SignalR Service trigger binding for Azure Functions

Use the SignalR trigger binding to respond to messages sent from Azure SignalR Service. When function is triggered, messages passed to the function is parsed as a json object.

In SignalR Service serverless mode, SignalR Service uses the Upstream feature to send messages from client to Function App. And Function App uses SignalR Service trigger binding to handle these messages. The general architecture is shown below: SignalR Trigger Architecture

For information on setup and configuration details, see the overview.


The following example shows a function that receives a message using the trigger binding and log the message.

SignalR Service trigger binding for C# has two programming models. Class based model and traditional model. Class based model can provide a consistent SignalR server-side programming experience. And traditional model provides more flexibility and similar with other function bindings.

With Class based model

See Class based model for details.

public class SignalRTestHub : ServerlessHub
    public async Task SendMessage([SignalRTrigger]InvocationContext invocationContext, string message, ILogger logger)
        logger.LogInformation($"Receive {message} from {invocationContext.ConnectionId}.");

With Traditional model

Traditional model obeys the convention of Azure Function developed by C#. If you're not familiar with it, you can learn from documents.

public static async Task Run([SignalRTrigger("SignalRTest", "messages", "SendMessage", parameterNames: new string[] {"message"})]InvocationContext invocationContext, string message, ILogger logger)
    logger.LogInformation($"Receive {message} from {invocationContext.ConnectionId}.");

Use attribute [SignalRParameter] to simplify ParameterNames

As it's bit cumbersome to use ParameterNames, SignalRParameter is provided to achieve the same purpose.

public static async Task Run([SignalRTrigger("SignalRTest", "messages", "SendMessage")]InvocationContext invocationContext, [SignalRParameter]string message, ILogger logger)
    logger.LogInformation($"Receive {message} from {invocationContext.ConnectionId}.");



The following table explains the binding configuration properties that you set in the function.json file and the SignalRTrigger attribute.

function.json property Attribute property Description
type n/a Must be set to SignalRTrigger.
direction n/a Must be set to in.
name n/a Variable name used in function code for trigger invocation context object.
hubName HubName This value must be set to the name of the SignalR hub for the function to be triggered.
category Category This value must be set as the category of messages for the function to be triggered. The category can be one of the following values:
  • connections: Including connected and disconnected events
  • messages: Including all other events except those in connections category
event Event This value must be set as the event of messages for the function to be triggered. For messages category, event is the target in invocation message that clients send. For connections category, only connected and disconnected is used.
parameterNames ParameterNames (Optional) A list of names that binds to the parameters.
connectionStringSetting ConnectionStringSetting The name of the app setting that contains the SignalR Service connection string (defaults to "AzureSignalRConnectionString")


The trigger input type is declared as either InvocationContext or a custom type. If you choose InvocationContext you get full access to the request content. For a custom type, the runtime tries to parse the JSON request body to set the object properties.


InvocationContext contains all the content in the message send from SignalR Service.

Property in InvocationContext Description
Arguments Available for messages category. Contains arguments in invocation message
Error Available for disconnected event. It can be Empty if the connection closed with no error, or it contains the error messages.
Hub The hub name which the message belongs to.
Category The category of the message.
Event The event of the message.
ConnectionId The connection ID of the client which sends the message.
UserId The user identity of the client which sends the message.
Headers The headers of the request.
Query The query of the request when clients connect to the service.
Claims The claims of the client.

Using ParameterNames

The property ParameterNames in SignalRTrigger allows you to bind arguments of invocation messages to the parameters of functions. The name you defined can be used as part of binding expressions in other binding or as parameters in your code. That gives you a more convenient way to access arguments of InvocationContext.

Say you have a JavaScript SignalR client trying to invoke method broadcast in Azure Function with two arguments message1, message2.

await connection.invoke("broadcast", message1, message2);

After you set parameterNames, the name you defined will respectively correspond to the arguments sent on the client side.

[SignalRTrigger(parameterNames: new string[] {"arg1, arg2"})]

Then, the arg1 will contain the content of message1, and arg2 will contain the content of message2.


For the parameter binding, the order matters. If you are using ParameterNames, the order in ParameterNames matches the order of the arguments you invoke in the client. If you are using attribute [SignalRParameter] in C#, the order of arguments in Azure Function methods matches the order of arguments in clients.

ParameterNames and attribute [SignalRParameter] cannot be used at the same time, or you will get an exception.

SignalR Service integration

SignalR Service needs a URL to access Function App when you're using SignalR Service trigger binding. The URL should be configured in Upstream Settings on the SignalR Service side.

Upstream Portal

When using SignalR Service trigger, the URL can be simple and formatted as shown below:


The Function_App_URL can be found on Function App's Overview page and The API_KEY is generated by Azure Function. You can get the API_KEY from signalr_extension in the App keys blade of Function App. API key

If you want to use more than one Function App together with one SignalR Service, upstream can also support complex routing rules. Find more details at Upstream settings.

Step by step sample

You can follow the sample in GitHub to deploy a chat room on Function App with SignalR Service trigger binding and upstream feature: Bidirectional chat room sample

Next steps