What Is Mempool?
Also known as: Memory Pool, Transaction Pool, Pending Transaction Pool
In short
The mempool (memory pool) is a temporary pool in which transactions that have been sent to the network but not yet added to a block are held separately on each node, waiting to be selected and confirmed by miners or validators.
When a user sends a transaction, it first reaches the node it is connected to, where it goes through validity checks such as signature, balance and format, and is propagated to other nodes over the peer-to-peer network. Each node stores pending transactions in its own mempool; for this reason, there is no single central mempool, and the transactions nodes see can differ slightly. Miners or validators select transactions from here when building a new block.
Because block space is limited, transaction selection is usually done by fee, and transactions paying higher fees are prioritized. When the network gets congested, the mempool grows, fees rise and low-fee transactions can wait a long time. On Bitcoin, the fee of a stuck transaction can be increased with methods such as RBF (Replace-by-Fee) and CPFP (Child Pays for Parent). Tools that visualize the mempool help users estimate the appropriate fee.
The mempool being publicly visible also creates some risks. On networks such as Ethereum in particular, bots that watch pending transactions can spot a profitable transaction and place their own ahead of it to carry out front-running or sandwich attacks. This is at the center of MEV debates; some users therefore send their transactions through private relay channels.
Last updated: · This content is for information purposes only and is not investment advice.