Mempool

Mempool is a memory-buffer that holds the transactions that are waiting to be executed.

Admission control (AC) module sends transactions to mempool. Mempool holds the transactions for a period of time, before consensus commits them. When a new transaction is added, mempool shares this transaction with other validators (validator nodes) in the system. Mempool is a “shared mempool,” as transactions between mempools are shared with other validators. This helps maintain a pseudoglobal ordering.

When a validator receives a transaction from another mempool, the transaction is ordered when it’s added to the ordered queue of the recipient validator. To reduce network consumption in the shared mempool, each validator is responsible for the delivery of its own transactions. We don't rebroadcast transactions originating from a peer validator.

We only broadcast transactions that have some probability of being included in the next block. This means that either the sequence number of the transaction is the next sequence number of the sender account, or it is sequential to it. For example, if the current sequence number for an account is 2 and local mempool contains transactions with sequence numbers 2, 4, 4, 7, 8, then only transactions 2, 4, and 4 will be broadcast.

The consensus module pulls transactions from mempool, mempool does not push transactions into consensus. This is to ensure that while consensus is not ready for transactions:

· Mempool can continue ordering transactions based on gas;

· and Consensus can allow transactions to build up in the mempool.

· This allows transactions to be grouped into a single consensus block, and prioritized by gas price.

Mempool doesn't keep track of transactions sent to consensus. On each get_block request (to pull a block of transaction from mempool), consensus sends a set of transactions that were pulled from mempool, but not committed. This allows the mempool to stay agnostic about different consensus proposal branches.

When a transaction is fully executed and written to storage, consensus notifies mempool. Mempool then drops this transaction from its internal state.