Friday, March 16, 2018

Dockerising both ASP.NET Core 2.0 and SQL Server Express in Windows containers

In a previous post I showed how you can have an ASP.NET Core 2.0 application work with a containerized SQL Server Express database server running in a Windows container. In this post I will take this one step further by also containerizing the ASP.NET Core 2.0 application.

Please go through my previous post before continuing with this tutorial.

We will generate the release version of the application by executing the following command from a terminal window in the root directory of your ASP.NET 2.0 project:

dotnet publish --framework netcoreapp2.0 --configuration Release --output dist

The above command instructs the dotnet utility to produce the release version of the application in the dist directory. This results in output similar to the following:

Microsoft (R) Build Engine version for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 340.14 ms for E:\_aspnet\_docker\AspCoreSqlExpress\AspCoreSqlExpress.csproj.
  Restore completed in 340.57 ms for E:\_aspnet\_docker\AspCoreSqlExpress\AspCoreSqlExpress.csproj.
  Restore completed in 339.79 ms for E:\_aspnet\_docker\AspCoreSqlExpress\AspCoreSqlExpress.csproj.
  Restore completed in 346.67 ms for E:\_aspnet\_docker\AspCoreSqlExpress\AspCoreSqlExpress.csproj.
  AspCoreSqlExpress -> E:\_aspnet\_docker\AspCoreSqlExpress\bin\Debug\netcoreapp2.0\AspCoreSqlExpress.dll
  AspCoreSqlExpress -> E:\_aspnet\_docker\AspCoreSqlExpress\dist\

If you inspect the dist directory, you will see content similar to the following:

The highlighted file in the above folder screen capture is my main DLL file that is the entry point into the web application.

Let us run the release version of the web application. To do this, change directory to the dist directory with the following terminal instruction:

cd dist

You can then run your main DLL file. In my case, this file is AspCoreSqlExpress.dll. I executed the following command:

dotnet AspCoreSqlExpress.dll

This generates an understandable error, if the SQL Server Express container is not running, because the application is unable to connect to a database:

Application startup exception: System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server) ---> System.ComponentModel.Win32Exception (0x80004005): The network path was not found

Contents of the /dist folder give us a good idea about ASP.NET Core 2.0 artifacts that need to be copied into a container. We shall simply copy contents of the dist directory into a Docker container that has the .NET Core 2.0 runtime.

Return to the root directory of your project by typing the following in a terminal window:

cd ..

Code in Startup.cs will need to be modified so that we can pass environment variables into the ASP.NET Core application. Therefore, open Startup.cs in your favorite editor and find method ConfigureServices(). Comment out (or delete) the following code:

services.AddDbContext<ApplicationDbContext>(options =>

Replaced the above code with:

var host = Configuration["DBHOST"] ?? "localhost";
var db = Configuration["DBNAME"] ?? "AspCoreSqlExpress";
var port = Configuration["DBPORT"] ?? "1433";
var username = Configuration["DBUSERNAME"] ?? "sa";
var password = Configuration["DBPASSWORD"] ?? "Sql!Expre55";

string connStr = $"Data Source={host},{port};Integrated Security=False;";
connStr += $"User ID={username};Password={password};Database={db};";
connStr += $"Connect Timeout=30;Encrypt=False;TrustServerCertificate=True;ApplicationIntent=ReadWrite;MultiSubnetFailover=False";

services.AddDbContext<ApplicationDbContext>(options =>

Five environment variables are used in the database connection string. These are: DBHOST, DBNAME, DBPORT, DBUSERNAME and DBPASSWORD. If these environment variables are not found then they will take up default values: localhost, AspCoreSqlExpress, 1433, sa and Sql!Expre55 respectively.

We need to create a Windows docker image that will contain the .NET Core 2.0 runtime. A suitable image for this purpose is: microsoft/aspnetcore:2.0-nanoserver-sac2016

Create a text file named and add to it the following content:

FROM microsoft/aspnetcore:2.0-nanoserver-sac2016
COPY dist /app
EXPOSE 80/tcp
ENTRYPOINT ["dotnet", "AspCoreSqlExpress.dll"]

Above are instructions to create a Windows Docker image that will contain our ASP.NET Core 2.0 application. I explain each line below:

FROM microsoft/aspnetcore:2.0-nanoserver-sac2016 Base Windows image microsoft/aspnetcore:2.0-nanoserver-sac2016 will be used
COPY dist /app Contents of the dist directory on the host computer will be copied to directory /app in the container
WORKDIR /app The working directory in the container is /app
EXPOSE 80/tcp Port 80 will be exposed in the container
ENV ASPNETCORE_URLS http://+:80 Expose port 80 for the Web API traffic
ENTRYPOINT ["dotnet", "AspCoreSqlExpress.dll"] The main ASP.NET Core 2.0 web application will be launched by executing "dotnet AspCore4Docker.dll"

There is a risk that the web application container (mvc) starts earlier than the SQL Server container. We need to figure out a way to have the web application wait for the database server to be ready. We can use a PowerShell script to accomplish this task. Create a PowerShell file named wait4port1433.ps1 in the root of the ASP.NET Core 2.0 web application. Add to it the following script:

do {
  Write-Host "waiting..."
  sleep 3      
} until(Test-NetConnection db -Port 1433 | ? { $_.TcpTestSucceeded } )

dotnet AspCoreSqlExpress.dll

The above PowerShell script waits until port 1433 is available on the db container. Once the port is available then it will start the web application by running dotnet demo-docker.dll.

Open the .csproj file and add this markup just before the closing </Project> tag:

   <Content Include="wait4port1433.ps1" CopyToOutputDirectory="Always" />

The above markup ensures that the PowerShell script file is published together with all other required files into the dist folder. After saving the .csproj file, remove the dist folder and re-publish your web application by running the following pair of commands from a terminal window:

rmdir dist /S /Q
dotnet publish --framework netcoreapp2.0 --configuration Release --output dist

We will next compose a docker yml file that orchestrates the entire system which involves two Windows containers: a SQL Server Express database container and a container that holds our web application. In the root folder of your application, create a text file named docker-compose.yml and add to it the following content:

version: '3'

    image: microsoft/mssql-server-windows-express:2016
       - sqldatavol:c:\sqldata
    restart: always
      - aspsqlnet
      - sa_password=Sql!Expre55

      context: .
      - db
      - aspsqlnet
      - "8888:80"
    #restart: always
      - DBHOST=db
      - DBNAME=sql-docker
      - DBPORT=1433
      - DBUSERNAME=sa
      - DBPASSWORD=Sql!Expre55
    entrypoint: powershell "c:\app\wait4port1433.ps1"



Below is an explanation of what this file does.

We will have two Windows containers. Each container is considered to be a service. The first service is named db and will host SQL Server Express. The second service is named mvc and will host our ASP.NET Core 2.0 web application.

The most current version of docker-compose is version 3. This is the first line in our docker-compose.yml file.

The SQL Server Express Container

Image microsoft/mssql-server-windows-express:2016 will be used for the SQL Server Express container.

A folder on the host machine named sqldatavol is mapped to folder c:\sqldata in the container. This is so that data is persisted on the host machine in case the container is destroyed as we do not want to lose valuable data.

restart: always is so that if the container stops, it is automatically restarted.

The db container will exist in a virtual network named aspsqlnet.

The sa password will be Sql!Expre55 when SQL Server Express is configured. This is set by the sa_password environment variable. Environment variable ACCEPT_EULA pertains to acceptance of the End-User Licensing Agreement, which is a required setting for the SQL Server image.

The ASP.NET Core 2.0 Web Application Container

The container will be built using the instructions in the file and the context used is the current directory.

depends_on indicates that the web application relies on the SQL Server Express container (db) to properly function.

The mvc container will also exist in a virtual network named aspsqlnet.

Port 80 in the mvc container is mapped to port 8888 on the host computer.

The environment variables needed by the web app are:

DBHOST pointing to the SQL Server Express service
- DBNAME is the name we wish to give to the database
- DBPORT is the port number that SQL Server Express is listening on
- DBUSERNAME is the database server user-name. This is set to the admin user-name 'sa'
- DBPASSWORD is the password for user-name 'sa'.
- ASPNETCORE_ENVIRONMENT is set to Development. In reality, you should change this to Production once you determine that your web application container works as expected.

We override the entry point declared in This entry point calls the PowerShell script file wait4port1433.ps1 that waits for port 1433 to exist. Once it becomes available, it then starts the web application by running dotnet demo-docker.dll.

Running the yml file

To find out if this all works, go to a terminal window in the root folder of the ASP.NET Core 2.0 application and run the following command:

docker-compose up

Point your browser to http://localhost:8888/ and you should see the main web page. To ensure that the database works properly, register a user by clicking on the Register link in the top right corner.

In my case, I received confirmation that a user was indeed registered:

As you can see in the top-right corner, the user with email a@a.a has been successfully registered.

To shutdown the two containers container, go to a terminal window in the root folder of the ASP.NET Core 2.0 application and run the following command:

docker-compose down

This opens up a whole new world for containerizing your ASP.NET Core 2.0 web applications in Windows Docker containers.


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