Scale SignalR Service with multiple instances

SignalR Service SDK supports multiple endpoints for SignalR Service instances. You can use this feature to scale the concurrent connections, or use it for cross-region messaging.

For ASP.NET Core

Add multiple endpoints from config

Configure with key Azure:SignalR:ConnectionString or Azure:SignalR:ConnectionString: for SignalR Service connection string.

If the key starts with Azure:SignalR:ConnectionString:, it should be in the format Azure:SignalR:ConnectionString:{Name}:{EndpointType}, where Name and EndpointType are properties of the ServiceEndpoint object, and are accessible from code.

You can add multiple instance connection strings using the following dotnet commands:

dotnet user-secrets set Azure:SignalR:ConnectionString:east-region-a <ConnectionString1>
dotnet user-secrets set Azure:SignalR:ConnectionString:east-region-b:primary <ConnectionString2>
dotnet user-secrets set Azure:SignalR:ConnectionString:backup:secondary <ConnectionString3>

Add multiple endpoints from code

A ServicEndpoint class describes the properties of an Azure SignalR Service endpoint. You can configure multiple instance endpoints when using Azure SignalR Service SDK through:

services.AddSignalR()
        .AddAzureSignalR(options => 
        {
            options.Endpoints = new ServiceEndpoint[]
            {
                // Note: this is just a demonstration of how to set options.Endpoints
                // Having ConnectionStrings explicitly set inside the code is not encouraged
                // You can fetch it from a safe place such as Azure KeyVault
                new ServiceEndpoint("<ConnectionString0>"),
                new ServiceEndpoint("<ConnectionString1>", type: EndpointType.Primary, name: "east-region-a"),
                new ServiceEndpoint("<ConnectionString2>", type: EndpointType.Primary, name: "east-region-b"),
                new ServiceEndpoint("<ConnectionString3>", type: EndpointType.Secondary, name: "backup"),
            };
        });

Customize endpoint router

By default, the SDK uses the DefaultEndpointRouter to pick up endpoints.

Default behavior

1. Client request routing:

   When client /negotiate with the app server. By default, SDK randomly selects one endpoint from the set of available service endpoints.

2. Server message routing:

   When sending a message to a specific connection and the target connection is routed to the current server, the message goes directly to that connected endpoint. Otherwise, the messages are broadcasted to every Azure SignalR endpoint.

Customize routing algorithm

You can create your own router when you have special knowledge to identify which endpoints the messages should go to.

The following example defines a custom router that routes messages with a group starting with east- to the endpoint named east:

private class CustomRouter : EndpointRouterDecorator
{
    public override IEnumerable<ServiceEndpoint> GetEndpointsForGroup(string groupName, IEnumerable<ServiceEndpoint> endpoints)
    {
        // Override the group broadcast behavior, if the group name starts with "east-", only send messages to endpoints inside east
        if (groupName.StartsWith("east-"))
        {
            return endpoints.Where(e => e.Name.StartsWith("east-"));
        }

        return base.GetEndpointsForGroup(groupName, endpoints);
    }
}

The following example overrides the default negotiate behavior and selects the endpoint depending on the location of the app server.

private class CustomRouter : EndpointRouterDecorator
{    public override ServiceEndpoint GetNegotiateEndpoint(HttpContext context, IEnumerable<ServiceEndpoint> endpoints)
    {
        // Override the negotiate behavior to get the endpoint from query string
        var endpointName = context.Request.Query["endpoint"];
        if (endpointName.Count == 0)
        {
            context.Response.StatusCode = 400;
            var response = Encoding.UTF8.GetBytes("Invalid request");
            context.Response.Body.Write(response, 0, response.Length);
            return null;
        }

        return endpoints.FirstOrDefault(s => s.Name == endpointName && s.Online) // Get the endpoint with name matching the incoming request
               ?? base.GetNegotiateEndpoint(context, endpoints); // Or fallback to the default behavior to randomly select one from primary endpoints, or fallback to secondary when no primary ones are online
    }
}

Don't forget to register the router to DI container using:

services.AddSingleton(typeof(IEndpointRouter), typeof(CustomRouter));
services.AddSignalR()
        .AddAzureSignalR(
            options => 
            {
                options.Endpoints = new ServiceEndpoint[]
                {
                    new ServiceEndpoint(name: "east", connectionString: "<connectionString1>"),
                    new ServiceEndpoint(name: "west", connectionString: "<connectionString2>"),
                    new ServiceEndpoint("<connectionString3>")
                };
            });

For ASP.NET

Add multiple endpoints from config

Configuration with key Azure:SignalR:ConnectionString or Azure:SignalR:ConnectionString: for SignalR Service connection string.

If the key starts with Azure:SignalR:ConnectionString:, it should be in format Azure:SignalR:ConnectionString:{Name}:{EndpointType}, where Name and EndpointType are properties of the ServiceEndpoint object, and are accessible from code.

You can add multiple instance connection strings to web.config:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name="Azure:SignalR:ConnectionString" connectionString="<ConnectionString1>"/>
    <add name="Azure:SignalR:ConnectionString:en-us" connectionString="<ConnectionString2>"/>
    <add name="Azure:SignalR:ConnectionString:zh-cn:secondary" connectionString="<ConnectionString3>"/>
    <add name="Azure:SignalR:ConnectionString:Backup:secondary" connectionString="<ConnectionString4>"/>
  </connectionStrings>
  ...
</configuration>

Add multiple endpoints from code

A ServiceEndpoint class describes the properties of an Azure SignalR Service endpoint. You can configure multiple instance endpoints when using Azure SignalR Service SDK through:

app.MapAzureSignalR(
    this.GetType().FullName, 
    options => {
            options.Endpoints = new ServiceEndpoint[]
            {
                // Note: this is just a demonstration of how to set options. Endpoints
                // Having ConnectionStrings explicitly set inside the code is not encouraged.
                // You can fetch it from a safe place such as Azure KeyVault
                new ServiceEndpoint("<ConnectionString1>"),
                new ServiceEndpoint("<ConnectionString2>"),
                new ServiceEndpoint("<ConnectionString3>"),
            }
        });

Customize a router

The only difference between ASP.NET SignalR and ASP.NET Core SignalR is the http context type for GetNegotiateEndpoint. For ASP.NET SignalR, it is of IOwinContext type.

The following code is a custom negotiate example for ASP.NET SignalR:

private class CustomRouter : EndpointRouterDecorator
{
    public override ServiceEndpoint GetNegotiateEndpoint(IOwinContext context, IEnumerable<ServiceEndpoint> endpoints)
    {
        // Override the negotiate behavior to get the endpoint from query string
        var endpointName = context.Request.Query["endpoint"];
        if (string.IsNullOrEmpty(endpointName))
        {
            context.Response.StatusCode = 400;
            context.Response.Write("Invalid request.");
            return null;
        }

        return endpoints.FirstOrDefault(s => s.Name == endpointName && s.Online) // Get the endpoint with name matching the incoming request
               ?? base.GetNegotiateEndpoint(context, endpoints); // Or fallback to the default behavior to randomly select one from primary endpoints, or fallback to secondary when no primary ones are online
    }
}

Don't forget to register the router to DI container using:

var hub = new HubConfiguration();
var router = new CustomRouter();
hub.Resolver.Register(typeof(IEndpointRouter), () => router);
app.MapAzureSignalR(GetType().FullName, hub, options => {
    options.Endpoints = new ServiceEndpoint[]
                {
                    new ServiceEndpoint(name: "east", connectionString: "<connectionString1>"),
                    new ServiceEndpoint(name: "west", connectionString: "<connectionString2>"),
                    new ServiceEndpoint("<connectionString3>")
                };
});

Service Endpoint Metrics

To enable an advanced router, SignalR server SDK provides multiple metrics to help server make smart decisions. The properties are under ServiceEndpoint.EndpointMetrics.

Metric Name	Description
ClientConnectionCount	Total count of concurrent client connections on all hubs for the service endpoint
ServerConnectionCount	Total count of concurrent server connections on all hubs for the service endpoint
ConnectionCapacity	Total connection quota for the service endpoint, including client and server connections

The following code is an example of customizing a router according to ClientConnectionCount.

private class CustomRouter : EndpointRouterDecorator
{
    public override ServiceEndpoint GetNegotiateEndpoint(HttpContext context, IEnumerable<ServiceEndpoint> endpoints)
    {
        return endpoints.OrderBy(x => x.EndpointMetrics.ClientConnectionCount).FirstOrDefault(x => x.Online) // Get the available endpoint with minimal clients load
               ?? base.GetNegotiateEndpoint(context, endpoints); // Or fallback to the default behavior to randomly select one from primary endpoints, or fallback to secondary when no primary ones are online
    }
}

Dynamic Scale ServiceEndpoints

From SDK version 1.5.0, we're enabling dynamic scale ServiceEndpoints for ASP.NET Core version first. So you don't have to restart app server when you need to add/remove a ServiceEndpoint. As ASP.NET Core is supporting a default configuration like appsettings.json with reloadOnChange: true, you don't need to change code, and it's supported by nature. And if you'd like to add some customized configuration and work with hot-reload, refer to Configuration in ASP.NET Core.

Note

Considering the time of connection set-up between server/service and client/service may be different, to ensure no message loss during the scale process, we have a staging period waiting for server connections to be ready before opening the new ServiceEndpoint to clients. Usually it takes seconds to complete and you'll be able to see a log message like Succeed in adding endpoint: '{endpoint}' which indicates the process complete.

In some expected situations, like cross-region network issues or configuration inconsistencies on different app servers, the staging period may not finish correctly. In these cases, it's suggested to restart the app server when you find the scaling process not working correctly.

The default timeout period for the scale is 5 minutes, and it can be customized by changing the value in ServiceOptions.ServiceScaleTimeout. If you have a lot of app servers, it's suggested to extend the value a little bit more.

Configuration in cross-region scenarios

The ServiceEndpoint object has an EndpointType property with value primary or secondary.

Primary endpoints are preferred endpoints to receive client traffic because they've have more reliable network connections. Secondary endpoints have less reliable network connections and are used only for server to client traffic. For example, secondary endpoints are used for broadcasting messages instead of client to server traffic.

In cross-region cases, the network can be unstable. For an app server located in East US, the SignalR Service endpoint located in the same East US region is primary and endpoints in other regions marked as secondary. In this configuration, service endpoints in other regions can receive messages from this East US app server, but no cross-region clients are routed to this app server. The following diagram shows the architecture:

When a client tries /negotiate with the app server with a default router, the SDK randomly selects one endpoint from the set of available primary endpoints. When the primary endpoint isn't available, the SDK then randomly selects from all available secondary endpoints. The endpoint is marked as available when the connection between server and the service endpoint is alive.

In a cross-region scenario, when a client tries /negotiate with the app server hosted in East US, by default it always returns the primary endpoint located in the same region. When all East US endpoints aren't available, the router redirects the client to endpoints in other regions. The following failover section describes the scenario in detail.

Failover

When no primary endpoint is available, the client's /negotiate picks from the available secondary endpoints. This failover mechanism requires that each endpoint serves as a primary endpoint to at least one app server.

Next steps

You can use multiple endpoints in high availability and disaster recovery scenarios.

Setup SignalR Service for disaster recovery and high availability