This subsystem is responsible for distribution availability data to peers.
Availability data are chunks,
AvailableData (which is
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 Storewhenever we find an occupied core on the chain, this is to ensure availability by at least 2/3+ of all validators, this happens after a candidate is backed.
PoVfrom validators, when requested via
FetchPoVmessage from backing (
The backing subsystem is responsible of making available data available in the
Availability Store upon validation. This subsystem will serve any
network requests by querying that store.
This subsystem does not handle any peer set messages, but the
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.
AvailabilityStore::QueryChunk(candidate_hash, index, response_channel)
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
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
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.
On the other side the subsystem will listen for incoming
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
We rely on the backing subsystem to make available data available locally in the
Availability Store after it has validated it.