Function retain_decision 

pub const fn retain_decision(
    rec: RecordMeta,
    batch: BatchMeta,
    is_newest_for_key: bool,
    txn: TxnDataState,
    now_ms: i64,
    delete_retention_ms: i64,
) -> RetainDecision

The single per-record KIP-534 retain decision.

Control batches, that is transaction commit and abort markers, are retained as long as the pass reads data of their transaction. Once the pass finds no data of it, the marker ages out through the delete horizon. Data records dedup newest-wins. A tombstone, that is a record with a null value, ages out through the delete horizon after it becomes the newest entry for its key.