# Using NServiceBus in Azure Functions (isolated worker)


This sample shows how to host NServiceBus within an Azure Function. In this case, a function is triggered by an incoming Azure Service Bus message. This enables hosting message handlers in Azure Functions, gaining the abstraction of message handlers implemented using `IHandleMessages<T>` and taking advantage of NServiceBus's extensible message-processing pipeline.

When hosting NServiceBus within an Azure Function, the function hosts an NServiceBus endpoint that is capable of processing different message types.



## Prerequisites

### Create queue

To use the sample, the endpoint queue must be created. It can be done manually in the Azure portal, or with the [Azure CLI](https://learn.microsoft.com/en-us/cli/azure/install-azure-cli). In this sample, the queue name is `ASBWorkerEndpoint`.

To create the queue with the Azure CLI, execute the following Azure CLI command:

```
az servicebus queue create --name ASBWorkerEndpoint --namespace-name <asb-namespace-to-use> --resource-group <resource-group-containing-namespace>
```

The queue can also be automatically created by adding a call to the `configuration.AdvancedConfiguration.EnableInstallers()` method.

### Configure Connection string

To use the sample, a valid Azure Service Bus connection string must be provided in the `local.settings.json` file.

## Sample structure

The sample contains the following projects:
- `AzureFunctions.ASBTrigger.Worker` - The Azure Functions host and NServiceBus endpoint
- `AzureFunctions.Messages` - message definitions

## Running the sample

The Functions project contains two functions:
1. An auto-generated Service Bus-triggered function. This function is generated by the `NServiceBusTriggerFunction` attribute in the `Program.cs` file.
1. An HTTP-triggered function to send a message to the Azure Service Bus queue.

Running the sample will launch the **Azure Functions runtime** window.

To try the Azure Function:

1. Open a browser and navigate to `http://localhost:7071/api/HttpSender`. The port number might be different and will be indicated when the function project is started.
1. The queue-triggered function will receive the `TriggerMessage` and process it with NServiceBus.
1. The NServiceBus message handler for `TriggerMessage` sends a `FollowUpMessage`.
1. The queue-triggered function will receive the `FollowUpMessage` and process it with NServiceBus.

## Code walk-through

The NServiceBus endpoint is configured as follows:

<!-- snippet: configuration-with-function-host-builder -->

```cs
[assembly: NServiceBusTriggerFunction("ASBWorkerEndpoint")]

var builder = FunctionsApplication.CreateBuilder(args);

builder.AddNServiceBus();

var host = builder.Build();

await host.RunAsync();
```

<!-- endsnippet -->

Note that `NServiceBusTriggerFunction` is used to automatically generate the Azure Functions trigger code needed to invoke NServiceBus.

### Handlers

These are the message handlers:

<!-- snippet: TriggerMessageHandler -->

```cs
public class TriggerMessageHandler(ILogger<TriggerMessageHandler> logger) : IHandleMessages<TriggerMessage>
{
    public Task Handle(TriggerMessage message, IMessageHandlerContext context)
    {
        logger.LogWarning("Handling {MessageType} in {HandlerType}", nameof(TriggerMessage), nameof(TriggerMessageHandler));

        return context.SendLocal(new FollowupMessage());
    }
}
```

<!-- endsnippet -->

<!-- snippet: FollowupMessageHandler -->

```cs
public class FollowupMessageHandler(ILogger<FollowupMessageHandler> logger) : IHandleMessages<FollowupMessage>
{
    public Task Handle(FollowupMessage message, IMessageHandlerContext context)
    {
        logger.LogWarning("Handling {MessageType} in {HandlerType}.", nameof(FollowupMessage), nameof(FollowupMessageHandler));

        return Task.CompletedTask;
    }
}
```

<!-- endsnippet -->