Mempool
A mempool is a node's temporary pool of valid, unconfirmed transactions waiting to be included in a block.
✦ Key Insight
Mempools explain why transactions can remain pending, be replaced, or be observed before confirmation. Publicly visible pending transactions can also create opportunities for front-running and other MEV strategies.
✕ Common Misconceptions
There is no single universal mempool shared identically by every node. Private order flow, local policies, propagation delays, and alternative account-abstraction mempools can change what different participants see.
Detailed Explanation
Aliases and acronyms: Memory pool; transaction pool; txpool; pending transaction pool
Plain-English explanation: When a blockchain transaction is broadcast, it may first sit in the transaction pool of participating nodes. Nodes can have slightly different views because the mempool is not a single global database. Block builders or validators select transactions from available pools according to network and fee rules.
Analogy: It is like a waiting area where submitted forms sit before a clerk chooses which ones to process into the official record.
How it works: A node receives a transaction, checks basic validity and local policy, and stores it while propagating it to peers. When a block includes the transaction, nodes remove it from their pools. Transactions may also be dropped if they become invalid, expire under policy, or are replaced.
Example:
Watch out for / common mistakes:
FAQs
Q: Is every pending transaction visible to everyone?
A: No. Many transactions enter public peer-to-peer pools, but private transaction relays and other systems can limit public visibility.
Q: Why does a transaction disappear from a mempool?
A: It may have been confirmed, replaced, invalidated, or dropped by a node's local policy.
Q: Is the mempool part of the blockchain?
A: No. It is temporary node state. Only confirmed transactions written into blocks become part of the chain history.
Sources
In Practice
Dig Deeper
Confirmation
A confirmation indicates that a blockchain transaction has been included in a block and recognised by the network. Additional blocks built afterward can provide further confidence that the transaction will remain part of the accepted blockchain history.
Transaction
A transaction is an action submitted to a blockchain that changes or records blockchain state. Transactions can transfer cryptocurrency, swap tokens, interact with smart contracts, stake assets, vote, mint tokens, or perform other network operations.