Mempool

When a user signs a transaction, it is broadcast to the network, nodes validate it (signature, nonce, balance and format) and, if accepted, store it in their mempool and propagate it to peers via gossip. Block producers (miners under proof of work, validators and proposers under proof of stake) select transactions from the mempool, usually prioritizing those with the highest fee per unit of gas, to include in the next block. Each node applies size limits and eviction policies for low-fee transactions, and many networks support replace-by-fee (RBF) to replace a pending transaction by raising its fee.

The mempool is the "limbo" between signing and confirmation. It explains why a transaction can take time or get stuck when the fee is too low, why it can be sped up by raising the fee, and it is where the risk of MEV (Maximal Extractable Value) arises: bots that watch pending transactions to front-run or sandwich them. As a good practice, this is the origin of private mempools and protected orderflow services. For issuers and tokenization infrastructure it matters for inclusion predictability and the finality of operations.