Background processing with .NET

Many apps need to do background processing outside of the context of a web request. This tutorial creates a web app that lets users input text to translate, and then displays a list of previous translations. The translation is done in a background process to avoid blocking the user's request.

The following diagram illustrates the translation request process.

Here is the sequence of events for how the tutorial app works:

Visit the web page to see a list of previous translations, stored in Firestore.
Request a translation of text by entering an HTML form.
The translation request is published to Pub/Sub.
A Cloud Run service subscribed to that Pub/Sub topic is triggered.
The Cloud Run service uses Cloud Translation to translate the text.
The Cloud Run service stores the result in Firestore.

This tutorial is intended for anyone who is interested in learning about background processing with Google Cloud. No prior experience is required with Pub/Sub, Firestore, App Engine, or Cloud Run functions. However, to understand all of the code, some experience with .NET, JavaScript, and HTML is helpful.

Preparing the app

In your terminal window, clone the sample app repository to your local machine:
```
git clone https://github.com/GoogleCloudPlatform/getting-started-dotnet.git
```
Alternatively, you can download the sample as a zip file and extract it.
Change to the directory that contains the background task sample code:
```
cd getting-started-dotnet/BackgroundProcessing
```

Understanding the `TranslateWorker` service

The service starts by importing several dependencies like Firestore and Translation.
```
using Google.Cloud.Firestore;
using Google.Cloud.Translation.V2;
```

The Firestore and Translation clients are initialized so they can be reused between handler invocations. That way, you don't have to initialize new clients for every invocation, which would slow down execution.

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<FirestoreDb>(provider =>
        FirestoreDb.Create(GetFirestoreProjectId()));
    services.AddSingleton<TranslationClient>(
        TranslationClient.Create());
    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

The Translation API translates the string to the language you selected.

var result = await _translator.TranslateTextAsync(sourceText, "es");

The controller's constructor receives the Firestore and Pub/Sub clients.

The Post method parses the Pub/Sub message to get the text to translate. It uses the message ID as a unique name for the translation request to make sure it doesn't store any duplicate translations.

using Google.Cloud.Firestore;
using Google.Cloud.Translation.V2;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;

namespace TranslateWorker.Controllers
{
    /// <summary>
    /// The message Pubsub posts to our controller.
    /// </summary>
    public class PostMessage
    {
        public PubsubMessage message { get; set; }
        public string subscription { get; set; }
    }

    /// <summary>
    /// Pubsub's inner message.
    /// </summary>
    public class PubsubMessage
    {
        public string data { get; set; }
        public string messageId { get; set; }
        public Dictionary<string, string> attributes { get; set; }
    }


    [Route("api/[controller]")]
    [ApiController]
    public class TranslateController : ControllerBase
    {
        private readonly ILogger<TranslateController> _logger;
        private readonly FirestoreDb _firestore;
        private readonly TranslationClient _translator;
        // The Firestore collection where we store translations.
        private readonly CollectionReference _translations;

        public TranslateController(ILogger<TranslateController> logger,
            FirestoreDb firestore,
            TranslationClient translator)
        {
            _logger = logger ?? throw new ArgumentNullException(nameof(logger));
            _firestore = firestore ?? throw new ArgumentNullException(
                nameof(firestore));
            _translator = translator ?? throw new ArgumentNullException(
                nameof(translator));
            _translations = _firestore.Collection("Translations");
        }

        /// <summary>
        /// Handle a posted message from Pubsub.
        /// </summary>
        /// <param name="request">The message Pubsub posts to this process.</param>
        /// <returns>NoContent on success.</returns>
        [HttpPost]
        public async Task<IActionResult> Post([FromBody] PostMessage request)
        {
            // Unpack the message from Pubsub.
            string sourceText;
            try
            {
                byte[] data = Convert.FromBase64String(request.message.data);
                sourceText = Encoding.UTF8.GetString(data);
            }
            catch (Exception e)
            {
                _logger.LogError(1, e, "Bad request");
                return BadRequest();
            }
            // Translate the source text.
            _logger.LogDebug(2, "Translating {0} to Spanish.", sourceText);
            var result = await _translator.TranslateTextAsync(sourceText, "es");
            // Store the result in Firestore.
            Translation translation = new Translation()
            {
                TimeStamp = DateTime.UtcNow,
                SourceText = sourceText,
                TranslatedText = result.TranslatedText
            };
            _logger.LogDebug(3, "Saving translation {0} to {1}.",
                translation.TranslatedText, _translations.Path);
            await _translations.Document(request.message.messageId)
                .SetAsync(translation);
            // Return a success code.
            return NoContent();
        }

        /// <summary>
        /// Serve a root page so Cloud Run knows this process is healthy.
        /// </summary>
        [Route("/")]
        public IActionResult Index()
        {
            return Content("Serving translate requests...");
        }
    }
}

Deploying the `TranslateWorker` service

In the BackgroundProcessing directory, run the PowerShell script to build and deploy the service to Cloud Run:
```
PublishTo-CloudRun.ps1
```

Understanding the `PublishTo-CloudRun.ps1` script

The PublishTo-CloudRun.ps1 script publishes the service to Cloud Run, and protects the TranslateWorker service from being abused. If the service permitted all incoming connections, then anyone could post translate requests to the controller and thereby incur costs. Therefore, you set up the service to only accept POST requests from Pub/Sub.

The script does the following:

Builds the app locally using dotnet publish.
Builds a container that runs the app using Cloud Build.
Deploys the app to Cloud Run.
Enables the project to create Pub/Sub authentication tokens.
Creates a service account to represent the Pub/Sub subscription identity.
Gives the service account permission to invoke TranslateWorker service.

Creates a Pub/Sub topic and subscription.

# 1. Build the application locally.
dotnet publish -c Release

# Collect some details about the project that we'll need later.
$projectId = gcloud config get-value project
$projectNumber = gcloud projects describe $projectId --format="get(projectNumber)"
$region = "us-central1"

# 2. Use Google Cloud Build to build the worker's container and publish to Google
# Container Registry.
gcloud builds submit --tag gcr.io/$projectId/translate-worker `
    TranslateWorker/bin/Release/netcoreapp2.1/publish

# 3. Run the container with Google Cloud Run.
gcloud beta run deploy translate-worker --region $region --platform managed `
    --image gcr.io/$projectId/translate-worker --no-allow-unauthenticated
$url = gcloud beta run services describe translate-worker --platform managed `
    --region $region --format="get(status.address.hostname)"

# 4. Enable the project to create pubsub authentication tokens.
gcloud projects add-iam-policy-binding $projectId `
     --member=serviceAccount:service-$projectNumber@gcp-sa-pubsub.iam.gserviceaccount.com `
     --role=roles/iam.serviceAccountTokenCreator

# 5. Create a service account to represent the Cloud Pub/Sub subscription identity.
$serviceAccountExists = gcloud iam service-accounts describe `
    cloud-run-pubsub-invoker@$projectId.iam.gserviceaccount.com 2> $null
if (-not $serviceAccountExists) {
    gcloud iam service-accounts create cloud-run-pubsub-invoker `
        --display-name "Cloud Run Pub/Sub Invoker"
}

# 6. For Cloud Run, give this service account permission to invoke 
# translate-worker service.
gcloud beta run services add-iam-policy-binding translate-worker `
     --member=serviceAccount:cloud-run-pubsub-invoker@$projectId.iam.gserviceaccount.com `
     --role=roles/run.invoker --region=$region

# 7. Create a pubsub topic and subscription, if they don't already exist.
$topicExists = gcloud pubsub topics describe translate-requests 2> $null 
if (-not $topicExists) {
    gcloud pubsub topics create translate-requests
}
$subscriptionExists = gcloud pubsub subscriptions describe translate-requests 2> $null
if ($subscriptionExists) {
    gcloud beta pubsub subscriptions modify-push-config translate-requests `
        --push-endpoint $url/api/translate `
        --push-auth-service-account cloud-run-pubsub-invoker@$projectId.iam.gserviceaccount.com
} else {
    gcloud beta pubsub subscriptions create translate-requests `
        --topic translate-requests --push-endpoint $url/api/translate `
        --push-auth-service-account cloud-run-pubsub-invoker@$projectId.iam.gserviceaccount.com
}

Understanding the `TranslateUI` service

The TranslateUI service renders a web page that displays recent translations, and accepts requests for new translations.

The StartUp class configures an ASP.NET app and creates Pub/Sub and Firestore clients.

using Google.Apis.Auth.OAuth2;
using Google.Cloud.Firestore;
using Google.Cloud.PubSub.V1;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using System;
using System.Net.Http;

namespace TranslateUI
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddSingleton<FirestoreDb>(
                provider => FirestoreDb.Create(GetFirestoreProjectId()));
            services.AddSingleton<PublisherClient>(
                provider => PublisherClient.CreateAsync(new TopicName(
                    GetProjectId(), GetTopicName())).Result);
            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();
            app.UseCookiePolicy();

            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });
        }

    }
}

The index handler Index gets all existing translations from Firestore and fills a ViewModel with the list:

using Google.Cloud.Firestore;
using Google.Cloud.PubSub.V1;
using Google.Protobuf;
using Microsoft.AspNetCore.Mvc;
using System.Diagnostics;
using System.Linq;
using System.Threading.Tasks;
using TranslateUI.Models;

namespace TranslateUI.Controllers
{
    public class HomeController : Controller
    {
        private readonly FirestoreDb _firestore;
        private readonly PublisherClient _publisher;
        private CollectionReference _translations;

        public HomeController(FirestoreDb firestore, PublisherClient publisher)
        {
            _firestore = firestore;
            _publisher = publisher;
            _translations = _firestore.Collection("Translations");
        }

        [HttpPost]
        [HttpGet]
        public async Task<IActionResult> Index(string SourceText)
        {
            // Look up the most recent 20 translations.
            var query = _translations.OrderByDescending("TimeStamp")
                .Limit(20);
            var snapshotTask = query.GetSnapshotAsync();

            if (!string.IsNullOrWhiteSpace(SourceText))
            {
                // Submit a new translation request.
                await _publisher.PublishAsync(new PubsubMessage()
                {
                    Data = ByteString.CopyFromUtf8(SourceText)
                });
            }

            // Render the page.
            var model = new HomeViewModel()
            {
                Translations = (await snapshotTask).Documents.Select(
                    doc => doc.ConvertTo<Translation>()).ToList(),
                SourceText = SourceText
            };
            return View(model);
        }

        [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
        public IActionResult Error()
        {
            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
        }
    }
}

New translations are requested by submitting an HTML form. The request translation handler validates the request, and publishes a message to Pub/Sub:

// Submit a new translation request.
await _publisher.PublishAsync(new PubsubMessage()
{
    Data = ByteString.CopyFromUtf8(SourceText)
});

Deploying the `TranslateUI` service

In the BackgroundProcessing directory, run the PowerShell script to build and deploy the service to Cloud Run:
```
./PublishTo-CloudRun.ps1
```

Understanding the `PublishTo-CloudRun.ps1` script

The PublishTo-CloudRun.ps1 script publishes the app to Cloud Run.

The script does the following:

Builds the app locally using dotnet publish.
Builds a container that runs the app by using Cloud Build.

Deploys the app to Cloud Run.

# 1. Build the application locally.
dotnet publish -c Release
# 2. Use Google Cloud Build to build the UI's container and publish to Google
# Container Registry. 
gcloud builds submit --tag gcr.io/$projectId/translate-ui `
    TranslateUI/bin/Release/netcoreapp2.1/publish

# 3. Run the container with Google Cloud Run.
gcloud beta run deploy translate-ui --region $region --platform managed `
    --image gcr.io/$projectId/translate-ui --allow-unauthenticated

Testing the app

After successfully running the PublishTo-CloudRun.ps1 script, try requesting a translation.

The final command in the PublishTo-CloudRun.ps1 script tells you the URL for your UI service. In your terminal window, find the URL for the TranslateUI service:
```
gcloud beta run services describe translate-ui --region $region --format="get(status.address.hostname)"
```
In your browser, go to the URL that you got from the previous step.

There is a page with an empty list of translations and a form to request new translations.
In the Text to translate field, enter some text to translate, for example, Hello, World.
Click Submit.
To refresh the page, click Refresh . There is a new row in the translation list. If you don't see a translation, wait a few more seconds and try again. If you still don't see a translation, see the next section about debugging the app.

Debugging the app

If you cannot connect to your Cloud Run service or don't see new translations, check the following:

Check that the PublishTo-CloudRun.ps1 script successfully completed and didn't output any errors. If there were errors (for example, message=Build failed), fix them, and try running again.
Check for errors in the logs:
1. In the Google Cloud console, go to the Cloud Run page.
  
  Go to Cloud Run page
2. Click the service name, translate-ui.
3. Click Logs.

Background processing with .NET Stay organized with collections Save and categorize content based on your preferences.

Preparing the app

Understanding the TranslateWorker service

Deploying the TranslateWorker service

Understanding the PublishTo-CloudRun.ps1 script

Understanding the TranslateUI service

Deploying the TranslateUI service

Understanding the PublishTo-CloudRun.ps1 script

Testing the app

Debugging the app

Background processing with .NET

Understanding the `TranslateWorker` service

Deploying the `TranslateWorker` service

Understanding the `PublishTo-CloudRun.ps1` script

Understanding the `TranslateUI` service

Deploying the `TranslateUI` service

Understanding the `PublishTo-CloudRun.ps1` script