribute the `PoV`. However, that's largely a mirage:
while the initial implementation distributes `PoV`s by gossip, that's inefficient, and will be replaced
with a system which fetches `PoV`s only when actually necessary.
> TODO: figure out more precisely the current status and plans; write them up
Therefore, we'll follow the `SignedStatement`. The `StatementDistribution` subsystem is largely concerned
with implementing a gossip protocol:
```mermaid
sequenceDiagram
participant SD as StatementDistribution
participant NB as NetworkBridge
alt On receipt of a
SignedStatement from CandidateBacking
% fn circulate_statement_and_dependents
SD ->> NB: SendValidationMessage
Note right of NB: Bridge sends validation message to all appropriate peers
else On receipt of peer validation message
NB ->> SD: NetworkBridgeUpdateV1
% fn handle_incoming_message
alt if we aren't already aware of the relay parent for this statement
SD ->> NB: ReportPeer
end
% fn circulate_statement
opt if we know of peers who haven't seen this message, gossip it
SD ->> NB: SendValidationMessage
end
end
```
But who are these `Listener`s who've asked to be notified about incoming `SignedStatement`s?
Nobody, as yet.
Let's pick back up with the PoV Distribution subsystem.
```mermaid
sequenceDiagram
participant CB as CandidateBacking
participant PD as PoVDistribution
participant Listener
participant NB as NetworkBridge
CB ->> PD: DistributePoV
Note over PD,Listener: Various subsystems can register listeners for when PoVs arrive
loop for each Listener
PD ->> Listener: Arc
end
Note over PD: Gossip to connected peers
PD ->> NB: SendPoV
Note over PD,NB: On receipt of a network PoV, PovDistribution forwards it to each Listener.
It also penalizes bad gossipers.
```
Unlike in the case of `StatementDistribution`, there is another subsystem which in various circumstances
already registers a listener to be notified when a new `PoV` arrives: `CandidateBacking`. Note that this
is the second time that `CandidateBacking` has gotten involved. The first instance was from the perspective
of the validator choosing to second a candidate via its `CandidateSelection` subsystem. This time, it's
from the perspective of some other validator, being informed that this foreign `PoV` has been received.
```mermaid
sequenceDiagram
participant SD as StatementDistribution
participant CB as CandidateBacking
participant PD as PoVDistribution
participant AS as AvailabilityStore
SD ->> CB: Statement
% CB::maybe_validate_and_import => CB::kick_off_validation_work
CB -->> PD: FetchPoV
Note over CB,PD: This call creates the Listener from the previous diagram
CB ->> AS: StoreAvailableData
```
At this point, things have gone a bit nonlinear. Let's pick up the thread again with `BitfieldSigning`. As
the `Overseer` activates each relay parent, it starts a `BitfieldSigningJob` which operates on an extremely
simple metric: after creation, it immediately goes to sleep for 1.5 seconds. On waking, it records the state
of the world pertaining to availability at that moment.
```mermaid
sequenceDiagram
participant OS as Overseer
participant BS as BitfieldSigning
participant RA as RuntimeApi
participant AS as AvailabilityStore
participant BD as BitfieldDistribution
OS ->> BS: ActiveLeavesUpdate
loop for each activated relay parent
Note over BS: Wait 1.5 seconds
BS -->> RA: Request::AvailabilityCores
loop for each availability core
BS -->> AS: QueryChunkAvailability
end
BS ->> BD: DistributeBitfield
end
```
`BitfieldDistribution` is, like the other `*Distribution` subsystems, primarily interested in implementing
a peer-to-peer gossip network propagating its particular messages. However, it also serves as an essential
relay passing the message along.
```mermaid
sequenceDiagram
participant BS as BitfieldSigning
participant BD as BitfieldDistribution
participant NB as NetworkBridge
participant PV as Provisioner
BS ->> BD: DistributeBitfield
BD ->> PV: ProvisionableData::Bitfield
BD ->> NB: SendValidationMessage::BitfieldDistribution::Bitfield
```
We've now seen the message flow to the `Provisioner`: both `CandidateBacking` and `BitfieldDistribution`
contribute provisionable data. Now, let's look at that subsystem.
Much like the `BitfieldSigning` subsystem, the `Provisioner` creates a new job for each newly-activated
leaf, and starts a timer. Unlike `BitfieldSigning`, we won't depict that part of the process, because
the `Provisioner` also has other things going on.
```mermaid
sequenceDiagram
participant A as Arbitrary
participant PV as Provisioner
participant CB as CandidateBacking
participant BD as BitfieldDistribution
participant RA as RuntimeApi
participant PO as Proposer
alt receive provisionable data
alt
CB ->> PV: ProvisionableData
else
BD ->> PV: ProvisionableData
end
loop over stored Senders
PV ->> A: ProvisionableData
end
Note over PV: store bitfields and backed candidates
else receive request for inherent data
PO ->> PV: RequestInherentData
alt we have already constructed the inherent data
PV ->> PO: send the inherent data
else we have not yet constructed the inherent data
Note over PV,PO: Store the return sender without sending immediately
end
else timer times out
note over PV: Waited 2 seconds
PV -->> RA: RuntimeApiRequest::AvailabilityCores
Note over PV: construct and store the inherent data
loop over stored inherent data requests
PV ->> PO: (SignedAvailabilityBitfields, BackedCandidates)
end
end
```
In principle, any arbitrary subsystem could send a `RequestInherentData` to the `Provisioner`. In practice,
only the `Proposer` does so.
The proposer is an atypical subsystem in that, unlike most of them, it is not primarily driven by
the `Overseer`, but instead by the `sp_consensus::Environment` and `sp_consensus::Proposer` traits
from Substrate. It doesn't make much sense to diagram this flow because it's very linear:
- Substrate creates a `Proposer` from the `ProposerFactory` once per upcoming block, using the `parent_header: Header`.
- At some later point, it calls `Proposer::propose(self, ...)`, consuming the proposer to generate a proposal
- `Proposer::propose` sends a `RequestInherentData` to the `Provisioner`. This has a fixed timeout of
2.5 seconds, meaning that the provisioner has approximately 0.5 seconds to generate and send the data.
The tuple `(SignedAvailabilityBitfields, BackedCandidates, ParentHeader)` is injected by the `Proposer`
into the inherent data. From that point on, control passes from the node to the runtime.