The mempool is the first place a Bitcoin transaction goes after it gets broadcasted to the network and before it is added to a block on the Bitcoin blockchain. Read on to learn about the Bitcoin mempool and how it works.

Understanding the Bitcoin Mempool

The mempool, short for “memory pool,” is a temporary storage for unconfirmed transactions in the Bitcoin network. It’s the first stop for all transactions before they are included in a block and added to the blockchain. 

Whenever a user initiates a transaction, it’s broadcast to the network first before entering the mempool. Here, the transaction waits to be picked up by a miner for inclusion in a block based on transaction fees. Miners prioritize transactions with higher fees. As such, users who want faster confirmations for their transactions may opt to pay higher fees.

Often, the mempool is a reflection of the Bitcoin network activity. It can become congested with a backlog of transactions during periods of high usage, leading to longer confirmation times and higher fees. The mempool tends to clear swiftly in quieter periods, resulting in faster transaction processing time and reduced fees. 

How Does the Bitcoin Mempool Work?

Sending BTC from one wallet address to another initiates a broadcast to the nodes on the network. The transaction is entered into the mempool and joins a queue of unconfirmed transactions. Then, if the transaction is valid, it’s held in the mempool, waiting to be selected by miners for inclusion in the next block to be added to the blockchain.

Bitcoin miners choose transactions from the mempool that they deem economically beneficial, prioritizing those with higher fees. 

As miners work to solve complex mathematical puzzles while adding new blocks, they include these chosen transactions in the block they are attempting to create. Transactions are confirmed once a miner successfully mines a block containing specific transactions. 

This confirmation marks the completion of a transaction, which then moves from the mempool and is added to the blockchain. The sender’s available balance reduces while the recipient’s BTC increases to reflect the completed transaction. 

Why Do Transactions Get Stuck in the Mempool?

Some transactions could get stuck in the mempool because of how miners pick which ones to include in a block. 

The mempool can become congested when there is a sudden surge in transactions being initiated. This incentivizes miners to prioritize transactions with higher fees because they stand to earn more by including them in the next block they mine. Therefore, transactions with lower fees may linger in the mempool, waiting to be picked up. 

This congestion leads to longer confirmation times for lower-fee transactions and may result in them appearing “stuck” in the mempool until the high network activity subsides.

Releasing Transactions From the Bitcoin Mempool

As a user, you can’t directly control the release of a transaction from the Bitcoin mempool. Here are a few ways to release transactions from the Bitcoin mempool.

  • Automatic removal: Unconfirmed transactions within a specific timeframe are automatically released from the mempool. The network’s mempool expiry rules govern this process to ensure the mempool isn’t indefinitely clogged with unconfirmed transactions.
  • Replaced transactions: You can create a new transaction with the same inputs but higher transaction fees. This new transaction is known as a “replace-by-fee” (RBF) transaction and is more likely to be included in a block. However, this feature is only available on some Bitcoin wallets.
  • Child-Pays-For-Parent (CPFP): A user can create a new “child” transaction with higher fees to speed up transactions stuck in the mempool because of low fees. Miners will then need to approve the child transaction with the parent, as the combined fees make it more attractive. Like RBF, this method also depends on the BTC wallet.

Although long transaction processing times on the Bitcoin network can be frustrating, the mempool is a vital part of the security of the blockchain’s security. It ensures the validation of transactions before being added to the network and supports the profitability of mining.