Mempool

A Mempool is a node's temporary storage for unconfirmed cryptocurrency transactions awaiting validation and inclusion in a blockchain block. Users also call it a memory pool or transaction pool. Each full node maintains its own mempool to track pending transactions on the network.

Users broadcast transactions to the network. Nodes receive these transactions, verify their validity (such as correct signatures and sufficient funds), and add valid ones to the mempool. Miners or validators then select transactions from the mempool. They prioritize those offering the highest fees per byte to maximize block rewards. When a new block confirms transactions, nodes remove them from the mempool. For example, in Bitcoin, miners assemble blocks from the mempool every 10 minutes on average.

The mempool matters for transaction speed, costs, and network health. During high demand, mempools fill up, delaying confirmations. Users compete by paying higher fees, which clears congestion. This fee market secures the network by incentivizing miners. Mempool data also reveals network activity and potential attacks, like spam floods.

Key characteristics include size limits—Bitcoin nodes default to 300 MB—to avoid memory overload. Nodes evict low-fee transactions when full. Replace-by-Fee (RBF) lets users bump fees on stuck transactions. Public mempools broadcast data openly, while some mining pools use private ones to prevent transaction front-running.