Docker Simplified: En praktisk vejledning til absolutte begyndere

Uanset om du planlægger at starte din karriere i DevOps, eller hvis du allerede er interesseret i det, hvis du ikke har Docker på dit CV, er det utvivlsomt tid for dig at tænke over det, da Docker er en af ​​de kritiske færdigheder for alle, der er i DevOps arena.

I dette indlæg vil jeg prøve mit bedste for at forklare Docker på den enkleste måde, jeg kan.

Før vi tager et dybt dyk og begynder at udforske Docker, lad os se på, hvilke emner vi vil dække som en del af denne nybegyndervejledning.

  • Hvad er Docker?
  • Problemet, som Docker løser
  • Fordele og ulemper ved at bruge Docker
  • Kernekomponenter i Docker
  • Docker Terminologi
  • Hvad er Docker Hub?
  • Docker-udgaver
  • Installation af Docker
  • Nogle vigtige Docker-kommandoer for at komme i gang
  • Afslutning

Lad os begynde med at forstå, hvad er Docker?

Enkelt sagt er Docker en softwareplatform, der forenkler processen med at opbygge, køre, administrere og distribuere applikationer. Det gør det ved at virtualisere operativsystemet på den computer, hvor det er installeret og kører.

Den første udgave af Docker blev udgivet i 2013.

Docker er udviklet ved hjælp af GO-programmeringssproget.

Når man ser på det rige sæt funktionalitet, som Docker har at tilbyde, er det blevet bredt accepteret af nogle af verdens førende organisationer og universiteter, såsom Visa, PayPal, Cornell University og Indiana University (bare for at nævne nogle få) for at køre og styre deres applikationer, der bruger Docker.

Lad os nu prøve at forstå problemet, og den løsning, som Docker har at tilbyde

Problemet

Lad os sige, at du har tre forskellige Python-baserede applikationer, som du planlægger at være vært på en enkelt server (som enten kan være en fysisk eller en virtuel maskine).

Hver af disse applikationer bruger en anden version af Python såvel som de tilknyttede biblioteker og afhængigheder, adskiller sig fra en applikation til en anden.

Da vi ikke kan have forskellige versioner af Python installeret på den samme maskine, forhindrer dette os i at være vært for alle tre applikationer på den samme computer.

Løsningen

Lad os se på, hvordan vi kunne løse dette problem uden at bruge Docker. I et sådant scenario kunne vi løse dette problem enten ved at have tre fysiske maskiner eller en enkelt fysisk maskine, som er kraftig nok til at være vært for og køre tre virtuelle maskiner på den.

Begge muligheder giver os mulighed for at installere forskellige versioner af Python på hver af disse maskiner sammen med deres tilknyttede afhængigheder.

Uanset hvilken løsning vi vælger, er omkostningerne forbundet med anskaffelse og vedligeholdelse af hardware ret dyre.

Lad os nu se, hvordan Docker kan være en effektiv og omkostningseffektiv løsning på dette problem.

For at forstå dette skal vi se på, hvordan Docker fungerer nøjagtigt.

Maskinen, hvorpå Docker er installeret og kører, kaldes normalt en Docker-vært eller vært i enkle vendinger.

Så når du planlægger at implementere en applikation på værten, opretter den en logisk enhed på den til at være vært for den applikation. I Docker-terminologi kalder vi denne logiske enhed en Container eller Docker Container for at være mere præcis.

En Docker Container har ikke noget operativsystem installeret og kører på det. Men det ville have en virtuel kopi af procestabellen, netværksgrænseflade (r) og filsystemets monteringspunkt (er). Disse er arvet fra operativsystemet til den vært, hvor containeren er hostet og kører.

Mens kernen i værtens operativsystem deles på tværs af alle containere, der kører på den.

Dette gør det muligt at isolere hver beholder fra den anden til stede på den samme vært. Således understøtter det flere containere med forskellige applikationskrav og afhængigheder til at køre på den samme vært, så længe de har de samme operativsystemkrav.

For at forstå, hvordan Docker har været gavnlig til at løse dette problem, skal du henvise til det næste afsnit, der diskuterer fordele og ulemper ved at bruge Docker.

Kort sagt, ville Docker virtualisere operativsystemet til den vært, som det er installeret og kører på, i stedet for at virtualisere hardwarekomponenterne.

Fordele og ulemper ved at bruge Docker

Fordele ved at bruge Docker

Nogle af de vigtigste fordele ved at bruge Docker er angivet nedenfor:

  • Docker understøtter flere applikationer med forskellige applikationskrav og afhængigheder, der skal hostes sammen på den samme vært, så længe de har de samme operativsystemkrav.
  • Opbevaring optimeret. Et stort antal applikationer kan hostes på den samme vært, da containere normalt er få megabyte i størrelse og bruger meget lidt diskplads.
  • Robusthed. En container har ikke et operativsystem installeret. Således bruger det meget lidt hukommelse i forhold til en virtuel maskine (som vil have et komplet operativsystem installeret og køre på det). Dette reducerer også opstartstiden til bare et par sekunder sammenlignet med et par minutter, der kræves for at starte en virtuel maskine.
  • Reducerer omkostninger. Docker er mindre krævende, når det kommer til den hardware, der kræves for at køre den.

Ulemper ved at bruge Docker

  • Applikationer med forskellige operativsystemkrav kan ikke hostes sammen på den samme Docker-vært. Lad os for eksempel sige, at vi har 4 forskellige applikationer, hvoraf 3 applikationer kræver et Linux-baseret operativsystem, og det andet program kræver et Windows-baseret operativsystem. I et sådant scenario kan de 3 applikationer, der kræver Linux-baseret operativsystem, hostes på en enkelt Docker-vært, mens applikationen, der kræver et Windows-baseret operativsystem, skal hostes på en anden Docker-vært.

Kernekomponenter i Docker

Docker Engine er en af ​​kernekomponenterne i Docker. Det er ansvarligt for den overordnede funktion af Docker-platformen.

Docker Engine er en klientserverbaseret applikation og består af 3 hovedkomponenter.

  1. Server
  2. REST API
  3. Klient

Den server kører en dæmon kendt som dockerd (Docker Daemon) , som ikke er andet end en proces. Det er ansvarligt for at oprette og administrere Docker-billeder, containere, netværk og volumener på Docker-platformen.

De REST API specificerer hvordan applikationerne kan interagere med serveren, og instruere den til at få deres job gjort.

Den klient er intet andet end en kommandolinje brugerflade, der giver brugerne mulighed for at interagere med Docker hjælp af kommandoerne.

Docker Terminologi

Lad os tage et hurtigt kig på nogle af terminologierne forbundet med Docker.

Docker Images and Docker Containers are the two essential things that you will come across daily while working with Docker.

In simple terms, a Docker Image is a template that contains the application, and all the dependencies required to run that application on Docker.

On the other hand, as stated earlier, a Docker Container is a logical entity. In more precise terms, it is a running instance of the Docker Image.

What is Docker Hub?

Docker Hub is the official online repository where you could find all the Docker Images that are available for us to use.

Docker Hub also allows us to store and distribute our custom images as well if we wish to do so. We could also make them either public or private, based on our requirements.

Please Note: Free users are only allowed to keep one Docker Image as private. If we wish to keep more than one Docker Image as private, we need to subscribe to a paid subscription plan.

Docker Editions

Docker is available in 2 different editions, as listed below:

  • Community Edition (CE)
  • Enterprise Edition (EE)

The Community Edition is suitable for individual developers and small teams. It offers limited functionality, in comparison to the Enterprise Edition.

The Enterprise Edition, on the other hand, is suitable for large teams and for using Docker in production environments.

The Enterprise Edition is further categorized into three different editions, as listed below:

  • Basic Edition
  • Standard Edition
  • Advanced Edition

Installing Docker

One last thing that we need to know before we go ahead and get our hands dirty with Docker is actually to have Docker installed.

Below are the links to the official Docker CE installation guides. You can follow these guides to install Docker on your machine, as they are simple and straightforward.

  • CentOS Linux
  • Debian Linux
  • Fedora Linux
  • Ubuntu Linux
  • Microsoft Windows
  • MacOS

Want to skip installation and head off straight to practicing Docker?

Just in case you are feeling too lazy to install Docker, or you don’t have enough resources available on your computer, you need not have to worry — here’s the solution to your problem.

You can head over to Play with Docker, which is an online playground for Docker. It allows users to practice Docker commands immediately, without having to install anything on your machine. The best part is it’s simple to use and available free of cost.

Docker Commands

Now it’s time to get our hands dirty with Docker commands, for which we all have been waiting till now.

docker create

The first command which we will be looking at is the docker create command.

This command allows us to create a new container.

The syntax for this command is as shown below:

docker create [options] IMAGE [commands] [arguments]

Please Note: Anything enclosed within the square brackets is optional. This is applicable to all the commands that you would see on this guide.

Some of the examples of using this command are shown below:

$ docker create fedora
02576e880a2ccbb4ce5c51032ea3b3bb8316e5b626861fc87d28627c810af03

In the above example, the docker create command would create a new container using the latest Fedora image.

Before creating the container, it will check if the latest official image of the Fedora is available on the Docker Host or not. If the latest image isn’t available on the Docker Host, it will then go ahead and download the Fedora image from the Docker Hub before creating the container. If the Fedora image is already present on the Docker Host, it will make use of that image and create the container.

If the container was created successfully, Docker will return the container ID. For instance, in the above example 02576e880a2ccbb4ce5c51032ea3b3bb8316e5b626861fc87d28627c810af03 is the container ID returned by Docker.

Each container has a unique container ID. We refer to the container using its container ID for performing various operations on the container, such as starting, stopping, restarting, and so on.

Now, let us refer to another example of docker create command, which has options and commands being passed to it.

$ docker create -t -i ubuntu bash
30986b73dc0022dbba81648d9e35e6e866b4356f026e75660460c3474f1ca005

In the above example, the docker create command creates a container using the Ubuntu image (As stated earlier, if the image isn’t available on the Docker Host, it will go ahead and download the latest image from the Docker Hub before creating the container).

The options -t and -i instruct Docker to allocate a terminal to the container so that the user can interact with the container. It also instructs Docker to execute the bash command whenever the container is started.

docker ps

The next command we will look at is the docker ps command.

The docker ps command allows us to view all the containers that are running on the Docker Host.

$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES30986b73dc00 ubuntu "bash" 45 minutes ago Up About a minute elated_franklin

It only displays the containers that are presently running on the Docker Host.

If you want to view all the containers that were created on this Docker Host, irrespective of their current status, such as whether they are running or exited, then you would need to include the option -a, which in turn would display all the containers that were created on this Docker Host.

$ docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES30986b73dc00 ubuntu “bash” About an hour ago Up 29 minutes elated_franklin02576e880a2c fedora “/bin/bash” About an hour ago Created hungry_sinoussi

Before we proceed further, let’s try to decode and understand the output of the docker ps command.

CONTAINER ID: A unique string consisting of alpha-numeric characters, associated with each container.

IMAGE: Name of the Docker Image used to create this container.

COMMAND: Any application specific command(s) that needs to be executed when the container is started.

CREATED: This shows the time elapsed since this container has been created.

STATUS: This shows the current status of the container, along with the time elapsed, in its present state.

If the container is running, it will display as Up along with the time period elapsed (for example, Up About an hour or Up 3 minutes).

If the container is stopped, then it will display as Exited followed by the exit status code within round brackets, along with the time period elapsed (for example, Exited (0) 3 weeks ago or Exited (137) 15 seconds ago, where 0 and 137 are the exit codes).

PORTS: This displays any port mappings defined for the container.

NAMES: Apart from the CONTAINER ID, each container is also assigned a unique name. We can refer to a container either using its container ID or its unique name. Docker automatically assigns a unique silly name to each container it creates. But if you want to specify your own name to the container, you can do that by including the — — name (double hyphen name) option to the docker create or the docker run (we will look at the docker run command later) command.

I hope this gives you a better understanding of the output of the docker ps command.

docker start

The next command we will look at, is the docker start command.

This command starts any stopped container(s).

The syntax for this command is as shown below:

docker start [options] CONTAINER ID/NAME [CONTAINER ID/NAME…]

We can start a container either by specifying the first few unique characters of its container ID or by specifying its name.

Some of the examples of using this command are shown below:

$ docker start 30986

In the above example, Docker starts the container beginning with the container ID 30986.

$ docker start elated_franklin

Whereas in this example, Docker starts the container named elated_franklin.

docker stop

The next command on the list is the docker stop command.

This command stops any running container(s).

The syntax for this command is as shown below:

docker stop [options] CONTAINER ID/NAME [CONTAINER ID/NAME…]

It is similar to the docker start command.

We can stop the container either by specifying the first few unique characters of its container ID or by specifying its name.

Some of the examples of using this command are shown below:

$ docker stop 30986

In the above example, Docker will stop the container beginning with the container ID 30986.

$ docker stop elated_franklin

Whereas in this example, Docker will stop the container named elated_franklin.

docker restart

The next command we will look at is the docker restart command.

This command restarts any running container(s).

The syntax for this command is as shown below:

docker restart [options] CONTAINER ID/NAME [CONTAINER ID/NAME…]

We can restart the container either by specifying the first few unique characters of its container ID or by specifying its name.

Some of the examples of using this command are shown below:

$ docker restart 30986

In the above example, Docker will restart the container beginning with the container ID 30986.

$ docker restart elated_franklin

Whereas in this example, Docker will restart the container named elated_franklin.

docker run

The next command we will be looking at is the docker run command.

This command first creates the container, and then it starts the container. In short, this command is a combination of the docker create and the docker start command.

The syntax for this command is as shown below:

docker run [options] IMAGE [commands] [arguments]

It has a syntax similar to that of the docker create command.

Some of the examples of using this command are shown below:

$ docker run ubuntu
30fa018c72682d78cf168626b5e6138bb3b3ae23015c5ec4bbcc2a088e67520

In the above example, Docker will create the container using the latest Ubuntu image and then immediately start the container.

If we execute the above command, it would start the container and immediately stop it — we wouldn’t get any chance to interact with the container at all.

If we want to interact with the container, then we need to specify the options: -it (hyphen followed by i and t) to the docker run command presents us with the terminal, using which we could interact with the container by typing in appropriate commands. Below is an example of the same.

$ docker run -it ubuntu
[email protected]:/#

In order to come out of the container, you need to type exit in the terminal.

docker rm

Moving on to the next command — if we want to delete a container, we use the docker rm command.

The syntax for this command is as shown below:

docker rm [options] CONTAINER ID/NAME [CONTAINER ID/NAME...]

Some of the examples of using this command are shown below:

$ docker rm 30fa elated_franklin

In the above example, we are instructing Docker to delete 2 containers within a single command. The first container to be deleted is specified using its container ID, and the second container to be deleted is specified using its name.

Please Note: The containers need to be in a stopped state in order to be deleted.

docker images

docker images is the next command on the list.

This command lists out all the Docker Images that are present on your Docker Host.

$ docker images
REPOSITORY TAG IMAGE CREATED SIZEmysql latest 7bb2586065cd 38 hours ago 477MBhttpd latest 5eace252f2f2 38 hours ago 132MBubuntu 16.04 9361ce633ff1 2 weeks ago 118MBubuntu trusty 390582d83ead 2 weeks ago 188MBfedora latest d09302f77cfc 2 weeks ago 275MBubuntu latest 94e814e2efa8 2 weeks ago 88.9MB

Let us decode the output of the docker images command.

REPOSITORY: This represents the unique name of the Docker Image.

TAG: Each image is associated with a unique tag. A tag basically represents a version of the image.

A tag is usually represented either using a word or set of numbers or a combination of alphanumeric characters.

IMAGE ID: A unique string consisting of alpha-numeric characters, associated with each image.

CREATED: This shows the time elapsed since this image has been created.

SIZE: This shows the size of the image.

docker rmi

The next command on the list is the docker rmi command.

The docker rmi command allows us to remove an image(s) from the Docker Host.

The syntax for this command is as shown below:

docker rmi [options] IMAGE NAME/ID [IMAGE NAME/ID...]

Some of the examples of using this command are shown below:

docker rmi mysql

The above command removes the image named mysql from the Docker Host.

docker rmi httpd fedora

The above command removes the images named httpd and fedora from the Docker Host.

docker rmi 94e81

The above command removes the image starting with the image ID 94e81 from the Docker Host.

docker rmi ubuntu:trusty

The above command removes the image named ubuntu, with the tag trusty from the Docker Host.

These were some of the basic Docker commands you will see. There are many more Docker commands to explore.

Wrap-Up

Containerization har for nylig fået den opmærksomhed, det fortjener, skønt det har eksisteret i lang tid. Nogle af de topteknologiske virksomheder som Google, Amazon Web Services (AWS), Intel, Tesla og Juniper Networks har deres egen brugerdefinerede version af containermotorer. De er stærkt afhængige af dem til at opbygge, køre, administrere og distribuere deres applikationer.

Docker er en ekstremt kraftig containeriseringsmotor, og den har meget at tilbyde, når det kommer til at opbygge, køre, administrere og distribuere dine applikationer effektivt.

Du har lige set Docker på et meget højt niveau. Der er meget mere at lære om Docker, såsom:

  • Docker-kommandoer (kraftigere kommandoer)
  • Docker-billeder (opbyg dine egne brugerdefinerede billeder)
  • Docker-netværk (opsæt og konfigurer netværk)
  • Docker Services (gruppering af containere, der bruger det samme billede)
  • Docker Stack (Grouping services required by an application)
  • Docker Compose (Tool for managing and running multiple containers)
  • Docker Swarm (Grouping and managing one or more machines on which docker is running)
  • And much more…

If you have found Docker to be fascinating, and are interested in learning more about it, then I would recommend that you enroll in the courses which are listed below. I found them to be very informative and straight to the point.

If you are an absolute beginner, then I would suggest you enroll in this course, which has been designed for beginners.

If you have some good knowledge about Docker, and are pretty much confident with the basic stuff and want to expand your knowledge, then I would suggest you should enroll into this course, which is aimed more towards advanced topics related to Docker.

Docker er en fremtidssikret færdighed og tager bare fart.

At investere din tid og penge i at lære Docker ville ikke være noget, du ville omvende dig.

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