Module fetcher

Component responsible for fetching transactions from [NewPooledTransactionHashes]. TransactionFetcher is responsible for rate limiting and retry logic for fetching transactions. Upon receiving an announcement, functionality of the TransactionFetcher is used for filtering out hashes 1) for which the tx is already known and 2) unknown but the hash is already seen in a previous announcement. The hashes that remain from an announcement are then packed into a request with respect to the [EthVersion] of the announcement. Any hashes that don’t fit into the request, are buffered in the TransactionFetcher. If on the other hand, space remains, hashes that the peer has previously announced are taken out of buffered hashes to fill the request up. The [GetPooledTransactions] request is then sent to the peer’s session, this marks the peer as active with respect to MAX_CONCURRENT_TX_REQUESTS_PER_PEER.

When a peer buffers hashes in the TransactionsManager::on_new_pooled_transaction_hashes pipeline, it is stored as fallback peer for those hashes. When TransactionsManager is polled, it checks if any of fallback peer is idle. If so, it packs a request for that peer, filling it from the buffered hashes. It does so until there are no more idle peers or until the hashes buffer is empty.

If a [GetPooledTransactions] request resolves with an error, the hashes in the request are buffered with respect to MAX_REQUEST_RETRIES_PER_TX_HASH. So is the case if the request resolves with partial success, that is some of the requested hashes are not in the response, these are then buffered.

Most healthy peers will send the same hashes in their announcements, as RLPx is a gossip protocol. This means it’s unlikely, that a valid hash, will be buffered for very long before it’s re-tried. Nonetheless, the capacity of the buffered hashes cache must be large enough to buffer many hashes during network failure, to allow for recovery.

Structs

• GetPooledTxRequest
  An inflight request for [PooledTransactions] from a peer.
• GetPooledTxRequestFut
  Stores the response receiver made by sending a [GetPooledTransactions] request to a peer’s session.
• GetPooledTxResponse
  Upon reception of a response, a GetPooledTxRequest is deconstructed to form a GetPooledTxResponse.
• TransactionFetcher
  The type responsible for fetching missing transactions from peers.
• TransactionFetcherInfo
  Tracks stats about the TransactionFetcher.
• TxFetchMetadata
  Metadata of a transaction hash that is yet to be fetched.
• UnverifiedPooledTransactions
  Wrapper of unverified [PooledTransactions].
• VerifiedPooledTransactions
  [PooledTransactions] that have been successfully verified.

Enums

• FetchEvent
  Represents possible events from fetching transactions.
• VerificationOutcome
  Outcome from verifying a [PooledTransactions] response. Signals to caller whether to penalize the sender of the response or not.