Inside the mempool

When your wallet broadcasts a transaction, it goes to a few nodes, which check it and pass it on to their peers. Within seconds it has crossed the network. Each node keeps the valid transactions it has received but not yet seen in a block: that waiting room is its mempool, short for memory pool.

There is no single mempool

Every node has its own. They are usually very similar, but not identical: a node may have joined late, set a smaller size, or refused a transaction its own policy dislikes. When people say the mempool, they mean the picture most nodes roughly share.

What a node checks before accepting

• The signatures are valid.
• The inputs exist and are not already spent.
• The outputs do not exceed the inputs.
• The fee meets the node's minimum, and the transaction follows its relay policy.

A full mempool drops the transactions paying the least. A transaction that waits too long, two weeks by default, is forgotten. Forgotten is not cancelled: the original can be broadcast again, and as long as its inputs are unspent it stays valid.