Mempool

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Pronunciation

[mem-pool]

Analogy

The mempool is like a waiting room where people (transactions) sit before being called into the main event (a ). During busy periods, the waiting room gets crowded, and those willing to pay higher entry (transaction ) get called in first while others wait longer.

Definition

The memory pool or pool where unconfirmed transactions wait before being included in a . The mempool is maintained by each independently as a staging area for transactions that have been broadcast to the network but not yet confirmed.

Key Points Intro

The mempool serves as the clearinghouse that bridges users and producers.

Key Points

Contains all valid but unconfirmed transactions broadcast to the network.

Varies between nodes—there is no single global mempool.

Acts as the marketplace where miners/validators select transactions for blocks.

Transactions with higher fees typically get selected from the mempool first.

Example

During a price surge, the network often experiences high volumes, resulting in a congested mempool with over 100,000 pending transactions. Users who need fast confirmations must pay higher to prioritize their transactions for selection from the crowded mempool.

Technical Deep Dive

Nodes manage mempools using various policies to maximize efficiency and prevent resource exhaustion. These typically include size limits (Bitcoin nodes default to 300MB), minimum fee thresholds to prevent spam, and replacement policies like Replace-by-Fee (RBF) that allow transactions to be superseded by versions with higher . Mempools implement sophisticated data structures for efficient lookup, validation, and prioritization based on feerate (fee per byte or unit of computation). During congestion, transactions with lower may remain in the mempool for extended periods or eventually be dropped if the mempool reaches capacity limits. Since each maintains its own mempool, propagation dynamics can lead to slightly different sets across the network, though these differences are typically reconciled as blocks are processed.

Security Warning

Monitor the current network mempool before sending time-sensitive transactions. During congestion, transactions with inadequate may remain unconfirmed for days or be dropped entirely from nodes' mempools after expiration periods (typically 14 days in ).

Caveat

While many explorers and wallets display 'the mempool,' these are actually snapshots from specific nodes or services and may not exactly match what every in the network sees, particularly during periods of high network activity or propagation delays.

Related Terms

Blockchain & Cryptocurrency Glossary

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