diff --git a/setup.py b/setup.py
index 86e3b29c3..ad4320116 100644
--- a/setup.py
+++ b/setup.py
@@ -26,7 +26,7 @@
     sys.exit(0)
 
 install_requires = [
-    'ZODB >= 5.0.0a1',
+    'ZODB >= 5.0.0a5',
     'six',
     'transaction >= 1.6.0',
     'persistent >= 4.1.0',
diff --git a/src/ZEO/ClientStorage.py b/src/ZEO/ClientStorage.py
index 292364157..b1b77368b 100644
--- a/src/ZEO/ClientStorage.py
+++ b/src/ZEO/ClientStorage.py
@@ -53,10 +53,7 @@
 # max signed 64-bit value ~ infinity :) Signed cuz LBTree and TimeStamp
 m64 = b'\x7f\xff\xff\xff\xff\xff\xff\xff'
 
-try:
-    from ZODB.ConflictResolution import ResolvedSerial
-except ImportError:
-    ResolvedSerial = 'rs'
+from ZODB.ConflictResolution import ResolvedSerial
 
 def tid2time(tid):
     return str(TimeStamp(tid))
@@ -77,6 +74,7 @@ def get_timestamp(prev_ts=None):
 
 MB = 1024**2
 
+@zope.interface.implementer(ZODB.interfaces.IMultiCommitStorage)
 class ClientStorage(object):
     """A storage class that is a network client to a remote storage.
 
@@ -724,18 +722,31 @@ def tpc_vote(self, txn):
         """
         tbuf = self._check_trans(txn, 'tpc_vote')
         try:
-            self._call('vote', id(txn))
+            for oid in self._call('vote', id(txn)) or ():
+                tbuf.serial(oid, ResolvedSerial)
         except POSException.StorageTransactionError:
             # Hm, we got disconnected and reconnected bwtween
             # _check_trans and voting. Let's chack the transaction again:
-            tbuf = self._check_trans(txn, 'tpc_vote')
+            self._check_trans(txn, 'tpc_vote')
+            raise
+        except POSException.ConflictError as err:
+            oid = getattr(err, 'oid', None)
+            if oid is not None:
+                # This is a band-aid to help recover from a situation
+                # that shouldn't happen.  A Client somehow misses some
+                # invalidations and has out of date data in its
+                # cache. We need some whay to invalidate the cache
+                # entry without invalidations. So, if we see a
+                # (unresolved) conflict error, we assume that the
+                # cache entry is bad and invalidate it.
+                self._cache.invalidate(oid, None)
             raise
 
         if tbuf.exception:
             raise tbuf.exception
 
-        if tbuf.serials:
-            return list(tbuf.serials.items())
+        if tbuf.resolved:
+            return list(tbuf.resolved)
         else:
             return None
 
@@ -830,6 +841,8 @@ def tpc_finish(self, txn, f=lambda tid: None):
 
         self._update_blob_cache(tbuf, tid)
 
+        return tid
+
     def _update_blob_cache(self, tbuf, tid):
         """Internal helper move blobs updated by a transaction to the cache.
         """
diff --git a/src/ZEO/StorageServer.py b/src/ZEO/StorageServer.py
index 5578376fb..4c807e0c4 100644
--- a/src/ZEO/StorageServer.py
+++ b/src/ZEO/StorageServer.py
@@ -84,7 +84,7 @@ class ZEOStorage:
     blob_tempfile = None
     log_label = 'unconnected'
     locked = False             # Don't have storage lock
-    verifying = store_failed = 0
+    verifying = 0
 
     def __init__(self, server, read_only=0):
         self.server = server
@@ -338,7 +338,6 @@ def tpc_begin(self, id, user, description, ext, tid=None, status=" "):
         self.blob_log = []
         self.tid = tid
         self.status = status
-        self.store_failed = 0
         self.stats.active_txns += 1
 
         # Assign the transaction attribute last. This is so we don't
@@ -426,38 +425,40 @@ def _try_to_vote(self, delay=None):
                     self.storage.tpc_begin(self.transaction)
 
                 for op, args in self.txnlog:
-                    if not getattr(self, op)(*args):
-                        break
-
+                    getattr(self, op)(*args)
 
                 # Blob support
-                while self.blob_log and not self.store_failed:
+                while self.blob_log:
                     oid, oldserial, data, blobfilename = self.blob_log.pop()
                     self._store(oid, oldserial, data, blobfilename)
 
-                if not self.store_failed:
-                    # Only call tpc_vote of no store call failed,
-                    # otherwise the serialnos() call will deliver an
-                    # exception that will be handled by the client in
-                    # its tpc_vote() method.
-                    serials = self.storage.tpc_vote(self.transaction)
-                    if serials:
-                        self.serials.extend(serials)
+                serials = self.storage.tpc_vote(self.transaction)
+                if serials:
+                    if not isinstance(serials[0], bytes):
+                        serials = (oid for (oid, serial) in serials
+                                   if serial == ResolvedSerial)
 
-                self.connection.async('serialnos', self.serials)
+                    self.serials.extend(serials)
 
-            except Exception:
+            except Exception as err:
                 self.storage.tpc_abort(self.transaction)
                 self._clear_transaction()
+
+                if isinstance(err, ConflictError):
+                    self.stats.conflicts += 1
+                    self.log("conflict error %s" % err, BLATHER)
+                if not isinstance(err, TransactionError):
+                    logger.exception("While voting")
+
                 if delay is not None:
                     delay.error(sys.exc_info())
                 else:
                     raise
             else:
                 if delay is not None:
-                    delay.reply(None)
+                    delay.reply(self.serials)
                 else:
-                    return None
+                    return self.serials
 
         else:
             return delay
@@ -549,120 +550,45 @@ def undoa(self, trans_id, tid):
         self._check_tid(tid, exc=StorageTransactionError)
         self.txnlog.undo(trans_id)
 
-    def _op_error(self, oid, err, op):
-        self.store_failed = 1
-        if isinstance(err, ConflictError):
-            self.stats.conflicts += 1
-            self.log("conflict error oid=%s msg=%s" %
-                     (oid_repr(oid), str(err)), BLATHER)
-        if not isinstance(err, TransactionError):
-            # Unexpected errors are logged and passed to the client
-            self.log("%s error: %s, %s" % ((op,)+ sys.exc_info()[:2]),
-                     logging.ERROR, exc_info=True)
-        err = self._marshal_error(err)
-        # The exception is reported back as newserial for this oid
-        self.serials.append((oid, err))
-
     def _delete(self, oid, serial):
-        err = None
-        try:
-            self.storage.deleteObject(oid, serial, self.transaction)
-        except (SystemExit, KeyboardInterrupt):
-            raise
-        except Exception as e:
-            err = e
-            self._op_error(oid, err, 'delete')
-
-        return err is None
+        self.storage.deleteObject(oid, serial, self.transaction)
 
     def _checkread(self, oid, serial):
-        err = None
-        try:
-            self.storage.checkCurrentSerialInTransaction(
-                oid, serial, self.transaction)
-        except (SystemExit, KeyboardInterrupt):
-            raise
-        except Exception as e:
-            err = e
-            self._op_error(oid, err, 'checkCurrentSerialInTransaction')
-
-        return err is None
+        self.storage.checkCurrentSerialInTransaction(
+            oid, serial, self.transaction)
 
     def _store(self, oid, serial, data, blobfile=None):
-        err = None
-        try:
-            if blobfile is None:
-                newserial = self.storage.store(
-                    oid, serial, data, '', self.transaction)
-            else:
-                newserial = self.storage.storeBlob(
-                    oid, serial, data, blobfile, '', self.transaction)
-        except (SystemExit, KeyboardInterrupt):
-            raise
-        except Exception as error:
-            self._op_error(oid, error, 'store')
-            err = error
+        if blobfile is None:
+            newserial = self.storage.store(
+                oid, serial, data, '', self.transaction)
         else:
-            if serial != b"\0\0\0\0\0\0\0\0":
-                self.invalidated.append(oid)
+            newserial = self.storage.storeBlob(
+                oid, serial, data, blobfile, '', self.transaction)
+
+        if serial != b"\0\0\0\0\0\0\0\0":
+            self.invalidated.append(oid)
+
+        if newserial:
 
             if isinstance(newserial, bytes):
                 newserial = [(oid, newserial)]
 
-            for oid, s in newserial or ():
+            for oid, s in newserial:
 
                 if s == ResolvedSerial:
                     self.stats.conflicts_resolved += 1
                     self.log("conflict resolved oid=%s"
                              % oid_repr(oid), BLATHER)
-
-                self.serials.append((oid, s))
-
-        return err is None
+                    self.serials.append(oid)
 
     def _restore(self, oid, serial, data, prev_txn):
-        err = None
-        try:
-            self.storage.restore(oid, serial, data, '', prev_txn,
-                                 self.transaction)
-        except (SystemExit, KeyboardInterrupt):
-            raise
-        except Exception as err:
-            self._op_error(oid, err, 'restore')
-
-        return err is None
+        self.storage.restore(oid, serial, data, '', prev_txn,
+                             self.transaction)
 
     def _undo(self, trans_id):
-        err = None
-        try:
-            tid, oids = self.storage.undo(trans_id, self.transaction)
-        except (SystemExit, KeyboardInterrupt):
-            raise
-        except Exception as e:
-            err = e
-            self._op_error(z64, err, 'undo')
-        else:
-            self.invalidated.extend(oids)
-            self.serials.extend((oid, ResolvedSerial) for oid in oids)
-
-        return err is None
-
-    def _marshal_error(self, error):
-        # Try to pickle the exception.  If it can't be pickled,
-        # the RPC response would fail, so use something that can be pickled.
-        if PY3:
-            pickler = Pickler(BytesIO(), 3)
-        else:
-            # The pure-python version requires at least one argument (PyPy)
-            pickler = Pickler(0)
-        pickler.fast = 1
-        try:
-            pickler.dump(error)
-        except:
-            msg = "Couldn't pickle storage exception: %s" % repr(error)
-            self.log(msg, logging.ERROR)
-            error = StorageServerError(msg)
-        return error
+        tid, oids = self.storage.undo(trans_id, self.transaction)
+        self.invalidated.extend(oids)
+        self.serials.extend(oids)
 
     # IStorageIteration support
 
@@ -1381,8 +1307,3 @@ class Serving(ServerEvent):
 
 class Closed(ServerEvent):
     pass
-
-default_cert_authenticate = 'SIGNED'
-def ssl_config(section):
-    from .sslconfig import ssl_config
-    return ssl_config(section, True)
diff --git a/src/ZEO/TransactionBuffer.py b/src/ZEO/TransactionBuffer.py
index bb3b50ec6..50471dfe5 100644
--- a/src/ZEO/TransactionBuffer.py
+++ b/src/ZEO/TransactionBuffer.py
@@ -46,7 +46,7 @@ def __init__(self, connection_generation):
         # stored are builtin types -- strings or None.
         self.pickler = Pickler(self.file, 1)
         self.pickler.fast = 1
-        self.serials = {} # processed { oid -> serial }
+        self.resolved = set() # {oid}
         self.exception = None
 
     def close(self):
@@ -61,10 +61,9 @@ def store(self, oid, data):
 
     def serial(self, oid, serial):
         if isinstance(serial, Exception):
-            self.exception = serial
-            self.serials[oid] = None
-        else:
-            self.serials[oid] = serial
+            self.exception = serial # This transaction will never be committed
+        elif serial == ResolvedSerial:
+            self.resolved.add(oid)
 
     def storeBlob(self, oid, blobfilename):
         self.blobs.append((oid, blobfilename))
@@ -72,7 +71,7 @@ def storeBlob(self, oid, blobfilename):
     def __iter__(self):
         self.file.seek(0)
         unpickler = Unpickler(self.file)
-        serials = self.serials
+        resolved = self.resolved
 
         # Gaaaa, this is awkward. There can be entries in serials that
         # aren't in the buffer, because undo.  Entries can be repeated
@@ -83,9 +82,9 @@ def __iter__(self):
         for i in range(self.count):
             oid, data = unpickler.load()
             seen.add(oid)
-            yield oid, data, serials.get(oid) == ResolvedSerial
+            yield oid, data, oid in resolved
 
-        # We may have leftover serials because undo
-        for oid, serial in serials.items():
+        # We may have leftover oids because undo
+        for oid in resolved:
             if oid not in seen:
-                yield oid, None, serial == ResolvedSerial
+                yield oid, None, True
diff --git a/src/ZEO/asyncio/server.py b/src/ZEO/asyncio/server.py
index 49f499c50..765344003 100644
--- a/src/ZEO/asyncio/server.py
+++ b/src/ZEO/asyncio/server.py
@@ -23,7 +23,7 @@ class ServerProtocol(base.Protocol):
     """asyncio low-level ZEO server interface
     """
 
-    protocols = b'Z4', b'Z5'
+    protocols = (b'Z5', )
 
     name = 'server protocol'
     methods = set(('register', ))
@@ -169,7 +169,7 @@ def reply(self, obj):
 
     def error(self, exc_info):
         self.sent = 'error'
-        log("Error raised in delayed method", logging.ERROR, exc_info=exc_info)
+        logger.error("Error raised in delayed method", exc_info=exc_info)
         self.protocol.send_error(self.msgid, exc_info[1])
 
     def __repr__(self):
@@ -206,7 +206,6 @@ def reply(self, obj):
 
     def error(self, exc_info):
         self.ready.wait()
-        log("Error raised in delayed method", logging.ERROR, exc_info=exc_info)
         self.protocol.call_soon_threadsafe(Delay.error, self, exc_info)
 
 
diff --git a/src/ZEO/asyncio/tests.py b/src/ZEO/asyncio/tests.py
index 8b4ddc980..4165ac389 100644
--- a/src/ZEO/asyncio/tests.py
+++ b/src/ZEO/asyncio/tests.py
@@ -757,7 +757,7 @@ def connect(self, finish=False):
         self.target = protocol.zeo_storage
         if finish:
             self.assertEqual(self.pop(parse=False), best_protocol_version)
-            protocol.data_received(sized(b'Z4'))
+            protocol.data_received(sized(b'Z5'))
         return protocol
 
     message_id = 0
@@ -795,9 +795,9 @@ def testServerBasics(self):
         self.assertEqual(self.pop(parse=False), best_protocol_version)
 
         # The client sends it's protocol:
-        protocol.data_received(sized(b'Z4'))
+        protocol.data_received(sized(b'Z5'))
 
-        self.assertEqual(protocol.protocol_version, b'Z4')
+        self.assertEqual(protocol.protocol_version, b'Z5')
 
         protocol.zeo_storage.notify_connected.assert_called_once_with(protocol)
 
diff --git a/src/ZEO/cache.py b/src/ZEO/cache.py
index 9a5b19721..553a8c24d 100644
--- a/src/ZEO/cache.py
+++ b/src/ZEO/cache.py
@@ -33,7 +33,7 @@
 
 import ZODB.fsIndex
 import zc.lockfile
-from ZODB.utils import p64, u64, z64
+from ZODB.utils import p64, u64, z64, RLock
 import six
 from ._compat import PYPY
 
@@ -182,6 +182,8 @@ def __init__(self, path=None, size=200*1024**2, rearrange=.8):
         # currentofs.
         self.currentofs = ZEC_HEADER_SIZE
 
+        self._lock = RLock()
+
         # self.f is the open file object.
         # When we're not reusing an existing file, self.f is left None
         # here -- the scan() method must be called then to open the file
@@ -239,9 +241,10 @@ def fc(self):
         return self
 
     def clear(self):
-        self.f.seek(ZEC_HEADER_SIZE)
-        self.f.truncate()
-        self._initfile(ZEC_HEADER_SIZE)
+        with self._lock:
+            self.f.seek(ZEC_HEADER_SIZE)
+            self.f.truncate()
+            self._initfile(ZEC_HEADER_SIZE)
 
     ##
     # Scan the current contents of the cache file, calling `install`
@@ -451,26 +454,28 @@ def _makeroom(self, nbytes):
     # new tid must be strictly greater than our current idea of the most
     # recent tid.
     def setLastTid(self, tid):
-        if (not tid) or (tid == z64):
-            return
-        if (tid <= self.tid) and self._len:
-            if tid == self.tid:
-                return                  # Be a little forgiving
-            raise ValueError("new last tid (%s) must be greater than "
-                             "previous one (%s)"
-                             % (u64(tid), u64(self.tid)))
-        assert isinstance(tid, bytes) and len(tid) == 8, tid
-        self.tid = tid
-        self.f.seek(len(magic))
-        self.f.write(tid)
-        self.f.flush()
+        with self._lock:
+            if (not tid) or (tid == z64):
+                return
+            if (tid <= self.tid) and self._len:
+                if tid == self.tid:
+                    return                  # Be a little forgiving
+                raise ValueError("new last tid (%s) must be greater than "
+                                 "previous one (%s)"
+                                 % (u64(tid), u64(self.tid)))
+            assert isinstance(tid, bytes) and len(tid) == 8, tid
+            self.tid = tid
+            self.f.seek(len(magic))
+            self.f.write(tid)
+            self.f.flush()
 
     ##
     # Return the last transaction seen by the cache.
     # @return a transaction id
     # @defreturn string, or 8 nulls if no transaction is yet known
     def getLastTid(self):
-        return self.tid
+        with self._lock:
+            return self.tid
 
     ##
     # Return the current data record for oid.
@@ -479,52 +484,54 @@ def getLastTid(self):
     #         in the cache
     # @defreturn 3-tuple: (string, string, string)
     def load(self, oid, before_tid=None):
-        ofs = self.current.get(oid)
-        if ofs is None:
-            self._trace(0x20, oid)
-            return None
-        self.f.seek(ofs)
-        read = self.f.read
-        status = read(1)
-        assert status == b'a', (ofs, self.f.tell(), oid)
-        size, saved_oid, tid, end_tid, lver, ldata = unpack(
-            ">I8s8s8sHI", read(34))
-        assert saved_oid == oid, (ofs, self.f.tell(), oid, saved_oid)
-        assert end_tid == z64, (ofs, self.f.tell(), oid, tid, end_tid)
-        assert lver == 0, "Versions aren't supported"
-
-        if before_tid and tid >= before_tid:
-            return None
-
-        data = read(ldata)
-        assert len(data) == ldata, (ofs, self.f.tell(), oid, len(data), ldata)
-
-        # WARNING: The following assert changes the file position.
-        # We must not depend on this below or we'll fail in optimized mode.
-        assert read(8) == oid, (ofs, self.f.tell(), oid)
-
-        self._n_accesses += 1
-        self._trace(0x22, oid, tid, end_tid, ldata)
-
-        ofsofs = self.currentofs - ofs
-        if ofsofs < 0:
-            ofsofs += self.maxsize
-
-        if (ofsofs > self.rearrange and
-            self.maxsize > 10*len(data) and
-            size > 4):
-            # The record is far back and might get evicted, but it's
-            # valuable, so move it forward.
-
-            # Remove fromn old loc:
-            del self.current[oid]
+        with self._lock:
+            ofs = self.current.get(oid)
+            if ofs is None:
+                self._trace(0x20, oid)
+                return None
             self.f.seek(ofs)
-            self.f.write(b'f'+pack(">I", size))
+            read = self.f.read
+            status = read(1)
+            assert status == b'a', (ofs, self.f.tell(), oid)
+            size, saved_oid, tid, end_tid, lver, ldata = unpack(
+                ">I8s8s8sHI", read(34))
+            assert saved_oid == oid, (ofs, self.f.tell(), oid, saved_oid)
+            assert end_tid == z64, (ofs, self.f.tell(), oid, tid, end_tid)
+            assert lver == 0, "Versions aren't supported"
+
+            if before_tid and tid >= before_tid:
+                return None
+
+            data = read(ldata)
+            assert len(data) == ldata, (
+                ofs, self.f.tell(), oid, len(data), ldata)
+
+            # WARNING: The following assert changes the file position.
+            # We must not depend on this below or we'll fail in optimized mode.
+            assert read(8) == oid, (ofs, self.f.tell(), oid)
+
+            self._n_accesses += 1
+            self._trace(0x22, oid, tid, end_tid, ldata)
 
-            # Write to new location:
-            self._store(oid, tid, None, data, size)
+            ofsofs = self.currentofs - ofs
+            if ofsofs < 0:
+                ofsofs += self.maxsize
 
-        return data, tid
+            if (ofsofs > self.rearrange and
+                self.maxsize > 10*len(data) and
+                size > 4):
+                # The record is far back and might get evicted, but it's
+                # valuable, so move it forward.
+
+                # Remove fromn old loc:
+                del self.current[oid]
+                self.f.seek(ofs)
+                self.f.write(b'f'+pack(">I", size))
+
+                # Write to new location:
+                self._store(oid, tid, None, data, size)
+
+            return data, tid
 
     ##
     # Return a non-current revision of oid that was current before tid.
@@ -533,54 +540,56 @@ def load(self, oid, before_tid=None):
     # @return data record, serial number, start tid, and end tid
     # @defreturn 4-tuple: (string, string, string, string)
     def loadBefore(self, oid, before_tid):
-        noncurrent_for_oid = self.noncurrent.get(u64(oid))
-        if noncurrent_for_oid is None:
-            result = self.load(oid, before_tid)
-            if result:
-                return result[0], result[1], None
-            else:
-                self._trace(0x24, oid, "", before_tid)
-                return result
-
-        items = noncurrent_for_oid.items(None, u64(before_tid)-1)
-        if not items:
-            result = self.load(oid, before_tid)
-            if result:
-                return result[0], result[1], None
-            else:
-                self._trace(0x24, oid, "", before_tid)
-                return result
+        with self._lock:
+            noncurrent_for_oid = self.noncurrent.get(u64(oid))
+            if noncurrent_for_oid is None:
+                result = self.load(oid, before_tid)
+                if result:
+                    return result[0], result[1], None
+                else:
+                    self._trace(0x24, oid, "", before_tid)
+                    return result
+
+            items = noncurrent_for_oid.items(None, u64(before_tid)-1)
+            if not items:
+                result = self.load(oid, before_tid)
+                if result:
+                    return result[0], result[1], None
+                else:
+                    self._trace(0x24, oid, "", before_tid)
+                    return result
 
-        tid, ofs = items[-1]
+            tid, ofs = items[-1]
 
-        self.f.seek(ofs)
-        read = self.f.read
-        status = read(1)
-        assert status == b'a', (ofs, self.f.tell(), oid, before_tid)
-        size, saved_oid, saved_tid, end_tid, lver, ldata = unpack(
-            ">I8s8s8sHI", read(34))
-        assert saved_oid == oid, (ofs, self.f.tell(), oid, saved_oid)
-        assert saved_tid == p64(tid), (ofs, self.f.tell(), oid, saved_tid, tid)
-        assert end_tid != z64, (ofs, self.f.tell(), oid)
-        assert lver == 0, "Versions aren't supported"
-        data = read(ldata)
-        assert len(data) == ldata, (ofs, self.f.tell())
-
-        # WARNING: The following assert changes the file position.
-        # We must not depend on this below or we'll fail in optimized mode.
-        assert read(8) == oid, (ofs, self.f.tell(), oid)
-
-        if end_tid < before_tid:
-            result = self.load(oid, before_tid)
-            if result:
-                return result[0], result[1], None
-            else:
-                self._trace(0x24, oid, "", before_tid)
-                return result
+            self.f.seek(ofs)
+            read = self.f.read
+            status = read(1)
+            assert status == b'a', (ofs, self.f.tell(), oid, before_tid)
+            size, saved_oid, saved_tid, end_tid, lver, ldata = unpack(
+                ">I8s8s8sHI", read(34))
+            assert saved_oid == oid, (ofs, self.f.tell(), oid, saved_oid)
+            assert saved_tid == p64(tid), (
+                ofs, self.f.tell(), oid, saved_tid, tid)
+            assert end_tid != z64, (ofs, self.f.tell(), oid)
+            assert lver == 0, "Versions aren't supported"
+            data = read(ldata)
+            assert len(data) == ldata, (ofs, self.f.tell())
+
+            # WARNING: The following assert changes the file position.
+            # We must not depend on this below or we'll fail in optimized mode.
+            assert read(8) == oid, (ofs, self.f.tell(), oid)
+
+            if end_tid < before_tid:
+                result = self.load(oid, before_tid)
+                if result:
+                    return result[0], result[1], None
+                else:
+                    self._trace(0x24, oid, "", before_tid)
+                    return result
 
-        self._n_accesses += 1
-        self._trace(0x26, oid, "", saved_tid)
-        return data, saved_tid, end_tid
+            self._n_accesses += 1
+            self._trace(0x26, oid, "", saved_tid)
+            return data, saved_tid, end_tid
 
     ##
     # Store a new data record in the cache.
@@ -591,45 +600,48 @@ def loadBefore(self, oid, before_tid):
     #                current.
     # @param data the actual data
     def store(self, oid, start_tid, end_tid, data):
-        seek = self.f.seek
-        if end_tid is None:
-            ofs = self.current.get(oid)
-            if ofs:
-                seek(ofs)
-                read = self.f.read
-                status = read(1)
-                assert status == b'a', (ofs, self.f.tell(), oid)
-                size, saved_oid, saved_tid, end_tid = unpack(
-                    ">I8s8s8s", read(28))
-                assert saved_oid == oid, (ofs, self.f.tell(), oid, saved_oid)
-                assert end_tid == z64, (ofs, self.f.tell(), oid)
-                if saved_tid == start_tid:
+        with self._lock:
+            seek = self.f.seek
+            if end_tid is None:
+                ofs = self.current.get(oid)
+                if ofs:
+                    seek(ofs)
+                    read = self.f.read
+                    status = read(1)
+                    assert status == b'a', (ofs, self.f.tell(), oid)
+                    size, saved_oid, saved_tid, end_tid = unpack(
+                        ">I8s8s8s", read(28))
+                    assert saved_oid == oid, (
+                        ofs, self.f.tell(), oid, saved_oid)
+                    assert end_tid == z64, (ofs, self.f.tell(), oid)
+                    if saved_tid == start_tid:
+                        return
+                    raise ValueError("already have current data for oid")
+            else:
+                noncurrent_for_oid = self.noncurrent.get(u64(oid))
+                if noncurrent_for_oid and (
+                    u64(start_tid) in noncurrent_for_oid):
                     return
-                raise ValueError("already have current data for oid")
-        else:
-            noncurrent_for_oid = self.noncurrent.get(u64(oid))
-            if noncurrent_for_oid and (u64(start_tid) in noncurrent_for_oid):
-                return
 
-        size = allocated_record_overhead + len(data)
+            size = allocated_record_overhead + len(data)
 
-        # A number of cache simulation experiments all concluded that the
-        # 2nd-level ZEO cache got a much higher hit rate if "very large"
-        # objects simply weren't cached.  For now, we ignore the request
-        # only if the entire cache file is too small to hold the object.
-        if size >= min(max_block_size, self.maxsize - ZEC_HEADER_SIZE):
-            return
+            # A number of cache simulation experiments all concluded that the
+            # 2nd-level ZEO cache got a much higher hit rate if "very large"
+            # objects simply weren't cached.  For now, we ignore the request
+            # only if the entire cache file is too small to hold the object.
+            if size >= min(max_block_size, self.maxsize - ZEC_HEADER_SIZE):
+                return
 
-        self._n_adds += 1
-        self._n_added_bytes += size
-        self._len += 1
+            self._n_adds += 1
+            self._n_added_bytes += size
+            self._len += 1
 
-        self._store(oid, start_tid, end_tid, data, size)
+            self._store(oid, start_tid, end_tid, data, size)
 
-        if end_tid:
-            self._trace(0x54, oid, start_tid, end_tid, dlen=len(data))
-        else:
-            self._trace(0x52, oid, start_tid, dlen=len(data))
+            if end_tid:
+                self._trace(0x54, oid, start_tid, end_tid, dlen=len(data))
+            else:
+                self._trace(0x52, oid, start_tid, dlen=len(data))
 
     def _store(self, oid, start_tid, end_tid, data, size):
         # Low-level store used by store and load
@@ -696,35 +708,37 @@ def _store(self, oid, start_tid, end_tid, data, size):
     # - tid the id of the transaction that wrote a new revision of oid,
     #        or None to forget all cached info about oid.
     def invalidate(self, oid, tid):
-        ofs = self.current.get(oid)
-        if ofs is None:
-            # 0x10 == invalidate (miss)
-            self._trace(0x10, oid, tid)
-            return
+        with self._lock:
+            ofs = self.current.get(oid)
+            if ofs is None:
+                # 0x10 == invalidate (miss)
+                self._trace(0x10, oid, tid)
+                return
 
-        self.f.seek(ofs)
-        read = self.f.read
-        status = read(1)
-        assert status == b'a', (ofs, self.f.tell(), oid)
-        size, saved_oid, saved_tid, end_tid = unpack(">I8s8s8s", read(28))
-        assert saved_oid == oid, (ofs, self.f.tell(), oid, saved_oid)
-        assert end_tid == z64, (ofs, self.f.tell(), oid)
-        del self.current[oid]
-        if tid is None:
             self.f.seek(ofs)
-            self.f.write(b'f'+pack(">I", size))
-            # 0x1E = invalidate (hit, discarding current or non-current)
-            self._trace(0x1E, oid, tid)
-            self._len -= 1
-        else:
-            if tid == saved_tid:
-                logger.warning("Ignoring invalidation with same tid as current")
-                return
-            self.f.seek(ofs+21)
-            self.f.write(tid)
-            self._set_noncurrent(oid, saved_tid, ofs)
-            # 0x1C = invalidate (hit, saving non-current)
-            self._trace(0x1C, oid, tid)
+            read = self.f.read
+            status = read(1)
+            assert status == b'a', (ofs, self.f.tell(), oid)
+            size, saved_oid, saved_tid, end_tid = unpack(">I8s8s8s", read(28))
+            assert saved_oid == oid, (ofs, self.f.tell(), oid, saved_oid)
+            assert end_tid == z64, (ofs, self.f.tell(), oid)
+            del self.current[oid]
+            if tid is None:
+                self.f.seek(ofs)
+                self.f.write(b'f'+pack(">I", size))
+                # 0x1E = invalidate (hit, discarding current or non-current)
+                self._trace(0x1E, oid, tid)
+                self._len -= 1
+            else:
+                if tid == saved_tid:
+                    logger.warning(
+                        "Ignoring invalidation with same tid as current")
+                    return
+                self.f.seek(ofs+21)
+                self.f.write(tid)
+                self._set_noncurrent(oid, saved_tid, ofs)
+                # 0x1C = invalidate (hit, saving non-current)
+                self._trace(0x1C, oid, tid)
 
     ##
     # Generates (oid, serial) oairs for all objects in the
diff --git a/src/ZEO/interfaces.py b/src/ZEO/interfaces.py
index 15d1c9e2f..83e48114a 100644
--- a/src/ZEO/interfaces.py
+++ b/src/ZEO/interfaces.py
@@ -24,8 +24,7 @@ def __init__(self, storage):
 class IClientCache(zope.interface.Interface):
     """Client cache interface.
 
-    Note that caches need not be thread safe, fpr the most part,
-    except for getLastTid, which may be called from multiple threads.
+    Note that caches need to be thread safe.
     """
 
     def close():
diff --git a/src/ZEO/tests/ConnectionTests.py b/src/ZEO/tests/ConnectionTests.py
index 0ca5006e0..6225cb303 100644
--- a/src/ZEO/tests/ConnectionTests.py
+++ b/src/ZEO/tests/ConnectionTests.py
@@ -1018,90 +1018,6 @@ def checkTimeoutAfterVote(self):
         # or the server.
         self.assertRaises(KeyError, storage.load, oid, '')
 
-    def checkTimeoutProvokingConflicts(self):
-        self._storage = storage = self.openClientStorage()
-        # Assert that the zeo cache is empty.
-        self.assert_(not list(storage._cache.contents()))
-        # Create the object
-        oid = storage.new_oid()
-        obj = MinPO(7)
-        # We need to successfully commit an object now so we have something to
-        # conflict about.
-        t = Transaction()
-        storage.tpc_begin(t)
-        revid1a = storage.store(oid, ZERO, zodb_pickle(obj), '', t)
-        revid1b = storage.tpc_vote(t)
-        revid1 = handle_serials(oid, revid1a, revid1b)
-        storage.tpc_finish(t)
-        # Now do a store, sleeping before the finish so as to cause a timeout.
-        obj.value = 8
-        t = Transaction()
-        old_connection_count = storage.connection_count_for_tests
-        storage.tpc_begin(t)
-        revid2a = storage.store(oid, revid1, zodb_pickle(obj), '', t)
-        revid2b = storage.tpc_vote(t)
-        revid2 = handle_serials(oid, revid2a, revid2b)
-
-        # Now sleep long enough for the storage to time out.
-        # This used to sleep for 3 seconds, and sometimes (but very rarely)
-        # failed then.  Now we try for a minute.  It typically succeeds
-        # on the second time thru the loop, and, since self.timeout is 1,
-        # it's typically faster now (2/1.8 ~= 1.11 seconds sleeping instead
-        # of 3).
-        deadline = time.time() + 60 # wait up to a minute
-        while time.time() < deadline:
-            if (storage.is_connected() and
-                (storage.connection_count_for_tests == old_connection_count)
-                ):
-                time.sleep(self.timeout / 1.8)
-            else:
-                break
-        self.assert_(
-            (not storage.is_connected())
-            or
-            (storage.connection_count_for_tests > old_connection_count)
-            )
-        storage._wait()
-        self.assert_(storage.is_connected())
-        # We expect finish to fail.
-        self.assertRaises(ClientDisconnected, storage.tpc_finish, t)
-        storage.tpc_abort(t)
-
-        # Now we think we've committed the second transaction, but we really
-        # haven't.  A third one should produce a POSKeyError on the server,
-        # which manifests as a ConflictError on the client.
-        obj.value = 9
-        t = Transaction()
-        storage.tpc_begin(t)
-        storage.store(oid, revid2, zodb_pickle(obj), '', t)
-        self.assertRaises(ConflictError, storage.tpc_vote, t)
-        # Even aborting won't help.
-        storage.tpc_abort(t)
-        self.assertRaises(ZODB.POSException.StorageTransactionError,
-                          storage.tpc_finish, t)
-        # Try again.
-        obj.value = 10
-        t = Transaction()
-        storage.tpc_begin(t)
-        storage.store(oid, revid2, zodb_pickle(obj), '', t)
-        # Even aborting won't help.
-        self.assertRaises(ConflictError, storage.tpc_vote, t)
-        # Abort this one and try a transaction that should succeed.
-        storage.tpc_abort(t)
-
-        # Now do a store.
-        obj.value = 11
-        t = Transaction()
-        storage.tpc_begin(t)
-        revid2a = storage.store(oid, revid1, zodb_pickle(obj), '', t)
-        revid2b = storage.tpc_vote(t)
-        revid2 = handle_serials(oid, revid2a, revid2b)
-        storage.tpc_finish(t)
-        # Now load the object and verify that it has a value of 11.
-        data, revid = storage.load(oid, '')
-        self.assertEqual(zodb_unpickle(data), MinPO(11))
-        self.assertEqual(revid, revid2)
-
 class MSTThread(threading.Thread):
 
     __super_init = threading.Thread.__init__
diff --git a/src/ZEO/tests/forker.py b/src/ZEO/tests/forker.py
index 6ee8ee3cd..353b216d7 100644
--- a/src/ZEO/tests/forker.py
+++ b/src/ZEO/tests/forker.py
@@ -95,6 +95,10 @@ def runner(config, qin, qout, timeout=None,
         import ZEO.asyncio.server
         old_protocol = ZEO.asyncio.server.best_protocol_version
         ZEO.asyncio.server.best_protocol_version = protocol
+        old_protocols = ZEO.asyncio.server.ServerProtocol.protocols
+        ZEO.asyncio.server.ServerProtocol.protocols = tuple(sorted(
+            set(old_protocols) | set([protocol])
+            ))
 
     try:
         import ZEO.runzeo, threading
@@ -142,8 +146,8 @@ def runner(config, qin, qout, timeout=None,
 
     finally:
         if old_protocol:
-            ZEO.asyncio.server.best_protocol_version = protocol
-
+            ZEO.asyncio.server.best_protocol_version = old_protocol
+            ZEO.asyncio.server.ServerProtocol.protocols = old_protocols
 
 def stop_runner(thread, config, qin, qout, stop_timeout=9, pid=None):
     qin.put('stop')
diff --git a/src/ZEO/tests/protocols.test b/src/ZEO/tests/protocols.test
index ab54a08c8..419a8b66c 100644
--- a/src/ZEO/tests/protocols.test
+++ b/src/ZEO/tests/protocols.test
@@ -5,7 +5,7 @@ A full test of all protocols isn't practical.  But we'll do a limited
 test that at least the current and previous protocols are supported in
 both directions.
 
-Let's start a Z309 server
+Let's start a Z4 server
 
     >>> storage_conf = '''
     ... <blobstorage>
@@ -94,82 +94,91 @@ A current client should be able to connect to a old server:
     >>> zope.testing.setupstack.rmtree('blobs')
     >>> zope.testing.setupstack.rmtree('server-blobs')
 
-And the other way around:
-
-    >>> addr, _ = start_server(storage_conf, dict(invalidation_queue_size=5))
-
-Note that we'll have to pull some hijinks:
-
-    >>> import ZEO.asyncio.client
-    >>> old_protocols = ZEO.asyncio.client.Protocol.protocols
-    >>> ZEO.asyncio.client.Protocol.protocols = [b'Z4']
-
-    >>> db = ZEO.DB(addr, client='client', blob_dir='blobs')
-    >>> str(db.storage.protocol_version.decode('ascii'))
-    'Z4'
-    >>> wait_connected(db.storage)
-    >>> conn = db.open()
-    >>> conn.root().x = 0
-    >>> transaction.commit()
-    >>> len(db.history(conn.root()._p_oid, 99))
-    2
-
-    >>> conn.root()['blob1'] = ZODB.blob.Blob()
-    >>> with conn.root()['blob1'].open('w') as f:
-    ...    r = f.write(b'blob data 1')
-    >>> transaction.commit()
-
-    >>> db2 = ZEO.DB(addr, blob_dir='server-blobs', shared_blob_dir=True)
-    >>> wait_connected(db2.storage)
-    >>> conn2 = db2.open()
-    >>> for i in range(5):
-    ...     conn2.root().x += 1
-    ...     transaction.commit()
-    >>> conn2.root()['blob2'] = ZODB.blob.Blob()
-    >>> with conn2.root()['blob2'].open('w') as f:
-    ...    r = f.write(b'blob data 2')
-    >>> transaction.commit()
-
-
-    >>> @wait_until()
-    ... def x_to_be_5():
-    ...     conn.sync()
-    ...     return conn.root().x == 5
-
-    >>> db.close()
-
-    >>> for i in range(2):
-    ...     conn2.root().x += 1
-    ...     transaction.commit()
-
-    >>> db = ZEO.DB(addr, client='client', blob_dir='blobs')
-    >>> wait_connected(db.storage)
-    >>> conn = db.open()
-    >>> conn.root().x
-    7
-
-    >>> db.close()
-
-    >>> for i in range(10):
-    ...     conn2.root().x += 1
-    ...     transaction.commit()
-
-    >>> db = ZEO.DB(addr, client='client', blob_dir='blobs')
-    >>> wait_connected(db.storage)
-    >>> conn = db.open()
-    >>> conn.root().x
-    17
-
-    >>> with conn.root()['blob1'].open() as f:
-    ...    f.read()
-    b'blob data 1'
-    >>> with conn.root()['blob2'].open() as f:
-    ...    f.read()
-    b'blob data 2'
-
-    >>> db2.close()
-    >>> db.close()
-
-Undo the hijinks:
-
-    >>> ZEO.asyncio.client.Protocol.protocols = old_protocols
+#############################################################################
+# Note that the ZEO 5.0 server only supports clients that use the Z5 protocol
+
+# And the other way around:
+
+#     >>> addr, _ = start_server(storage_conf, dict(invalidation_queue_size=5))
+
+# Note that we'll have to pull some hijinks:
+
+#     >>> db = ZEO.DB(addr, client='client', blob_dir='blobs')
+#     >>> str(db.storage.protocol_version.decode('ascii'))
+#     'Z4'
+#     >>> wait_connected(db.storage)
+#     >>> conn = db.open()
+#     >>> conn.root().x = 0
+#     >>> transaction.commit()
+#     >>> len(db.history(conn.root()._p_oid, 99))
+#     2
+
+#     >>> db = ZEO.DB(addr, client='client', blob_dir='blobs')
+#     >>> db.storage.protocol_version
+#     b'Z4'
+#     >>> wait_connected(db.storage)
+#     >>> conn = db.open()
+#     >>> conn.root().x = 0
+#     >>> transaction.commit()
+#     >>> len(db.history(conn.root()._p_oid, 99))
+#     2
+
+#     >>> conn.root()['blob1'] = ZODB.blob.Blob()
+#     >>> with conn.root()['blob1'].open('w') as f:
+#     ...    r = f.write(b'blob data 1')
+#     >>> transaction.commit()
+
+#     >>> db2 = ZEO.DB(addr, blob_dir='server-blobs', shared_blob_dir=True)
+#     >>> wait_connected(db2.storage)
+#     >>> conn2 = db2.open()
+#     >>> for i in range(5):
+#     ...     conn2.root().x += 1
+#     ...     transaction.commit()
+#     >>> conn2.root()['blob2'] = ZODB.blob.Blob()
+#     >>> with conn2.root()['blob2'].open('w') as f:
+#     ...    r = f.write(b'blob data 2')
+#     >>> transaction.commit()
+
+
+#     >>> @wait_until()
+#     ... def x_to_be_5():
+#     ...     conn.sync()
+#     ...     return conn.root().x == 5
+
+#     >>> db.close()
+
+#     >>> for i in range(2):
+#     ...     conn2.root().x += 1
+#     ...     transaction.commit()
+
+#     >>> db = ZEO.DB(addr, client='client', blob_dir='blobs')
+#     >>> wait_connected(db.storage)
+#     >>> conn = db.open()
+#     >>> conn.root().x
+#     7
+
+#     >>> db.close()
+
+#     >>> for i in range(10):
+#     ...     conn2.root().x += 1
+#     ...     transaction.commit()
+
+#     >>> db = ZEO.DB(addr, client='client', blob_dir='blobs')
+#     >>> wait_connected(db.storage)
+#     >>> conn = db.open()
+#     >>> conn.root().x
+#     17
+
+#     >>> with conn.root()['blob1'].open() as f:
+#     ...    f.read()
+#     b'blob data 1'
+#     >>> with conn.root()['blob2'].open() as f:
+#     ...    f.read()
+#     b'blob data 2'
+
+#     >>> db2.close()
+#     >>> db.close()
+
+# Undo the hijinks:
+
+#     >>> ZEO.asyncio.client.Protocol.protocols = old_protocols
diff --git a/src/ZEO/tests/testZEO.py b/src/ZEO/tests/testZEO.py
index b366a4790..0474760e9 100644
--- a/src/ZEO/tests/testZEO.py
+++ b/src/ZEO/tests/testZEO.py
@@ -745,24 +745,23 @@ def tpc_begin(self, transaction):
         self.server.tpc_begin(id(transaction), '', '', {}, None, ' ')
 
     def tpc_vote(self, transaction):
-        vote_result = self.server.vote(id(transaction))
-        assert vote_result is None
-        result = self.server.connection.serials[:]
+        result = self.server.vote(id(transaction))
+        assert result == self.server.connection.serials[:]
         del self.server.connection.serials[:]
         return result
 
     def store(self, oid, serial, data, version_ignored, transaction):
         self.server.storea(oid, serial, data, id(transaction))
 
-    def send_reply(self, *args):        # Masquerade as conn
-        pass
+    def send_reply(self, _, result):        # Masquerade as conn
+        self._result = result
 
     def tpc_abort(self, transaction):
         self.server.tpc_abort(id(transaction))
 
     def tpc_finish(self, transaction, func = lambda: None):
         self.server.tpc_finish(id(transaction)).set_sender(0, self)
-
+        return self._result
 
 def multiple_storages_invalidation_queue_is_not_insane():
     """
@@ -929,14 +928,14 @@ def tpc_finish_error():
     buffer, sadly, using implementation details:
 
     >>> tbuf = t.data(client)
-    >>> tbuf.serials = None
+    >>> tbuf.resolved = None
 
     tpc_finish will fail:
 
     >>> client.tpc_finish(t) # doctest: +ELLIPSIS
     Traceback (most recent call last):
     ...
-    AttributeError: ...
+    TypeError: ...
 
     >>> client.tpc_abort(t)
     >>> t.abort()
diff --git a/src/ZEO/tests/testZEO2.py b/src/ZEO/tests/testZEO2.py
index 7b4fad8d8..e0c36626c 100644
--- a/src/ZEO/tests/testZEO2.py
+++ b/src/ZEO/tests/testZEO2.py
@@ -78,6 +78,8 @@ def proper_handling_of_blob_conflicts():
     >>> class Sender:
     ...     def send_reply(self, id, reply):
     ...         print('reply', id, reply)
+    ...     def send_error(self, id, err):
+    ...         print('error', id, err)
     >>> delay.set_sender(1, Sender())
 
     >>> logger = logging.getLogger('ZEO')
@@ -87,13 +89,20 @@ def proper_handling_of_blob_conflicts():
 
 Now, when we abort the transaction for the first client. The second
 client will be restarted.  It will get a conflict error, that is
-handled correctly:
+raised to the client:
 
     >>> zs1.tpc_abort('0') # doctest: +ELLIPSIS
-    reply 1 None
+    Error raised in delayed method
+    Traceback (most recent call last):
+    ...
+    ZODB.POSException.ConflictError: ...
+    error 1 database conflict error ...
+
+The transaction is aborted by the server:
 
-    >>> fs.tpc_transaction() is not None
+    >>> fs.tpc_transaction() is None
     True
+
     >>> zs2.connected
     True
 
@@ -116,7 +125,7 @@ def proper_handling_of_errors_in_restart():
 
     >>> zs1 = ZEO.tests.servertesting.client(server, 1)
     >>> zs1.tpc_begin('0', '', '', {})
-    >>> zs1.storea(ZODB.utils.p64(99), ZODB.utils.z64, 'x', '0')
+    >>> zs1.storea(ZODB.utils.p64(99), ZODB.utils.z64, b'x', '0')
 
 Intentionally break zs1:
 
@@ -135,7 +144,7 @@ def proper_handling_of_errors_in_restart():
 
     >>> zs1 = ZEO.tests.servertesting.client(server, 1)
     >>> zs1.tpc_begin('1', '', '', {})
-    >>> zs1.storea(ZODB.utils.p64(99), ZODB.utils.z64, 'x', '1')
+    >>> zs1.storea(ZODB.utils.p64(99), ZODB.utils.z64, b'x', '1')
     >>> _ = zs1.vote('1') # doctest: +ELLIPSIS
 
     >>> zs1.tpc_finish('1').set_sender(0, zs1.connection)
@@ -220,7 +229,7 @@ def some_basic_locking_tests():
     ZEO.asyncio.server INFO
     received handshake 'Z5'
     >>> tid1 = start_trans(zs1)
-    >>> zs1.vote(tid1) # doctest: +ELLIPSIS
+    >>> resolved1 = zs1.vote(tid1) # doctest: +ELLIPSIS
     ZEO.StorageServer DEBUG
     (test-addr-1) ('1') lock: transactions waiting: 0
     ZEO.StorageServer BLATHER
@@ -477,7 +486,7 @@ def lock_sanity_check():
     ZEO.asyncio.server INFO
     received handshake 'Z5'
     >>> tid1 = start_trans(zs1)
-    >>> zs1.vote(tid1) # doctest: +ELLIPSIS
+    >>> resolved1 = zs1.vote(tid1) # doctest: +ELLIPSIS
     ZEO.StorageServer DEBUG
     (test-addr-1) ('1') lock: transactions waiting: 0
     ZEO.StorageServer BLATHER
@@ -493,7 +502,7 @@ def lock_sanity_check():
     ZEO.asyncio.server INFO
     received handshake 'Z5'
     >>> tid2 = start_trans(zs2)
-    >>> zs2.vote(tid2) # doctest: +ELLIPSIS
+    >>> resolved2 = zs2.vote(tid2) # doctest: +ELLIPSIS
     ZEO.StorageServer DEBUG
     (test-addr-2) ('1') lock: transactions waiting: 0
     ZEO.StorageServer BLATHER