What you should know about Windows containers before you start using them

/ 15 Aug, 2017

A lot of articles and blogs have already been written about the benefits of using containers for your application delivery[1], also called containerized delivery. While the serverless trend is considered the containers’ successor in application delivery, it still is not the best fit for all situations. For example, containerized delivery is a perfect fit for organizations that do not want to go into the cloud (for now) or that have to deal with their existing application landscape. In this article, we’ll look at the most important things you as an architect/lead developer have to know about Windows containers before you start using them. If you are interested in more practical manuals about configuring a containerized pipeline, setting up a container registry and other tips and tricks, please have a look at our blog posts[2].

Containers as the next step in application delivery

Containerized delivery can be seen as the next step in application delivery for many organizations. If we look at the way in which we have been delivering our software during the past 10 years, we see a shift from manually installing our applications towards an automated way of pushing the applications into production. Driven by the mindset that the reliability, speed and efficiency of delivering our applications to our customers could be improved, various tools and frameworks have been introduced over time, e.g. WiX, PowerShell, PowerShell DSC, Chocolatey, Chef, and Puppet.

Figure 1 – Deployment maturity anno 2017

In the earlier years, we baked an MSI by using WiX and the installation of our applications was performed manually by one of our colleagues. Because this way of working was error-prone and it took us a lot of time to find and fix mistakes, we decided to make use of new scripting languages like PowerShell to fully automate the installation of our applications. Although this scripted way of installing our applications brought us more reliability and speed, still some deployments went wrong over different environments (DTAP) because of missing libraries, configurations and tool versions.

[1] http://xpir.it/mag4-wincon1, http://xpir.it/mag4-wincon2

[2] https://xpirit.com/blogs

So, we decided to make use of Desired State technologies like PowerShell DSC, Chocolatey, Chef, and Puppet to ensure that our environments will end up in the correct state. We succeeded in creating an automated and reliable way of delivering our applications to production, however we still have to reboot machines after de-installations to ensure that all registry keys, caches and file system files are really cleared. Moreover, we experience a lot of waiting- and downtimes caused by (de-)installations and are often not able to run multiple versions of a given application on the same host. That’s a pity because we see a lot of VM resources not being consumed by our various applications. Sounds familiar? Deploying your applications in containers can help to solve these issues.

The origin and benefits of containers

Containers are the resulting artifacts of a new level of virtualization that is implemented on Windows. Looking at the history of virtualization, it started with concepts like virtual memory and virtual machines. Containers are the next level of this virtualization trend. Where VMs are a result of hardware virtualization, containers are the result of OS virtualization. Where hardware virtualization lets the VM believe that its hardware resources are dedicated to that instance, OS virtualization lets the container believe that the OS instance is dedicated to that container, although it shares the OS with other containers. The host machine on which different containers can run is called a container host.

Working with containers for your application delivery will give you some benefits as mentioned in the article of Alex Thissen. Instead of installing the application during deployment, containerized delivery will install the application during build and will create a container image out of it. Once you ship your containers to new environments you’ll get instant startup times. The image format that is used does not only contain the running application, but also contains a snapshot of the context around the application such as the file system, registry and running processes. This mechanism ensures that our application will run in the same way in different environments. Another benefit is that the container technology helps us to utilize more of the machine resources because it makes it possible to run multiple versions of the same application/conflicting applications on a single host.

How container isolation works

While containers are not VMs, both artifacts support resource governance and an isolated environment[3]. Similar to VMs, each container has an isolated view on its running processes, environment variables, registry and file system. This means that it is possible to change a file within one container while this change is not detected by another container. This isolation characteristic is important to notice. By default, it is not possible to share any files between different containers. Even the container host cannot see the file system changes that are made within the container. The same goes for the registry and environment variables isolation.

The process isolation part is slightly different. While Windows Server Containers cannot see each other’s running processes, it is possible to see the running processes of a given Windows Server Containers from the container host. While this is also the case for Linux containers, this is not an appropriate option for multi-tenancy situations where the container host or different containers are not in the same trust boundary. Because of this security implication, Microsoft decided to introduce an extra container type called the Hyper-V container[4]. While it is possible to see the running processes from within the container host for Windows Server Containers, Hyper-V containers run a normal Windows Server Container within a minimalized (utility) Hyper-V “VM”. Because of this extra Hyper-V virtualization layer around the normal Windows Server Container it is not possible to see any content of this container type from the container host. Actually, the Hyper-V container is a hybrid model between a VM and a container.

[3] http://xpir.it/mag4-wincon3

How this impacts the way we share data between our applications

By default, containers are not designed to share any resources. To be able to share data across your containerized applications, you need to store data in remote data storage solutions such as Redis or SQL. However, in some scenarios these solutions are not appropriate because of an existing application architecture you have to deal with. For example, when you have a windows service generating PDFs from files on a given file system location and a website that is putting those files onto this location. How to deal with this inter-container data sharing? We can certainly extend our applications to communicate over HTTP/TCP to exchange important information, but luckily there are also other options available.

Data Volumes

One option is that you make use of Data Volumes. Data Volumes are artifacts that differ from containers. They  have their own lifecycle and you have to manage them with separate commands. For example, when you delete a container, the Data Volume will still exist. Once you have made a Data Volume, you can map it as a directory on your containers. Within the container these mapped directories will bypass the normal Union File System implementation and by doing so, all changes in this directory are always persisted on the file system of the container host. Moreover, changes that are made within this volume are directly available for other containers that consume the same Data Volume. An example script of consuming Data Volumes in your containers can be found below.

[4] https://xpir.it/mag4-wincon4

# Create the DataVolume and give it a name
docker volume create –name loggingvol

# Map the DataVolume with name loggingvol to directory c:/logging within the container of 
Consumer1 and c:/otherlogginglocation within the container of Consumer2.
docker run -v loggingvol:C:/logging –name Consumer1 microsoft/windowsservercore
docker run -v loggingvol:C:/otherlogginglocation –name Customer2 microsoft/windowsservercore

Figure 2 – Data Volume concept and example script

Data Volume Containers

Another option is that you make use of Data Volume containers. Sometimes you need to map multiple Data Volumes for most of the containers you run (for example a logging and data directory). For this purpose, you can make use of Data Volume Containers.

Data Volumes Containers are just containers that you’ve created and mapped on different Data Volumes. Where normally you must specify the separate volume mappings for each container you run, in the case of Data Volume Containers you just have to add the “–volumes-from” switch to the Docker run command. For each Data Volume that is mapped within the Data Volume Container, Docker will create a new volume mapping on the same file system directories within your consuming containers. While you may expect that deletion of the Volume Container will result in deletion of the Volumes, this is not the case. It just works as normal Data Volumes, except that we have an extra option to easily manage consistency of multiple Volume mappings over different containers.

Volume Plugins

By default, Data Volumes are stored on a single container host. But what if I’m running my containers on different container hosts in a cluster and still want to share the data from my Data Volumes on different container hosts? Or what happens with my volume data when my single container host crashes? It’s exactly for this reason that Volume Plugins[5] were introduced. Volume Plugins enable you to make Data Volumes hosts independent by integrating their storage with external storage systems such as Amazon EBS. For each Data Volume you can specify whether it should use a different Volume Plugin implementation by specifying the “–driver” switch on volume creation.

Learn about the concepts: images and image layers

When you start building your own containers, you must define what your container will look like. You can achieve this by specifying the various instructions[6] that have to be executed during the build process to define/bake your container. One instruction could be that you copy some files from the build machine to a given location within the container. When using Docker, this file is called a Dockerfile.

During the Docker build process each individual instruction is executed and its resulting file system changes are persisted in an image layer as shown in Figure 3 – Image layers, Images and Containers. This image layer will be stored locally on the build server. The reason for this is that we can have multiple containers that share the same base set of stacked image layers. Instead of rebuilding all these layers again, the Docker build process will look for existing image layers in this local store before it starts building a new layer.

The order in which you specify the instructions makes sense for the image layers that are generated, for example, the Dockerfiles below will create different image layers while they have the same set of instructions. The concept of having a stack of image layers where each image layer represents a set of file system changes is also called a Union File System[7].

[5] https://xpir.it/mag4-wincon5

[6] https://xpir.it/mag4-wincon6

[7] https://xpir.it/mag4-wincon7

Figure 3 – Image layers, Images and Containers (source: https://docs.docker.com/engine/userguide/storagedriver/imagesandcontainers/)

A container image is delivered at the end of the Docker build process. This image consists of a stacked set of image layers and is the blueprint for all containers you will run on the basis of this image. The difference between a container image and image layers, is that a container image “tags” a given image layer with a user-friendly name. All image layers that are under this tagged layer will be part of the container image. A container image is not a living thing and does not have a state. It is no more than a sealed artifact from which we can create multiple runtime instances: the containers.

To keep track of changes that are made within the container from the moment it was created, each container gets an extra Read/Write layer on top of the stacked image layers of the original container image. Where we might expect a totally different implementation between containers and container images, the only real difference between both objects is just this extra thin R/W layer for each container instance! During the lifecycle of the container, the changes that have been made within the container (actually this R/W layer) are stored temporarily on the container host. Once you delete the container, all those changes are lost. You can prevent this by using the Docker commit command to persist the changes that are part of this thin R/W layer into a separate image layer in the local store. From that moment on you can tag a new image based on this newly created image layer and you can initiate multiple containers based on this image.

Dealing with sensitive information – short peek

Containerized delivery will force you to deal with sensitive information in a different way. Once you create a container image out of a running container or from scratch, others can see all of its content just by running a container from that image locally. They can even make use of docker history to see the commands that have run on the container or they can look at the content of the versioned Dockerfile. Hence it is important to be sure that no sensitive information is stored within the container image or Dockerfile. But how to ensure this? One way of working is that you make use of the Docker Secrets Manager[8] to manage all secrets. Follow our blogs to see how to make this work and what other options are available here.

How these new concepts change the way you’ll look at maintaining your applications

Containers are designed as stateless artifacts. You should avoid storing data in your container as much as possible. As mentioned earlier, containers make use of a Read/Write layer to keep track of any changes since the initialization of the container. Once a container is deleted, these changes are lost. The objective of this implementation is to force users to persist all important changes in container images. By doing this, we can always reproduce a given state and are always able to selectively scale different parts of our system.

Another important characteristic of containers is that they are intended to be immutable. Thanks to the isolation part of containers we don’t need to upgrade running applications because we can run different versions of the same container side by side on a single container host. Moreover, because containers also contain the context around the applications, we can be sure that initializing a container on different container hosts will ensure that our application will run in exactly the same way on those different container hosts. Once we accept the stateless character of containers and create container images for all different versions of our applications, there is no longer any need for in-place upgrades. Instead of treating our applications as pets, we can now treat our applications as a flock of applications. If one container gets ill we will not nurse it back to health, but we will remove it and just initiate a new container based on the same genealogical register (the container image).

Reaching Nirvana: Environment-as-Code

Many of us know about Infrastructure-as-Code[9]. The concept of scripting the creation of the infrastructure you need. Containerization will add a layer on top of this Infrastructure-as-Code layer, and this is called Environment-as-Code.

Once we have created the necessary container infrastructure like the Container Registry, a cluster of Container Hosts and the VSTS pipeline, it is time to deliver our applications via, and on top of, this infrastructure. Normally we would perform a deployment of our applications individually, and this is still possible within containerized delivery. However, wouldn’t it be better from an immutability and quality (repeatability) perspective to always deliver the entire application stack as one? To think about complete environments as deployable items instead of single applications?

Figure 6 – Environment-as-code

[8] https://xpir.it/mag4-wincon8

[9]  https://xpir.it/mag4-wincon9 

This is what we call Environment-as-Code. Not only are your containers immutable and stateless, your complete environment (except your data) should be immutable and stateless as well. And in the same way as we version individual container images, we should also version environment templates. Luckily this is possible with containerized delivery by making use of Docker Compose[10].

version: '2'
    image: xpirit/data:1.0.0
      - 'logging:C:/Xpirit/Logging'
      - 'export:C:/Xpirit/Export'

    image: xpirit/website:1.0.3
      context: ./Xpirit-website
      dockerfile: Dockerfile
      - data
      - data
      - "80:80"

    image: xpirit/service:2.0.2
      context: ./Xpirit-agentservice
      dockerfile: Dockerfile
      - data
      - data

      driver: "local"
      driver: "local"

      name: nat

Figure 7 – Example compose file

Within the compose file you specify the containers (services section), volumes and network related details that are part of your environment. By running the docker-compose up command on your docker-compose.yml file, with some environment specific variables specified in an environment[11] file, the Docker engine will eventually create the elements that you have specified in your Docker compose file (including Docker networking).

Applying Environment-as-Code in practice means that you have to adapt your delivery pipeline based on this concept. All applications/containers will still have a separate commit stage in which compilation, unit testing and acceptance testing on code units takes place. After this commit stage and after running the necessary acceptance tests on our individual containers, you will change the version of the container you just built in the Docker-compose file. Instead of just having a single delivery pipeline for each container, you are also able to deliver the complete environment via a combined delivery pipeline to production.

Figure 8 – Combined delivery pipeline

Where to find more information

We have just looked at a number of important concepts and principles that you need to know about before you start performing containerized delivery on the Microsoft stack. There is a lot more to be told about containers and containerized delivery, so please follow our blog posts on https://xpirit.com/blogs or on my personal blog on http://www.solidalm.com to remain up-to-date about working with containers on Windows. A good starting point to learn the necessary commands and tools of working with Docker can be found on https://docs.docker.com/engine/getstarted/. Brighten your life, and start working with containers today

[10] https://xpir.it/mag4-wincon10

[11] https://xpir.it/mag4-wincon11

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