.

z.txt

+ZODB5
+-----
+
+zconn.db._storage always provide IMVCCStorage - either raw storage provides it,
+or DB wraps raw storage with MVCCAdapter.
+
+MVCCAdapter in turn uses either MVCCAdapterInstance (current) or
+HistoricalStorageAdapter, or UndoAdapterInstance. Retriving zconn.at from those:
+
+MVCCAdapterInstance
+    ._start
+
+HistoricalStorageAdapter
+    ._before
+
+UndoAdapterInstance
+    # no way to retrieve `at`, but .undo_instance() through which
+    # UndoAdapterInstance is returnerd, is not used anywhere.
+
+
+For the reference: FileStorage, ZEO and NEO do not provide IMVCCStorage, thus
+for them we can rely on MVCCAdapterInstance.
+
+RelStorage is IMVCCStorage - TODO: how to extract at.
+
+
+ZODB4
+-----
+
+TODO
+
+
+
+ZODB3
+-----
+
+TODO
+
+
+
+
+
+
+
+
+
+
+
+
+----------------------------------------
+
+
+
+
+
+zconn._storage
+
+
+
+
+
+
+
+
+DB.getTid(at, before)