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Showing content from https://learn.microsoft.com/azure/azure-functions/functions-bindings-signalr-service-trigger below:

Azure Functions SignalR Service trigger binding

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:

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

You can create a C# function by using one of the following C# modes:

• Isolated worker model: Compiled C# function that runs in a worker process that's isolated from the runtime. An isolated worker process is required to support C# functions running on long-term support (LTS) and non-LTS versions for .NET and the .NET Framework.
• In-process model: Compiled C# function that runs in the same process as the Azure Functions runtime.
• C# script: Used primarily when you create C# functions in the Azure portal.

• Isolated worker model
• In-process model

The following sample shows a C# function that receives a message event from clients and logs the message content.

[Function(nameof(OnClientMessage))]
public static void OnClientMessage(
    [SignalRTrigger("Hub", "messages", "sendMessage", "content", ConnectionStringSetting = "SignalRConnection")]
        SignalRInvocationContext invocationContext, string content, FunctionContext functionContext)
{
    var logger = functionContext.GetLogger(nameof(OnClientMessage));
    logger.LogInformation("Connection {connectionId} sent a message. Message content: {content}", invocationContext.ConnectionId, content);
}

Important

Class based model of SignalR Service bindings in C# isolated worker doesn't optimize how you write SignalR triggers due to the limitation of C# worker model. For more information about class based model, see Class based model.

SignalR Service trigger binding for C# in-process model has two programming models. Class based model and traditional model. Class based model provides a consistent SignalR server-side programming experience. Traditional model provides more flexibility and is similar to other function bindings.

See Class based model for details.

public class HubName1 : ServerlessHub
{
    [FunctionName("SignalRTest")]
    public Task SendMessage([SignalRTrigger]InvocationContext invocationContext, string message, ILogger logger)
    {
        logger.LogInformation($"Receive {message} from {invocationContext.ConnectionId}.");
    }
}

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

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

Because it can be hard to use ParameterNames in the trigger, the following example shows you how to use the SignalRParameter attribute to define the message attribute.

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

SignalR trigger isn't currently supported for Java.

Here's binding data in the function.json file:

{
    "type": "signalRTrigger",
    "name": "invocation",
    "hubName": "hubName1",
    "category": "messages",
    "event": "SendMessage",
    "parameterNames": [
        "message"
    ],
    "direction": "in"
}

• Model v4
• Model v3

app.generic("function1",
    {
        trigger: { "type": "signalRTrigger", "name": "invocation", "direction": "in", "hubName": "hubName1", "event": "SendMessage", "category": "messages" },
        handler: (triggerInput, context) => {
            context.log(`Receive ${triggerInput.Arguments[0]} from ${triggerInput.ConnectionId}.`)
        }
    })

Here's binding data in the function.json file:

{
    "type": "signalRTrigger",
    "name": "invocation",
    "hubName": "hubName1",
    "category": "messages",
    "event": "SendMessage",
    "parameterNames": [
        "message"
    ],
    "direction": "in"
}

Here's the JavaScript code:

module.exports = function (context, invocation) {
    context.log(`Receive ${context.bindingData.message} from ${invocation.ConnectionId}.`)
};

Complete PowerShell examples are pending.

Here's the Python code:

import logging
import json
import azure.functions as func

def main(invocation) -> None:
    invocation_json = json.loads(invocation)
    logging.info("Receive {0} from {1}".format(invocation_json['Arguments'][0], invocation_json['ConnectionId']))

Both in-process and isolated worker process C# libraries use the SignalRTrigger attribute to define the function. C# script instead uses a function.json configuration file.

• Isolated worker model
• In-process model

The following table explains the properties of the SignalRTrigger attribute.

• connections: Including connected and disconnected events
• messages: Including all other events except those in connections category

The following table explains the properties of the SignalRTrigger attribute.

• connections: Including connected and disconnected events
• messages: Including all other events except those in connections category

There isn't currently a supported Java annotation for a SignalR trigger.

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

• connections: Including connected and disconnected events
• messages: Including all other events except those in connections category

See the Example section for complete examples.

For optimal security, your function app should use managed identities when connecting to the Azure SignalR service instead of using a connection string, which contains a shared secret key. For more information, see Authorize requests to Azure SignalR Service resources with Microsoft Entra managed identities.

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 sent from a SignalR service, which includes the following properties:

The property ParameterNames in SignalRTrigger lets you bind arguments of invocation messages to the parameters of functions. You can use the name you defined 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 names you defined correspond to the arguments sent on the client side.

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

Then, the arg1 contains the content of message1, and arg2 contains the content of message2.

For the parameter binding, the order matters. If you're using ParameterNames, the order in ParameterNames matches the order of the arguments you invoke in the client. If you're 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'll get an exception.

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.

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

<Function_App_URL>/runtime/webhooks/signalr?code=<API_KEY>

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.

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.

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

• Azure Functions development and configuration with Azure SignalR Service
• SignalR Service Trigger binding sample
• SignalR Service Trigger binding sample in isolated worker process

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