Why Linux Containers and Docker are the Runtime for the Software Defined Data Center (SDDC)..(4/7)

The third and previous blog in this seven part series (@ http://www.vamsitalkstech.com/?p=4659)  discussed Apache Mesos, a project that aims to abstract away various system resources – CPU, memory, network and disk resources to provide consuming digital applications with a giant cluster from which they can utilize capacity – a key requirement of the Software Defined Datacenter (SDDC). In this fourth blog, we will discuss another important ecosystem technology & project – Linux Containers and Docker – which forms the foundational runtime component in the SDDC. The next blog will discuss Kubernetes – Google’s container orchestration platform.

Much like shipping goods in Containers over Oceans, Linux Containers offer a portable, lightweight & convenient way to ship business applications. (Image Credit – WallPapers 13)

Executive Summary…

We can agree that the Digital application is inherently a distributed application. Such applications have historically been extremely hard to develop, setup and manage across a large fleet of data center servers that are a mix of platforms and technologies. Thus it is no surprise that one of the most disruptive developments in the last five years has been the innovation in the Linux container space. Containers now enable the running distributed applications at scale. 

Due to business reasons, Digital applications demand constant updates, changes and incremental revisions in response to changing customer needs. The Software Defined Datacenter (SDDC) thus needs a runtime paradigm that enables not just efficient hardware usage but also supports standardized application environments that are portable simplified and consistent across hybrid clouds and hypervisors.  Containers fill this need and are thus emerging to be the natural unit of deployment across the SDDC. Much has been written on the topic of Docker and Linux Container technology. My goal for this blog post is to distill key insights in the container ecosystem.

The Technologies of Linux Containers & Docker

Unlike Virtual Machines, Container Engines such as Docker share a common OS (Image Credit – MSFT Azure)

Linux Containers are alike and yet different from virtual machines. They are alike in the sense that each Container shares system resources on the underlying hardware platform – CPU, RAM, and Network – as with VMs. However, while each VM maintains its separate copy of the Operating System (OS), containers share the same OS kernel while keeping themselves separate from other containers running on the same OS.  How do they do that?

Though the terms ‘Docker’ and ‘Container’ have become almost synonymous – it needs to be noted that Docker is a company focused on developing technology enablement around containers in areas such as orchestration, networking, and management. Docker was an open source project (now renamed to Moby [1]) that provided capabilities such as a standard description of container formats, utilities for application packaging, deployment & lifecycle management of applications inside Linux Containers. It provides a Docker CLI command line tool for the lifecycle management of image-based containers.

Prior to the explosion of interest in Linux containers & the founding of Docker, traditional Linux distributions (with a minimum kernel level of 3.8) supported two foundational paradigms – control groups (cgroups) and kernel namespaces.  Linux containers use both these features to achieve their goal of isolation and portability. Cgroups enables the host to limit the resources each container process can use from a CPU, Memory, Filesystem, User ID components and Network standpoint. This ensures that containers running on a host cannot starve others of resources thus avoiding the “Noisy Neighbor” problem that bedeviled a lot of cloud deployments.

Kernel Namespaces ensure another kind of isolation for process interactions within the OS. Containers can only view and modify resources in the same namespace. This ensures a security mechanism where other containers and processes on the host cannot launch attacks on a given application running on a tenant container or on the host itself. Thus the combination of both these technologies ensures that multiple applications running within their individual containers can share CPU and Memory without needing the overhead of virtualization. Docker also grants each container its own networking implementation thus ensuring that resources such as socket and interfaces can also be protected.

Companies including Red Hat, IBM, Google, Cisco, VMware, and CoreOS have greatly aided with the development of and accessibility of containers in their platforms and products.

Layered Filesystems..

Various Image Layers in Docker. Each layer in the file system is mounted on the previous.The topmost is the Writable Container. (Image Credit – Docker)

We discussed how Container Images are Immutable. This is the key advantage of using container technology such as Docker & is made possible by the notion of a Union filesystem. What are Union filesystems and how do they enforce immutability? Much like the image in a Virtual Machine sense, Containers also run from an image, which typically are a snapshot of a filesystem but tend to be much smaller than VM images since the Container is installed on a host kernel.

Union filesystems are best described as a layered architecture – in that each layer is created independently and then added atop of the previous layer.  An example of a Union filesystem is a Linux Kernel – an OS – then a data base like Oracle – then Tomcat – and a web application on it. The top layer is always the Writable layer. The real advantage in using a union filesystem is that using these images becomes super efficient from a storage and execution standpoint. Union filesystems also help in sharing portions of the OS across containers. Simply put, an image contains everything an application needs – from it’s dependencies and external libraries. When an Image is run, it is called a Container. In the case of Docker, it uses a layered copy on write filesystem called AUFS (Another Union Filesystem).

Containers and Developers..

Containers are possibly the first infrastructure software category created by developers in mind. The prominence of Linux Containers has Docker coincided with the onset of agile development practices under the DevOps umbrella – CI/CD etc. Containers are an excellent choice to create agile delivery pipelines and continuous deployment. At their core, Containers enable the creation of multiple self-contained execution environments over the same operating system.

Developers are naturally excited about Linux Containers for five specific reasons –

1. Containers allow for image consistency across OS environments. This is a huge help in accelerating the development process from development to debugging to production. Developers can just focus on building their applications (in dev environments that match the test and prod) and packaging them in containers. This just takes a lot of the inefficiency around environment dissimilarities out of the equation.
2. Containers are treated as a standard linux process by the kernel & thus are orders of magnitude quicker from a startup time when compared to VMs. This means that developers can start their applications in a manner of seconds as long as they run them in a container.
3. Containers also provide development organizations the ability to standardize application development workflows and update processes. This solves the scalability problem that digital applications have caused large organizations.
4. Digital applications are leading the move to adopt microservices. Microservices offer a way to build applications as a collection of discrete services as opposed to a monolithic architecture. By there very nature, microservices can be built and managed by different teams. Containerization affords a lightweight way of building and deploying microservices.
5. Containers offer a portable way of delivering applications (across Operating Systems) as well as provide horizontal scalability.
   

   Digital Application development using Containers..

Digital Application Development and Deployment Workflow using Containers.

There are a few key runtime components involved in operationalizing a small to medium to large scale container infrastructure as the above illustration depicts.

1. Firstly, developers create container images. These images describe an application and it’s dependencies. An easy way to conceptualize an image is to think of it as a basic deployment template. Image are also immutable in that they are read only and any changes happen in the top most layer which is writable. Modifying an image is to create a new one. Images thus have a Parent Child relationship. Developers create images by building their applications on their developer environments, performing unit tests and then pushing to a repository. Once the container is built with the necessary dependencies, these tools run a battery of tests to validate business functionality. A large part of this process is usually best automated using CI/CD tools like Jenkins, CruiseControl or Buildbot etc.
2. The built images are then made available in a Container Registry. This is either maintained internally or sourced from a trusted external source. As the name suggests, Registries maintain a catalog of container images of frequently used software – e.g. Custom applications and other software packages such as WordPress, Relational databases, Web Servers, Big Data technologies and Application Servers etc
3. The next step is to create and deploy (runtime) containers from these images on a set of servers. Once images are released as a result of application development, sys admins work on the provisioning of the servers to run these images. Once a Container engine is installed on the server, images are loaded on and they take the runtime shape of containers. The mode of getting these images on these servers follows either a push/pull mechanism.
4. Scheduling of containers on servers is also a process that usually done by Sys Admins. This involves running containers of certain kinds on servers that match up to certain CPU, I/O and Network capacity requirements
5. To create complex real world deployments, not only do the servers and networking have to be created but these containers are also interconnected (e.g. a web server container to an application server) using Discovery mechanisms. These containers then need to also connect to a host of enterprise services. Customer traffic is then routed to the clustered containers running on these servers. Monitor the logs and performance of these containers and the microservices running on them.
6. The process repeats from step #1 above.

Industry Adoption of Containers.

In a few years, containers will deliver the bulk of compute workloads across public cloud providers such as Amazon AWS, Google Compute Engine and Microsoft Azure. Given that the VM options on these clouds can run multiple containers which can scale on demand, the industry will begin to gravitate to higher utilization density. The SDDC has already begun incorporating hybrid architectures that run both containers and VMs in a complementary fashion.The Software Defined Datacenter will incorporate a hybrid model consisting of applications running on both Linux Containers and Virtual Machines.

Customers also have choices of traditional enterprise operating systems such as Red Hat Enterprise Linux or Microsoft Windows or can also run containers on OS’s developed for the purpose of hosting containers at hyper scale. These OS’s just provide tools to manage containers and nothing else. Examples include Red Hat Atomic Platform and CoreOS. Moving up the stack, pioneers such as Google and Red Hat have added core support for containers in projects such as OpenStack, Kubernetes, Mesos, OpenShift & CloudFoundry by helping with networking and persistent storage. Kubernetes (which we will cover in the next post) also handles provisioning on multiple public cloud platforms. Config Mgmt platforms such as Ansible, Chef and Puppet now support containerized deployments.

Technical Considerations for Container Adoption

Some key considerations that industry players are tackling from the standpoint of running containers at scale –

1. Container Orchestration –  Organize groups of containers into compassable applications, scheduling them on servers that match their resource requirements, placement of containers based on network topology etc
2. Container Networking – Containers follow a pluggable model and the network is no different. Key considerations – an enterprise network connectivity stack is needed to not only provide the interconnect between different containers but also to integrate them with existing Layer 2/3 networks. Additionally, network isolation needs to be provided for microservices running on these containers using either a dedicated IP address for each or an overlay network.
3. Management and Monitoring -Life cycle processes ranging from Management and Monitoring encompass a range of questions – application patching with low downtime, graceful failures in cloud native applications, container scale up & scale down based on traffic patterns etc.

Containers and your Enterprise…

So what is the best way to adopt containers across a large enterprise?

• Develop your container strategy in the context of the Nexus of Forces (i.e., information, mobile, social and cloud) initiatives in your organization — Containers are at the junction of these technologies.
• Institute an organizational process to examine the business value of any initiative to adopt Containers. Understand what tools and platforms to adopt that will abstract away the complexities of using containers.
• Understanding skills required to leverage containers. Containers are a new way for both developers and SysOps. Dependency management moves to the developers but they realize tremendous benefits in adopting these for high-velocity Digital applications
• Identifying, measuring and benchmarking key success metrics that measure the ROI of the overall container investments.

Conclusion..

To sum up, the Linux (and Windows) container space is exploding both from a mindshare as well as an adoption standpoint. What is hugely encouraging is that a host of next generation platform technologies (ranging from IaaS to PaaS) are not just choosing to support containers as their basic runtime unit but are also focusing on becoming the defacto solution supporting a host of container ecosystem usecases – provisioning, orchestration, management, CI/CD et al. The next two blogs will respectively discuss how Google Kubernetes and Red Hat OpenShift overcome these challenges and abstract away much of the complexity around container deployments.

The next blog post in this series will discuss Google Kubernetes, the dominant project in the container orchestration space.

References

[1] Introducing Moby Project –  https://blog.docker.com/2017/04/introducing-the-moby-project/