Mempools: Everything to Know

By  Beluga Research August 12, 2023

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  • Cryptocurrency mempools are repositories where pending transactions are temporarily stored before being included in a block
  • A memory pool, also known as a mempool, is a data structure that stores unconfirmed cryptocurrency transactions before a miner adds the next block to the blockchain
  • Crypto miners prioritize transactions in a mempool according to the transactions' fees, size, and extent to which they incur network congestion
  • Mempools provide efficient transaction processing, fee flexibility, transparency, and security against double spending


Cryptocurrency mempools are repositories where pending transactions are temporarily stored before miners include them in a block. A mempool serves as a waiting area for transactions, so miners can ascertain they are valid and ready to be added to the blockchain. The mempool is a bridge between the moment a transaction is broadcasted by a user and the point at which a miner permanently adds the transaction to the blockchain.

A Brief History

The concept of mempools emerged with the advent of Bitcoin in 2009. In the early days of Bitcoin, there was no explicit mempool. Miners typically processed transactions directly into As the Bitcoin network grew in popularity, it became clear that the community needed a mechanism to manage pending transactions. This led to the creation of the Bitcoin mempool, which has become an integral part of the process to confirm transactions.

Mempools: Everything to Know

When a user initiates a cryptocurrency transaction, the transaction is broadcast to the network. The transaction then enters the mempool of all participating nodes.

Each mempool has its own set of rules for accepting or rejecting transactions. These rules are called a mempool policy. A mempool policy determines factors about transactions, including transaction fees, sizes, and types. Higher transaction fees incentivize miners to choose these transactions first. When they choose a transaction quickly, it is processed faster than other transactions.

The list of transactions and fees attached to them is called the fee market. Mempools are in constant flux. Transactions enter or leave the mempool as they are confirmed or rejected. If a transaction remains unconfirmed for an extended period, it may be dropped from the mempool. This allows the user to resend it with a higher fee if desired. Flexibility ensures that the mempool remains efficient and responsive to changing network conditions.

Getting Started

  • Basic understanding. Each mempool is a data structure residing in the memory of every participating node in the blockchain.
  • Mempools are necessary to ensure that transactions get processed. Miners rely on mempools to select transactions to include in the next block that they mine.
  • Mempools help miners earn transaction fees. This supports the growth of the blockchain.

Unique Aspects

  • Transactions in the mempool are unconfirmed. They have not yet been included in a block. This means there is a level of uncertainty regarding their finality. Miners decide which transactions to prioritize based on the transactions' fees and sizes, as well as network congestion.
  • Transaction fees play a significant role in determining the priority of a transaction within the mempool. Higher fees incentivize miners to include the transaction in their blocks promptly. Lower fees typically result in longer waiting times, as a transaction competes with others for inclusion. The fee market ensures that the limited block space is allocated to transactions with the highest fees. This promotes efficiency and discourages spam.
  • Prevention of double-spending. Since transactions are added to the mempool before being confirmed, the network can detect and reject attempts to spend the same funds twice. Nodes in the network maintain a comprehensive view of the mempool. They actively check for conflicting transactions. This consensus mechanism helps maintain the integrity and security of the blockchain.
  • Challenges related to scalability. As cryptocurrency adoption grows and transaction volumes increase, mempools can become congested. Congestion typically results in longer confirmation times and higher fees. Developers and researchers address these concerns by exploring innovative solutions, including transaction batching, fee estimation algorithms, and layer-two scaling solutions like the Lightning Network.


  • Efficient Transaction Processing - Mempools enable efficient transaction processing by providing a mechanism to prioritize transactions based on their fee rates.
  • Flexibility in Fee Selection - Users can choose the fee they are willing to pay for their transaction. Flexibility allows users to adjust their fees according to their urgency. This ensures faster confirmation during periods of network congestion or higher transaction volumes.
  • Potential for Fee Optimization - By monitoring the mempool, users can analyze the fee rates of transactions that are getting confirmed and adjust their fees accordingly. Optimization helps users save costs by avoiding overpaying for transaction fees while maintaining reasonable confirmation times.
  • Transparency - Mempools provide transparency by allowing users to monitor the status of their transactions. Users can see the position of their transaction in the mempool. They can also estimate the time it might take for their transaction to be confirmed.
  • Security Against Double Spending - A transaction in the mempool is considered pending and not yet confirmed. This prevents the same funds from being used in multiple transactions.


  • Potential for Transaction Delays - During periods of high network activity, the mempool can become congested. This results in increased competition for block space. This congestion may lead to delays in transaction confirmations, especially for transactions with lower fee rates. Users who do not prioritize their fees appropriately may experience longer confirmation times.
  • Fee Volatility - The fee rates required for timely transaction confirmations can vary depending on network conditions. Fluctuations in network activity and the number of pending transactions can lead to fee rate spikes. These make it challenging to predict the optimal fee for fast confirmation. Volatility can be a disadvantage for users who require consistent and predictable transaction costs.
  • Limited Block Size - Mempools are limited by the maximum block size. This determines the number of transactions that can be included in a block. When the mempool exceeds this limit, transactions with lower fee rates may be delayed or dropped from the mempool. This limitation can create a competitive environment where users need to pay higher fees to secure faster confirmation.
  • Potential for Transaction Censorship - In centralized blockchain networks, the mempool can be controlled by a central authority, allowing them to selectively include or exclude transactions. This centralized control introduces the risk of transaction censorship. It undermines the decentralized nature of cryptocurrencies and compromises the cryptocurrencies' inclusivity and openness.