Subtree Updater

The Subtree Updater is an offchain actor that saves the protocol gas by inserting batches of new notes into the commitment tree through "ZK Subtree Updates". The role is permissionless, so anyone can run a Subtree Updater.
You can read more about the subtree update mechanism here, but the TL;DR is that the subtree updater polls the contracts for new merkle tree insertions and keeps a local, up-to-date copy of the commitment tree in-memory. When enough insertions occur, the subtree updater will compute the new root using its local copy of the tree, publish it on-chain, and prove that it's correct using a zero-knowledge proof.