Reresolve DNS as fallback when all hosts are unreachable

If all nodes in the cluster change their IPs at one time, driver used to
no longer be able to ever contact the cluster; the only solution was to
restart the driver. A fallback is added to the control connection
logic so that when no known host is reachable, Cluster one again
resolves all the known hostnames and ControlConnection tries to connect them.

Author: sylwiaszunejko

cassandra/cluster.py

@@ -1220,30 +1220,7 @@ def __init__(self,
         self.endpoint_factory = endpoint_factory or DefaultEndPointFactory(port=self.port)
         self.endpoint_factory.configure(self)
 
-        raw_contact_points = []
-        for cp in [cp for cp in self.contact_points if not isinstance(cp, EndPoint)]:
-            raw_contact_points.append(cp if isinstance(cp, tuple) else (cp, port))
-
-        self.endpoints_resolved = [cp for cp in self.contact_points if isinstance(cp, EndPoint)]
-        self._endpoint_map_for_insights = {repr(ep): '{ip}:{port}'.format(ip=ep.address, port=ep.port)
-                                           for ep in self.endpoints_resolved}
-
-        strs_resolved_map = _resolve_contact_points_to_string_map(raw_contact_points)
-        self.endpoints_resolved.extend(list(chain(
-            *[
-                [DefaultEndPoint(ip, port) for ip, port in xs if ip is not None]
-                for xs in strs_resolved_map.values() if xs is not None
-            ]
-        )))
-
-        self._endpoint_map_for_insights.update(
-            {key: ['{ip}:{port}'.format(ip=ip, port=port) for ip, port in value]
-             for key, value in strs_resolved_map.items() if value is not None}
-        )
-
-        if contact_points and (not self.endpoints_resolved):
-            # only want to raise here if the user specified CPs but resolution failed
-            raise UnresolvableContactPoints(self._endpoint_map_for_insights)
+        self._resolve_hostnames()
 
         self.compression = compression
 
@@ -1427,6 +1404,31 @@ def __init__(self,
         if application_version is not None:
             self.application_version = application_version
 
+    def _resolve_hostnames(self):
+        raw_contact_points = []
+        for cp in [cp for cp in self.contact_points if not isinstance(cp, EndPoint)]:
+            raw_contact_points.append(cp if isinstance(cp, tuple) else (cp, self.port))
+
+        self.endpoints_resolved = [cp for cp in self.contact_points if isinstance(cp, EndPoint)]
+        self._endpoint_map_for_insights = {repr(ep): '{ip}:{port}'.format(ip=ep.address, port=ep.port)
+                                           for ep in self.endpoints_resolved}
+        strs_resolved_map = _resolve_contact_points_to_string_map(raw_contact_points)
+        self.endpoints_resolved.extend(list(chain(
+            *[
+                [DefaultEndPoint(ip, port) for ip, port in xs if ip is not None]
+                for xs in strs_resolved_map.values() if xs is not None
+            ]
+        )))
+
+        self._endpoint_map_for_insights.update(
+            {key: ['{ip}:{port}'.format(ip=ip, port=port) for ip, port in value]
+             for key, value in strs_resolved_map.items() if value is not None}
+        )
+
+        if self.contact_points and (not self.endpoints_resolved):
+            # only want to raise here if the user specified CPs but resolution failed
+            raise UnresolvableContactPoints(self._endpoint_map_for_insights)
+
     def _create_thread_pool_executor(self, **kwargs):
         """
         Create a ThreadPoolExecutor for the cluster. In most cases, the built-in
@@ -1720,6 +1722,20 @@ def protocol_downgrade(self, host_endpoint, previous_version):
                     "http://datastax.github.io/python-driver/api/cassandra/cluster.html#cassandra.cluster.Cluster.protocol_version", self.protocol_version, new_version, host_endpoint)
         self.protocol_version = new_version
 
+    def _add_resolved_hosts(self):
+        for endpoint in self.endpoints_resolved:
+            host, new = self.add_host(endpoint, signal=False)
+            if new:
+                host.set_up()
+                for listener in self.listeners:
+                    listener.on_add(host)
+
+        self.profile_manager.populate(
+            weakref.proxy(self), self.metadata.all_hosts())
+        self.load_balancing_policy.populate(
+            weakref.proxy(self), self.metadata.all_hosts()
+        )
+
     def connect(self, keyspace=None, wait_for_all_pools=False):
         """
         Creates and returns a new :class:`~.Session` object.
@@ -1740,18 +1756,8 @@ def connect(self, keyspace=None, wait_for_all_pools=False):
                           self.contact_points, self.protocol_version)
                 self.connection_class.initialize_reactor()
                 _register_cluster_shutdown(self)
-                for endpoint in self.endpoints_resolved:
-                    host, new = self.add_host(endpoint, signal=False)
-                    if new:
-                        host.set_up()
-                        for listener in self.listeners:
-                            listener.on_add(host)
-
-                self.profile_manager.populate(
-                    weakref.proxy(self), self.metadata.all_hosts())
-                self.load_balancing_policy.populate(
-                    weakref.proxy(self), self.metadata.all_hosts()
-                )
+                
+                self._add_resolved_hosts()
 
                 try:
                     self.control_connection.connect()
@@ -3585,16 +3591,8 @@ def _set_new_connection(self, conn):
         if old:
             log.debug("[control connection] Closing old connection %r, replacing with %r", old, conn)
             old.close()
-
-    def _reconnect_internal(self):
-        """
-        Tries to connect to each host in the query plan until one succeeds
-        or every attempt fails. If successful, a new Connection will be
-        returned.  Otherwise, :exc:`NoHostAvailable` will be raised
-        with an "errors" arg that is a dict mapping host addresses
-        to the exception that was raised when an attempt was made to open
-        a connection to that host.
-        """
+    
+    def _connect_host_in_lbp(self):
         errors = {}
         lbp = (
             self._cluster.load_balancing_policy
@@ -3604,7 +3602,7 @@ def _reconnect_internal(self):
 
         for host in lbp.make_query_plan():
             try:
-                return self._try_connect(host)
+                return (self._try_connect(host), None)
             except ConnectionException as exc:
                 errors[str(host.endpoint)] = exc
                 log.warning("[control connection] Error connecting to %s:", host, exc_info=True)
@@ -3614,7 +3612,31 @@ def _reconnect_internal(self):
                 log.warning("[control connection] Error connecting to %s:", host, exc_info=True)
             if self._is_shutdown:
                 raise DriverException("[control connection] Reconnection in progress during shutdown")
+        
+        return (None, errors)
 
+    def _reconnect_internal(self):
+        """
+        Tries to connect to each host in the query plan until one succeeds
+        or every attempt fails. If successful, a new Connection will be
+        returned.  Otherwise, :exc:`NoHostAvailable` will be raised
+        with an "errors" arg that is a dict mapping host addresses
+        to the exception that was raised when an attempt was made to open
+        a connection to that host.
+        """
+        (conn, _) = self._connect_host_in_lbp()
+        if conn is not None:
+            return conn
+        
+        # Try to re-resolve hostnames as a fallback when all hosts are unreachable
+        self._cluster._resolve_hostnames()
+
+        self._cluster._add_resolved_hosts()
+
+        (conn, errors) = self._connect_host_in_lbp()
+        if conn is not None:
+            return conn
+        
         raise NoHostAvailable("Unable to connect to any servers", errors)
 
     def _try_connect(self, host):