In this lesson, you will learn how to design load-balancing strategies to maintain session stickiness for persistent WebSocket connections in horizontally scaled SignalR environments.
When you scale your SignalR application across multiple server instances, the initial HTTP handshake and the subsequent persistent WebSocket connection are technically two separate events. If a client performs the handshake on Server A but the resulting WebSocket request is routed to Server B by your load balancer, the connection will fail because Server B lacks the internal state context of that specific negotiation.
To resolve this, your infrastructure must implement Sticky Sessions (also known as Session Affinity).
Configuring Load Balancer Affinity
At the infrastructure layer, your load balancer must be configured to inspect a cookie (often injected by the load balancer itself) to ensure that once a client connects to a specific node, all future traffic—including the WebSocket upgrade request—is pinned to that same node.
In C#, you can enforce this requirement by ensuring your Hub context remains aware of the connection lifecycle, but the routing itself must happen at the gateway. If you are using Nginx as a reverse proxy, you would define an upstream block with ip_hash or cookie-based persistence:
upstream signalr_nodes {
ip_hash; # Ensures a client IP always hits the same server
server server1.example.com;
server server2.example.com;
}
Why Stickiness Matters for SignalR
SignalR utilizes the ConnectionId to track the state of the hub lifecycle. When you have multiple servers, each server keeps its own in-memory collection of HubContext connections. If the connection isn't sticky, the "Negotiation" phase—which establishes the protocol and transport—will mismatch with the "Connection" phase.
Without stickiness, the client will experience intermittent 404 Not Found errors or 401 Unauthorized responses during the transport upgrade process, as the target server will have no record of the negotiation performed on its peer. By pinning the client to a specific node for the duration of the initial connection lifecycle, you guarantee the state integrity required for the SignalR Hub to function correctly.
Key Takeaway: Horizontal scaling for WebSockets requires Sticky Sessions at the load balancer level to ensure the handshake and the persistent connection remain routed to the same server node.