EC
ASK
RedisINFONotableClusterHIGH confidence

Redirect for cluster slot migration

Production Risk

Low if clients are cluster-aware. High if they are not.

What this means

This is a temporary redirection used during cluster resharding. It indicates that the slot for a key is being migrated from one node to another. The client must send an ASKING command to the destination node before sending the actual command.

Why it happens
  1. 1A cluster resharding operation is in progress (`CLUSTER SETSLOT MIGRATING`).
  2. 2The specific key being requested has not yet been moved to the destination node.
  3. 3The client is attempting to access a key in a slot that is currently being migrated.
How to reproduce

A client requests a key 'mykey' from Node A, but the slot for 'mykey' is being migrated to Node B. The key itself hasn't moved yet, so Node A issues an ASK redirect.

trigger — this will error
trigger — this will error
# On Node A, while slot for 'mykey' is migrating to B
GET mykey

expected output

(error) ASK 12345 192.168.1.101:6379

Fix 1

Use a cluster-aware client library

WHEN Connecting to any Redis Cluster

Use a cluster-aware client library
// Client handles this automatically. The logic is:
// 1. Receive ASK redirection to Node B
// 2. Connect to Node B
// 3. Send ASKING command
// 4. Send original command (e.g., GET mykey)
// 5. Receive reply from Node B

Why this works

Proper cluster clients understand the ASK protocol. Unlike MOVED, an ASK redirect is for a single command only. The client must prefix its next command to the target node with the ASKING command, which signals that it is aware of the ongoing migration.

Fix 2

Use redis-cli with the -c flag

WHEN Using the command-line interface

Use redis-cli with the -c flag
redis-cli -c -p 6379

Why this works

The `-c` flag enables cluster mode in the CLI, which automatically handles both MOVED and ASK redirections by sending the `ASKING` command when required.

What not to do

✕ Treat ASK like MOVED

ASK is a one-time redirection for a migrating slot. If you update your client's slot map permanently (as you would for MOVED), you will incorrectly send all future traffic for that slot to the destination node, even for keys that haven't migrated yet.

Sources
Official documentation ↗

Redis Cluster resharding and migration logic.

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