Create a new more specific error for the cases where a slot is not covered by the cluster. If this happens it will attempt to rebuild the cluster until TTL is expired and then raise that exception back to the user. Fixes #350

Author: Grokzen

docs/release-notes.rst

@@ -27,6 +27,7 @@ Release Notes
     * Nodemanager initiailize should now handle usernames properly (#365)
     * PubSub tests has been all been disabled
     * New feature, host_port_remap. Send in a remapping configuration to RedisCluster instance where the nodes configuration recieved from the redis cluster can be altered to allow for connection in certain circumstances. See new section in clients.rst in docs/ for usage example.
+    * When a slot is not covered by the cluster, it will not raise SlotNotCoveredError instead of the old generic RedisClusterException. The client will not attempt to rebuild the cluster layout a few times before giving up and raising that exception to the user. (#350)
 
 
 2.0.0 (Aug 12, 2019)

docs/upgrading.rst

@@ -8,6 +8,11 @@ This document describes what must be done when upgrading between different versi
 
 Python3 version must now be one of 3.5, 3.6, 3.7, 3.8
 
+The following exception example has now a new more specific exception class that will be attempted to be caught and the client to resolve the cluster layout. If enough attempts has been made then SlotNotCoveredError will be raised with the same message as before. If you have catch for RedisClusterException you either remove it and let the client try to resolve the cluster layout itself, or start to catch SlotNotCoveredError. This error usually happens during failover if you run skip_full_coverage_check=True when running on AWS ElasticCache for example.
+
+	## Example exception
+	rediscluster.exceptions.RedisClusterException: Slot "6986" not covered by the cluster. "skip_full_coverage_check=True"
+
 
 1.3.x --> 2.0.0
 ---------------

rediscluster/client.py

@@ -21,6 +21,7 @@
     ClusterError,
     MovedError,
     RedisClusterException,
+    SlotNotCoveredError,
     TryAgainError,
 )
 from .pubsub import ClusterPubSub
@@ -595,6 +596,16 @@ def _execute_command(self, *args, **kwargs):
 
                 connection.send_command(*args)
                 return self.parse_response(connection, command, **kwargs)
+            except SlotNotCoveredError as e:
+                # In some cases during failover to a replica is happening
+                # a slot sometimes is not covered by the cluster layout and
+                # we need to attempt to refresh the cluster layout and try again
+                self.refresh_table_asap = True
+                time.sleep(0.05)
+
+                # This is the last attempt before we run out of TTL, raise the exception
+                if ttl == 1:
+                    raise e
             except (RedisClusterException, BusyLoadingError):
                 raise
             except ConnectionError:

rediscluster/connection.py

@@ -14,7 +14,7 @@
 from .exceptions import (
     RedisClusterException, AskError, MovedError,
     TryAgainError, ClusterDownError, ClusterCrossSlotError,
-    MasterDownError,
+    MasterDownError, SlotNotCoveredError,
 )
 
 # 3rd party imports
@@ -329,7 +329,7 @@ def get_master_node_by_slot(self, slot):
         try:
             return self.nodes.slots[slot][0]
         except KeyError as ke:
-            raise RedisClusterException('Slot "{slot}" not covered by the cluster. "skip_full_coverage_check={skip_full_coverage_check}"'.format(
+            raise SlotNotCoveredError('Slot "{slot}" not covered by the cluster. "skip_full_coverage_check={skip_full_coverage_check}"'.format(
                 slot=slot, skip_full_coverage_check=self.nodes._skip_full_coverage_check,
             ))
 

rediscluster/exceptions.py

@@ -85,3 +85,14 @@ class MasterDownError(ClusterDownError):
     """
     """
     pass
+
+
+class SlotNotCoveredError(RedisClusterException):
+    """
+    This error only happens in the case where the connection pool will try to
+    fetch what node that is covered by a given slot.
+
+    If this error is raised the client should drop the current node layout and
+    attempt to reconnect and refresh the node layout again
+    """
+    pass

tests/test_cluster_connection_pool.py

@@ -11,7 +11,7 @@
 from rediscluster.connection import (
     ClusterConnectionPool, ClusterBlockingConnectionPool, ClusterReadOnlyConnectionPool,
     ClusterConnection, UnixDomainSocketConnection)
-from rediscluster.exceptions import RedisClusterException
+from rediscluster.exceptions import RedisClusterException, SlotNotCoveredError
 from .conftest import (skip_if_server_version_lt, skip_for_no_cluster_impl)
 
 # 3rd party imports
@@ -205,6 +205,11 @@ def test_master_node_by_slot(self):
         node = pool.get_master_node_by_slot(12182)
         node['port'] = 7002
 
+        pool = self.get_pool(connection_kwargs={})
+        pool.nodes.slots = {}
+        with pytest.raises(SlotNotCoveredError):
+            pool.get_master_node_by_slot(12182)
+
     def test_from_url_connection_classes(self):
         from rediscluster.client import RedisCluster
         from rediscluster.connection import ClusterConnectionPool, ClusterConnection, SSLClusterConnection