What is the mempool?

A transaction is sent to the network and passed from node to node. Each node keeps it in its mempool only if the transaction follows that node's rules.

Miners build each new block from the transactions in their mempool. Transactions with higher fees are generally included first, so a transaction with a low fee can wait a long time.

• A waiting transaction shows the same inputs, outputs and amounts that it will show once it is mined.
• Nodes relay waiting transactions to their peers, so a transaction spreads across the network before any block includes it.
• Different nodes can hold different sets of waiting transactions, so one observer does not always see what another sees.
• Other parties can act on a waiting transaction, for example by broadcasting a competing replacement.

Anyone running a node sees a payment shortly after it is broadcast, which gives them its timing before any block records it.