111 lines
4.6 KiB
Markdown
111 lines
4.6 KiB
Markdown
---
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tags: [docker, containerization, Linux]
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---
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# Containerizaton
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> In this entry we look at containerization as a general technology that is
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> distinct from its particular implementation by Docker.
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We can understand the role and significance of containers and containerization
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by analogy with shipping containers. Before the invention of shipping containers
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as a **standardised modular unit** for transporting goods, shipping and supply
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chains were chaotic: inefficient, complex, frictional. With the advent of the
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shipping container, the same artefact could be used for transportation via
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ships, trains, lorries etc.
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With containers we can share and transport software via a single standardised
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unit that works regardless of the host operating system or server. We can also
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iterate different software environments easily: local, production, test etc. As
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with the shipping of goods, standardisation has driven down running costs and
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increased the speed of transport.
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## Containers and software
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- Containers are a technology that allow you to package and isolate applications
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with their entire runtime environment. This makes it easy to move the
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contained application between various environments whilst retaining full
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functionality.
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- The technology of containerization is distinct from _Docker_. Docker is a
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particular implementation of containerization that simplifies the process and
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bases it on a standardised specification.
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- Containers are native to the Linux [kernal](The_kernel.md)
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and are key part of how it works. Thus when you run containers on Linux, you
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are using native capability. When you use containers on Windows or Mac you
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have to run a virtual version of Linux in order to exploit the capabilities of
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that kernel. (Docker provides this)
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## How containers work
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A key feature of the Linux kernal is the existence of **cgroups** (control
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groups).
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> Containerization = resource control + process isolation
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- A cgroup **groups a series of processes together and contrains their access to
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system resources (CPU, memory, disk)**
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- Cgroups use namespaces to provide **process isolation**: groups of processes
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are isolated so that they cannot access other processes on the system. For
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example a container is ignorant of the underlying operating system and
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network, by default.
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In ordinary [user space](User_Space.md) applications share
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the _same_ processor, memory and file system resources. This increases the
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likelihood of resourcing challenges, dependency conflicts and security threats.
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Without modularisation and the titration of resources, you are opened up to much
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greater possibility of failure.
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For example one application could fill up the harddrive preventing other
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applications from writing to it. One application can "bring down" another
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applicaton.
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Prior to containerization, in an enterprise environment most of the system
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administration consisted in managing resouces to avoid excessive resource
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expenditure and security challenges.
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_Standard userspace_
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_Userspace with containerization_
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## Differences with virtual machines
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Containers are similar in many regards to VMs. They both offer a virtualized
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environment in which to run software and they both provide isolation of
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resources. (When you start up a VM you specify the available RAM and disk
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space.)
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The central difference is that VMs require the existence of a full operating
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system. Containers can run without an OS although it is often convenient to run
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them with stripped-down Linux distributions.
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A VM contains a complete installation of a guest OS and requires a hypervisor on
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the host OS to manage the different operating environments. This makes them
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slower and consume more resources.
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A containerized system radically reduces this overhead. It just starts up the
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container and after that it's job is done since the Linux kernel handles the
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rest as it would any other process on the OS.
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| Virtual Machine | Container |
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| ------------------------------------------- | --------------------------------------- |
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| Isolation of resources and operating system | Isolation of resources only |
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| Require a hypervisor to run | Require a container runtime to run |
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| Slower due to additional abstraction | Faster due to less overhead |
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| Less portable | More portable |
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| Slower and more difficult to run | Scale rapidly due to lightweight nature |
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## Why use containers?
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- portability
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- scalability
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- efficiency
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- isolation
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- DevOps agility
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