Mempool: What Is It And How Does It Work?

Mempool: What Is It And How Does It Work?

The mempool, short for "memory pool" or "transaction pool", serves as a gateway to the blockchain and is a vital component of its infrastructure, particularly in the world of cryptocurrencies like Bitcoin. This concept is similar to 'Staging' environments in traditional software development, where testing and quality control are conducted before the release to production. However, unlike the centralized staging environments, each node in a blockchain network has its own mempool to manage.

Transactions in a blockchain begin in the mempool, a list of pending transactions awaiting validation from a node before being committed to a block on the blockchain. The mempool's role is often overlooked, yet it is crucial for understanding how transactions are processed and confirmed on-chain. By analyzing the mempool, we can gain a deeper insight into the workings of blockchains and the status of in-flight transactions.

What Is A Mempool?

A mempool, or memory pool, is a fundamental mechanism within blockchain technology, initially popularized by Bitcoin and subsequently adopted by Ethereum and other blockchain networks. It acts as a dynamic staging area or "waiting room" for unconfirmed transactions, playing a crucial role in determining the sequence and inclusion of transactions in a blockchain's ledger.

Each node in a blockchain network maintains its own mempool, storing information related to unconfirmed transactions. This decentralized approach means that there are as many mempools as there are nodes, with each node receiving and storing transactions at different times and having varying capacities based on its hardware. Consequently, different nodes may have different sets of pending transactions at any given time, leading to variations in mempool sizes and transaction counts across the network.

In the Bitcoin network, when a user sends a transaction, it is broadcast to the network and stored in the mempools of individual nodes until a miner includes it in a block. This process is integral to the network's transaction processing and validation system. Similarly, in the Ethereum ecosystem, the mempool refers to in-memory data structures within a node that store candidate transactions before they are mined. Ethereum nodes, such as Geth and Parity, refer to this as the “transaction pool” or “transaction queue” respectively.

The mempool is not just a singular entity but a collection of individual mempools across nodes, each striving to synchronize with others over the network. Due to the inherent unreliability and latency in network communication, each node's mempool can differ, sometimes significantly. Nodes also have different criteria for accepting transactions, such as minimum gas price and mempool size limits.


Transactions typically leave a node's mempool when they are included in a block, but they can also be removed if they are replaced, canceled, or dropped due to the node's mempool configuration. This decentralized and dynamic system ensures transaction ordering, fee prioritization, and efficient block construction, essential for the robust functioning of blockchain networks like Bitcoin and Ethereum.

How Does Mempool Work?

Whenever a transaction is initiated, it begins its journey by being broadcast from one node to others in the network. Nodes assess these transactions against several criteria, including verifying the signature, ensuring the spender owns the coins, and confirming the output does not exceed the input. Once a transaction passes these checks, it is shared across the network, eventually landing in the mempool, a holding area until a miner selects it for inclusion in a block.

Key Points about Mempools

The concept of a universal, shared mempool is a misconception. In reality, each node operates its unique mempool, receiving and processing transactions at different times. The capacity of a node's mempool varies based on its resources; nodes with limited memory allocate smaller space for transaction logs, while more powerful nodes can handle larger volumes of transaction data.

Miners, driven by profit, tend to prioritize transactions with higher fees. This dynamic makes fee estimation challenging, particularly during times of high demand and limited block space. Users can gauge fee trends by observing the range of fees in the current pool of unconfirmed transactions, enabling them to avoid overpaying during low network activity or underpaying for urgent transactions.

Mempool Dynamics in Transaction Processing

The journey of a transaction through the mempool involves several steps:

  1. A transaction is initiated from a wallet, aimed at another wallet or smart contract.
  2. The user's wallet digitally signs the transaction.
  3. The signed transaction is sent to a gateway node on a blockchain network (like Ethereum or Bitcoin).
  4. This node validates the transaction and adds it to its mempool.
  5. The transaction is then broadcast to other nodes, which also validate and add it to their mempools, further propagating it across the network.
  6. Miners pick up the transaction from the mempool and incorporate it into a new block on the blockchain.
  7. This block, containing the transaction, is broadcast, and receiving nodes then remove the transaction from their mempools.

Impact of Mempool on Transaction Fees

The mempool's size significantly influences transaction speeds and fees in blockchain networks. Miners prioritize transactions with higher fees, as these fees form part of their rewards for mining new blocks. Consequently, a crowded mempool creates a competitive environment, driving up transaction fees and extending confirmation times. Conversely, during periods of low network activity, when the mempool is less congested, transaction fees decrease, and confirmation times are shorter. This ebb and flow in the mempool's activity level is a critical factor in the blockchain's operational dynamics.

How To Monitor The Mempool

Numerous tools and services are available for keeping track of the mempool across various cryptocurrencies like Bitcoin, offering insights into network activity and transaction dynamics.

  • Blockchain Explorers for Mempool Monitoring

Blockchain explorers like Blockstream, Blockchair, or Blockchain serve as valuable resources for observing the mempool's current status. They provide data such as the number of unconfirmed transactions, the mempool's total size, and the prevailing average transaction fee.

  • Specialized Mempool Monitoring Services

For more granular information, services like or are useful. They delve deeper into mempool analytics, offering details on the distribution of transactions by fee rate, the duration of the oldest transactions in the mempool, and predictions on confirmation times for various fee rates.

  • APIs for Custom Mempool Data Access

Blockchain data providers such as Bitquery or Coinmetrics offer APIs that allow for the programmatic retrieval of mempool data. This functionality can be integrated into custom applications or scripts, enabling tailored analysis and usage of mempool information.

The value of monitoring the mempool lies in its ability to guide decision-making regarding transaction timing and fee rates, optimizing for quicker confirmations. Additionally, it offers a window into the broader network's condition, including congestion levels and overall activity, aiding users and developers in navigating the cryptocurrency landscape more effectively.

Mempool Transactions: How to Roll Back

Suppose you've initiated a Bitcoin transaction with a fee of about 10 satoshis per virtual byte (sat/vb). After transmitting it to the Bitcoin network, you find it stalled, unconfirmed due to a surge in the fees required for current block confirmations. Typically, you're faced with two choices:

Patiently Wait for Fee Decrease: One option is to wait it out, hoping fees will drop. This period can range from days to weeks, or even months, during which your transaction remains unprocessed in the mempool.

Opt for Fee Enhancement with RBF: Alternatively, you can opt to increase your transaction fee using methods like Replace by Fee (RBF) or other fee bumping strategies, enhancing its chances for inclusion in a block.

But what if you want a different approach, like extracting your transaction from the mempool entirely? Theoretically, transactions with low fees should be expelled from mempools over time as they undergo purging.

However, this outcome isn't a given, due to the decentralized nature of Bitcoin's network. Each node worldwide operates independently, maintaining its own mempool. Your node might drop the transaction after a set period, but others might hold onto it, leaving it adrift in their mempools. In periods of low transaction volume, such a transaction might even get confirmed unexpectedly.

It's critical to accept that a transaction might never be entirely purged from all mempools. There's always a possibility that someone could archive it and reintroduce it to the network later. If you find your transaction in limbo, the most viable solutions are to either adjust the fee upwards to expedite its processing or create a new transaction sending the same Unspent Transaction Outputs (UTXOs) to yourself. This latter method effectively invalidates the original transaction, resolving the stalemate. Additionally, understanding the dynamics of mempool management across different nodes is crucial in making informed decisions about such transactions.

The Mempool and Bitcoin: Scalability Challenges and Future Strategies

The ongoing evolution of the Mempool and the scalability of Bitcoin is a focal point of research and innovation within the cryptocurrency sector. Addressing scalability challenges is crucial for the broader acceptance and usability of Bitcoin and other cryptocurrencies.

Key strategies include the further adoption of Segregated Witness (SegWit) and the deployment of the Lightning Network. SegWit enhances transaction processing by decoupling the signature data from transaction data, enabling more transactions to fit within each block. The Lightning Network, as a second-layer solution, facilitates transactions off the main blockchain, thereby alleviating congestion in the mempool.

The integration of Schnorr signatures is also being explored. These signatures are anticipated to compact transaction data sizes significantly, contributing to scalability improvements. Schnorr signatures allow for the combination of multiple signatures into one, effectively shrinking the transaction footprint.

Moreover, proposals to expand the block size limit are under discussion. This could potentially quicken transaction processing and enhance efficiency. However, this approach sparks contention within the community due to concerns that a larger block size might lead to increased centralization risks and potentially compromise network security.

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