# What is the mempool?

The mempool is the waiting area for valid transactions that no block has included. Each node keeps its own copy, so anyone running a node can see the transactions in it before they are confirmed.

Also called memory pool.

## How a transaction waits

A transaction is sent to the network and passed from node to node. Each node keeps it in its mempool only if the transaction follows that node's rules.

Miners build each new block from the transactions in their mempool. Transactions with higher fees are generally included first, so a transaction with a low fee can wait a long time.

## What observers see

- A waiting transaction shows the same inputs, outputs and amounts that it will show once it is mined.
- Nodes relay waiting transactions to their peers, so a transaction spreads across the network before any block includes it.
- Different nodes can hold different sets of waiting transactions, so one observer does not always see what another sees.
- Other parties can act on a waiting transaction, for example by broadcasting a competing replacement.

## Why it matters for privacy

Anyone running a node sees a payment shortly after it is broadcast, which gives them its timing before any block records it.

## Related terms

- [Confirmation](https://redacted.gg/glossary/confirmation.md): A transaction gets one confirmation when a block includes it, and each later block adds one more. THORChain waits for more confirmations on larger transfers, up to a maximum set for each chain.
- [Front-running](https://redacted.gg/glossary/front-running.md): Front-running is trading ahead of an order you can see coming. On public chains, bots and traders watch visible orders and wallets and act first, at the expense of the person whose order they saw.
- [UTXO](https://redacted.gg/glossary/utxo.md): Bitcoin keeps no account balances. It keeps unspent transaction outputs, or UTXOs, which are the coins a wallet can spend, and each payment spends whole UTXOs and creates new ones.
- [Chain analysis](https://redacted.gg/glossary/chain-analysis.md): Chain analysis traces funds on public blockchains and groups addresses into likely owners, using the public record together with outside data such as exchange records, address labels and leaks.
- [Bitcoin privacy](https://redacted.gg/glossary/bitcoin-privacy.md): Bitcoin is pseudonymous. Addresses carry no names, but every transaction is public and permanent, so once one of your addresses is tied to you, anyone can read its history and the addresses linked to it.

Sources: [Memory pool, Bitcoin Wiki](https://en.bitcoin.it/wiki/Memory_pool), [Transaction broadcasting, Bitcoin developer guide](https://developer.bitcoin.org/devguide/p2p_network.html), [Transactions, Bitcoin developer guide](https://developer.bitcoin.org/devguide/transactions.html)
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Page: https://redacted.gg/glossary/mempool/