Showing posts with label Linux. Show all posts
Showing posts with label Linux. Show all posts

Thursday, March 17, 2022

Why use Vim

Depending on your preconceptions, Vim may look exotic or sexy. Let's review those assumptions and provide rational reasons to use this fantastic text editor.
Photo by Alex Knight on Unsplash

It may be possible that you heard about Vim. It may be possible that you didn't. Depending on your background, it may even be possible that have preconceptions about it. On this post, let's try to review all assumptions and provide concrete reasons to use this fantastic text editor.

This article is an adaptation of another publication made by me on Vim4us. I'm re-publishing here to a wider audience with a few tweaks.

Vim is ubiquitous

Vim has been around for almost thirty years. Due to its simplicity, ubiquity  and low resource requirements, it's the preferred editor by sysadmins worldwide.

Easy to install

Vim is also easy to install on Windows and Macs and is packaged in most Linux distros meaning that, even if it isn't installed in your system, Vim is one line from the terminal and two clicks from your software manager.

Vim is lightweight

Differently from most editors, Vim is very lightweight. The installation package is only 10 Mb and depending on your setup, memory consumption reaches  20 Mb. Compare that with most text editors, especially the Electron-based editors like Visual Studio. Install size is not less than 200Mb, memory consumption quickly 1Gb (50 times more!) while requiring 1.5Gb of storage, making it slow, even on modern hardware.

If you're running a Mac, a low end computer, a phone, or even a Raspberry Pi, Vim is definitely a good option for you.

Vim is stable

As previously said, Vim has been around for almost 30 years. And will probably be for at two more decades. Learning Vim is an excellent investment as you will be able to use your knowledge for the next two decades at least.

Compare that to the editor you use today (Eclipse, Visual Studio, Sublime Text, Visual Studio Code) - can you really guarantee you'll be using them ten years from now?

Vim is language-independent

Vim works well with anything you want, as long as it's text. Vim works by default with most file formats, has locales, can be localized, supports eastern typography such as Arabic and Hebrew and comes with built-in support (including highlighting) for most languages.

Vim respects your freedom

Vim does not contain any built-in telemetry. It's (unfortunately) common theses days the companies are abusing your statistics in favor of improvements in their system. Sysadmins trust that Vim will not be reaching the network to run ad-hoc requests.

Vim is efficient

Vim is brilliant in how it optimizes your use of the keyboard. We'll talk about that later but for now, understand that its combination of multiple modes, motions, macros and other brilliant features makes it literally light-years ahead of other text editors.

Thriving Ecosystem

Stop for a second and think about which feature you couldn't live without today on your current text editor? The answer you probably be that Python or Go extension, meaning that what you'll miss is not actually about the editor but about its ecosystem.

Vim has a brilliant ecosystem. You'll find thousands of extensions covering anything you need. You can also host your extensions anywhere (on GitHub, for example) without being locked by any vendor. You could also host them in private/corporate repos just for your team or share on public directories like Vim Awesome.

Vim is ultra-customizable

Even if by default Vim has most of what you need, it's important to understand that Vim lets you change pretty much everything. For example, you can make temporary/local customizations (by using the Ex mode), permanent customizations (by changing your .vimrc) or even customizations based on file type.

Vim is always getting better

Vim is actively developed meaning that it keeps getting better. Vim users get security patches and new features all the time. Vim is also updated to accommodate the latest upgrades on modern operating systems while also supporting older systems too!

Huge Community

Vim's community is huge and you can get help easily. These days, the most active discussions happen on Vim's mailing lists, Stack Exchange, IRC, YouTube and of course, Reddit.

Extensive documentation

Learning how to learn Vim is the key to a continuous understanding of the tool and not getting frustrated. There are many ways to get help on Vim: using its built-in help system, using the man pages and obviously, accessing the communities listed above.

Vim is free

These days it may be odd to say that Vim's free. Vim's freedom goes beyond its price, but also your freedom to modify it to your needs and deploy it wherever you want. Vim developers also have a strong commitment to helping needed people around the world.

GUI-less

Vim also runs GUI-less, meaning it runs on your terminal. So you get a full featured text-editor on any system you're working on, regardless if it's a local desktop or remote supercomputer. This feature is essential for sysadmins and developers who often need to modify text files on remote machines trough an SSH connection.

Rich out-of-the-box toolset

Vim comes with fantastic tooling by default: powerful search, regular expression support, syntax highlighting, text sort, integrated terminal, integrated file manager, cryptography, color schemes, plugin management and much more. All without a single plugin installed!

Vim integrates into your workflow

Differently from other text editors which force you into their thing, Vim adjusts seamlessly to your workflow via powerful customization, extension support, integrated shell support and ability to pipe data in/out from it. 

Vim can be programmed

Want to go the extra mile? Vim also has its own language, called VimL. With it you can create your own plugins and optimize even further the system to your needs.

Vim will boost your productivity

There are multiple ways Vim will boost your productivity. First, Vim's extensive use of the home row of the keyboard saves you from having to reach the arrow keys (or even worse, the mouse) to do your work. Second, with Vim you can quickly create macros to reproduce repetitive operations, third, the combination of motions, plugins, custom shortcuts and shell integration will definitely boost your productivity way more than you could imagine.

Vim will make you type better and faster

Being keyboard based, Vim's workflow based on the home row will definitely help force you to type better. With Vim you'll realize that you probably move your hands way more than you should and will significantly increase your typing speed.

Vim will make you learn more

Most editors these days do too much. Yes, part of that is imposed on us by languages that require a lot of metadata (Java and C# for example). One problem with that is that you end up relying on the text editor much more than you need. Without access to Eclipse or Visual Studio it may be possible that you'll feel the impostor syndrome. 

With Vim, despite being able to, you'll feel closer to your work, resulting in a better understanding of what you're doing. You'll also realize that you will learn more and memorize better the contents of what you're working on.

Conclusion

On this post we provided many tips why one should learn Vim. Vim is stable, ubiquitous and is supported by an engaged, growing community. Given all its features, Vim is definitely a good tool to learn now and harvest the benefits for decades to come.

References

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Monday, January 3, 2022

Why use the terminal

The command-line (aka terminal) is a scary thing for most users. But understanding it can be a huge step in your learning journey and add a significant boost to your career in tech.

Photo by Tianyi Ma on Unsplash

Depending on your technical skills, the command-line interface (also known as CLI or terminal) may look scary. But it shouldn't! The CLI is a powerful and resourceful tool that every person aspiring greater tech skills should learn and be comfortable with. On this article, let's review many reasons why you should learn and use the command line, commonly (and often incorrectly) referred to as terminal, shell, bash and CLI. 

This article is an adaptation of another one originally published by me on Linux4us. I'm re-publishing here to a wider audience with a few tweaks.

Ubiquitous

The command-line interface (CLI) is available in every operating system, not only in Linux. Very frequently, this is where developers and system administrators spend a lot of time. But, if you want to work with Linux, development, the cloud or with technology in general, better start learning it.

Terminals are available in every operating system including Linux, Windows and Macs

Powerful

CLI-based apps are much more powerful than their GUI-based equivalents. That happens because usually GUIs are usually wrappers around libraries that power both the GUIs and the terminal apps. Very frequently, these libraries contain way more functionality than what's available in the graphical interface because, as you might expect, since software development takes time and costs money to produce, developers only add to GUI apps the most popular features. 

For example, take a look at the plethora of options that the GNU find tool provides us:

Does your GUI-based find tool has all those options?

Quicker

Common and repetitive tasks are also faster in the terminal with the advantage that you will be able to repeat and even schedule these tasks so they run automatically, releasing you to do actual work, leaving the repetitive tasks to computer.

For example, consider this standard development workflow:

  1. download code from GitHub
  2. make changes
  3. commit code locally
  4. push changes back to GitHub

If you were doing the above using a GUI-based git client (for example, Tortoise Git), the workflow would be similar to the below, taking you approximately 20 minutes to complete:

  1. Open Tortoise Git's web page
  2. Click Download
  3. Next -> Next -> Next -> Finish
  4. Right-click a folder in Windows Explorer (or Nautilus, or Finder) -> Select clone -> Paster the Url -> Click OK
  5. Wait for the download to Complete -> Click OK
  6. Back to Windows Explorer -> Find File -> Open it
  7. Make your changes (by probably using GEdit, KEdit or Visual Studio Code) -> Save
  8. Back to Windows Explorer
  9. Right Click -> Commit
  10. Right Click -> Push
  11. Take a deep breath

In the terminal (for example, in Ubuntu), the workflow would be equivalent to the below and could be completed in less than 2 minutes:

sudo apt update && sudo apt install git -y   # install git
git clone <url>     # clone the GitHub repo locally
vim/nano file -> save  # edit the file using a text-based editor
git commit -m <msg> # commits the file locally
git push  # push the changes back to our GitHub repo

Automation

Terminal/CLI-based tasks can be scripted (automated) and easily repeated, meaning that you will be able to optimize a big part of your workflow. Another benefit is that these scripts can be easily shared, exactly as business and professional developers do!

So let's continue the above example. Our developer realized she is wasting too much time in the GUI and would like to speed up her workflow even more. She learned some bash scripting and wrote the function below:

gcp ()
{
    msg="More updates";
    if [ -n "$1" ]; then
        msg=$1;
    fi;
    git add ./ && git commit -m "$msg" && git push
} 

She's happy because now she can run from the terminal, the below command as soon as she finishes her changes:

gcp <commit-msg>

What previously took 5 minutes is now is done in 2 seconds (1.8 seconds to write the commit message and 0.2 to push the code upstream). A significant improvement in her workflow. Imagine how much more productive she would be during the course of her career!

It's important to always think how can you optimize your workflow. These small optimizations add up to your productivity significantly over time.

Lightweight

Not only the CLI is faster and more lightweight than equivalent GUI-based applications but it's quicker to run the same commands. For example, consider a Git client like Tortoise Git. It was supposed to be lightweight (what most GUI apps aren't) but it takes 3s to completely load and uses 10Mb of memory:

Our GUI-based git client TortoiseGit

Now take a look at its CLI equivalent. git status runs in 0.3s and consumes less than 1Mb. In other words, 20 times more efficient memory-wise and 10 times faster. 

A simple CLI command is 20x more efficient and 10x faster then its GUI equivalent

Disk Space Efficient

Another advantage of terminal apps over their GUI-equivalents is reduced disk space. For example, contrast these two popular apps. Can you spot the differences?

Application    Installation Size       Total Size       Memory Usage   
Visual Studio Code        80Mb 300Mb 500Mb (on sunny days)
Nano 0.2 Mb 0.8 Mb 3 Mb
400x more efficient 375x more efficient 160x more efficient

Extensible

Another important aspect is that the CLI is extensible. From it, skilled users could easily either extend its basic functionality using its built-in features like pipes and redirections combining inputs and outputs from different tools.

For example, sysadmins could list the first two users in the system who use Bash as a shell, ordered alphabetically with:

cat /etc/passwd | grep bash | cut -d : -f 1 | sort | head -2

What's interesting from the above command is how we combined 5 different tools to get the results we need. Once you master the Linux terminal, you'll too will be able to utilize these tools effectively to get work done significantly faster!

This is a more advanced topic. We'll see in future posts more details about it.

Customizable

As you might expect, the terminal is extremely customizable. Everything from the prompt to functions (as seen above) and even custom keybindings can be customized. For example, In Linux, binding the shortcut Ctrl+V to open the Vim text editor on the terminal is simple. Add this to your .bashrc file:

bind '"\C-V":"vim\n"'

Extensive range of Apps

Contrary to what most newcomers thing, the terminal has apps too! You will find apps for pretty much any use case. For example:

The above list is far from comprehensive. It's just to give you an idea of what you'd be able to find in there

For example, here's the Castero Podcast app running on a terminal:

Source; GitHub

Professional Development

Want to work with Linux, as a developer or with the cloud? Another important aspect of using the terminal is that it will make you more ready for the job market. Since servers usually run Linux and don't have GUIs, you will end up having to use some of the above tools on your day-to-day work. Developers frequently use it to run repetitive tasks, becoming way more productive. So why not start now?

Learn more about your System

Hopefully at this point you realize that you will learn way more about your system and computers in general when you use the terminal. And I'm not talking solely to Linux users. Windows and Mac users will learn a lot too! This is the secret sauce that the most productive developers want you to know!

It's also a huge win for testing new tools, maintaining your system, installing software, fixing issues and tweaking as you wish.

Getting Started

Ready to get started on your terminal/CLI journey? Here's a video that may serve as a good intro: 

Conclusion

Every modern computer has a terminal. Learning it will save you time, allow you to automate common actions, make you learn more about your system, grow professionally and be more productive. Well worth the effort, isn't it?

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Monday, November 1, 2021

Docker and Containers - Everything you should know

Much has been discussed about Docker, containers, virtualization, microservices and distributed applications. On this post let's recap the essential concepts and review related technologies.
Photo by chuttersnap on Unsplash

Much has been discussed about Docker, microservices, virtualization and containerized applications. So much, that most people probably didn't catch up. As the ecosystem matures and new technologies and standards come and go, the container ecosystem can be confusing at times. On this post we will recap the essential concepts and a solid reference for the future.

Virtualization

So let's start with a bit of history. More a less 20 years ago the industry saw a big growth in processing power, memory, storage and a significant decrease in hardware prices. Engineers realized that their applications weren't utilizing the resources effectively so they developed Virtual machines (VMs) and hypervisors to run multiple operating systems in parallel on the same server.
Source: Resellers Panel
A hypervisor is computer software, firmware or hardware that creates and runs virtual machines. The computer where the hypervisor runs is called the host, and the VM is called a guest.

The first container technologies

As virtualization grew, engineers realized that VMs were difficult to scale, hard to secure, utilized a lot of redundant resources and maxed out at a dozen per server. Those limitations led to the first containerization tools listed below.
  • FreeBSD Jails: FreeBSD jails appeared in 2000 allowing the partitioning of a FreeBSD system into multiple subsystems. Jails was developed so that the same server could be sharded with multiple users without securely. 
  • Google's lmctfy: Google also had their own container implementation called lmcty (Let Me Contain That For You). According to the project page, lmctfy used to be Google’s container stack which now seems to be moved to runc. 
  • rkt: rkt was another container engine for Linux. rkt has ended and with CoreOS transitioning into Fedora CoreOS. Most of the efforts on that front should be happening into Podman now. 
  • LXC: released on 2008, the Linux Containers project (LXC) is another container solution for Linux. LXC provides a CLI, tools, libraries and a reference specification that's followed by Docker, LXD, systemd-nspawn and Podman/Buildah. 
  • Podman/Buildah: Podman and Buildah are also tools to create and manage containers. Podman provides an equivalent Docker CLI and improves on Docker by neither requiring a daemon (service) nor requiring root privileges. Podman's available by default on RH-based distros (RHEL, CentOS and Fedora). 
  • LXD: LXD is another system container manager. Developed by Canonical, Ubuntu's parent company, it offers pre-made images for multiple Linux distributions and is built around a REST API. Clients, such as the command line tool provided with LXD itself then do everything through that REST API. 

Docker

Docker first appeared in 2008 as dotCloud and became open-source in 2013. Docker is by far the most used container implementation. According to Docker Inc., more than 3.5 million Docker applications have been deployed and over 37 billion containerized applications downloaded.

Docker grew so fast because it allowed developers to easily pull, run and share containers remotely on Docker Hub as simple as:
docker run -it nginx /bin/bash

Differences between containers and VMs

So what's the difference between containers and VMs? While each VM has to have their own kernel, applications, libraries and services, containers don't as they share some of the host's resources. VMs are also slower to build, provision, deploy and restore. Since containers also provide a way to run isolated services, are lightweight (some are only a few MBs), start fast and are easier to deploy and scale, containers became the standard today.

The image below shows a visual comparison between VMs and Containers:
Source: ZDNnet

Why Containers?

Here are guidelines that could help you decide if you should be using containers instead of VMs:
  • containers share the operating system's kernel with other containers
  • containers are designed to run one main process, VMs manage multiple sets of processes
  • containers maximize the host's resource utilization 
  • containers faster to run, download and start
  • containers are easier to scale
  • containers are more portable than VMs
  • containers are usually more secure due to the reduced attack surface
  • containers are easier to deploy 
  • containers can be very lightweight (some are just a few MBs)
Containers are not only advantages. They also bring many technical challenges and will require you to not only rethink how your system is designed but also to use different tools. Look at the Ecosystem section below to understand.

Usage of Containers

And how much are containers being used? According to the a Cloud Native Computing Foundation survey, 84% of companies today use containers in production, a 15% increase from last year. Another good metric is provided by the Docker Index:

Open Collaboration

As the ecosystem stabilized, companies such as Amazon, Google, Microsoft and Red Hat collaborated on a shared format under Open Container Initiative (OCI). OCI was created from standards and technologies developed by Docker such as libcontainer. The standardization means that today you can run Docker and other LXC-based containers such as Podman on any OS.

The Cloud Native Computing Foundation (CNCF), part of the Linux Foundation is another significant entity in the area. CNF hosts many of the fastest-growing open source projects, including Kubernetes, Prometheus, and Envoy. CNCF's mission is to promote, monitor and hosts critical components of the global technology infrastructure.

The Technologies

Now let's dive into the technologies used by Docker (and OCI containers in general). The image below shows a detailed overview of the internals of a container. For clarity, we'll break the discussion in user and kernel space.

User space technologies

In usersland, Docker and other OCI containers utilize essentially these technologies:
  • runc: runc is a CLI tool for spawning and running containers. runc is a fork of libcontainer, a library developed by Docker that was donated to the OCI and includes all modifications needed to make it run independently of Docker. 
  • containerd: containerd is a project developed by Docker and donated to the CNCF that builds on top of runc adding features, such as image transfer, storage, execution, network and more.
  • CRI: CRI is the containerd plugin for the Kubernetes Container Runtime Interface. With it, you could run Kubernetes using containerd as the container runtime. 
  • Prometheus: Prometheus is an open-source systems monitoring and alerting toolkit. Prometheus is an independent project and member of the Cloud Native Computing Foundation.
  • gRPC: gRPC is an open source remote procedure call system developed by Google. It uses HTTP/2 for transport, Protocol Buffers as the interface description language, and provides features such as authentication, bidirectional streaming and flow control, blocking or nonblocking bindings, and cancellation and timeouts.
  • Go: yes, some of the tools are developed in C but Go shines in the area. Most of the open-source projects around containers use Go including: runc, runtime-tools, Docker CE, containerd, Kubernetes, libcontainer, Podman, Buildah, rkt, CoreDNS, LXD, Prometheus, CRI, etc. 

Kernel space technologies

In order to provide isolation, security and resource management, Docker relies on the following features from the Linux Kernel:
  • Union Filesystem (or UnionFS, UFS): UnionFS is a filesystem that allows files and directories of separate file systems to be transparently overlaid, forming a single file system. Docker implements some of them including brtfs and zfs.
  • Namespaces: Namespaces are a feature of the Linux kernel that partitions kernel resources so that one set of processes sees one set of resources while another set of processes sees a different set of resources. Specifically for Docker, PID, net, ipc, mnt and ufs are required.
  • Cgroups: Cgroups allow you to allocate resources — such as CPU time, system memory, network bandwidth, or combinations of these resources — among groups of processes running on a system. 
  • chroot:  chroot changes the apparent root directory for the current running process and its children. A program that is run in such a modified environment cannot name files outside the designated directory tree. 

Docker Overview

You probably installed Docker on your machine, pulled images and executed them. Three distinct tools participated on that operation: two local Docker tools and a remote container registry. On your local machine the two tools are:
  • Docker client: this is the CLI tool you use to run your commands. The CLI is essentially a wrapper to interact with the daemon (service) via a REST API.
  • Docker daemon (service): the daemon is a backend service that runs on your machine. The Docker daemon is the tool that performs most of the jobs such as downloading, running and creating resources on your machine.
The image below shows how the client and the daemon interact with each other:
Source: Docker Overview

Remote Registry

And what happens when you push your images to a container registry such as Docker Hub? The next image shows the relationship between client, dameon and the remote registry.
Source: Docker Overview

Images and Containers

Moving lower on the stack, it's time to take a quick look at Docker images. Internally, a Docker image can look like this:

Important concepts about images and containers that you should know:
  • Images are built on layers, utilizing the the union file system.
  • Images are readonly. Modifications made by the user are stored on a separate docker volume managed by the Docker daemon. They are removed as soon as you remove the container.
  • Images are managed using  docker image <operation> <imageid>
  • An instance of an image is called a container.
  • Containers are managed with the  docker container <operation> <containerid>
  • You can inspect details about your image with docker image inspect <imageid>
  • Images can be created with docker commit, docker build or Dockerfiles
  • Every image has to have a base image. scratch is the base empty image.
  • Dockerfiles are templates to script images. Developed by Docker, they became the standard for the industry.
  • The docker tool allows you to not only create and run images but also to create volumes, networks and much more.
For more information about how to build your images, check the official documentation.

Container Security

Due to the new practices of containers new security measures had to be applied. By default, containers are very reliable on some of the security measures of the host operating system kernel. Docker applies the principle of least privilege to provide isolation and reduce the attack surface. In essence, the best practices around container practice are:
  • signing containers 
  • only used images from trusted registries
  • harden the host operating system
  • enforce the principle of least privilege and do not elevate access to access devices
  • offer centralized logging and monitoring
  • run automated vulnerability scanning

The Ecosystem

Since this post is primarily about containers I'll defer the discussion of some the ecosystem for the future. However, it's important to list the main areas people working with containers, microservices and distributed applications should learn:
  • Container Registries: remote registries that allow you to push and share your own images.
  • Orchestration: orchestration tools deploy, manage and monitor your microservices.
  • DNS and Service Discovery: with containers and microservices, you'll probably need DNS and service discovery so that your services can see and talk to each onther.
  • Key-Value Stores: provide a reliable way to store data that needs to be accessed by a distributed system or cluster.
  • Routing: routes the communication between microservices.
  • Load Balancing: load balancing in a distributed system is a complex problem. Consider specific tooling for your app.
  • Logging: microservices and distributed applications will require you to rethink your logging strategy so they're available on a central location.
  • Communication Bus: your applications will need to communicate and using a Bus is the preferred way.
  • Redundancy: necessary to guarantee that your system can sustain load and keep operating on crashes.
  • Health Checking: consistent health checking is necessary to guarantee all services are operating.
  • Self-healing: microservices will fail. Self-healing is the process of redeploying services when they crash.
  • Deployments, CI, CD: redeploying microservices is different than the traditional deployment. You'll probably have to rethink your deployments, CI and CD.
  • Monitoring: monitoring should be centralized for distributed applications.
  • Alerting: it's a good practice to have alerting systems on events triggered from your system.
  • Serverless: allows you to build and run applications and services without running the servers..
  • FaaS - Functions as a service: allows you to develop, run, and manage application functionalities without maintaining the infrastructure.

Conclusion

On this post we reviewed the most important concepts about Docker containers, virtualization and the whole ecosystem. As you probably realized by the lenght of this post, the ecosystem around containers and microservices is huge - and keeps growing! We will cover in more detail much of the topic addressed here on future posts.

In the next posts, we will start divining in the details of some of these technologies.

References

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Wednesday, September 1, 2021

Docker - 28 facts you should know

Docker is a very mature technology at this point but there's a lot of information that's still confused or ignored. Let's review some facts that everyone working with Docker should know.
Photo by Shunya Koide on Unsplash

Docker is a pretty established technology at this point and most people should know what it is. There are however important facts that everyone should know about. Let's see them.

Docker is not just a Container

In 2013, Docker introduced what would become the industry standard for containers. For millions of developers today, Docker is the standard way to build apps. However, Docker is much more than that command that you use on the terminal. Docker is a set of platform as a service products that uses OS-level virtualization to deliver software in packages called containers.

Today, apart from Docker (the containerization tool) Docker Inc (the company) offers:
  • Docker Engine: an open source containerization technology for building and containerizing your applications. Available for Linux and Windows.
  • Docker Compose: a tool for running and orchestrating containers on a single host.
  • Docker Swarm: A toolkit for orchestrating distributed systems at any scale. It includes primitives for node discovery, raft-based consensus, task scheduling and more
  • Docker Desktop: tools to run Docker on Windows and Mac.
  • Docker Hub: the public container registry
  • Docker Registry: server side application that stores and lets you distribute Docker images
  • Docker Desktop Enterprise: offers enterprise tools for the desktop
  • Docker Enterprise: a full set of tools for enterprise customers.
  • Docker Universal Control Plane (UCP): a cluster management solution
  • Docker Kubernetes Service: a full Kubernetes orchestration feature set
  • Security Scans: available on Docker Enterprise.

The second most loved platform

Developers love Docker (the tool 😉). Docker was elected the second most loved platform according to StackOverflow's 2019 survey. In fact, the company has made huge contributions to the development ecosystem and is a bliss to use. Well deserved!
Source: StackOverflow's 2019 survey

Images != Containers

A lot of people confuses this and interchangeability mix images and containers. The correct way to think about it is by using an Object-Oriented programming paradigm of Class/Instance. A Docker image is your class whereas the container is your instance. Continuing on the analogy with OO you can create multiple instances (containers) of your class (image).

As per Docker themselves,
A container is a standard unit of software that packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another. A Docker container image is a lightweight, standalone, executable package of software that includes everything needed to run an application: code, runtime, system tools, system libraries and settings.

Docker was once named dotCloud

What you know today as Docker Inc., once was called dotCloud Inc.. dotCloud Inc. changed its name to Docker Inc. to grow the ecosystem by establishing Docker as a new standard for containerization, an alternative approach to virtualization which rapidly gained adoption. The project became one of the fastest-growing open source projects on GitHub. Surely it worked!

Docker is neither the first nor the only tool to run containers

Docker is neither the first nor the only tool to run containers. In fact, one of the key technologies Docker is based of, the chroot syscall was released in 1979 for Unix v7. Next, the first known container technology was FreeBSD jails (2000), Solaris Zones (2004), LXC (2008) and  Google's mcty. However, Docker made significant contributions to the segment since the establishment of Open Container Initiative (OCI) in 2015. Today the open standards allow tools such as Podman to offer an equivalent Docker CLI.

Containers are the new unit of deployment

In the past, applications included tens, and on some cases, hundreds of distinct business units and a huge number of lines of code. Developing, maintaining, deploying and even scaling out those big monoliths required a huge effort. Developers were frequently frustrated that their code would fail on production but would work locally. Containers alleviate this pain as they are deployed exactly as intended, are easier to deploy and can be easily scalable.

Containers run everywhere

Due to that fact that containers run on top of the container framework, they abstract the platform they're running in. That's a huge enhancement from the past where IT had to replicate the exact same setup on different environments. It also simplifies due to the fact that today you can deploy your images to your own datacenter, cloud service or even better, to a managed Kubernetes service with confidence that they'll run as they ran on your machine.
Source: Docker - What's a Container?

Docker Hub

Docker Hub is Docker's official container repository. Docker Hub is the most popular container registry in the world and one of the catalysts for the enourmous growth of Docker and containers themselves. Users and companies share their images online and everyone can download and run these images as simple as running the dock run command such as:
docker run -it alpine /bin/bash
Docker Hub is also the official repo for some of the world's most popular (and awesome!) technologies including:

Docker Hub Alternatives

But Docker Hub's not the only container registry out there. Multiple vendors including AWS, Google, Microsoft and Red Hat have their offerings. Currently, the most popular alternatives to Docker Hub are Google Container Registry (GCR), Amazon Elastic Container Registry (ECR), Azure Container Registry (ACR), Quay and Red Hat Container Registry. All of them offer public and public repos.

Speaking of private repos, Docker also has a similar offering called Docker Trusted Registry. Available on Docker Enterprise, you can install it on your intranet and securely store, serve and manage your company's images.

Docker Images

As per Docker, an image is a lightweight, standalone, executable package of software that includes everything needed to run an application: code, runtime, system tools, system libraries and settings.

There are important concepts about images and containers that are worth repeating:
  • Images are built on layers, utilizing a technology called UnionFS (union filesystem).
  • Images are readonly. Modifications made by the user are stored on a separate docker volume managed by the Docker daemon. They are removed as soon as the container is removed.
  • Images are managed using  docker image <operation> <imageid>
  • An instance of an image is called a container.
  • Containers are managed with the  docker container <operation> <containerid>
  • You can inspect details about your image with docker image inspect <imageid>
  • Images can be created with docker commit, docker build or Dockerfiles
  • Every image has to have a base image. scratch is the base empty image.
  • Dockerfiles are templates to script images. Developed by Docker, they became the standard for the industry.
  • The docker tool allows you to not only create and run images but also to create volumes, networks and much more.
For more information about how to build your images, check the official documentation.

A Layered Architecture

Docker images are a compilation of read-only layers. The below image shows an example of the multiple layers a Docker image can have. The upper layer is the writeable portion: modifications made by the user are stored on a separate docker volume managed by the Docker daemon. They are removed as soon as you remove the container.

Volumes are your disks

Because images and containers are readonly, and because the temporary volume created for your image is lost as soon as the image is removed the recommended way to persist the data for your container is volumes. As per Docker:
Volumes are the preferred mechanism for persisting data generated by and used by Docker containers. While bind mounts are dependent on the directory structure of the host machine, volumes are completely managed by Docker.
Some advantages of Volumes over bindmounts are:
  • Volumes are easier to back up or migrate than bind mounts.
  • You can manage volumes using Docker CLI commands or the Docker API.
  • Volumes work on both Linux and Windows containers.
  • Volumes can be more safely shared among multiple containers.
Creating volumes is as simple as:
docker volume create myvol
And using them with your container should be as simple as:
docker run -it -v myvol:/data alpine:latest /bin/sh
Other commands of interest for volumes are:
  • docker volume inspect <vol>:  inspects the volume
  • docker volume rm <vol>:  removes the volume
You can also have read-only volumes by appending :ro to your command

Dockerfiles

Dockerfile is a text file that contains all the commands a user could call on the command line to assemble an image. Using docker build users can create an automated build that executes several command-line instructions in succession.

The most common commands in Dockerfiles are:
  • FROM <image_name>[:<tag>]: specifies the base the current image on <image_name>
  • LABEL <key>=<value> [<key>=value>...]: adds metadata to the image
  • EXPOSE <port>: indicates which port should be mapped into the container
  • WORKDIR <path>: sets the current directory for the following commands
  • RUN <command> [ && <command>... ]: executes one or more shell commands
  • ENV <name>=<value>: sets an environment variable to a specific value
  • VOLUME <path>: indicates that the <path> should be externally mounted volume
  • COPY <src> <dest>: copies a local file, a group of files, or a folder into the container
  • ADD <src> <dest>: same as COPY but can handle URIs and local archives
  • USER <user | uid>: sets the runtime context to <user> or <uid> for commands after this one
  • CMD ["<path>", "<arg1>", ...]: defines the command to run when the container is started

.dockerignore

You may be familiar with .gitignore. Docker also accepts a .dockerignore file that can be used to ignore files and directories when building your image. Despite what you heard before, Docker recommends using .dockerignore files:
To increase the build’s performance, exclude files and directories by adding a .dockerignore file to the context directory.

Client-Server Architecture

The Docker tool is split into two parts: a daemon with a RESTful API and a client that interacts to the daemon. The docker command you run on the command line is a frontend and essentially interacts with the daemon. The daemon is a server and a service that listens and responds to requests from the client or services authorized using the HTTP protocol. The daemon also manages your images, containers and all operations including image transfer, storage, execution, network and more.

runc and containerd

A technical overview of the internals of Docker can be seen on the below image. Since already discussed some of the technologies on the Platform layer, let's focus now on the technologies found on the platform layer: containerd and runc.
runc is a CLI tool for spawning and running containers according to the OCI specification. runc was created from libcontainer, a library developed by Docker and donated to the OCI. libcontainer was open sourced by Docker in 2013 and donated by Docker to the Open Container Initiative (OCI).

containerd is an open source project project and the industry-standard container runtime. Developed by Docker and donated to the CNCF, containerd builds on top of runc, is available as a daemon for Linux and Windows and adds features, such as image transfer, storage, execution, network and more. containerd is by far the most popular container runtime and is the default runtime of Kubernetes 1.8 + and Docker.

Linux kernel features

In order to provide isolation, security and resource management, Docker relies on the following features from the Linux Kernel:
  • Union Filesystem (or UnionFS, UFS): UnionFS is a filesystem that allows files and directories of separate file systems to be transparently overlaid, forming a single coherent file system.
  • Namespaces: Namespaces are a feature of the Linux kernel that partitions kernel resources so that one set of processes sees one set of resources while another set of processes sees a different set of resources. Specifically for Docker, PID, net, ipc, mnt and ufs are required.
  • Cgroups: Cgroups allow you to allocate resources — such as CPU time, system memory, network bandwidth, or combinations of these resources — among groups of processes running on a system. 
  • chroot:  chroot changes the apparent root directory for the current running process and its children. A program that is run in such a modified environment cannot name files outside the designated directory tree.

A huge Ecosystem

The ecosystem around containers just keep growing. The image below lists some of the tools and services in the area.

Today the ecosystem around containers encompasses:
  • Container Registries: remote registries that allow you to push and share your own images.
  • Orchestration: orchestration tools deploy, manage and monitor your microservices.
  • DNS and Service Discovery: with containers and microservices, you'll probably need DNS and service discovery so that your services can see and talk to each onther.
  • Key-Value Stores: provide a reliable way to store data that needs to be accessed by a distributed system or cluster.
  • Routing: routes the communication between microservices.
  • Load Balancing: load balancing in a distributed system is a complex problem. Consider specific tooling for your app.
  • Logging: microservices and distributed applications will require you to rethink your logging strategy so they're available on a central location.
  • Communication Bus: your applications will need to communicate and using a Bus is the preferred way.
  • Redundancy: necessary to guarantee that your system can sustain load and keep operating on crashes.
  • Health Checking: consistent health checking is necessary to guarantee all services are operating.
  • Self-healing: microservices will fail. Self-healing is the process of redeploying them when they crash.
  • Deployments, CI, CD: redeploying microservices is different than the traditional deployment. You'll probably have to rethink your deployments, CI and CD.
  • Monitoring: monitoring should be centralized for distributed applications.
  • Alerting: it's a good practice to have alerting systems on events triggered from your system.
  • Serverless: servless technologies are also growing year over year. Today you can even find solid alternatives clouds such as AWS, Google Cloud and Azure.

Containers are way more effective than VMs

While each VM has to have their own kernel, applications, libraries and services, containers do not since they share the resources of the host. VMs are also slower to provision, deploy and restore. So since containers also provide a way to run isolated services, can be lightweight (some are only a few MBs), start quickly, are quicker to deploy and scale, containers are usually the preferred unit of scale these days.
Source: ZDNnet

Containers are booming

According to the latest Cloud Native Computing Foundation survey, 84% of companies use containers in production with 78% using Kubernetes. The Docker Index also provides impressive numbers reporting more than 130 billion pulls just from Docker Hub.
Source: Docker

Open standards

Companies such as Amazon, IBM, Google, Microsoft and Red Hat collaborate under the Open Container Initiative (OCI) which was created from standards and technologies developed by Docker such libcontainer. The standardization allows you to run Docker and other LXC-based containers on third-party tools such as Podman in any operating system.

Go is the container language

You won't see this mentioned elsewhere but I'll make this bold statement: Go is the container language. Apart from kernel features (written in C) and lower level services (written in C++), most of the open-source projects in the container space use Go, including:  runc, runtime-tools, Docker CE, containerd, Kubernetes, libcontainer, Podman, Buildah, rkt, CoreDNS, LXD, Prometheus, CRI, etc. 

gRPC is the standard protocol for synchronous communication

gRPC is an open source remote procedure call system developed by Google. It uses HTTP/2 for transport, Protocol Buffers as the interface description language, and provides features such as authentication, bidirectional streaming and flow control, blocking or nonblocking bindings, and cancellation and timeouts. gRPC is also the preferred protocol when communicating between containers.

Orchestration technologies emerge

The most deployed orchestration tool today is Kubernetes with 78% of the market share. Kubernetes was developed at Google then donated to the the Cloud Native Computing Foundation (CNCF). There are however other container orchestration products are Apache Mesos, Rancher, Open Shift, Docker Cluster on Docker Enterprise, and more.

Kubernetes is the Container Operating System

It's impossible talk Docker these days without mentioning Kubernetes. Kubernetes is an open source orchestration system for automating the management, placement, scaling and routing of containers that has become popular with developers and IT operations teams in recent years. It was first developed by Google and contributed to Open Source in 2014, and is now maintained by the Cloud Native Computing Foundation. Since version 1.9 Kubernetes uses containerd sas its container runtime so it can use other container runtimes such CRI-O. A container runtime is responsible for managing and running the individual containers of a pod.

Today Kubernetes is embedded with Docker Desktop so developers can develop Docker and Kubernetes at the comfort of their desktops. Plus, because Docker containers implement the OCI specification, you can build your containers using a tools such as Buildah / LXD and run in on Kubernetes.

Security Scans

Docker also offers a automated scans via Docker Enterprise Platform. Because images are readonly, automated scans are simple as it becomes simply checking the sha of your image. The image below details how this happens. More information is available here.

Windows also has native container

As a Windows user, you might already be aware that there exists so-called Windows containers that run natively on Windows. And you are right. Recently, Microsoft has ported the Docker engine to Windows and it is now possible to run Windows containers directly on a Windows Server 2016 without the need for a VM. So, now we have two flavors of containers, Linux containers and Windows containers. The former only run on Linux host and the latter only run on a Windows Server. In this book, we are exclusively discussing Linux containers, but most of the things we learn also apply to Windows containers.  

Today you can run Windows-based or Linux-based containers on Windows 10 for development and testing using Docker Desktop, which makes use of containers functionality built-in to Windows. You can also run containers natively on Windows Server.

Service meshes

More recently, the trend goes towards a service mesh. This is the new buzz word. As we containerize more and more applications, and as we refactor those applications into more microservice-oriented applications, we run into problems that simple orchestration software cannot solve anymore in a reliable and scalable way. Topics in this area are service discovery, monitoring, tracing, and log aggregation. Many new projects have emerged in this area, the most popular one at this time being Istio, which is also part of the CNCF.

Conclusion

On this post we reviewed 28 about Docker everyone should know. Hope this article was interesting and that you learned something new today. Docker is a fantastic tool and given it's popularity, it's ecosystem will only grow bigger thus, it's important to learn it well and understand its internals.

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Bruno Hildenbrand