Redis - Cluster

Redis Cluster

Redis Cluster is a distributed implementation of Redis that automatically partitions data across multiple nodes and provides fault tolerance. Here’s a guide to understanding various Redis Cluster commands and what they do.

1. CLUSTER NODES

This command provides detailed information about all the nodes in the cluster. When you run it, Redis returns a list of all cluster nodes, their roles, and their states.

Example

127.0.0.1:6379> cluster nodes
004a2ba5d1a19fe28ef3f6397261ee7246f7cbe0 172.27.0.3:6379@16379 master - 0 1734437046074 3 connected 10923-16383
59a39b18368aac76e9e373d928f4dd10783193f5 172.27.0.6:6379@16379 slave 004a2ba5d1a19fe28ef3f6397261ee7246f7cbe0 0 1734437045000 3 connected
2a23e44ab6ec167219b74da9f3c7dc7af79a6ea8 172.27.0.7:6379@16379 slave a7425fa5cb1c211642276cfa90e54930da8568bf 0 1734437048146 2 connected
5a6a43f5a5f6216c10a5fcee212accfa1cebd365 172.27.0.5:6379@16379 myself,master - 0 1734437047000 1 connected 0-5460
e60f7cefdc90eb721944009e22d30b8417ff1553 172.27.0.4:6379@16379 slave 5a6a43f5a5f6216c10a5fcee212accfa1cebd365 0 1734437045038 1 connected
a7425fa5cb1c211642276cfa90e54930da8568bf 172.27.0.2:6379@16379 master - 0 1734437047109 2 connected 5461-10922

Where:

• Node ID: A unique identifier for each node (e.g., c37ba469d276...).
• Address: The IP and port of the node (e.g., 127.0.0.1:6379).
  • The address of the node. @16379 represents the inter-node communication port.
• Role: Whether the node is a master or slave (replica).
• Slots: Range of hash slots the node is responsible for (e.g., 0-5460).

Use Cases

• To check the cluster topology and the role of each node.
• To debug issues like unreachable nodes or incorrect configuration.

2. redis-cli --cluster check 127.0.0.1:6379

This command is part of the Redis CLI’s cluster management utility. It verifies the health of the cluster by connecting to a specific node and performing a series of checks.

Key points

• Ensures all nodes in the cluster are reachable.
• Verifies that slots are properly distributed and covered.
• Checks for misconfigurations, such as missing replicas.

Output

❯ redis-cli --cluster check 127.0.0.1:6379
127.0.0.1:6379 (5a6a43f5...) -> 0 keys | 5461 slots | 1 slaves.
172.27.0.3:6379 (004a2ba5...) -> 0 keys | 5461 slots | 1 slaves.
172.27.0.2:6379 (a7425fa5...) -> 0 keys | 5462 slots | 1 slaves.
[OK] 0 keys in 3 masters.
0.00 keys per slot on average.
>>> Performing Cluster Check (using node 127.0.0.1:6379)
M: 5a6a43f5a5f6216c10a5fcee212accfa1cebd365 127.0.0.1:6379
   slots:[0-5460] (5461 slots) master
   1 additional replica(s)
M: 004a2ba5d1a19fe28ef3f6397261ee7246f7cbe0 172.27.0.3:6379
   slots:[10923-16383] (5461 slots) master
   1 additional replica(s)
S: 59a39b18368aac76e9e373d928f4dd10783193f5 172.27.0.6:6379
   slots: (0 slots) slave
   replicates 004a2ba5d1a19fe28ef3f6397261ee7246f7cbe0
S: 2a23e44ab6ec167219b74da9f3c7dc7af79a6ea8 172.27.0.7:6379
   slots: (0 slots) slave
   replicates a7425fa5cb1c211642276cfa90e54930da8568bf
S: e60f7cefdc90eb721944009e22d30b8417ff1553 172.27.0.4:6379
   slots: (0 slots) slave
   replicates 5a6a43f5a5f6216c10a5fcee212accfa1cebd365
M: a7425fa5cb1c211642276cfa90e54930da8568bf 172.27.0.2:6379
   slots:[5461-10922] (5462 slots) master
   1 additional replica(s)
[OK] All nodes agree about slots configuration.
>>> Check for open slots...
>>> Check slots coverage...
[OK] All 16384 slots covered.

Use Cases

• To confirm that your cluster is healthy after setup.
• To troubleshoot slot misconfigurations or network issues.

3. CLUSTER SLOTS

This command displays how hash slots are distributed among the nodes in the cluster. It’s an overview of which slots belong to which master and their associated replicas.

Example

127.0.0.1:6379> cluster slots
1) 1) (integer) 0
   2) (integer) 5460
   3) 1) "172.27.0.5"
      2) (integer) 6379
      3) "5a6a43f5a5f6216c10a5fcee212accfa1cebd365"
      4) (empty array)
   4) 1) "172.27.0.4"
      2) (integer) 6379
      3) "e60f7cefdc90eb721944009e22d30b8417ff1553"
      4) (empty array)
...

Where

• Slot Range: The hash slots managed by each master node (e.g., 0-5460).
• Node Address: The master node responsible for the slots.
• Replica Nodes: If replicas are set up, their addresses are also listed.

Use Cases

• To visualize how data is distributed across the cluster.
• To locate specific slots for debugging.

4. CLUSTER INFO

This command provides statistics and status information about the entire cluster.

Example

127.0.0.1:6379> cluster info
cluster_state:ok
cluster_slots_assigned:16384
cluster_slots_ok:16384
cluster_slots_pfail:0
cluster_slots_fail:0
cluster_known_nodes:6
cluster_size:3
cluster_current_epoch:6
cluster_my_epoch:1
cluster_stats_messages_ping_sent:297
cluster_stats_messages_pong_sent:294
cluster_stats_messages_sent:591
cluster_stats_messages_ping_received:289
cluster_stats_messages_pong_received:297
cluster_stats_messages_meet_received:5
cluster_stats_messages_received:591
total_cluster_links_buffer_limit_exceeded:0

Key Metrics

• cluster_state: Shows if the cluster is healthy (ok) or has issues (fail).
• cluster_slots_assigned: Total hash slots assigned to nodes (should always be 16384 in a functional cluster).
• cluster_slots_pfail/fail: Slots with nodes in pre-failure or failure state.
• cluster_known_nodes: Number of nodes in the cluster.
• cluster_size: Number of master nodes.

Use Cases

• To monitor cluster health.
• To quickly see if any slots or nodes are failing.

5. CLUSTER MYID

This command returns the unique ID of the node you’re connected to.

Example

127.0.0.1:6379> cluster myid
"5a6a43f5a5f6216c10a5fcee212accfa1cebd365"

Use Cases

• To identify the current node’s role in the cluster.
• Useful in debugging to match the node ID with the output of CLUSTER NODES.

6. CLUSTER REPLICAS node_id

This command lists all the replicas for a specific master node in the cluster. You provide the master’s node ID as an argument.

Example

127.0.0.1:6379> cluster replicas 5a6a43f5a5f6216c10a5fcee212accfa1cebd365
1) "e60f7cefdc90eb721944009e22d30b8417ff1553 172.27.0.4:6379@16379 slave 5a6a43f5a5f6216c10a5fcee212accfa1cebd365 0 1734437164427 1 connected"

Where

• Replica Node ID: The unique identifier for the replica.
• Address: The IP and port of the replica.

Use cases

• To identify the replicas of a specific master.
• Useful for setting up monitoring or verifying replication.

7. CLUSTER KEYSLOT key_name

This command calculates the hash slot for a specific key. Redis Cluster uses the hash slot to determine which node stores the key.

Example

127.0.0.1:6379> set name cloudsimplify
-> Redirected to slot [5798] located at 172.27.0.2:6379
OK
172.27.0.2:6379> cluster keyslot name
(integer) 5798

Redis calculates a CRC16 hash of the key, which is then mapped to one of the 16,384 slots.

Use Cases

• To debug where a key is stored in the cluster.
• To understand how Redis partitions data.

8. CLUSTER GETKEYSINSLOT slot_num count

This command lists keys that belong to a specific hash slot. You also specify how many keys to return (count).

Example

172.27.0.2:6379> set age 1
-> Redirected to slot [741] located at 172.27.0.5:6379
OK
172.27.0.5:6379> keys *
1) "age"
172.27.0.5:6379> cluster getkeysinslot 741 1
1) "age"

Use Cases

• To find out which keys are stored in a particular slot.
• Useful for debugging slot-specific data or migration issues.

Redis Cluster commands provide powerful tools to manage and monitor distributed data. By understanding these commands:

• You can ensure your cluster is properly configured and healthy.
• You can locate specific keys, slots, and nodes for troubleshooting.
• You can gain deeper insights into how Redis partitions and replicates data.