Redis Cluster: hash slots, MOVED/ASK redirects và cách sharding hoạt động?

Redis Cluster chia key space thành 16384 hash slots (0-16383).

• Mỗi key được assign slot = CRC16(key) % 16384.
• Cluster có N master nodes, mỗi master sở hữu một range slot (ví dụ: 3 masters mỗi master có ~5461 slots). MOVED redirect: khi client gửi request đến node sai (key không thuộc slot của node đó), node trả về MOVED <slot> <ip:port> — client phải resend đến đúng node.
• Smart client cache slot map để tránh redirect (redis-py, Jedis, ioredis tự handle). ASK redirect: trong quá trình resharding (migrate slot giữa nodes), một số key đã migrate, số khác chưa.
• Node cũ trả ASK <slot> <new_node> cho key đã migrate — client gửi ASKING + request đến node mới một lần (không cache như MOVED). Key tags: {user_id}.orders — chỉ hash user_id phần trong {}, đảm bảo keys liên quan cùng slot để dùng multi-key commands.
• Cluster yêu cầu ít nhất 6 nodes (3 master + 3 replica) để đảm bảo HA.

Redis Cluster divides the key space into 16384 hash slots (0–16383).

• Each key is assigned a slot = CRC16(key) % 16384.
• The cluster has N master nodes, each owning a range of slots (e.g., 3 masters each own ~5461 slots). MOVED redirect: when a client sends a request to the wrong node (the key's slot is not owned by that node), the node returns MOVED <slot> <ip:port> — the client must resend to the correct node.
• Smart clients cache the slot map to avoid redirects (redis-py, Jedis, ioredis handle this automatically). ASK redirect: during resharding (migrating a slot between nodes), some keys have already moved and some haven't.
• The source node returns ASK <slot> <new_node> for a migrated key — the client sends ASKING + the request to the new node once (not cached like MOVED). Key tags: {user_id}.orders — only user_id (the part inside {}) is hashed, ensuring related keys land on the same slot so multi-key commands can be used.
• Cluster requires at least 6 nodes (3 masters + 3 replicas) to guarantee HA.