**When building scalable backend systems, one common question arises: “If we scale our application using multiple instances, how should caching work?” This becomes especially important when using Redis in distributed architectures. Let’s understand the problem step by step and then move toward advanced scaling concepts like sharding and replication .** 🚨 The Problem with Local Redis per Instance Suppose we scale our backend application: Load Balancer ↓ ┌───────────┐ │ Instance1 │ └───────────┘ │ Local Redis 1 ┌───────────┐ │ Instance2 │ └───────────┘ │ Local Redis 2 Enter fullscreen mode Exit fullscreen mode At first glance, this looks fine. But here’s the issue.…