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Home Interviews How do you scale a SignalR application across multiple servers (e.g., using Redis backplane)? – MindStick
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ICSM Computer
ICSM Computer 22 May 2025
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To scale a SignalR application across multiple servers, you need to share connection state and messages between instances. This is done using a backplane, with Redis being the most popular and efficient choice.

Why Use a Redis Backplane?

  1. SignalR stores connection info in-memory per server, so by default:
    1. A message sent on Server A won't reach a client connected to Server B.
    2. Redis solves this by syncing messages across servers.

Steps to Set Up SignalR with Redis Backplane

1. Install Redis and Required NuGet Package

Install the Redis backplane package:

dotnet add package Microsoft.AspNetCore.SignalR.StackExchangeRedis

Ensure you have a Redis server running (locally, on Docker, or hosted).

2. Configure SignalR to Use Redis in Startup.cs (or Program.cs)

For ASP.NET Core:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSignalR().AddStackExchangeRedis("localhost:6379", options =>
    {
        options.Configuration.ChannelPrefix = "MyAppSignalR"; // optional, for multiple apps
    });

    // ... other services
}

Replace localhost:6379 with your actual Redis connection string.

3. Use SignalR as Usual

No changes needed in your Hub code or client code — Redis handles syncing behind the scenes.

public class ChatHub : Hub
{
    public async Task SendMessageToRoom(string roomName, string message)
    {
        await Clients.Group(roomName).SendAsync("ReceiveMessage", Context.UserIdentifier, message);
    }
}

When to Use Redis Backplane

Use Redis if:

  1. You’re hosting your app on multiple servers/instances.
  2. You need reliable group messaging across instances.
  3. You want to scale horizontally with a load balancer.

What Redis Backplane Does Not Do

  1. It does not share connection state — it just forwards messages.
  2. You still need to re-add group memberships on reconnect.
  3. It does not persist messages (not a message queue).

Additional Tips

  1. Use a load balancer with sticky sessions (optional) for more predictable connection mapping.
  2. For high-scale production, consider:
    1. Azure SignalR Service (fully managed)
    2. Scaling Redis (using Redis Cluster or Sentinel)
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