Definition
Cluster mempool is a major reworking of how Bitcoin Core organises unconfirmed transactions. Instead of tracking loose ancestor and descendant relationships, it places every transaction that is connected through spending into a single group called a cluster, then sorts ("linearizes") each cluster into fee-rate-ordered chunks. This lets a node reason precisely about which transactions a rational miner would include first.
Why bounded clusters help
By capping the size of any cluster, the implementation bounds how much work must be redone when a new transaction arrives. That makes mempool-wide decisions, such as comparing a replacement against everything it would evict, fast enough to run inside the P2P validation path. The result is more accurate block templates and Replace-by-Fee logic that is harder to game, because the node can directly compare the mining value of competing transaction sets.
Relationship to TRUC and pinning
Cluster mempool generalises the idea behind TRUC: where TRUC bounds topology only for transactions that opt in, cluster mempool bounds cluster size for all transactions, but far less restrictively. Linearization research drew on a maximum-ratio-closure algorithm and on linear-programming approaches to find near-optimal orderings efficiently. The implementation was staged for release in Bitcoin Core 31.0.
Cluster mempool is the engine that makes modern anti-pinning policy robust. For the opt-in version of the same idea and the attack it counters, see TRUC transactions and transaction pinning.
In Simple Terms
Cluster mempool is a major reworking of how Bitcoin Core organises unconfirmed transactions. Instead of tracking loose ancestor and descendant relationships, it places every transaction…