A Backplane allows multiple SignalR servers to communicate with each other to ensure messages reach the right user, regardless of which server they are on.
When Server 1 wants to send a message to 'All', it publishes that message to a **Redis Pub/Sub** channel. Server 2 and Server 3 are also subscribed to that same channel. They receive the message from Redis and then broadcast it to their own locally connected users. This 'Pub/Sub' dance ensures every user gets the update.
It's incredibly simple in .NET:
builder.Services.AddSignalR().AddStackExchangeRedis("connection_string");.
SignalR handles all the complex message serialization and pub/sub logic for you. Your code doesn't change; the infra does.
Q: "Is Redis the best backplane?"
Architect Answer: "It's the most common for on-premise or self-hosted environments. However, Redis backplanes can become a bottleneck if you have millions of messages per second. For massive scale in the cloud, the **Azure SignalR Service** is a much more robust and managed option."