.

lib/zodb.py

@@ -146,6 +146,27 @@ def zconn_at(zconn): # -> tid
         raise POSException.ConnectionStateError("database connection is closed")
 
     # ZODB5 uses MVCC uniformly
+    #
+    # zconn.db._storage always provides 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.
     if zmajor >= 5:
         zstor = zconn._storage
         if isinstance(zstor, ZODB.mvccadapter.MVCCAdapterInstance):
@@ -156,7 +177,24 @@ def zconn_at(zconn): # -> tid
 
         raise AssertionError("zconn_at: TODO: add support for zstor %r" % zstor)
 
-    raise AssertionError("zconn_at: TODO: add support for ZODB34")
+    # ZODB4
+    #
+    # Connection:
+    #     .before     !None for historic connections
+    #
+    #     ._txn_time  - if !None - set to tid of _next_ transaction
+    #                   XXX set to None initially - what to do?
+    #
+    #     # XXX do something like that ZODB5 is doing:
+    #     zconn._start = zconn._storage.lastTransaction() + 1
+    #     # XXX _and_ check out queued invalidations
+    elif zmajor == 4:
+        raise AssertionError("zconn_at: TODO: add support for ZODB4")
+
+
+    # ZODB3
+    else:
+        raise AssertionError("zconn_at: TODO: add support for ZODB3")
 
 
 # before2at converts tid that specifies database state as "before" into tid that

z.txt

-ZODB5
------
-
-zconn.db._storage always provides 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
------
-
-Connection:
-    .before     !None for historic connections
-
-    ._txn_time  - if !None - set to tid of _next_ transaction
-                  XXX set to None initially - what to do?
-
-    # XXX do something like that ZODB5 is doing:
-    zconn._start = zconn._storage.lastTransaction() + 1
-    # XXX _and_ check out queued invalidations
-
-
-ZODB3
------
-
-TODO
-
-
-
-
-
-
-
-
-
-
-
-
-----------------------------------------
-
-
-
-
-
-zconn._storage
-
-
-
-
-
-
-
-
-DB.getTid(at, before)