{"id":"mvcc-active-set-frozen-at-begin","text":"`MVCCDatabase.begin_transaction` captures `active_at_start` as a frozen set of all currently active transaction IDs at that moment; it is never modified after creation and serves as the transaction's immutable snapshot boundary","truth_value":"IN","source":"entries/2026/05/29/topic-snapshot-vs-ssi-tradeoffs.md","source_url":"","source_hash":"","justifications":[],"dependents":["mvcc-isolation-survives-restart"],"metadata":{"example":"tx.active_at_start = {\n    tid for tid, t in self._transactions.items() if t.is_active}"},"created_at":"","updated_at":"","reviewed_at":"","verified_at":"","retracted_at":"","explanation":{"steps":[{"node":"mvcc-active-set-frozen-at-begin","truth_value":"IN","reason":"premise"}]}}