Added a replacement for the default cluster node in the event of failure. Handles failovers better.

Author: barshaul

CHANGES

@@ -28,6 +28,7 @@
     * Fixed "cannot pickle '_thread.lock' object" bug (#2354, #2297)
     * Added CredentialsProvider class to support password rotation
     * Enable Lock for asyncio cluster mode
+    * Added a replacement for the default cluster node in the event of failure (#2463)
 
 * 4.1.3 (Feb 8, 2022)
     * Fix flushdb and flushall (#1926)

redis/asyncio/cluster.py

@@ -516,35 +516,44 @@ def set_response_callback(self, command: str, callback: ResponseCallbackT) -> No
 
     async def _determine_nodes(
         self, command: str, *args: Any, node_flag: Optional[str] = None
-    ) -> List["ClusterNode"]:
+    ) -> tuple[list["ClusterNode"], bool]:
+        """Determine which nodes should be executed the command on
+
+        Returns:
+            tuple[list[Type[ClusterNode]], bool]:
+                A tuple containing a list of target nodes and a bool indicating
+                if the return node was chosen because it is the default node
+        """
         if not node_flag:
             # get the nodes group for this command if it was predefined
             node_flag = self.command_flags.get(command)
 
         if node_flag in self.node_flags:
             if node_flag == self.__class__.DEFAULT_NODE:
                 # return the cluster's default node
-                return [self.nodes_manager.default_node]
+                return [self.nodes_manager.default_node], True
             if node_flag == self.__class__.PRIMARIES:
                 # return all primaries
-                return self.nodes_manager.get_nodes_by_server_type(PRIMARY)
+                return self.nodes_manager.get_nodes_by_server_type(PRIMARY), False
             if node_flag == self.__class__.REPLICAS:
                 # return all replicas
-                return self.nodes_manager.get_nodes_by_server_type(REPLICA)
+                return self.nodes_manager.get_nodes_by_server_type(REPLICA), False
             if node_flag == self.__class__.ALL_NODES:
                 # return all nodes
-                return list(self.nodes_manager.nodes_cache.values())
+                return list(self.nodes_manager.nodes_cache.values()), False
             if node_flag == self.__class__.RANDOM:
                 # return a random node
-                return [random.choice(list(self.nodes_manager.nodes_cache.values()))]
+                return [
+                    random.choice(list(self.nodes_manager.nodes_cache.values()))
+                ], False
 
         # get the node that holds the key's slot
         return [
             self.nodes_manager.get_node_from_slot(
                 await self._determine_slot(command, *args),
                 self.read_from_replicas and command in READ_COMMANDS,
             )
-        ]
+        ], False
 
     async def _determine_slot(self, command: str, *args: Any) -> int:
         if self.command_flags.get(command) == SLOT_ID:
@@ -641,6 +650,7 @@ async def execute_command(self, *args: EncodableT, **kwargs: Any) -> Any:
         command = args[0]
         target_nodes = []
         target_nodes_specified = False
+        is_default_node = False
         retry_attempts = self.cluster_error_retry_attempts
 
         passed_targets = kwargs.pop("target_nodes", None)
@@ -654,10 +664,13 @@ async def execute_command(self, *args: EncodableT, **kwargs: Any) -> Any:
         for _ in range(execute_attempts):
             if self._initialize:
                 await self.initialize()
+                if is_default_node:
+                    # Replace the default cluster node
+                    self.replace_default_node()
             try:
                 if not target_nodes_specified:
                     # Determine the nodes to execute the command on
-                    target_nodes = await self._determine_nodes(
+                    target_nodes, is_default_node = await self._determine_nodes(
                         *args, node_flag=passed_targets
                     )
                     if not target_nodes:
@@ -1436,12 +1449,13 @@ async def _execute(
         ]
 
         nodes = {}
+        is_default_node = False
         for cmd in todo:
             passed_targets = cmd.kwargs.pop("target_nodes", None)
             if passed_targets and not client._is_node_flag(passed_targets):
                 target_nodes = client._parse_target_nodes(passed_targets)
             else:
-                target_nodes = await client._determine_nodes(
+                target_nodes, is_default_node = await client._determine_nodes(
                     *cmd.args, node_flag=passed_targets
                 )
                 if not target_nodes:
@@ -1487,6 +1501,9 @@ async def _execute(
                         result.args = (msg,) + result.args[1:]
                         raise result
 
+            if is_default_node:
+                self.replace_default_node()
+
         return [cmd.result for cmd in stack]
 
     def _split_command_across_slots(

redis/cluster.py

@@ -379,6 +379,30 @@ class AbstractRedisCluster:
 
     ERRORS_ALLOW_RETRY = (ConnectionError, TimeoutError, ClusterDownError)
 
+    def replace_default_node(self, target_node: "ClusterNode" = None) -> None:
+        """Replace the default cluster node.
+        A random cluster node will be chosen if target_node isn't passed, and primaries
+        will be prioritized. The default node will not be changed if there are no other
+        nodes in the cluster.
+
+        Args:
+            target_node (ClusterNode, optional): Target node to replace the default
+            node. Defaults to None.
+        """
+        if target_node:
+            self.nodes_manager.default_node = target_node
+        else:
+            curr_node = self.get_default_node()
+            primaries = [node for node in self.get_primaries() if node != curr_node]
+            if primaries:
+                # Choose a primary if the cluster contains different primaries
+                self.nodes_manager.default_node = random.choice(primaries)
+            else:
+                # Otherwise, hoose a primary if the cluster contains different primaries
+                replicas = [node for node in self.get_replicas() if node != curr_node]
+                if replicas:
+                    self.nodes_manager.default_node = random.choice(replicas)
+
 
 class RedisCluster(AbstractRedisCluster, RedisClusterCommands):
     @classmethod
@@ -811,7 +835,14 @@ def set_response_callback(self, command, callback):
         """Set a custom Response Callback"""
         self.cluster_response_callbacks[command] = callback
 
-    def _determine_nodes(self, *args, **kwargs):
+    def _determine_nodes(self, *args, **kwargs) -> tuple[list["ClusterNode"], bool]:
+        """Determine which nodes should be executed the command on
+
+        Returns:
+            tuple[list[Type[ClusterNode]], bool]:
+                A tuple containing a list of target nodes and a bool indicating
+                if the return node was chosen because it is the default node
+        """
         command = args[0].upper()
         if len(args) >= 2 and f"{args[0]} {args[1]}".upper() in self.command_flags:
             command = f"{args[0]} {args[1]}".upper()
@@ -825,28 +856,28 @@ def _determine_nodes(self, *args, **kwargs):
             command_flag = self.command_flags.get(command)
         if command_flag == self.__class__.RANDOM:
             # return a random node
-            return [self.get_random_node()]
+            return [self.get_random_node()], False
         elif command_flag == self.__class__.PRIMARIES:
             # return all primaries
-            return self.get_primaries()
+            return self.get_primaries(), False
         elif command_flag == self.__class__.REPLICAS:
             # return all replicas
-            return self.get_replicas()
+            return self.get_replicas(), False
         elif command_flag == self.__class__.ALL_NODES:
             # return all nodes
-            return self.get_nodes()
+            return self.get_nodes(), False
         elif command_flag == self.__class__.DEFAULT_NODE:
             # return the cluster's default node
-            return [self.nodes_manager.default_node]
+            return [self.nodes_manager.default_node], True
         elif command in self.__class__.SEARCH_COMMANDS[0]:
-            return [self.nodes_manager.default_node]
+            return [self.nodes_manager.default_node], True
         else:
             # get the node that holds the key's slot
             slot = self.determine_slot(*args)
             node = self.nodes_manager.get_node_from_slot(
                 slot, self.read_from_replicas and command in READ_COMMANDS
             )
-            return [node]
+            return [node], False
 
     def _should_reinitialized(self):
         # To reinitialize the cluster on every MOVED error,
@@ -990,6 +1021,7 @@ def execute_command(self, *args, **kwargs):
             dict<Any, ClusterNode>
         """
         target_nodes_specified = False
+        is_default_node = False
         target_nodes = None
         passed_targets = kwargs.pop("target_nodes", None)
         if passed_targets is not None and not self._is_nodes_flag(passed_targets):
@@ -1013,7 +1045,7 @@ def execute_command(self, *args, **kwargs):
                 res = {}
                 if not target_nodes_specified:
                     # Determine the nodes to execute the command on
-                    target_nodes = self._determine_nodes(
+                    target_nodes, is_default_node = self._determine_nodes(
                         *args, **kwargs, nodes_flag=passed_targets
                     )
                     if not target_nodes:
@@ -1025,6 +1057,9 @@ def execute_command(self, *args, **kwargs):
                 # Return the processed result
                 return self._process_result(args[0], res, **kwargs)
             except Exception as e:
+                if is_default_node:
+                    # Replace the default cluster node
+                    self.replace_default_node()
                 if retry_attempts > 0 and type(e) in self.__class__.ERRORS_ALLOW_RETRY:
                     # The nodes and slots cache were reinitialized.
                     # Try again with the new cluster setup.
@@ -1883,7 +1918,7 @@ def _send_cluster_commands(
         # if we have to run through it again, we only retry
         # the commands that failed.
         attempt = sorted(stack, key=lambda x: x.position)
-
+        is_default_node = False
         # build a list of node objects based on node names we need to
         nodes = {}
 
@@ -1900,7 +1935,7 @@ def _send_cluster_commands(
                 if passed_targets and not self._is_nodes_flag(passed_targets):
                     target_nodes = self._parse_target_nodes(passed_targets)
                 else:
-                    target_nodes = self._determine_nodes(
+                    target_nodes, is_default_node = self._determine_nodes(
                         *c.args, node_flag=passed_targets
                     )
                     if not target_nodes:
@@ -1926,6 +1961,8 @@ def _send_cluster_commands(
                         # Connection retries are being handled in the node's
                         # Retry object. Reinitialize the node -> slot table.
                         self.nodes_manager.initialize()
+                        if is_default_node:
+                            self.replace_default_node()
                         raise
                     nodes[node_name] = NodeCommands(
                         redis_node.parse_response,
@@ -2007,6 +2044,8 @@ def _send_cluster_commands(
             self.reinitialize_counter += 1
             if self._should_reinitialized():
                 self.nodes_manager.initialize()
+                if is_default_node:
+                    self.replace_default_node()
             for c in attempt:
                 try:
                     # send each command individually like we

tests/test_asyncio/test_cluster.py

@@ -788,6 +788,26 @@ async def test_can_run_concurrent_commands(self, request: FixtureRequest) -> Non
         )
         await rc.close()
 
+    def test_replace_cluster_node(self, r: RedisCluster) -> None:
+        prev_default_node = r.get_default_node()
+        r.replace_default_node()
+        assert r.get_default_node() != prev_default_node
+        r.replace_default_node(prev_default_node)
+        assert r.get_default_node() == prev_default_node
+
+    async def test_default_node_is_replaced_after_exception(self, r):
+        curr_default_node = r.get_default_node()
+        # CLUSTER NODES command is being executed on the default node
+        nodes = await r.cluster_nodes()
+        assert "myself" in nodes.get(curr_default_node.name).get("flags")
+
+        # Mock connection error for the default node
+        mock_node_resp_exc(curr_default_node, ConnectionError("error"))
+        # Test that the command succeed from a different node
+        nodes = await r.cluster_nodes()
+        assert "myself" not in nodes.get(curr_default_node.name).get("flags")
+        assert r.get_default_node() != curr_default_node
+
 
 class TestClusterRedisCommands:
     """

tests/test_cluster.py

@@ -791,6 +791,29 @@ def test_cluster_retry_object(self, r) -> None:
             == retry._retries
         )
 
+    def test_replace_cluster_node(self, r) -> None:
+        prev_default_node = r.get_default_node()
+        r.replace_default_node()
+        assert r.get_default_node() != prev_default_node
+        r.replace_default_node(prev_default_node)
+        assert r.get_default_node() == prev_default_node
+
+    def test_default_node_is_replaced_after_exception(self, r):
+        curr_default_node = r.get_default_node()
+        # CLUSTER NODES command is being executed on the default node
+        nodes = r.cluster_nodes()
+        assert "myself" in nodes.get(curr_default_node.name).get("flags")
+
+        def raise_connection_error():
+            raise ConnectionError("error")
+
+        # Mock connection error for the default node
+        mock_node_resp_func(curr_default_node, raise_connection_error)
+        # Test that the command succeed from a different node
+        nodes = r.cluster_nodes()
+        assert "myself" not in nodes.get(curr_default_node.name).get("flags")
+        assert r.get_default_node() != curr_default_node
+
 
 @pytest.mark.onlycluster
 class TestClusterRedisCommands: