A mempool, or memory pool, is where unconfirmed transactions wait after you send them and before they are added to a blockchain. You can think of it as a temporary inbox that nodes use while the network chooses which transactions to put in the next block.
When you send a transaction, your wallet passes it to one or more nodes. Each node checks if the transaction is valid and, if it is, adds it to its own mempool. Miners or validators look through these pools to pick transactions for the next block. Since each node keeps its own mempool, the list of waiting transactions can be a bit different on each one.
After a miner or validator picks which transactions to include, those transactions are added to a new block. When the network accepts the block, the transactions are confirmed and taken out of mempools. Until then, they stay in the mempool and may be replaced or dropped if the network has certain rules.
Transaction fees become important in the mempool. When the network is busy, miners usually choose transactions with higher fees first. This means users who want faster confirmation often pay more. As a result, transactions with lower fees may wait longer. Tools like fee estimators and explorers track mempool activity to help users pick the right fee.
When there is a lot of activity, mempools can get crowded, causing delays and higher fees. Sometimes, transactions are dropped if they wait too long or if there are conflicts, like trying to spend the same coins twice. Each network and node software has its own rules for removing or replacing transactions.
If a transaction stays in the mempool for a long time, users can try to speed it up by raising the fee. Some blockchains let you increase the fee directly, while others use methods like child-pays-for-parent, where a new transaction pays a higher total fee to encourage miners to include both. Wallets and mempool tracking sites help users see the status of their transactions and suggest what fee might help them get confirmed faster.
Public tools and explorers display mempool size, the number of pending transactions, and fee statistics. By checking these, users and developers can see how busy the network is and decide when to send transactions or what fee to use.
Different blockchains handle mempools in their own ways. Some have quick confirmations and small mempools, while others can build up many waiting transactions during busy times. The rules for replacing, keeping, and sharing transactions depend on the protocol and the node software.
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