Availability Distribution

This subsystem is responsible for distribution availability data to peers. Availability data are chunks, PoVs and AvailableData (which is PoV + PersistedValidationData). It does so via request response protocols.

In particular this subsystem is responsible for:

• Respond to network requests requesting availability data by querying the Availability Store.
• Request chunks from backing validators to put them in the local Availability Store whenever we find an occupied core on any fresh leaf, this is to ensure availability by at least 2/3+ of all validators, this happens after a candidate is backed.
• Fetch PoV from validators, when requested via FetchPoV message from backing (pov_requester module).

The backing subsystem is responsible of making available data available in the local Availability Store upon validation. This subsystem will serve any network requests by querying that store.

Protocol

This subsystem does not handle any peer set messages, but the pov_requester does connect to validators of the same backing group on the validation peer set, to ensure fast propagation of statements between those validators and for ensuring already established connections for requesting PoVs. Other than that this subsystem drives request/response protocols.

Input:

• OverseerSignal::ActiveLeaves(ActiveLeavesUpdate)
• AvailabilityDistributionMessage{msg: ChunkFetchingRequest}
• AvailabilityDistributionMessage{msg: PoVFetchingRequest}
• AvailabilityDistributionMessage{msg: FetchPoV}

Output:

• NetworkBridgeMessage::SendRequests(Requests, IfDisconnected::TryConnect)
• AvailabilityStore::QueryChunk(candidate_hash, index, response_channel)
• AvailabilityStore::StoreChunk(candidate_hash, chunk)
• AvailabilityStore::QueryAvailableData(candidate_hash, response_channel)
• RuntimeApiRequest::SessionIndexForChild
• RuntimeApiRequest::SessionInfo
• RuntimeApiRequest::AvailabilityCores

Functionality

PoV Requester

The PoV requester in the pov_requester module takes care of staying connected to validators of the current backing group of this very validator on the Validation peer set and it will handle FetchPoV requests by issuing network requests to those validators. It will check the hash of the received PoV, but will not do any further validation. That needs to be done by the original FetchPoV sender (backing subsystem).

Chunk Requester

After a candidate is backed, the availability of the PoV block must be confirmed by 2/3+ of all validators. The chunk requester is responsible of making that availability a reality.

It does that by querying checking occupied cores for all active leaves. For each occupied core it will spawn a task fetching the erasure chunk which has the ValidatorIndex of the node. For this an ChunkFetchingRequest is issued, via Substrate's generic request/response protocol.

The spawned task will start trying to fetch the chunk from validators in responsible group of the occupied core, in a random order. For ensuring that we use already open TCP connections wherever possible, the requester maintains a cache and preserves that random order for the entire session.

Note however that, because not all validators in a group have to be actual backers, not all of them are required to have the needed chunk. This in turn could lead to low throughput, as we have to wait for fetches to fail, before reaching a validator finally having our chunk. We do rank back validators not delivering our chunk, but as backers could vary from block to block on a perfectly legitimate basis, this is still not ideal. See issues 2509 and 2512 for more information.

The current implementation also only fetches chunks for occupied cores in blocks in active leaves. This means though, if active leaves skips a block or we are particularly slow in fetching our chunk, we might not fetch our chunk if availability reached 2/3 fast enough (slot becomes free). This is not desirable as we would like as many validators as possible to have their chunk. See this issue for more details.

Serving

On the other side the subsystem will listen for incoming ChunkFetchingRequests and PoVFetchingRequests from the network bridge and will respond to queries, by looking the requested chunks and PoVs up in the availability store, this happens in the responder module.

We rely on the backing subsystem to make available data available locally in the Availability Store after it has validated it.