Mempool

What Is a Mempool?

The name is a contraction of "memory" and "pool." It acts as a waiting room for transactions that have been broadcast to the network but have not yet been confirmed on the blockchain.

How the Mempool Works

Before accepting a transaction, nodes run a series of checks to confirm it is valid. This includes verifying that signatures are correct, that outputs do not exceed inputs, and that the funds have not already been spent. If a transaction fails these checks, it is rejected.

There is no single, universal mempool shared by every node. Each node is configured differently and receives transactions at slightly different times. Lower-end devices with limited resources may dedicate only a small amount of memory to storing pending transactions, while higher-end nodes may devote considerably more.

Why the Mempool Matters

Miners and validators are generally motivated by rewards, so transactions with higher fees attached tend to be selected first. When network activity is high and block space is limited, transactions with low fees may wait a long time or, in some cases, be dropped from the mempool entirely.

The mempool can also help with estimating fees. By looking at the current set of unconfirmed transactions, a user can make a more educated guess about how quickly a transaction is likely to be confirmed at a given fee level. In periods of low activity, there is usually no need to overpay, while time-sensitive transactions during busy periods may require a higher fee. If a transaction becomes stuck with a fee that is too low, some networks allow the sender to resubmit it with a higher fee using a method known as Replace-by-Fee (RBF).