Added a ClientStorage server-sync configuration option and

server_sync constructor argument to force a server round trip at
the beginning of transactions to wait for any outstanding
invalidations at the start of the transaction to be delivered.

CHANGES.rst

 Changelog
 =========
 
+- Added a ``ClientStorage`` ``server-sync`` configuration option and
+  ``server_sync`` constructor argument to force a server round trip at
+  the beginning of transactions to wait for any outstanding
+  invalidations at the start of the transaction to be delivered.
+
 - Fixed bugs in using the ZEO 5 client with ZEO 4 servers.
 
 5.0.0a2 (2016-07-30)

README.rst

@@ -456,6 +456,23 @@ read_only_fallback
 
    If ``read_only_fallback`` is set, then ``read_only`` is ignored.
 
+server_sync
+   Flag, false by default, indicating whether the ``sync`` method
+   should make a server request.  The ``sync`` method is called at the
+   start of explcitly begin transactions.  Making a server requests assures
+   that any invalidations outstanding at the beginning of a
+   transaction are processed.
+
+   Setting this to True is important when application activity is
+   spread over multiple ZEO clients. The classic example of this is
+   when a web browser makes a request to an application server (ZEO
+   client) that makes a change and then makes a request to another
+   application server that depends on the change.
+
+   Setting this to True makes transactions a little slower because of
+   the added server round trip.  For transactions that don't otherwise
+   need to access the storage server, the impact can be significant.
+
 wait_timeout
    How long to wait for an initial connection, defaulting to 30
    seconds.  If an initial connection can't be made within this time
@@ -567,6 +584,9 @@ read-only-fallback
 
    If ``read_only_fallback`` is set, then ``read_only`` is ignored.
 
+server-sync
+   Sets thr ``server_sync`` option described above.
+
 wait_timeout
    How long to wait for an initial connection, defaulting to 30
    seconds.  If an initial connection can't be made within this time

src/ZEO/ClientStorage.py

@@ -95,8 +95,10 @@ class ClientStorage(ZODB.ConflictResolution.ConflictResolvingStorage):
                  min_disconnect_poll=1, max_disconnect_poll=None,
                  wait=True,
                  drop_cache_rather_verify=True,
-                 username=None, password=None, realm=None,
                  credentials=None,
+                 server_sync=False,
+                 # The ZODB-define ZConfig support may ball these:
+                 username=None, password=None, realm=None,
                  # For tests:
                  _client_factory=ZEO.asyncio.client.ClientThread,
                  ):
@@ -181,6 +183,8 @@ class ClientStorage(ZODB.ConflictResolution.ConflictResolvingStorage):
 
         """
 
+        assert not username or password or realm
+
         if isinstance(addr, int):
             addr = ('127.0.0.1', addr)
 
@@ -268,6 +272,8 @@ class ClientStorage(ZODB.ConflictResolution.ConflictResolvingStorage):
 
         self._commit_lock = threading.Lock()
 
+        self.server_sync = server_sync
+
         if wait:
             try:
                 self._wait()
@@ -377,6 +383,14 @@ class ClientStorage(ZODB.ConflictResolution.ConflictResolvingStorage):
                 'interfaces', ()):
                 zope.interface.alsoProvides(self, iface)
 
+        if self.protocol_version >= b'Z5':
+            self.ping = lambda : self._call('ping')
+        else:
+            self.ping = lambda : self._call('lastTransaction')
+
+        if self.server_sync:
+            self.sync = self.ping
+
     def set_server_addr(self, addr):
         # Normalize server address and convert to string
         if isinstance(addr, str):

src/ZEO/StorageServer.py

@@ -73,7 +73,7 @@ registered_methods = set(( 'get_info', 'lastTransaction',
     'history', 'record_iternext', 'sendBlob', 'getTid', 'loadSerial',
     'new_oid', 'undoa', 'undoLog', 'undoInfo', 'iterator_start',
     'iterator_next', 'iterator_record_start', 'iterator_record_next',
-    'iterator_gc', 'server_status', 'set_client_label'))
+    'iterator_gc', 'server_status', 'set_client_label', 'ping'))
 
 class ZEOStorage:
     """Proxy to underlying storage for a single remote client."""
@@ -614,6 +614,9 @@ class ZEOStorage:
     def ruok(self):
         return self.server.ruok()
 
+    def ping(self):
+        pass
+
 class StorageServerDB:
     """Adapter from StorageServerDB to ZODB.interfaces.IStorageWrapper
 
@@ -949,7 +952,6 @@ class StorageServer:
         return dict((storage_id, self.server_status(storage_id))
                     for storage_id in self.storages)
 
-
 class StubTimeoutThread:
 
     def begin(self, client):

src/ZEO/component.xml

@@ -120,6 +120,15 @@
       </description>
     </key>
 
+    <key name="server-sync" datatype="boolean" default="off">
+      <description>
+        A flag indicating whether calls to sync() should make a server
+        request, thus causing the storage to wait for any outstanding
+        invalidations. The sync method is called when transactions are
+        explicitly begun.
+      </description>
+    </key>
+
     <key name="wait-timeout" datatype="integer" default="30">
       <description>
          How long to wait for an initial connection, defaulting to 30

src/ZEO/tests/ConnectionTests.py

@@ -769,7 +769,7 @@ class ReconnectionTests(CommonSetupTearDown):
 
         # Accesses should fail now
         with short_timeout(self):
-            self.assertRaises(ClientDisconnected, self._storage.history, ZERO)
+            self.assertRaises(ClientDisconnected, self._storage.ping)
 
         # Restart the server, this time read-write
         self.startServer(create=0, keep=0)

src/ZEO/tests/testConfig.py

@@ -62,6 +62,7 @@ class ZEOConfigTestBase(setupstack.TestCase):
         blob_cache_size_check=10,
         read_only=False,
         read_only_fallback=False,
+        server_sync=False,
         wait_timeout=30,
         client_label=None,
         storage='1',
@@ -106,6 +107,7 @@ class ZEOConfigTest(ZEOConfigTestBase):
             blob_cache_size=424242,
             read_only=True,
             read_only_fallback=True,
+            server_sync=True,
             wait_timeout=33,
             client_label='test_client',
             name='Test'

src/ZEO/zconfig.py

@@ -85,5 +85,6 @@ class ClientStorageConfig:
             name=config.name,
             read_only=config.read_only,
             read_only_fallback=config.read_only_fallback,
+            server_sync = config.server_sync,
             wait_timeout=config.wait_timeout,
             **options)