21 BTCMempool
In plain words
The mempool is the waiting room for Bitcoin payments: transactions sent but not yet in a block. Miners usually pick the ones paying the highest fee per byte first.
Short definition
The waiting room of unconfirmed transactions.
How it works
There is no single mempool: every node keeps its own, filled with valid transactions it has heard about and that pass its relay policy. Policy (not consensus) sets limits such as the minimum relay fee or how much data an OP_RETURN output may carry, which is why nodes running Bitcoin Core and Bitcoin Knots can see slightly different mempools.
When blocks are full, fees rise. A stuck transaction can be bumped with Replace-By-Fee (BIP 125) or with a child that pays for the parent (CPFP).
See also
- Fee rate (sat/vB)Fee per virtual byte of transaction size; higher rates confirm sooner.
- BlockA block is a batch of Bitcoin transactions added to the chain about every ten minutes. The miner who finds it collects the new coins and the fees.
- ConfirmationA transaction is confirmed once it is in a block; each new block on top adds one more.
- UTXO (unspent transaction output)Bitcoin has no account balances. Your wallet holds separate “coins” called UTXOs, like notes in a purse; a payment spends some of them and creates new ones, including change back to you.
- Bitcoin KnotsBitcoin Knots is an alternative node program based on Bitcoin Core and maintained by Luke Dashjr. It follows the same consensus rules but filters more non-payment data by default.
References
- mempool.space mempool.space
- BIP 125 (github.com/bitcoin/bips) github.com