backup: Teach cluster in BACKUPING state to also serve regular ZODB clients in read-only mode

A backup cluster for tids <= backup_tid has all data to provide regular
read-only ZODB service. Having regular ZODB access to the data can be
handy e.g. for externally verifying data for consistency between
main and backup clusters. Peeking around without disturbing main
cluster might be also useful sometimes.

In this patch:

- master & storage nodes are taught:

    * to instantiate read-only or regular client service handler depending on cluster state:
      RUNNING   -> regular
      BACKINGUP -> read-only

    * in read-only client handler:
      + to reject write-related operations
      + to provide read operations but adjust semantic as last_tid in the database
        would be = backup_tid

- new READ_ONLY_ACCESS protocol error code is introduced so that client can
  raise POSException.ReadOnlyError upon receiving it.

I have not implemented back-channel for invalidations in read-only mode (yet ?).
This way once a client connects to cluster in backup state, it won't see
new data fetched by backup cluster from upstream after client connected.

The reasons invalidations are not implemented is that for now (imho)
there is no off-hand ready infrastructure to get updates from
replicating node on transaction-by-transaction basis (it currently only
notifies when whole batch is done). For consistency verification (main
reason for this patch) we also don't need invalidations to work, as in
that task we always connect afresh to backup. So I simply only put
relevant TODOs about invalidations for now.

The patch is not very polished but should work.

/reviewed-on nexedi/neoppod!4

neo/client/handlers/__init__.py

@@ -15,9 +15,12 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from neo.lib import handler
-from ZODB.POSException import StorageError
+from ZODB.POSException import StorageError, ReadOnlyError
 
 class AnswerBaseHandler(handler.AnswerBaseHandler): # XXX
 
     def protocolError(self, conn, message):
         raise StorageError("protocol error: %s" % message)
+
+    def readOnlyAccess(self, conn, message):
+        raise ReadOnlyError(message)

neo/lib/protocol.py

@@ -75,6 +75,7 @@ def ErrorCodes():
     REPLICATION_ERROR
     CHECKING_ERROR
     BACKEND_NOT_IMPLEMENTED
+    READ_ONLY_ACCESS
 
 @Enum
 def ClusterStates():

neo/master/app.py

@@ -103,6 +103,7 @@ class Application(BaseApplication):
             self)
         self.secondary_master_handler = secondary.SecondaryMasterHandler(self)
         self.client_service_handler = client.ClientServiceHandler(self)
+        self.client_ro_service_handler = client.ClientReadOnlyServiceHandler(self)
         self.storage_service_handler = storage.StorageServiceHandler(self)
 
         registerLiveDebugger(on_log=self.log)
@@ -418,7 +419,6 @@ class Application(BaseApplication):
             return
 
         # select the storage handler
-        client_handler = self.client_service_handler
         if state in (ClusterStates.RUNNING, ClusterStates.STARTING_BACKUP,
                      ClusterStates.BACKINGUP, ClusterStates.STOPPING_BACKUP):
             storage_handler = self.storage_service_handler
@@ -435,10 +435,13 @@ class Application(BaseApplication):
             conn = node.getConnection()
             conn.notify(notification_packet)
             if node.isClient():
-                if state != ClusterStates.RUNNING:
+                if state == ClusterStates.RUNNING:
+                    handler = self.client_service_handler
+                elif state == ClusterStates.BACKINGUP:
+                    handler = self.client_ro_service_handler
+                else:
                     conn.abort()
                     continue
-                handler = client_handler
             elif node.isStorage() and storage_handler:
                 handler = storage_handler
             else:

neo/master/backup_app.py

@@ -310,6 +310,8 @@ class BackupApplication(object):
         logging.debug("partition %u: updating backup_tid of %r to %s",
                       offset, cell, dump(tid))
         cell.backup_tid = tid
+        # TODO provide invalidation feedback about new txns to read-only clients connected to backup cluster
+        # NOTE ^^^ not only here but also hooked to in-progress feedback from fetchObjects (storage)
         # Forget tids we won't need anymore.
         cell_list = app.pt.getCellList(offset, readable=True)
         del tid_list[:bisect(tid_list, min(x.backup_tid for x in cell_list))]

neo/master/handlers/client.py

@@ -14,7 +14,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from neo.lib.protocol import NodeStates, Packets, ProtocolError, MAX_TID
+from neo.lib.protocol import NodeStates, Packets, ProtocolError, MAX_TID, Errors
 from . import MasterHandler
 
 class ClientServiceHandler(MasterHandler):
@@ -118,3 +118,27 @@ class ClientServiceHandler(MasterHandler):
         # BUG: The replicator may wait this transaction to be finished.
         self.app.tm.abort(tid, conn.getUUID())
 
+
+# like ClientServiceHandler but read-only & only for tid <= backup_tid
+class ClientReadOnlyServiceHandler(ClientServiceHandler):
+
+    def _readOnly(self, conn, *args, **kw):
+        conn.answer(Errors.ReadOnlyAccess(
+            'read-only access because cluster is in backuping mode'))
+
+    askBeginTransaction     = _readOnly
+    askNewOIDs              = _readOnly
+    askFinishTransaction    = _readOnly
+    askFinalTID             = _readOnly
+    askPack                 = _readOnly
+    abortTransaction        = _readOnly
+
+    # XXX LastIDs is not used by client at all, and it requires work to determine
+    # last_oid up to backup_tid, so just make it non-functional for client.
+    askLastIDs              = _readOnly
+
+    # like in MasterHandler but returns backup_tid instead of last_tid
+    def askLastTransaction(self, conn):
+        assert self.app.backup_tid is not None   # we are in BACKUPING mode
+        backup_tid = self.app.pt.getBackupTid()
+        conn.answer(Packets.AnswerLastTransaction(backup_tid))

neo/master/handlers/identification.py

@@ -41,9 +41,12 @@ class IdentificationHandler(MasterHandler):
 
         state = NodeStates.RUNNING
         if node_type == NodeTypes.CLIENT:
-            if app.cluster_state != ClusterStates.RUNNING:
-                raise NotReadyError
+            if app.cluster_state == ClusterStates.RUNNING:
                 handler = app.client_service_handler
+            elif app.cluster_state == ClusterStates.BACKINGUP:
+                handler = app.client_ro_service_handler
+            else:
+                raise NotReadyError
             human_readable_node_type = ' client '
         elif node_type == NodeTypes.STORAGE:
             if app.cluster_state == ClusterStates.STOPPING_BACKUP:

neo/storage/handlers/client.py

@@ -16,7 +16,7 @@
 
 from neo.lib import logging
 from neo.lib.handler import EventHandler
-from neo.lib.util import dump, makeChecksum
+from neo.lib.util import dump, makeChecksum, add64
 from neo.lib.protocol import Packets, LockState, Errors, ProtocolError, \
     ZERO_HASH, INVALID_PARTITION
 from ..transactions import ConflictError, DelayedError, NotRegisteredError
@@ -130,7 +130,7 @@ class ClientOperationHandler(EventHandler):
         conn.answer(Packets.AnswerTIDsFrom(self.app.dm.getReplicationTIDList(
             min_tid, max_tid, length, partition)))
 
-    def askTIDs(self, conn, first, last, partition):
+    def _askTIDs(self, first, last, partition):
         # This method is complicated, because I must return TIDs only
         # about usable partitions assigned to me.
         if first >= last:
@@ -142,8 +142,10 @@ class ClientOperationHandler(EventHandler):
         else:
             partition_list = [partition]
 
-        tid_list = app.dm.getTIDList(first, last - first, partition_list)
-        conn.answer(Packets.AnswerTIDs(tid_list))
+        return app.dm.getTIDList(first, last - first, partition_list)
+
+    def askTIDs(self, conn, *args):
+        conn.answer(Packets.AnswerTIDs(self._askTIDs(*args)))
 
     def askFinalTID(self, conn, ttid):
         conn.answer(Packets.AnswerFinalTID(self.app.tm.getFinalTID(ttid)))
@@ -222,3 +224,67 @@ class ClientOperationHandler(EventHandler):
                     logging.info('CheckCurrentSerial delay: %.02fs', duration)
             conn.answer(Packets.AnswerCheckCurrentSerial(0, oid, serial))
 
+
+# like ClientOperationHandler but read-only & only for tid <= backup_tid
+class ClientReadOnlyOperationHandler(ClientOperationHandler):
+
+    def _readOnly(self, conn, *args, **kw):
+        conn.answer(Errors.ReadOnlyAccess(
+            'read-only access because cluster is in backuping mode'))
+
+    abortTransaction        = _readOnly
+    askStoreTransaction     = _readOnly
+    askVoteTransaction      = _readOnly
+    askStoreObject          = _readOnly
+    askFinalTID             = _readOnly
+    # takes write lock & is only used when going to commit
+    askCheckCurrentSerial   = _readOnly
+
+    # below operations: like in ClientOperationHandler but cut tid <= backup_tid
+
+    def askTransactionInformation(self, conn, tid):
+        backup_tid = self.app.dm.getBackupTID()
+        if tid > backup_tid:
+            conn.answer(Errors.TidNotFound(
+                'tids > %s are not fully fetched yet' % dump(backup_tid)))
+            return
+        super(ClientReadOnlyOperationHandler, self).askTransactionInformation(
+            conn, tid)
+
+    def askObject(self, conn, oid, serial, tid):
+        backup_tid = self.app.dm.getBackupTID()
+        if serial:
+            if serial > backup_tid:
+                # obj lookup will find nothing, but return properly either
+                # OidDoesNotExist or OidNotFound
+                serial = ZERO_TID
+        elif tid:
+            tid = min(tid, add64(backup_tid, 1))
+
+        # limit "latest obj" query to tid <= backup_tid
+        else:
+            tid = add64(backup_tid, 1)
+
+        super(ClientReadOnlyOperationHandler, self).askObject(
+            conn, oid, serial, tid)
+
+    def askTIDsFrom(self, conn, min_tid, max_tid, length, partition):
+        backup_tid = self.app.dm.getBackupTID()
+        max_tid = min(max_tid, backup_tid)
+        # NOTE we don't need to adjust min_tid: if min_tid > max_tid
+        #      db.getReplicationTIDList will return empty [], which is correct
+        super(ClientReadOnlyOperationHandler, self).askTIDsFrom(
+                conn, min_tid, max_tid, length, partition)
+
+    def askTIDs(self, conn, first, last, partition):
+        backup_tid = self.app.dm.getBackupTID()
+        tid_list = self._askTIDs(first, last, partition)
+        tid_list = filter(lambda tid: tid <= backup_tid, tid_list)
+        conn.answer(Packets.AnswerTIDs(tid_list))
+
+    # FIXME askObjectUndoSerial to limit tid <= backup_tid
+    # (askObjectUndoSerial is used in undo() but itself is read-only query)
+
+    # FIXME askObjectHistory to limit tid <= backup_tid
+    # TODO dm.getObjectHistory has to be first fixed for this
+    #def askObjectHistory(self, conn, oid, first, last):

neo/storage/handlers/identification.py

@@ -19,7 +19,7 @@ from neo.lib.handler import EventHandler
 from neo.lib.protocol import uuid_str, NodeTypes, NotReadyError, Packets
 from neo.lib.protocol import ProtocolError, BrokenNodeDisallowedError
 from .storage import StorageOperationHandler
-from .client import ClientOperationHandler
+from .client import ClientOperationHandler, ClientReadOnlyOperationHandler
 
 class IdentificationHandler(EventHandler):
     """ Handler used for incoming connections during operation state """
@@ -48,6 +48,9 @@ class IdentificationHandler(EventHandler):
                 raise BrokenNodeDisallowedError
             # choose the handler according to the node type
             if node_type == NodeTypes.CLIENT:
+                if app.dm.getBackupTID():
+                    handler = ClientReadOnlyOperationHandler
+                else:
                     handler = ClientOperationHandler
                 if node is None:
                     node = app.nm.createClient(uuid=uuid)

neo/storage/handlers/storage.py

@@ -105,6 +105,7 @@ class StorageOperationHandler(EventHandler):
         self.app.dm.commit()
         assert not pack_tid, "TODO"
         if next_tid:
+            # TODO also provide feedback to master about current replication state (tid)
             self.app.replicator.fetchObjects(next_tid, next_oid)
         else:
             self.app.replicator.finish()

neo/tests/threaded/testReplication.py

@@ -17,10 +17,12 @@
 import random
 import time
 import transaction
+from ZODB.POSException import ReadOnlyError, POSKeyError
 import unittest
 from collections import defaultdict
 from functools import wraps
 from neo.lib import logging
+from neo.client.exception import NEOStorageError
 from neo.storage.checker import CHECK_COUNT
 from neo.storage.replicator import Replicator
 from neo.lib.connector import SocketConnector
@@ -461,5 +463,99 @@ class ReplicationTests(NEOThreadedTest):
             checker.CHECK_COUNT = CHECK_COUNT
             cluster.stop()
 
+    @backup_test()
+    def testBackupReadOnlyAccess(self, backup):
+        """Check backup cluster can be used in read-only mode by ZODB clients"""
+        B = backup
+        U = B.upstream
+        Z = U.getZODBStorage()
+        #Zb = B.getZODBStorage()    # XXX see below about invalidations
+
+        oid_list = []
+        tid_list = []
+
+        # S -> Sb link stops working during [cutoff, recover) test iterations
+        cutoff  = 4
+        recover = 7
+        def delayReplication(conn, packet):
+            return isinstance(packet, Packets.AnswerFetchTransactions)
+
+        with ConnectionFilter() as f:
+            for i in xrange(10):
+                if i == cutoff:
+                    f.add(delayReplication)
+                if i == recover:
+                    # .remove() removes the filter and retransmits all packets
+                    # that were queued once first filtered packed was detected
+                    # on a connection.
+                    f.remove(delayReplication)
+
+                # commit new data to U
+                txn = transaction.Transaction()
+                txn.note('test transaction %i' % i)
+                Z.tpc_begin(txn)
+                oid = Z.new_oid()
+                Z.store(oid, None, '%s-%i' % (oid, i), '', txn)
+                Z.tpc_vote(txn)
+                tid = Z.tpc_finish(txn)
+                oid_list.append(oid)
+                tid_list.append(tid)
+
+                # make sure data propagated to B  (depending on cutoff)
+                self.tic()
+                if cutoff <= i < recover:
+                    self.assertLess(B.backup_tid, U.last_tid)
+                else:
+                    self.assertEqual(B.backup_tid, U.last_tid)
+                self.assertEqual(B.last_tid,   U.last_tid)
+                self.assertEqual(1, self.checkBackup(B, max_tid=B.backup_tid))
+
+                # read data from B and verify it is what it should be
+                # XXX we open new ZODB storage every time because invalidations
+                # are not yet implemented in read-only mode.
+                Zb = B.getZODBStorage()
+                for j, oid in enumerate(oid_list):
+                    if cutoff <= i < recover and j >= cutoff:
+                        self.assertRaises(POSKeyError, Zb.load, oid, '')
+                    else:
+                        data, serial = Zb.load(oid, '')
+                        self.assertEqual(data, '%s-%s' % (oid, j))
+                        self.assertEqual(serial, tid_list[j])
+
+                # verify how transaction log & friends behave under potentially
+                # not-yet-fully fetched backup state (transactions committed at
+                # [cutoff, recover) should not be there; otherwise transactions
+                # should be fully there)
+                Zb = B.getZODBStorage()
+                Btxn_list = list(Zb.iterator())
+                self.assertEqual(len(Btxn_list), cutoff if cutoff <= i < recover
+                                                 else i+1)
+                for j, txn in enumerate(Btxn_list):
+                    self.assertEqual(txn.tid, tid_list[j])
+                    self.assertEqual(txn.description, 'test transaction %i' % j)
+                    obj, = txn
+                    self.assertEqual(obj.oid, oid_list[j])
+                    self.assertEqual(obj.data, '%s-%s' % (obj.oid, j))
+
+                # TODO test askObjectHistory once it is fixed
+
+                # try to commit something to backup storage and make sure it is
+                # really read-only
+                Zb._cache._max_size = 0     # make store() do work in sync way
+                txn = transaction.Transaction()
+                self.assertRaises(ReadOnlyError, Zb.tpc_begin, txn)
+                self.assertRaises(ReadOnlyError, Zb.new_oid)
+                self.assertRaises(ReadOnlyError, Zb.store, oid_list[-1],
+                                            tid_list[-1], 'somedata', '', txn)
+                # tpc_vote first checks whether there were store replies -
+                # thus not ReadOnlyError
+                self.assertRaises(NEOStorageError, Zb.tpc_vote, txn)
+
+                # close storage because client app is otherwise shared in
+                # threaded tests and we need to refresh last_tid on next run
+                # (XXX see above about invalidations not working)
+                Zb.close()
+
+
 if __name__ == "__main__":
     unittest.main()