Migrating to Docker

So, I was thinking. What's the alternative here to having this big cost increase? The previous App Service Plan was running on a Windows machine. And there's nothing wrong with that. But a Windows machine costs a lot more than a Linux machine, which provides the same functionality anyway. We'll come to the creation of the Linux App Service Plan later.

So, how do we handle migrating the ASP.NET website to Docker? Well, it's actually pretty easy. First of all, you need to install Docker on your machine. Since I'm working on a Windows machine, I installed Docker Desktop for Windows. You can find the software to download here.

Docker is integrated in Visual Studio, so that makes thing a lot easier.

I worked with Docker in the past, but only with the command line version. It seems now that the version at the time of writing this article, version 4.14.1, also has a GUI. Also the subscription service had changed, but it's still free from small companies, so no worries.

Ok, now that we've got that out of the way, the next step is Dockerizing the application so we can deploy it to the App Service. For that, we need a Docker file. When I generated the ASP.NET website from the Visual Studio templates, I didn't check the Enable Docker Support checkbox. So there's no Docker file present in the project. But no worries, there's no need to start all over again.

• Just right-click on the Web Application project in your Solution
• Add, Docker Support
• Select Linux

The default Docker file is added to your project. For now, I'm going to leave it as is.

Visual Studio also supports building the Docker image. Just right-click the Dockerfile in your project and select Build Docker Image. It's that easy. Check the output window in Visual Studio to see the build steps and the actual command that was executed. If your prefer, or in case it's required, you can still execute your own Docker commands in the Command Line. For now I'm happy with the results.

Although you can use the Docker Desktop app for everything Docker related, most operations are also available in Visual Studio. Open the Containers window by navigating the main menu , View, Other Windows, Containers. This way you can see all your local images as well as remote images in Azure for example.

Running the container is also integrated in Visual Studio, by just hitting the Play button on the Docker option. And voila, you can debug the Docker container. So cool.

Now that we have a way to generate and to run the Docker images, there's also a need to publish them remotely. You can publish them in several places like Docker Hub or in Azure Container Registry, which I'll be using from here on.

Azure Container Registry also comes with a cost.

For me, the price will be 5€/month, which I'm willing to pay to store the images for all my (future) applications. One thing to keep in mind is to keep the images as small as possible, beacuase you pay for the storage.

In the article Publishing the ASP.NET website to Azure, we saw how to publish an ASP.NET to and Azure App Service hosted in Windows. Now we're going to create a new Publish Profile to publish the Docker image to the Azure Container Registry. This will be used afterwards to deploy the image to the designated App Service.

The following steps were taken:

1. In Visual Studio, navigate to the Web Application project in the Solution Explorer and right-click it. Select the option Publish in the context menu.
2. You are presented several ways of publishing the application. This time I'm going to publish a Docker image, so I selected Docker Container Registry.
   
3. As specific target, select Azure Container Registry.
   
4. In the App Service section, select the target Subscription. Make sure you're signed in into an Azure account.
5. Select Resource group in the View control and you will see your Container Registry instances appear in the list. Select your Container Registry you wish to publish the website to. I selected the previously created Container Registry.
   
   Select Finish to create the Publish Profile.
6. You get a detailed overview of Publish Profile. Notice that the Image tag property is set to latest. I'll come back to that later. I'm leaving everything as is for now.
   

Click Publish. The image is built and then afterwards published to the Azure Container Registry. Notice that the image is not directly pushed to the Registry itself, but into a repository.

The name of this repository has been chosen by Visual Studio. For now I'm ok with that.

Click the repository to check it out. It now contains the image with the tag latest we just published.

All is nice and well, but this doesn't accomplish much. The image is just sitting there, being idle. The next thing we need to do is host the image in an App Service.

In one of the pervious articles Hosting the ASP.NET website in Azure, we saw how to host an ASP.NET website in an Azure App Service on a Windows App Service Plan. This time we are going to host our application in a Docker container on a Linux App Service. And as you can guess, a Linux App Service doesn't run on a Windows App Service Plan. So there's nothing else we can do than delete the previously created App Service and App Service Plan that goes with it. And then we start all over again.

We need to create a new App Service and App Service Plan to host the Dockerized application. The process is similar to the one in the Hosting the ASP.NET website in Azure. But take the following steps:

An error will probably arise when trying to create the App Service. The following message is shown:
Cannot perform credential operations for /subscriptions/{YOUR_SUBSCRIPTION}/resourceGroups/{YOUR_RESOURCEGROUP}/providers/Microsoft.ContainerRegistry/registries/{YOUR_REGISTRY} as admin user is disabled. Kindly enable admin user as per docs: https://docs.microsoft.com/en-us/azure/container-registry/container-registry-authentication#admin-account

To be able to deploy a Docker image from the Azure Container Registry, the App Service needs to have permissions to access the ACR. The quick and dirty solution to allow this is to enable an admin user as follows:

1. Navigate to your Azure Container Registry instance
2. In the Settings section, select Acces keys
3. There you will find a checkbox Admin user, enable it.
   
4. Reproduce the previous steps to create the Web App. This time, it should work. The Web App Service is created, the image spins up and the website works correctly.

Later we will see how to allow the App Service to have access to the ACR without the Admin user option. For now, we will keep it as is. But notice that the Docker Registry settings are now stored in the App Service Configuration in the Settings section:

Beacuase we deleted the previously created Windows App Service, we lost all the changes we made to have a Custom Domain and TL Certificate. So these steps need to be reproduced again. Check out the articles Map a custom domain to your App Service and Secure the domain using a TLS certificate again. There's no difference to configuring these in either a Windows or Linux App Service.

Ok, my website is up and running again, just the way it was before. This time in a Linux App Service using a Docker image instead of a regular web application running in IIS on a Windows machine. But there's still something bothering me. When I publish the image from Visual Studio to the Azure Container Registry, it doesn't really do anything to the App Service. The App Service is not aware that a new version of the image has been pushed. This means that the App Service needs to be restarted in order for the new image to be spun up when the App Service starts. That's unacceptable when I deploy my website.

When you push an image to an Azure Container Registry, a container needs to be actively spun up in the App Service using this new image. The mechanism for this action is called Continuous Deployment. There are several ways of making sure this happens automatically. For now I'm going to describe the easiest way for the simple use case of my website. I activated this as follows:

• Navigate to your App Service. In my case this is 'random-awesome-memory'
• In the left menu Deployment section, navigate to Deployment Center
• In the default selected Settings tab, you will see the Docker settings we configured earlier when creating the Web App Service. At the bottom, there's a setting called Continuous deployment, make sure to check the On value. Don't forget to save the changes in the top menu.

When the Continuous Deployment is activate and saved, a webhook is created in your Container Registry. Navigate to your Azure Container Registry instance, then select Webhooks in the left menu in the Services section. You will get an overview of all the webhooks configured in the Container Registry. In my case, there's only one and that's the one we just activated in the Continuous Deployment.

Navigate further down the list to see more details of the webhook. In short, what it does is, it reacts to the push action of the image defined in the scope, which in my case is the image with the tag latest. This can be configured more granularly but for now I'm going to leave it the ways it is. If you want to dig any further, check out the details of the webhook's request and response objects.

So, now that the Continuous Deployment has been configured, the App Service will restart automatically each time a new image has been pushed into the Container Registry.

I was able to get rid of the costly B1 Basic App Service plan in Windows by replacing it with the B1 Basic plan in Linux. The cost was reduced from 46€/month to 11€/month. An additional cost of 5€/month was also added for the Azure Container Registry. So the total cost of running the website is 16€/month. But, beacuase I scaled the App Service Plan to Basic B1, I'm also able to add additional applications or websites to it without extra charges. Also the Azure Container Registry is not just limited to this website. It will be used to store images for other websites and applications as well.

• Docker - Get started with Docker for Windows
• Azure Container Registry Documentation - Azure Container Registry
• Azure Container Registry pricing Documentation - Container Registry pricing.
• Azure Container Registry Documentation - Azure Container Registry service tiers
• Azure Pricing Documentation - App Service pricing
• Azure App Service Documentation - App Service on Linux
• Visual Studio Documentation - Container Tools in Visual Studio
• Visual Studio Documentation - How to customize Docker containers in Visual Studio
• Azure App Service Documentation - Migrate custom software to Azure App Service using a custom container
• Azure App Service Documentation - Continuous deployment with custom containers in Azure App Service