Handle -READONLY as a redirection signal for Redis Cluster (AWS ElastiCache support) (#1656)

* feature: Added support for READONLY responses from AWS Elastic Cache

* chore: improve -READONLY handler comment and add changelog entry (#1656)

* tests: add unit test demonstrating ServerException behavior with READONLY error

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 ### Added
+- Handle `-READONLY` responses in Redis Cluster for AWS ElastiCache Redis OSS failover events
 ### Changed
 - Include command name in unsupported container command error messages (#1653)
 ### Fixed

src/Connection/Cluster/RedisCluster.php

@@ -465,6 +465,9 @@ class RedisCluster extends AbstractAggregateConnection implements ClusterInterfa
         $details = explode(' ', $error->getMessage(), 2);
 
         switch ($details[0]) {
+            case 'READONLY':
+                return $this->onReadOnlyResponse($command);
+
             case 'MOVED':
                 return $this->onMovedResponse($command, $details[1]);
 
@@ -476,6 +479,30 @@ class RedisCluster extends AbstractAggregateConnection implements ClusterInterfa
         }
     }
 
+    /**
+     * Handles -READONLY responses by disconnecting the current node's connection
+     * and refreshing the slots map (when cluster slots are enabled), then
+     * re-executing the command so it is routed to the updated primary node.
+     *
+     * This is a workaround for AWS ElastiCache Redis OSS, which may return
+     * -READONLY errors during failover events. Standard Redis clusters do not
+     * exhibit this behavior.
+     *
+     * @param CommandInterface $command Command that generated the -READONLY response.
+     *
+     * @return mixed
+     */
+    protected function onReadOnlyResponse(CommandInterface $command)
+    {
+        if ($this->useClusterSlots) {
+            $connection = $this->getConnectionByCommand($command);
+            $connection->disconnect();
+            $this->askSlotMap();
+        }
+
+        return $this->executeCommand($command);
+    }
+
     /**
      * Handles -MOVED responses by executing again the command against the node
      * indicated by the Redis response.

tests/Predis/Connection/Cluster/RedisClusterTest.php

@@ -738,6 +738,45 @@ class RedisClusterTest extends PredisTestCase
         ));
     }
 
+    /**
+     * @group disconnected
+     */
+    public function testRetriesExecutingCommandOnReadonlyServerExceptionDoesNotUpdateSlotMap(): void
+    {
+        $serverException = new Response\ServerException('READONLY You can\'t write against a read only replica.');
+        $command = $this->getCommandFactory()->create('get', ['node:1001']);
+
+        $connection1 = $this->getMockConnection('tcp://127.0.0.1:6381?slots=0-5460');
+        $connection1
+            ->expects($this->exactly(2))
+            ->method('executeCommand')
+            ->with($this->isRedisCommand($command))
+            ->willThrowException($serverException);
+
+        $connection1
+            ->expects($this->never())
+            ->method('disconnect');
+
+        /** @var FactoryInterface|MockObject */
+        $factory = $this->getMockBuilder('Predis\Connection\FactoryInterface')->getMock();
+        $factory
+            ->expects($this->never())
+            ->method('create');
+
+        $cluster = new RedisCluster($factory, new Parameters());
+        $cluster->useClusterSlots(true);
+        $cluster->setRetryInterval(0);
+        $cluster->setRetryLimit(1);
+
+        $cluster->add($connection1);
+        $cluster->getSlotMap()->setSlots(0, 5460, '127.0.0.1:6381');
+
+        $this->expectException('Predis\Response\ServerException');
+        $this->expectExceptionMessage('READONLY You can\'t write against a read only replica.');
+
+        $cluster->executeCommand($command);
+    }
+
     /**
      * @group disconnected
      * @group slow