📋

Description

The Redis ASK error indicates that the slot your client is attempting to access is currently migrating to a different node in the cluster. This means the node you contacted knows the data is moving and is redirecting you.

💬

Error Message

ASK slot node

🔍

Known Causes

3 known causes

⚠️

Ongoing Resharding

The cluster is actively resharding, moving slots from one node to another to rebalance data distribution. ⚙

⚠️

Node Restart

A node recently restarted and is temporarily redirecting requests while it catches up with the cluster state. 💻

⚠️

Manual Slot Migration

An administrator is manually migrating slots between nodes for maintenance or optimization purposes. ⚙

🛠️

Solutions

3 solutions available

1. Client-Side Redirection (Recommended) easy

Most modern Redis clients are designed to automatically handle ASK redirects by querying the cluster for the correct node. Ensure your client is configured correctly and updated.

1

Verify your Redis client library is up-to-date. Older versions might not support the cluster redirect mechanism.

2

Check your client's configuration for cluster mode. Ensure it's properly connected to a seed node in the Redis cluster and that cluster support is enabled.

Example configuration for `redis-py` (Python):
```python
from redis.cluster import RedisCluster

startup_nodes = [{"host": "192.168.1.100", "port": "7000"}]
rc = RedisCluster(startup_nodes=startup_nodes, decode_responses=True)

# Now you can interact with the cluster, the client will handle ASK redirects
rc.set('mykey', 'myvalue')
print(rc.get('mykey'))
```

3

If the ASK error persists, it might indicate a temporary issue with the cluster's state. A simple retry with a short delay is often sufficient.

Example retry logic in Python:
```python
import time
from redis.exceptions import AskRedirectError

for _ in range(3):
    try:
        rc.set('anotherkey', 'anothervalue')
        print(rc.get('anotherkey'))
        break  # Success, exit loop
    except AskRedirectError as e:
        print(f"ASK redirect encountered: {e}. Retrying in 1 second...")
        time.sleep(1)
    except Exception as e:
        print(f"An unexpected error occurred: {e}")
        break
```

2. Identify and Monitor Slot Migration medium

Understand which slot is migrating and why. This involves querying the cluster state to identify the source and target nodes involved in the migration.

1

Connect to any node in the Redis cluster using `redis-cli`.

redis-cli -c -h <node_ip> -p <node_port>

2

Execute the `CLUSTER SLOTS` command to get the current state of slot assignments and migrations.

CLUSTER SLOTS

3

Analyze the output of `CLUSTER SLOTS`. Look for slots that are listed with two different node configurations, indicating a migration is in progress. The `ASK` error message will specify the slot number. For example:
```
1) 1) (integer) 10000 # Start slot
2) (integer) 11999 # End slot
3) 1) "192.168.1.101" (string) # Master node
2) (integer) 7001
4) 1) "192.168.1.102" (string) # Slave node (or target for migration)
2) (integer) 7002
```
If slot `X` is causing an `ASK` error, and `CLUSTER SLOTS` shows it being migrated from node A to node B, the ASK error means you are currently connected to node A (the source) and it's telling you to go to node B (the target).

4

Once you identify the target node from `CLUSTER SLOTS`, you can try directing your client's connection to that specific node for the affected keys.

If the ASK error is for slot 10000, and `CLUSTER SLOTS` shows it migrating to 192.168.1.102:7002, you might temporarily configure your client to connect to that IP and port for operations involving keys that hash to slot 10000. This is usually handled automatically by clients supporting cluster mode.

3. Force Resharding or Manual Slot Reassignment (Advanced) advanced

If slot migrations are causing persistent issues or you need to manually control the data distribution, you can initiate resharding operations or directly reassign slots using cluster commands. This is a more intrusive operation and requires careful planning.

1

Ensure you have a good understanding of your cluster topology and the impact of moving slots.

2

Connect to the Redis cluster's master node using `redis-cli` in cluster mode.

redis-cli -c -h <master_node_ip> -p <master_node_port>

3

To initiate a resharding operation, you can use the `CLUSTER SETSLOT` command. This is a powerful command and should be used with extreme caution.

Example: To move slot 5000 from node ID `abcdef123456` to node ID `fedcba654321`:
```bash
CLUSTER SETSLOT 5000 IMPORTING <target_node_id>
CLUSTER SETSLOT 5000 NODE <target_node_id>
```

**Important Considerations:**
- **`IMPORTING` state:** This makes the target node accept writes for the slot from the source node.
- **`NODE` state:** This assigns the slot to the target node as its primary owner.
- **Migrating data:** After setting the slot to `IMPORTING`, you'll typically need to run `CLUSTER GETKEYSINSLOT <slot_id>` on the source node to identify keys and then use `MIGRATE` to move them to the target node. This is a complex process and often automated by cluster management tools.
- **`CLUSTER SETSLOT <slot_id> MIGRATING <target_node_id>`:** This command is used on the source node to indicate that it's migrating the slot. The client will then be redirected to the target node using the `ASK` error.

4

Alternatively, use Redis Cluster's built-in resharding tools or external cluster management utilities (like `redis-trib.rb` or cloud provider tools) for a more automated and safer resharding process. These tools manage the state transitions and data migration more gracefully.

Using `redis-trib.rb` (example):
```bash
./redis-trib.rb reshard <node_ip>:<node_port>
```
Follow the prompts to specify source and target nodes and the number of slots to move.

Fix Redis Error ASK: Slot Migration