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README

Mempool

Mempool represents a pool of valid transactions pushed by the network, ready to be included in the next candidate block. It also prioritizes transactions by fee.

Main responsibilities:

  • Queue transactions received from the network, ready to be verified
  • Verify any received transaction
  • Store transactions in the mempool state that do pass fully acceptance criteria
  • Sort transactions in the mempool state by fee.
  • On block acceptance, remove all accepted transactions from the mempool state
  • On request from block generator, provide a set of transactions with highest fee, up to a specified total size.

Transaction acceptance criteria:

  • Decodable
  • Passes rusk.Preverify
  • Does not exist in the mempool state
  • Does not exist in the blockchain state
  • Does not contain a nullifier already used by another transaction in mempool state.

Underlying storage

Mempool is storage-agnostic. An underlying storage must implement interface Pool to be applicable. At that stage, we support two types of stores:

  • hashmap - based on golang map that implements in-memory key/value store.
  • buntdb - based on buntdb, a low-level, in-memory and ACID compliant key/value store that persists to disk.

Implementation details

Exposed methods

Mempool exposes ProcessTx method that is concurrent-safe, implements transaction acceptance criteria and adds valid transactions to the mempool state.

Background goroutines

Mempool is driven by two goroutines:

Main Loop goroutine handles:

  • GetMempoolTxsBySize request from a Block Generator component.
  • GetMempoolTxs request from GraphQL component.
  • Block Accepted event triggered by Chain component.

PropagateLoop goroutine handles:

  • any request for transaction repropagation sent by ProcessTx and repropagates at proper rate. It also should prioritize propagation by transaction fee (not implemented, see also #1240).