PMMR

PMMR stands for Pruned Merkle Mountain Range, a data structure used to store histories of data in an append-only system while allowing selective pruning of older content. It is a variant of the Merkle Mountain Range designed to maintain a compact, verifiable state, and it has been applied in the Monero cryptocurrency to manage the growing blockchain more efficiently.

In a PMMR, leaves correspond to data elements such as transaction outputs. As new leaves are appended, the structure updates a set of peaks that together encode the current state. Pruning marks certain older leaves and their internal nodes as discarded to save disk space. Despite pruning, the PMMR preserves enough cryptographic information to prove membership of non-pruned leaves using the remaining root hashes and proof paths.

PMMRs are used in Monero to store the set of outputs and related commitments. The design enables full nodes to prune spent outputs and other historic data while preserving verifiability for the current state. It also supports light clients by providing succinct proofs and a stable root that can be checked without accessing the entire history.

Advantages include reduced storage requirements and support for light-client verification. Limitations involve increased complexity in maintenance, potential need for periodic reconstruction to ensure integrity, and reliance on the integrity of the pruning metadata.

Related concepts include Merkle Mountain Range and blockchain pruning in privacy-focused cryptocurrencies.