Snapshot

A fixed view of the store, taken at a sequence number.

It pins three things, and each for its own reason. The sequence number is the filter: nothing committed after it is visible. The Version keeps the segments the view needs from being deleted by a compaction that has already replaced them. The StoreState keeps the memtables reachable, which matters precisely when a flush has moved their contents into a segment the newer version knows about and this one does not.

A snapshot also holds back compaction: an older version of a key may not be dropped while a snapshot that could still ask for it exists. Holding one open for a long time therefore costs disk space, which is the honest trade and the reason this is AutoCloseable rather than something the collector tidies up.

store.snapshot().use { snapshot ->
    store.get(key, snapshot)          // unchanged by anything written from here on
}