Understanding the Mempool
The Mempool, also known as the memory pool, is a storage mechanism within a cryptocurrency node that holds unconfirmed transactions. It acts as a waiting room for transactions that have not yet been included in a block.
When a transaction is submitted to miners, it needs to be relayed to other nodes in the network. During this process, the Mempool serves as a holding area for all pending transactions.
Each node has its own Mempool, which contains a collection of unconfirmed transactions encountered by that particular node. This allows the node to decide whether or not to relay a new transaction.
Since nodes can run on different hardware, the capacity for storing unconfirmed transactions varies. As a result, Mempool sizes and transaction counts differ across different sources.
When a node receives a new valid block, it removes all transactions included in that block from its Mempool. This includes transactions with conflicting inputs, resulting in a significant decrease in Mempool size.
Running a node does not provide financial incentives, and the dedicated hardware is often limited. In older versions of Bitcoin, a node’s Mempool could use up all its RAM, causing the node to crash and restart with an empty Mempool. However, recent updates have introduced a minimal fee threshold. If the Mempool size approaches the RAM capacity, transactions with fees per kB lower than this threshold are immediately removed from the Mempool. Only new transactions with a sufficient fee per kB are allowed to remain.