Unit and Integration Test for Azure Function with ServiceBusTrigger

Question

I have an Azure Function which is triggered by an Azure Service Bus Queue.

The function is below.

1. How this Run method can be unit tested?
2. And how an integration test can be done by starting with AddContact trigger, checking the logic in the method and the data being sent to a blob using the output binding?

    public static class AddContactFunction
    {
        [FunctionName("AddContactFunction")]
        public static void Run([ServiceBusTrigger("AddContact", Connection = "AddContactFunctionConnectionString")]string myQueueItem, ILogger log)
        {
            log.LogInformation($"C# ServiceBus queue trigger function processed message: {myQueueItem}");
        }
    }

Answer 1

I had the exact same doubts. Adding Unit Tests is not too complicated, at the end of the day, its a function, so all we got to do is to call the Azure Function with the correct string, for parameter string myQueueItem.

Adding Integration tests needs some additional ground work. In the Github project, the author uses the TestFunctionHost class from Azure/azure-functions-host project.

I tried following this strategy, but the amount of code needed to setup all these is uncomfortably high for my liking. Not a lot of it is well documented, and some of the stuff needs developers to use Azure App Services myGet feed.

I wanted a simpler approach, and thankfully I found one. Azure Functions is built on top of the Azure WebJobs SDK package, and leverages its JobHost class to run. So in our integration tests, all we need to do, is to setup this Host, and tell it where to look for the Azure Functions to load and run.

IHost host = new HostBuilder()
        .ConfigureWebJobs()
        .ConfigureDefaultTestHost<CLASS_CONTAINING_THE_AZURE_FUNCTIONS>(webjobsBuilder => {
            webjobsBuilder.AddAzureStorage();
            webjobsBuilder.AddServiceBus();
        })
        .ConfigureServices(services => {
            services.AddSingleton<INameResolver>(resolver);
        })
        .Build();

using (host) {
    await host.StartAsync();
    // ..
}
...

Once this is done, we can send messages to ServiceBus and our Azure Functions will get triggered. Once can even set breakpoints in the Functions getting tested and debug issues!

I have blogged about the whole process here and I have also created a github repository at this link, to showcase test driven development with Azure Functions.

Answer 2

How this Run method can be unit tested?

The method is a static public method. You can unit test it by invoking the static method AddContactFunction.Run(/* parameters /*); You will not need a Service Bus namespace or a message for that matter as your function expects to receive a string from the SDK. Which you can provide and verify the logic works as expected.

And how an integration test can be done by starting with AddContact trigger, checking the logic in the method and the data being sent to a blob using the output binding?

This would be a much more sophisticated scenario. This would require to run Functions runtime and generate a real Service Bus message to trigger the functions as well as validate that the blob was written. There's no integration/end-to-end testing framework that is shipped with Functions and you'd need to come up with something custom. Azure Functions Core Tools could be helpful to achieve that.