pezkuwi-subxt/polkadot/roadmap/implementers-guide/src/node/availability/availability-recovery.md

Availability Recovery

This subsystem is the inverse of the Availability Distribution subsystem: validators will serve the availability chunks kept in the availability store to nodes who connect to them. And the subsystem will also implement the other side: the logic for nodes to connect to validators, request availability pieces, and reconstruct the AvailableData.

This version of the availability recovery subsystem is based off of direct connections to validators. In order to recover any given AvailableData, we must recover at least f + 1 pieces from validators of the session. Thus, we will connect to and query randomly chosen validators until we have received f + 1 pieces.

Protocol

PeerSet: Validation

Input:

• NetworkBridgeUpdateV1(update)
• AvailabilityRecoveryMessage::RecoverAvailableData(candidate, session, backing_group, response)

Output:

• NetworkBridge::SendValidationMessage
• NetworkBridge::ReportPeer
• AvailabilityStore::QueryChunk

Functionality

We hold a state which tracks the current recovery interactions we have live, as well as which request IDs correspond to which interactions. An interaction is a structure encapsulating all interaction with the network necessary to recover the available data.

struct State {
    /// Each interaction is implemented as its own remote async task, and these handles are remote
    /// for it.
    interactions: FuturesUnordered<InteractionHandle>,
    /// A multiplexer over receivers from live interactions.
    interaction_receivers: FuturesUnordered<ResponseReceiver<Concluded>>,
    /// A recent block hash for which state should be available.
    live_block_hash: Hash,
    // An LRU cache of recently recovered data.
    availability_lru: LruCache<CandidateHash, Result<AvailableData, RecoveryError>>,
}

/// This is a future, which concludes either when a response is received from the interaction,
/// or all the `awaiting` channels have closed.
struct InteractionHandle {
    candidate_hash: CandidateHash,
    interaction_response: RemoteHandle<Concluded>,
    awaiting: Vec<ResponseChannel<Result<AvailableData, RecoveryError>>>,
}

struct Unavailable;
struct Concluded(CandidateHash, Result<AvailableData, RecoveryError>);

struct InteractionParams {
    validator_authority_keys: Vec<AuthorityId>,
    validators: Vec<ValidatorId>,
    // The number of pieces needed.
    threshold: usize,
    candidate_hash: Hash,
    erasure_root: Hash,
}

enum InteractionPhase {
    RequestFromBackers {
        // a random shuffling of the validators from the backing group which indicates the order
        // in which we connect to them and request the chunk.
        shuffled_backers: Vec<ValidatorIndex>,
    }
    RequestChunks {
        // a random shuffling of the validators which indicates the order in which we connect to the validators and
        // request the chunk from them.
        shuffling: Vec<ValidatorIndex>,
        received_chunks: Map<ValidatorIndex, ErasureChunk>,
        requesting_chunks: FuturesUnordered<Receiver<ErasureChunkRequestResponse>>,
    }
}

struct Interaction {
    to_subsystems: SubsystemSender,
    params: InteractionParams,
    phase: InteractionPhase,
}

Signal Handling

On ActiveLeavesUpdate, if activated is non-empty, set state.live_block_hash to the first block in Activated.

Ignore BlockFinalized signals.

On Conclude, shut down the subsystem.

AvailabilityRecoveryMessage::RecoverAvailableData(receipt, session, Option<backing_group_index>, response)

1. Check the availability_lru for the candidate and return the data if so.
2. Check if there is already an interaction handle for the request. If so, add the response handle to it.
3. Otherwise, load the session info for the given session under the state of live_block_hash, and initiate an interaction with launch_interaction. Add an interaction handle to the state and add the response channel to it.
4. If the session info is not available, return RecoveryError::Unavailable on the response channel.

From-interaction logic

FromInteraction::Concluded

1. Load the entry from the interactions map. It should always exist, if not for logic errors. Send the result to each member of awaiting.
2. Add the entry to the availability_lru.

Interaction logic

launch_interaction(session_index, session_info, candidate_receipt, candidate_hash, Option<backing_group_index>)

1. Compute the threshold from the session info. It should be f + 1, where n = 3f + k, where k in {1, 2, 3}, and n is the number of validators.
2. Set the various fields of InteractionParams based on the validator lists in session_info and information about the candidate.
3. If the backing_group_index is Some, start in the RequestFromBackers phase with a shuffling of the backing group validator indices and a None requesting value.
4. Otherwise, start in the RequestChunks phase with received_chunks,requesting_chunks, and next_shuffling all empty.
5. Set the to_subsystems sender to be equal to a clone of the SubsystemContext's sender.
6. Initialize received_chunks to an empty set, as well as requesting_chunks.

Launch the interaction as a background task running interaction_loop(interaction).

interaction_loop(interaction) -> Result<AvailableData, RecoeryError>

// How many parallel requests to have going at once.
const N_PARALLEL: usize = 50;

• Request AvailabilityStoreMessage::QueryAvailableData. If it exists, return that.

• If the phase is InteractionPhase::RequestFromBackers

  • Loop:
    • If the requesting_pov is Some, poll for updates on it. If it concludes, set requesting_pov to None.
    • If the requesting_pov is None, take the next backer off the shuffled_backers.
      • If the backer is Some, issue a NetworkBridgeMessage::Requests with a network request for the AvailableData and wait for the response.
      • If it concludes with a None result, return to beginning.
      • If it concludes with available data, attempt a re-encoding.
        • If it has the correct erasure-root, break and issue a Ok(available_data).
        • If it has an incorrect erasure-root, return to beginning.
      • If the backer is None, set the phase to InteractionPhase::RequestChunks with a random shuffling of validators and empty next_shuffling, received_chunks, and requesting_chunks and break the loop.

• If the phase is InteractionPhase::RequestChunks:

  • Request AvailabilityStoreMessage::QueryAllChunks. For each chunk that exists, add it to received_chunks and remote the validator from shuffling.
  • Loop:
    • If received_chunks + requesting_chunks + shuffling lengths are less than the threshold, break and return Err(Unavailable).
    • Poll for new updates from requesting_chunks. Check merkle proofs of any received chunks. If the request simply fails due to network issues, insert into the front of shuffling to be retried.
    • If received_chunks has more than threshold entries, attempt to recover the data. If that fails, or a re-encoding produces an incorrect erasure-root, break and issue a Err(RecoveryError::Invalid). If correct, break and issue Ok(available_data).
    • While there are fewer than N_PARALLEL entries in requesting_chunks,
      • Pop the next item from shuffling. If it's empty and requesting_chunks is empty, return Err(RecoveryError::Unavailable).
      • Issue a NetworkBridgeMessage::Requests and wait for the response in requesting_chunks.