Modernizing Traditional .NET Framework Applications

Modernizing Traditional .NET Framework Applications

Introduction

Docker containers have long been used to enable the development of new applications leveraging modern application architectural patterns like microservices, but Docker containers are not just for new applications. Traditional or Brownfield applications can also be migrated to containers and Docker Enterprise Edition to take advantage of the benefits that Docker Enterprise provides.

What You Will Learn

This reference architecture provides guidance and examples for modernizing traditional .NET Framework applications to Docker Enterprise Edition. You will learn to identify the types of .NET Framework applications that are good candidates for containerization, the “lift-and-shift” approach to containerization with little to no code changes, how to get started, and guidance around various .NET Framework applications and Windows Server containers, including handling Windows Integrated Authentication, networking, logging, and monitoring.

This document focuses primarily on custom .NET Framework applications. It does not cover commercial off-the-shelf (COTS) .NET Framework applications such as SharePoint and Sitecore. Although it may be possible to run these COTS applications in Docker Enterprise, guidance on how to do so for these applications are beyond the scope of this reference architecture. Also, .NET Core is not covered. All references to .NET applications refer to .NET Framework applications and not .NET Core applications.

Refactoring to microservices architectures is also not covered in this document. At the end of the containerization process discussed in this reference architecture, your .NET Framework application will be ready should you decide to refactor parts of the application to microservices.

Note

Before continuing, please become familiar with the reference architecture Design Considerations and Best Practices to Modernize Traditional Apps

See the caveats section for additional important information to be aware of.

Caveats

Before you begin there are some things to be aware of that will impact your deployment of applications on Docker Enterprise.

Note

Windows Server 2019 is the recommended platform to run Windows containerized applications. Versions prior to Windows Server 2016 do not support running containers of any type. Windows 2016, while capable of supporting containers, is not Microsoft’s recommended container host platform.

Desktop based apps with graphical user interfaces (GUIs) cannot yet be containerized

Due to the unique nature of certain Windows features (e.g. networking, security, file system) there are several items of note regarding the deployment of a Docker service. Below is a list of these issues including the current “best practices” used to work around them.


  • Networking (see Example compose file for a service running on Windows nodes below)

  • For services that need to be reachable outside the swarm, Linux containers are able to use Docker swarm’s ingress routing mesh. However, Windows Server 2016 does not currently support the ingress routing mesh. Therefore Docker services scheduled for Windows Server 2016 nodes that need to be accessed outside of swarm need to be configured to bypass Docker’s routing mesh. This is done by publishing ports using host mode which publishes the service’s port directly on the node where it is running.

    Additionally, Docker’s DNS Round Robin is the only load balancing strategy supported by Windows Server 2016 today; therefore, for every Docker service scheduled to these nodes, the --endpoint-mode parameter must also be specified with a value of dnsrr.

  • When running Docker for Windows there is an issue related to container IP addresses. The IP address shown when using the docker inspect command for a container is incorrect. To browse a web site or api running in a container you must use the docker exec command and query the IP address from within the container (e.g. ipconfig). Also, port assignments are ignored by Docker for Windows when running Windows containers (e.g. docker run -p 8080:80). Run the example app to illustrate this issue.

  • Docker Objects

  • Configs use the SYSTEM and ADMINISTRATOR permissions

    • When using a Docker Config object to replace the web.config file (ASP.Net apps), IIS will not be able to consume the file. IIS requires (by default) BUILTIN\IIS_IUSRS credentials applied to files it will read/write to.

    • Due to the fact that Docker Config objects are attached after the container is created, assigning rights to the application folder during a docker build will not solve this problem. Files added by the Config will retain their original credentials (ADMINISTRATOR & SYSTEM).

  • Secrets stored on node temporarily

    • Microsoft Windows has no built-in driver for managing RAM disks, so within running Windows containers, secrets are persisted in clear text to the container’s root disk. However, the secrets are explicitly removed when a container stops. In addition, Windows does not support persisting a running container as an image using docker commit or similar commands.

    • On Windows, we recommend enabling BitLocker on the volume containing the Docker root directory on the host machine to ensure that secrets for running containers are encrypted at rest.

    • Secret files with custom targets are not directly bind-mounted into Windows containers, since Windows does not support non-directory file bind-mounts. Instead, secrets for a container are all mounted in C:\ProgramData\Docker\internal\secrets (an implementation detail which should not be relied upon by applications) within the container. Symbolic links are used to point from there to the desired target of the secret within the container. The default target is C:\ProgramData\Docker\secrets.

    • When creating a service which uses Windows containers, the options to specify UID, GID, and mode are not supported for secrets. Secrets are currently only accessible by administrators and users with system access within the container.

  • AD authentication requires use of Integrated Windows Authentication

  • Windows node must be joined to the AD domain

  • Common base images for Windows applications

    • Additional Windows Features may be required depending on app requirements

  • ASP.Net applications: microsoft/aspnet

  • WCF Services: microsoft/iis

  • Console Applications: microsoft/dotnet-framework

  • .Net build tools: microsoft/dotnet-framework

    • Used for multi-stage builds (use the SDK variants)

  • ASP.Net Core applications: microsoft/aspnetcore

  • ASP.Net Core build tools: microsoft/aspnetcore-build

    • Used for multi-stage builds

  • Windows base OS images: microsoft-windows-base-os-images


Example compose file for a service running on Windows nodes

version: '3.3'

services:
  website:
    image: mcr.microsoft.com/windows/servercore/iis:windowsservercore-ltsc2019    # serves a default site on port 80
    ports:
      - mode: host    # host mode networking
    deploy:
      replicas: 1
      placement:
          constraints:
            - engine.labels.os == windows   # place service only on Windows nodes
      labels:
        com.docker.lb.hosts: app.example.org    # Replace with a real URL
        com.docker.lb.network: mystack_myoverlay # the network that the layer 7 mesh will hand-off to
        com.docker.lb.port: 80 # the port the service expects traffic on
      endpoint_mode: dnsrr    # dns round robin load balancing
    networks:
        - myoverlay   # custom overlay network the service will use

networks:
  myoverlay:  # the custom service definition
    driver: overlay

Application Selection

Before diving in, it’s important to understand there are different types of .NET Framework applications. Although not intended to be exhaustive, this section describes the most common types of .NET Framework applications and considerations that need to be made for these applications before proceeding with containerization.

Application Type

Considerations

ASP.NET Framework Applications

  • Web-based applications and web services

  • Built with ASP.NET MVC, ASP.NET Web Forms, ASP.NET Web Services, or Web API

  • Commonly hosted on Internet Information Services (IIS)

  • Good candidate for containerization

WCF Services

  • Service-oriented applications built with Windows Communication Framework

  • Often hosted in IIS as well but can also be hosted inside other applications and services (for example, inside another Web application, Windows Forms app, a Windows service, etc.)

  • Should be a good candidate for containerization, except for services hosted in a desktop application

Windows Services

  • Applications that run as background services in Windows

  • Can be containerized but since services run in the background, a foreground process needs to be created to keep the container running

Desktop Applications

  • Desktop applications built as Windows Forms or Windows Presentation Foundation (WPF) apps

Console Applications

  • Applications that run from the command line

  • Should be easy to containerize and is a good candidate for containerization

COTS Applications

  • Short for Commercial Off the Shelf Applications

  • Examples include SharePoint, Sitecore, and DNN

  • Generally best to avoid containerizing COTS applications until the vendor officially supports running these applications in containers

Application Dependencies

When initially getting started with the app containerization process, avoid applications that have many dependencies, components, and/or many tiers. Begin with a 2-3 tier application first until you are comfortable with the containerization process before moving to more complex applications.

Additionally, for applications that have component dependencies, ensure that the components can be installed without interaction (i.e., unattended installation or scripted). Components that require interaction during installation can’t be added to the Dockerfile.

Lastly, for applications that have dependencies to services or external systems (e.g. databases, file shares, web services, etc.) ensure that the addresses/endpoints for those services are stored in configuration files and are resolvable from the Docker Enterprise Windows Server hosts. Any hard-coded service references will need to be refactored prior to containerization.

Application Containerization

When containerizing an application it is important to determine what the desired outcome state is for the application. It is recommended that applications be divided into two categories.

  • Applications that will be rearchitected to be microservices, horizontally scalable, geo-redundant, highly available, stateless, etc…

  • Applications that will not be rearchitected but will take advantage of an improved development pipeline.

For the first scenario (rearchitected) the applications should be built as microservices and should deployed in a container native fashion.

For the second scenario a “lift and shift” approach should be applied to allow for the agility and portability of containers without significant rewriting of the application.

With a “lift and shift” approach, some rules of thumb are:

  • Keep the .NET Framework version the same

  • Keep the existing application architecture

  • Keep the same versions of components and application dependencies

  • Keep the deployment simple: static and not elastic

Once the application is successfully containerized it should then be easier and faster to change, for example:

  • Upgrade to a newer version of application server

  • Integrate into a simplified CI/CD pipeline

  • Deploy the application against any Docker Enterprise environment regardless of location

  • Reduction of technical debt

With a rearchitecting approach containers can provide the same benefits as for lift and shift with the addition of:

  • Higher flexibility and agility of developing more targeted services

  • Ease of unit testing

  • Higher velocity of pipeline execution

  • Increased frequency of deployments.

The following sections discuss the application containerization process.

Creating the Dockerfile

Note

Refer to Best practices for writing Dockerfiles for information on creating the Dockerfile.

The first step in a lift and shift approach is to create the Dockerfile, and the first step in creating the Dockerfile is choosing the right base Docker image to use. All containerized .NET Framework applications use an image that is based on Microsoft’s Windows Server Core base OS image.

Microsoft Base Images

Depending on the type of .NET Framework application, consider using the following as base images to start:

Application Type

Image

Notes

ASP.NET Applications

microsoft/aspnet

IIS and ASP.NET Framework pre installed

WCF Services

microsoft/servercore-iis

Assumes the WCF service is hosted in IIS. If hosted in another application, another base image may be more appropriate.

Windows Services

microsoft/dotnet-framework

.NET Framework pre installed

Console Applications

microsoft/dotnet-framework

.NET Framework pre installed

Windows Features

It’s important to enable windows features required by your application. This is done using Powershell commands in your Dockerfile To optimize your image, don’t include any unnecessary Windows features that aren’t being used by your application.

Application Pools

You can use the default settings,or use your own customized application pool for your web app. Note that if you use a domain account or service account for your application pool identity, you cannot just specify a domain account in your Dockerfile. You need to set the identity to one of the built-in types and then use a Group Managed Service Account (gMSA) via a Credential Spec when running the container. See the section Integrated Windows Authentication for more details.

Web Configuration Settings

Any settings that have been configured manually for the web application through IIS (e.g. authentication settings, etc.) must be added to your Dockerfile manually. Note that IIS management console should not be used to apply changes to running containers.

Final Dockerfile

The following Dockerfile is an example of a final Dockerfile:

# escape=`
FROM mcr.microsoft.com/dotnet/framework/aspnet:3.5-windowsservercore-ltsc2019
SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue';"]

# used only for gMSA authentication. Remove if using integrated auth.
RUN Enable-WindowsOptionalFeature -Online -FeatureName IIS-WindowsAuthentication

# Create the App Pool - not needed if you’re using default App pool
RUN Import-Module WebAdministration; `
    New-Item -Path IIS:\AppPools\MyAppPool; `
    Set-ItemProperty -Path IIS:\AppPools\MyAppPool -Name managedRuntimeVersion -Value 'v4.0'; `
    Set-ItemProperty -Path IIS:\AppPools\MyAppPool -Name processModel -value @{identitytype='ApplicationPoolIdentity'}

# Set up website: MyApp
RUN New-Item -Path 'C:\MyApp' -Type Directory -Force;
# Not needed if you use the default web site.
RUN New-Website -Name 'MyApp' -PhysicalPath 'C:\MyApp' -Port 80 -ApplicationPool 'MyAppPool' -Force;

# This disables Anonymous Authentication and enables Windows Authentication
RUN $siteName='MyApp'; `
    Set-WebConfigurationProperty -filter /system.WebServer/security/authentication/AnonymousAuthentication -name enabled -value false -location $sitename; `
    Set-WebConfigurationProperty -filter /system.WebServer/security/authentication/windowsAuthentication -name enabled -value true -location $sitename;

EXPOSE 80

COPY ["MyApp", "/MyApp"]

RUN $path='C:\MyApp'; `
    $acl = Get-Acl $path; `
    $newOwner = [System.Security.Principal.NTAccount]('BUILTIN\IIS_IUSRS'); `
    $acl.SetOwner($newOwner); `
    dir -r $path | Set-Acl -aclobject  $acl

In the above Dockerfile, a new app pool was explicitly created and configuration was added to disable Anonymous Authentication and enable Windows Authentication. This image can now be built and pushed to Mirantis Secure Registry:

docker image build -t msr.example.com/demos/myapp:1.0-10.0.14393.1715 .
docker image push msr.example.com/demos/myapp:1.0-10.0.14393.1715

During the build and debugging process, for IIS-hosted applications such as the above, you may also want to build a second Dockerfile that enables remote IIS management:

# escape=`
FROM msr.example.com/demos/myapp:1.0-10.0.14393.1715
SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue';"]

# Enable Remote IIS Management
RUN Install-WindowsFeature Web-Mgmt-Service; `
    NET USER dockertester 'Docker1234' /ADD; `
    NET LOCALGROUP 'Administrators' 'testing' /add; `
    Configure-SMRemoting.exe -enable; `
    sc.exe config WMSVC start=auto; `
    Set-ItemProperty -Path HKLM:\SOFTWARE\Microsoft\WebManagement\Server -Name EnableRemoteManagement -Value 1

EXPOSE 80 5985

With the above Dockerfile, the container’s IIS is available at <container-ip>:5985 and can be reviewed remotely on another machine with IIS management console installed. The user is dockertester with a password of Docker1234. Note that IIS management console should not be used to apply changes to running containers. It should only be used to troubleshoot and determine if the instructions in the Dockerfile have been properly applied.

The above Dockerfile also represents a typical Dockerfile created for .NET Framework applications. The high level steps in such a Dockerfile are:

  1. Select a base image

  2. Install Windows features and other dependencies

  3. Install and configure your application

  4. Expose ports

CMD and ENTRYPOINT

One step that is often in a Dockerfile but not in the above example is the use of CMD or ENTRYPOINT.

The ASP.NET Framework base image used in the above example already contains an entrypoint that was sufficient for this application. You can choose to create your own entrypoint for your application so you can change or add additional functionality. One scenario to use an entrypoint for is when your application needs to wait for services that it requires. Typically, a Powershell script is created to handle the wait logic:

# PowerShell entrypoint.ps1
while ((Get-Service "MyWindowsService").Status -ne "Running") {
  Start-Sleep -Seconds 10;
}

and the Dockerfile contains an ENTRYPOINT entry that points to that Powershell file:

ENTRYPOINT ["powershell", ".\\entrypoint.ps1"]

Image Tags and Windows Versions

When using one of the previously mentioned Microsoft Base Images, it is important to use the right tag. With default settings, Microsoft only supports containers whose base image version exactly matches the host’s operating system version as described in Windows container requirements on docs.microsoft.com. Although a container may start or even appear to work even if its base version doesn’t match the host’s version, Microsoft cannot guarantee full functionality so it’s best to always match the versions.

To determine the Windows Server version of the Docker Windows Server host, use the following Powershell command:

Get-ItemProperty "HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion" | % {"{0}.{1}.{2}.{3}" -f $_.CurrentMajorVersionNumber,$_.CurrentMinorVersionNumber,$_.CurrentBuildNumber,$_.UBR}

The output will be something like 10.0.17763.678. When using one of Microsoft’s base images, use an image tagged with the full version number outputted by the above command. For example, a Dockerfile for an ASP.NET 3.5 web application would start with the following:

# escape=`
FROM mcr.microsoft.com/dotnet/framework/aspnet:3.5-windowsservercore-ltsc2019

When tagging your own images, it’s a good practice with Windows Server containers to also indicate the full Windows Server version number.

Note

For containers started with Hyper-V isolation --isolation=hyperv, the version match requirement is not necessary.

Integrated Windows Authentication

One of the unique aspects often found in Windows-based applications is the use of Integrated Windows Authentication (IWA). It is often used with Windows-based applications to validate a client’s identity, where the client’s identity/account is maintained in Active Directory. A client, in this case, may be an end user, a computer, an application, or a service.

A common pattern is to use Integrated Windows Authentication for applications hosted in IIS to authenticate the application’s end users. With this approach, the application authenticates with the credentials of the user currently logged in, eliminating the need for the application and the user to maintain another set of credentials for authentication purposes. Another common use of IWA is to use it for service-to-service authentication, such as the authentication that happens between an ASP.NET Framework application (more specifically, the application’s process identity) and a backend service like a SQL Server service.

Because containers cannot currently be joined to an Active Directory domain as required for Integrated Windows Authentication to work, some additional configuration is required for applications that require IWA as these applications are migrated to containers. The following sections provide the configuration steps needed to enable IWA.

Group Managed Service Accounts

A Group Managed Service Account (gMSA), introduced in Windows Server 2012, is similar to a Managed Service Account (MSA). Like a MSA, gMSAs are managed domain accounts that can be used by applications and services as a specific user principal used to connect to and access network resources. Unlike MSAs, which can only be used by a single instance of a service, a gMSA can be used by multiple instances of a service running across multiple computers, such as in a server farm or in load-balanced services. Similarly, containerized applications and services use the gMSA when access to domain resources (file shares, databases, directory services, etc.) from the container are needed.

Prior to creating a Group Managed Service Account for a containerized application or service, ensure that Windows Server worker nodes that are part of your Docker Swarm cluster are joined to your Active Directory domain. This is required to access and use the gMSA. Additionally, it is highly recommended to create an Active Directory group specifically for managing the Windows Server hosts in your Docker Swarm cluster.

To create an Active Directory group called Container Hosts, the following Powershell command can be used:

New-ADGroup "Container Hosts" -Group Global

To add your Windows Server worker nodes to this group:

$group = Get-ADGroup "Container Hosts";
$host = Get-ADComputer "Windows Worker Node Name";
Add-ADGroupMember $group -Members $host;

For the Active Directory domain controller (DC) to begin managing the passwords for Group Managed Service Accounts, a root key for the Key Distribution Service (KDS) is first needed. This step is only required once for the domain.

The Powershell cmdlet Get-KDSRootKey can be used to check if a root key already exists. If not, a new root key can be added with the following:

Add-KDSRootKey -EffectiveImmediately

Note that although the -EffectiveImmediately parameter is used, the key is not immediately replicated to all domain controllers. Additional information on creating KDS root keys that are effective immediately for test environments can be found at Create the Key Distribution Services KDS Root Key.

Once the KDS root key is created and the Windows Server worker nodes are joined to the domain, a Group Managed Service Account can then be created for use by the containerized application. The Powershell cmdlet New-ADServiceAccount is used to create a gMSA. At a minimum, to ensure that the gMSA will work properly in a container, the -Name, -ServicePrincipalName, and -PrincipalsAllowedToRetrieveManagedPasswords options should be used:

New-ADServiceAccount -Name mySvcAcct -DNSHostName myapp.example.com `
  -ServicePrincipalName HTTP/myapp.example.com `
  -PrincipalsAllowedToRetrieveManagedPasswords 'Container Hosts'
  • Name - the account name that is given to the gMSA in Active Directory.

  • DNSHostName - the DNS host name of the service.

  • ServicePrincipalName - the unique identifier(s) for the service that will be using the gMSA account.

  • PrincipalsAllowedToRetrieveManagedPasswords - the principals that are allowed to use the gMSA. In this example, Container Hosts is the name of the Active Directory group where all Windows Server worker nodes in the Swarm have been been added to.

Once the Group Managed Service Account has been created, you can test to see if the gMSA can be used on the Windows Server worker node by executing the following Powershell commands on that node:

Add-WindowsFeature RSAT-AD-Powershell;
Import-Module ActiveDirectory;
Install-ADServiceAccount mySvcAcct;
Test-ADServiceAccount mySvcAcct;

Credential Specs

Once a Group Managed Service Account is created, the next step is to create a credential spec. A credential spec is a file that resides on the Windows Server worker node and stores information about a gMSA. When a container is created, you can specify a credential spec for a container to use, which then uses the associated gMSA to access network resources.

To create a credential spec, open a Powershell session on one of the Windows Server worker nodes in the Swarm and execute the following commands:

Invoke-WebRequest https://raw.githubusercontent.com/Microsoft/Virtualization-Documentation/live/windows-server-container-tools/ServiceAccounts/CredentialSpec.psm1 -OutFile CredentialSpec.psm1
Import-Module .\CredentialSpec.psm1;
New-CredentialSpec -Name myapp -AccountName mySvcAcct;

The first two lines simply downloads and imports into the session a Powershell module from Microsoft’s virtualization team that contains Powershell functions for creating and managing credential specs.

The New-CredentialSpec function is used on the last line to create a credential spec. The -Name parameter indicates the name for the credential spec (and is used to name the credential spec JSON file), and the -AccountName parameter indicates the name of the Group Managed Service Account to use.

Credential specs are created and stored in the C:\ProgramData\docker\CredentialSpecs\ directory by default. The Get-CredentialSpec Powershell function can be used to list all credential specs on the current system. For each credential spec file you create, copy the file to the same directory on the other Windows Server worker nodes that are part of the cluster.

The contents of a credential spec file should look similar to the following:

{
    "CmsPlugins":  [
        "ActiveDirectory"
    ],
    "DomainJoinConfig":  {
        "Sid":  "S-1-5-21-2718210484-3565342085-4281728074",
        "MachineAccountName":  "mySvcAcct",
        "Guid":  "274490ad-0f72-4bdd-af6b-d8283ca3fa69",
        "DnsTreeName":  "example.com",
        "DnsName":  "example.com",
        "NetBiosName":  "DCKR"
    },
    "ActiveDirectoryConfig":  {
        "GroupManagedServiceAccounts":  [
            {
                "Name":  "mySvcAcct",
                "Scope":  "example.com"
            },
            {
                "Name":  "mySvcAcct",
                "Scope":  "DCKR"
            }
        ]
    }
}

Once the credential spec file is created, it can be used by a container by specifying it as the value of the --security-opt parameter passed to the docker run command:

docker run --security-opt "credentialspec=file://myapp.json" `
  -d -p 80:80 --hostname myapp.example.com `
  msr.example.com/demos/myapp:1.0-10.0.14393.1715

Notice in the above example, the --hostname value specified matches the Service Principal Name that was assigned when the Group Managed Service Account was created. This is also required for Integrated Windows Authentication to function properly.

When configuring for use in a Docker stack, the credential_spec and hostname keys can be used in the Docker Compose YAML file as in the following example:

version: "3.3"
services:
  web:
    image: msr.example.com/demos/myapp:1.0-10.0.14393.1715
    credential_spec:
      file: myapp.json
    hostname: myapp.example.com

Networking

Networking is another aspect to consider when containerizing your Windows application’s services and components. For services that need to be available outside the swarm, Linux containers are able to use Docker swarm’s ingress routing mesh. However, Windows Server 2016 does not currently support the ingress routing mesh. Therefore Docker services scheduled for Windows Server 2016 nodes that need to be accessed outside of swarm need to be configured to bypass Docker’s routing mesh. This is done by publishing ports using host mode which publishes the service’s port directly on the node where it is running.

Additionally, Docker’s DNS Round Robin is the only load balancing strategy supported by Windows Server 2016 today; therefore, for every Docker service scheduled to these nodes, the --endpoint-mode parameter must also be specified with a value of dnsrr. For example:

docker service create `
  --publish mode=host,target=80,port=80 `
  --endpoint-mode dnsrr `
  --constraint "node.os.platform == windows" `
  msr.example.com/demos/myapp:1.0-10.0.14393.1715

Because ingress routing mesh is not being used, an error could occur should a client attempt to access the service on a node where the service isn’t currently deployed. One approach to ensure the service is accessible from multiple nodes is to deploy the service in global mode which places a single instance of the service on each node:

docker service create `
  --publish mode=host,target=80,port=80 `
  --endpoint-mode dnsrr `
  --mode global `
  --constraint "node.os.platform == windows" `
  msr.example.com/demos/myapp:1.0-10.0.14393.1715

Creating a global service ensures that one and only one instance of that service runs on each node. However, if replicated deployment mode is what is desired, additional considerations and configurations need to be made to properly handle load balancing and service discovery. With host publishing mode, it is your responsibility to provide a list of IP addresses and ports to your load balancer. Doing so typically requires a custom registrator service on each Windows Server host that uses Docker events to monitor containers starting and stopping. Implementation of the custom registrator service is out of scope for this article.

Note that Docker’s routing and service discovery for services on the same overlay network works without additional configuration.

For more details about swarm networking in general, see the Exploring Scalable, Portable Docker Swarm Container Networks reference architectures.

HTTP Routing Mesh

Another option to consider for services available outside the swarm is Mirantis Kubernetes Engine’s (MKE) HTTP Routing Mesh (HRM). HRM works at the application layer (L7) and uses the Host HTTP request header found in HTTP requests to route incoming requests to the corresponding service. Docker services can participate in the HRM by adding a com.docker.ucp.mesh.http label and attaching it to an HRM network (ucp-hrm is a default network):

docker service create `
  --name aspnet_app `
  --port 80 `
  --network ucp-hrm `
  --label com.docker.ucp.mesh.http.demoappweb: "external_route=http://mydemoapp.example.com,internal_port=80" `
  --placement "node.os.platform == windows" `
  msr.example.com/demos/myapp:1.0-10.0.14393.1715

In the above example, because of the value for the com.docker.ucp.mesh.http.demoappweb label, inbound HTTP traffic received with mydemoapp.example.com Host HTTP request header will be routed to a container for this service on the container’s port 80. More details on how to use HTTP Routing Mesh can be found in the Mirantis Kubernetes Engine Service Discovery and Load Balancing for Swarm

Logging

There are many different approaches to logging in traditional .NET Framework applications. Simpler applications log to the console (standard out or standard error), if available. Some applications will output logs to the file system or will log to Windows Event logs. Other applications will send its logs to a centralized location, such as a database or a logging service.

In Docker, logs are captured by default to a JSON file. The log entries in the file are usually whatever the console output is of the application or service. For .NET Framework applications that already write to standard output or standard error, these messages will appear in the JSON log file as well when the Docker command docker container logs <containerid> is issued. Some refactoring of your application may be required if your application does not currently send messages to standard out or standard error.

For .NET Framework applications that write to a log file, the entries in the log file can be relayed or redirected to the console in order to output them into Docker’s logs. This approach is outlined in this post from my colleague, Elton Stoneman, who uses a Powershell script and the Get-Content ... PowerShell cmdlet to relay IIS logs to Docker. This same approach can be taken with your own application’s custom log files.

For applications that centralize its logs to a database, no refactoring should be necessary as long as the application in the container continues to have access to the logging database that’s used. You may, however, want to do at least some refactoring to capture container-specific information in the logging DB such as container IDs, host, etc.

For applications that are sending logs to a centralized logging service, there may or may not be some refactoring required, depending on the service that is used. Additionally, Docker has several logging drivers available for Windows Server, including drivers that work with centralized logging services such as Amazon or Splunk. You can configure the logging driver that is used for each container or at the host level.

The logging drivers available for Windows Server are:

Driver

Description

json-file

Logs are formatted as JSON. Default logging driver for Docker.

awslogs

Writes log messages to Amazon CloudWatch logs.

etwlogs

Writes log messages as Event Tracing for Windows (ETW) events.

fluentd

Writes log messages to fluentd (forward input). The fluentd daemon must be running on the host machine.

logentries

Writes log messages to Rapid7 Logentries.

splunk

Writes log messages to splunk using the HTTP Event Collector.

syslog

Writes logging messages to the syslog facility. The syslog daemon must be running on the host machine.

More information about the logging drivers above can be found in the Docker docs.

If you are not already using a centralized logging service, consider running a container-based centralized logging service running in Docker MKE. One logging service stack that is often used with Docker is ELK (Elasticsearch, Logstash and Kibana). Each component of the ELK stack can be run in a Linux container. Various Beats can then be used on the Windows Server hosts/containers to ship the appropriate logs to ELK services. A Beat, such as Winlogbeat Filebeat, can be installed on the Docker Windows Server host and configured to monitor and ship different log files. The Beat may even be containerized and run as a global service on each Windows Server host. An example of Filebeat running in Windows Server containers and shipping container logs on the host to a Docker MKE hosted ELK service can be found at https://github.com/bxtp4p/docker-logging-win.

Monitoring

Like logging, monitoring is another aspect of .NET Framework applications where different approaches can be used, though most applications use a monitoring service such as AppDynamics, New Relic, or Microsoft Operations Management Suite (OMS). Like centralized logging services, depending on the monitoring service used, some refactoring or application configuration changes may be necessary when moving your application to a container.

If a monitoring solution isn’t currently in place or you are just looking to get started and experiment with .NET Framework container monitoring, Prometheus may be worth considering. Prometheus is an open source monitoring solution that can be run in a container. An example of running Prometheus in a container and monitoring an ASP.NET Framework application can be found at https://github.com/dockersamples/aspnet-monitoring.

Summary

This document provided an approach and guidance on how to containerize Brownfield .NET Framework applications. It covers how to start the containerization process, introduces dockerfiles that can be used to assist in the process, and identifies key points to consider and directions on how to properly run .NET Framework applications on Docker. Follow the items outlined in this document to effectively migrate your .NET Framework applications to Docker.