Introduction

Redis replica lag high when master write rate exceeds replication bandwidth. This guide provides step-by-step diagnosis and resolution.

Symptoms

Typical error output:

bash
Warning: Replication lag high
master_repl_offset: 100000000, slave_repl_offset: 99900000
Lag: 1000000 bytes

Common Causes

  1. 1.Network bandwidth between master and replica insufficient
  2. 2.Master write rate exceeding replication speed
  3. 3.TCP buffer or timeout misconfigured
  4. 4.Disk I/O on replica slow

Step-by-Step Fix

Step 1: Check Current State

bash
redis-cli INFO replication
redis-cli --latency
redis-cli --latency-history

Step 2: Identify Root Cause

bash
redis-cli INFO
redis-cli CONFIG GET "*"
redis-cli SLOWLOG GET 10

Step 3: Apply Primary Fix

```bash # Increase output buffer for replica redis-cli CONFIG SET client-output-buffer-limit "replica 256mb 64mb 60"

# Increase TCP buffer redis-cli CONFIG SET repl-backlog-size 256mb

# Check network latency redis-cli --latency -h <master-ip> ```

Step 4: Apply Alternative Fix

```bash # Alternative fix: Check configuration redis-cli CONFIG GET "*memory*" redis-cli CONFIG GET "*timeout*"

# Update settings redis-cli CONFIG SET parameter value

# Verify the fix redis-cli INFO ```

Step 5: Verify the Fix

bash
redis-cli INFO replication | grep lag
# lag should be 0 or minimal

Common Pitfalls

  • Not using connection pooling for high-throughput applications
  • Setting maxmemory without eviction policy
  • Using KEYS command in production
  • Not monitoring replication lag

Best Practices

  • Monitor Redis memory and fragmentation regularly
  • Use SCAN instead of KEYS for key enumeration
  • Configure appropriate eviction policy for workload
  • Implement proper backup strategy (RDB or AOF)
  • Redis Connection Refused
  • Redis High Latency
  • Redis Cluster Down
  • Redis Persistence Failed