Understanding MOVED and ASK Redirection in Redis Cluster

In a Redis cluster, when a client requests a key whose hash slot is not served by the current node, the node returns a redirection error directing the client to the correct node.

There are two redirection commands: MOVED and ASK . Both cause redirection but are used at different times. MOVED is returned when the slot has permanently moved to another node, while ASK is a temporary redirect during slot migration, such as when re‑sharding.

1. MOVED redirection

Trigger condition: The requested key’s hash slot is permanently assigned to another node.

Typical scenario: Cluster topology changes like node scaling or slot reallocation.

Example flow:

1. Client sends GET key1 to node A.

2. Node A checks the slot; if key1’s slot (e.g., 5000) is outside A’s range (0‑4000), it returns a MOVED error.

3. Node A replies MOVED 5000 127.0.0.1:7002 .

4. The client caches the mapping of slot 5000 to node B and directs subsequent requests to node B.

Example CLI output:

127.0.0.1:7000> get love (error) MOVED 16198 127.0.0.1:7002

2. ASK redirection

Trigger condition: The key’s hash slot is in the process of migrating, with data split between the old and new nodes.

Typical scenario: During slot migration before it is fully completed.

Example flow:

1. Slot 5000 is moving from node B to node C; key1 has already moved to node C, but node B’s metadata is not updated.

2. Client sends GET key1 to node B.

3. Node B replies ASK 5000 127.0.0.1:7003 .

4. The client temporarily redirects by sending an ASKING command to node C, then executes GET key1 without updating its slot cache, because the slot is still owned by node B until migration finishes.