A mempool, short for memory pool, is a node’s temporary holding area for valid transactions it has heard about but that have not yet been included in a block known to that node. There is no single universal queue shared by every computer. Each node applies the network’s rules and local policies, so two nodes can hold different pending transactions at the same time.
How It Works
A wallet signs a transaction and broadcasts it to peers. Receiving nodes check it against protocol rules and local policy, then may relay it to other peers. A miner or validator later selects transactions for a candidate block. Fee bidding can influence selection, but the details differ: Bitcoin commonly expresses a transaction fee rate in satoshis per virtual byte, while Ethereum uses gas and a fee market with base and priority components. A transaction can be replaced under certain rules, remain pending, or be dropped from a node’s pool; seeing it in one explorer does not guarantee confirmation.
History
The idea appears in Bitcoin’s original peer-to-peer software: nodes needed temporary memory to hold and relay transactions before block inclusion. Other networks implement their own transaction-pool policies. Ethereum’s public documentation describes pending transactions being broadcast and later executed by validators, while clients maintain their own local views of pending activity.
Common Misconceptions
A mempool is not a permanent queue, a block, or a guarantee that a transaction will be confirmed next. Pool size and contents vary by node, client, network, and moment. A low-fee transaction may wait, be replaced, or disappear from a node’s pool without being reversed on-chain, because it was never confirmed there. A transaction can also be pending for reasons other than network congestion, including an invalid nonce or wallet settings.
Criticisms
Public pending transactions can expose trade intent before confirmation and create opportunities for transaction reordering or front-running on some networks. Fee competition can make inclusion unpredictable and expensive during demand spikes. At the same time, open propagation lets independent nodes relay activity without a single central operator. Private transaction channels and different ordering mechanisms can reduce some exposure, but each introduces its own trust, access, or decentralization trade-offs.
Social Media Sentiment
In an August 2026 Reddit thread, Bitcoin users discussing slow confirmations disagree about whether the network is unusually busy and suggest waiting or using replace-by-fee where supported. It illustrates user frustration and differing troubleshooting experiences, not a live measure of current congestion. Reddit discussion.
Last updated: 2026-10
Related Terms
See Also
These articles connect pending transactions with ordering, scaling, and cross-chain execution:
- What Is MEV — And Why Every Ethereum Transaction Is a Game
- Does Ethereum TPS Include Layer 2 Transactions?
- How Do Crypto Bridges Work — And Why Are They Such a Hack Target?
Sources
- Bitcoin Core: getmempoolinfo — The RPC reference describes information about the local node’s current transaction memory pool.
- Bitcoin.org fee guidance — This guide explains fee rates, confirmation uncertainty, and why low-fee transactions may wait or be replaced.
- Ethereum transactions — Ethereum.org describes transaction broadcasting, validation, and inclusion in blocks.
- Ethereum gas and fees — This documentation explains Ethereum gas and the network’s fee components.