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Fix Redis Cluster Slot Assignment Failed

Resolve Redis cluster slot assignment failures by checking cluster state, node connectivity, and resharding process.

Published: Apr 7, 20269 min readBy FixWikiHub Editorial Team

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Introduction

Redis Cluster fails to assign hash slots to nodes. Cluster remains in partially initialized state with some slots unassigned.

Symptoms

```bash $ redis-cli --cluster add-node new-node:6379 existing:6379

[ERR] Node new-node:6379 is not empty. Either the node already knows other nodes or contains some data. ```

Slot assignment error:

```bash $ redis-cli cluster addslots 0 1 2

(error) ERR Slot 0 is already busy ```

Cluster check failure:

```bash $ redis-cli --cluster check node:6379

[WARNING] Node node:6379 has slots in migrating state [WARNING] The following slots are not covered: 5000-6000 ```

Cluster state:

```bash $ redis-cli cluster info

cluster_state:fail cluster_slots_assigned:15384 # Should be 16384 ```

Common Causes

1.Slot already assigned - Hash slot owned by another node
2.Node not empty - New node contains existing data
3.Cluster state inconsistent - Nodes disagree on slot ownership
4.Resharding in progress - Ongoing migration not completed
5.Insufficient masters - Not enough master nodes for slots
6.Network partition - Nodes cannot communicate

Step-by-Step Fix

1.Check logs for specific error messages
2.Verify configuration settings
3.Test network connectivity
4.Review recent changes
5.Apply corrective action
6.Verify the fix

Step 1: Check Cluster Status

```bash # Check cluster info: redis-cli cluster info

# Output: # cluster_state:ok # Should be ok # cluster_slots_assigned:16384 # Must be all 16384 # cluster_slots_ok:16384

# Check cluster nodes: redis-cli cluster nodes

# Output format: # node_id ip:port@bus flags master_id link_state slots

# Check cluster health: redis-cli --cluster check redis1:6379

# Check specific node: redis-cli -h node -p 6379 cluster info

# Count assigned slots: redis-cli cluster nodes | grep -o '[[0-9-]*]' | wc -l ```

Step 2: Check Slot Coverage

```bash # Check which slots are assigned: redis-cli cluster slots

# Output: # 1) 1) 0 # Start slot # 2) 1000 # End slot # 3) 1) "10.0.0.1" # Master IP # 2) 6379 # Master port

# Find unassigned slots: redis-cli --cluster check redis1:6379 2>&1 | grep "not covered"

# Check specific slot: redis-cli cluster keyslot mykey # Returns slot number

# Check slot owner: redis-cli cluster nodes | grep "[1234]"

# List all slots by node: redis-cli cluster nodes | awk '{print $1, $NF}' | grep "[" ```

Step 3: Check Node Connectivity

```bash # Check node connectivity: redis-cli -h redis1 ping redis-cli -h redis2 ping redis-cli -h redis3 ping

# Check cluster bus port (port + 10000): nc -zv redis1 16379 nc -zv redis2 16379

# Check all nodes can reach each other: for node in redis1 redis2 redis3; do echo "Testing $node:" redis-cli -h $node cluster nodes | grep -c "connected" done

# Check handshake status: redis-cli cluster nodes | grep handshake

# Meet nodes: redis-cli cluster meet redis2 6379

# Check link state: redis-cli cluster nodes | awk '{print $1, $NF}' # connected or disconnected ```

Step 4: Fix Empty Node Requirement

```bash # For new cluster node, must be empty:

# Check if node has data: redis-cli -h new-node DBSIZE # Should be 0

# Check if node knows other nodes: redis-cli -h new-node cluster nodes # Should show only itself

# Flush existing data: redis-cli -h new-node FLUSHALL

# Reset cluster config: redis-cli -h new-node CLUSTER RESET HARD

# Verify node is empty: redis-cli -h new-node cluster info redis-cli -h new-node DBSIZE

# Now add to cluster: redis-cli --cluster add-node new-node:6379 existing-master:6379 ```

Step 5: Assign Slots to New Master

```bash # For new master without slots:

# Option 1: Reshard from existing master: redis-cli --cluster reshard redis1:6379 \ --cluster-from <source-node-id> \ --cluster-to <target-node-id> \ --cluster-slots 4096 \ --cluster-yes

# Option 2: Interactive reshard: redis-cli --cluster reshard redis1:6379 # Follow prompts: # How many slots? 4096 # Receiving node ID? <target-node-id> # Source node ID? all

# Option 3: Add specific slots manually: redis-cli cluster addslots {0..4095}

# Verify slots assigned: redis-cli cluster nodes | grep <node-id> ```

Step 6: Fix Slot Migration Issues

```bash # Check for migrating/importing slots: redis-cli cluster nodes | grep -E "[|->"

# Output: # [5000-<-node-id] # Importing # [5000->-node-id] # Migrating

# Check specific slot status: redis-cli cluster getkeysinslot 5000 10

# Fix stuck migration:

# 1. Cancel import on target: redis-cli -h target cluster setslot 5000 stable

# 2. Cancel export on source: redis-cli -h source cluster setslot 5000 stable

# 3. Continue migration: redis-cli --cluster reshard redis1:6379

# Or use fix: redis-cli --cluster fix redis1:6379 --cluster-search-multiple-owners ```

Step 7: Add Slots to Empty Master

```bash # If master has no slots:

# Check master node ID: redis-cli cluster nodes | grep master

# Add slots (must be unassigned): redis-cli cluster addslots {0..4095}

# Error if slots already assigned: # (error) ERR Slot 0 is already busy

# Check who owns slots: redis-cli cluster slots | grep "0]"

# Delete slots from wrong owner first: redis-cli -h wrong-owner cluster delslots {0..4095}

# Then assign to correct node: redis-cli -h correct-node cluster addslots {0..4095}

# Verify: redis-cli cluster nodes | grep correct-node-id ```

Step 8: Rebalance Cluster

```bash # Rebalance slots across all masters:

redis-cli --cluster rebalance redis1:6379

# With threshold: redis-cli --cluster rebalance redis1:6379 --cluster-threshold 1

# Simulate first: redis-cli --cluster rebalance redis1:6379 --cluster-simulate

# Use pipeline for speed: redis-cli --cluster rebalance redis1:6379 --cluster-pipeline 100

# Check balance after: redis-cli cluster nodes | awk '/master/{print $NF}' | sort

# Output shows slot ranges per master: # [0-5460] # [5461-10922] # [10923-16383] ```

Step 9: Fix Cluster State Fail

```bash # If cluster_state:fail

# Check why: redis-cli --cluster check redis1:6379

# Common issues: # 1. Slots not covered # 2. Majority of masters not reachable # 3. Slots in migrating state

# Fix slots not covered: redis-cli --cluster fix redis1:6379

# Fix with multiple owners: redis-cli --cluster fix redis1:6379 --cluster-search-multiple-owners

# Force cluster to OK (dangerous): redis-cli cluster setslot 5000 stable # Repeat for all problem slots

# If majority lost (emergency): # Create new cluster from remaining: redis-cli --cluster create redis1:6379 redis2:6379 --cluster-replicas 1 --cluster-force

# This destroys data on failed nodes! ```

Step 10: Redis Cluster Verification Script

```bash # Create verification script: cat << 'EOF' > /usr/local/bin/check-redis-cluster.sh #!/bin/bash

NODE=${1:-"localhost:6379"}

echo "" echo "=== Cluster Nodes ===" redis-cli -h $(echo $NODE | cut -d: -f1) -p $(echo $NODE | cut -d: -f2) cluster nodes | awk '{print $2, $3, $8}' | column -t

echo "" echo "=== Slot Coverage ===" redis-cli -h $(echo $NODE | cut -d: -f1) -p $(echo $NODE | cut -d: -f2) cluster slots | wc -l echo "Expected: 16384 slots"

echo "" echo "=== Cluster Check ===" redis-cli --cluster check $NODE 2>&1 | grep -E "[OK]|[ERR]|[WARNING]"

echo "" echo "=== Node Connectivity ===" for node in $(redis-cli -h $(echo $NODE | cut -d: -f1) cluster nodes | awk '{print $2}' | cut -d@ -f1); do host=$(echo $node | cut -d: -f1) port=$(echo $node | cut -d: -f2) redis-cli -h $host -p $port ping done

echo "" echo "=== Unassigned Slots ===" redis-cli --cluster check $NODE 2>&1 | grep "not covered" EOF

chmod +x /usr/local/bin/check-redis-cluster.sh

# Usage: /usr/local/bin/check-redis-cluster.sh redis1:6379

# Monitor: watch -n 5 'redis-cli cluster info | grep cluster_state' ```

Redis Cluster Checklist

Check	Command	Expected
Cluster state	cluster info	ok
Slots assigned	cluster info	16384
All nodes connected	cluster nodes	All connected
No migrations	cluster nodes	No -> or <-
Balanced slots	rebalance	Even distribution
Node connectivity	ping	PONG

Verification

```bash # After fixing slot assignment

# 1. Check cluster info redis-cli cluster info | grep cluster_state // cluster_state:ok

# 2. Verify slots redis-cli cluster info | grep cluster_slots_assigned // cluster_slots_assigned:16384

# 3. Check nodes redis-cli cluster nodes // All nodes connected, slots assigned

# 4. Test key storage redis-cli set testkey value // OK

# 5. Check key slot redis-cli cluster keyslot testkey // Shows slot number

# 6. Run cluster check redis-cli --cluster check redis1:6379 // All checks pass ```

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Last updated: Apr 7, 2026

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Fix Redis Cluster Slot Assignment Failed

Introduction

Symptoms

Common Causes

Step-by-Step Fix

Step 1: Check Cluster Status

Step 2: Check Slot Coverage

Step 3: Check Node Connectivity

Step 4: Fix Empty Node Requirement

Step 5: Assign Slots to New Master

Step 6: Fix Slot Migration Issues

Step 7: Add Slots to Empty Master

Step 8: Rebalance Cluster

Step 9: Fix Cluster State Fail

Step 10: Redis Cluster Verification Script

Redis Cluster Checklist

Verification

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Fix Redis Cluster Slot Assignment Failed

Introduction

Symptoms

Common Causes

Step-by-Step Fix

Step 1: Check Cluster Status

Step 2: Check Slot Coverage

Step 3: Check Node Connectivity

Step 4: Fix Empty Node Requirement

Step 5: Assign Slots to New Master

Step 6: Fix Slot Migration Issues

Step 7: Add Slots to Empty Master

Step 8: Rebalance Cluster

Step 9: Fix Cluster State Fail

Step 10: Redis Cluster Verification Script

Redis Cluster Checklist

Verification

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