Magnus K Karlsson: What is Docker and Why is it Hot?

August 11, 2015

What is Docker and Why is it Hot?

What is Docker?

It is the possible replacement for Virtualization.

What is the Difference between Virtualization and Docker?

Below is a good picture from https://www.docker.com/whatisdocker.

So the difference between Virtualization and Docker is that everything runs locally on the same server, without the Virtualization overhead.

Example. Lets download a ready image from the Docker Hub (https://hub.docker.com/). You can either browse the Docker Hub via web browser or via command line.

$ sudo docker search tomcat
...
NAME                                        DESCRIPTION                                     STARS     OFFICIAL   AUTOMATED
tomcat                                      Apache Tomcat is an open source implementa...   203       [OK]       
consol/tomcat-7.0                           Tomcat 7.0.57, 8080, "admin/admin"              12                   [OK]
consol/tomcat-8.0                           Tomcat 8.0.15, 8080, "admin/admin"              10                   [OK]
...

Lets pick the official docker image. The first time you run a new Docker Image, it will download it and that will take a little while.

$ sudo docker run -it --rm -p 8888:8080 tomcat:8.0
...
10-Aug-2015 18:52:33.409 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["http-nio-8080"]
10-Aug-2015 18:52:33.413 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["ajp-nio-8009"]
10-Aug-2015 18:52:33.413 INFO [main] org.apache.catalina.startup.Catalina.start Server startup in 658 ms

And now try it by open http://localhost:8888.

So what happened? We started a new process.

$ ps -aux | grep -i docker
...
root     29569  0.0  0.0 222512 10728 pts/25   Sl+  21:19   0:00 docker run -it --rm -p 8888:8080 tomcat:8.0
...

And a new tomcat process.

$ ps -aux | grep -i java
...
root     29578  1.7  1.2 7172840 200552 pts/1  Ssl+ 21:19   0:04 /usr/bin/java -Djava.util.logging.config.file=/usr/local/tomcat/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djava.endorsed.dirs=/usr/local/tomcat/endorsed -classpath /usr/local/tomcat/bin/bootstrap.jar:/usr/local/tomcat/bin/tomcat-juli.jar -Dcatalina.base=/usr/local/tomcat -Dcatalina.home=/usr/local/tomcat -Djava.io.tmpdir=/usr/local/tomcat/temp org.apache.catalina.startup.Bootstrap start
...

And for network. A new virtual bridge was created, which is basically a virtual router.

$ ifconfig
...
docker0   Link encap:Ethernet  HWaddr 56:84:7a:fe:97:99
...

Why is Docker Better?

Lower licensing costs. You only need ONE license for the virtualized server for ALL its virtual guests.
More RAM and CPU. With Docker all process runs on the server directly. No virtualization layer that steals resources.
Less disks. A virtual guest is at least 20 GB and a Docker image is around 300 MB, since no duplication of OS and all its libraries are needed.

How Does Docker Work?

Docker makes use of proven mature Linux technologies:

CPU and RAM - cgroups. With cgroups you can restrict how much a process can use of the CPU and RAM, much like the same way CPU and RAM is handled in virtualization.
Disks - namespaces. With namespaces you can isolate local file system and divide it much like a network filesystem.
Network - virtual ethernet bridge.

Common Docker Commands

# Lists all docker process
$ sudo docker ps -a

# Print detailed information of a Docker proccess
$ sudo docker inspect <ContainerID>

# List all local Docker images
$ sudo docker images

# Run Docker image
$ sudo docker run -p <host_port1>:<container_port1> -p <host_port2>:<container_port2>

# Search for a ready Docker image at Docker Hub (https://registry.hub.docker.com/)
$ sudo docker search <SearchString>

Installation on Ubuntu 14.04

Install docker.

$ sudo apt-get install docker.io

Test the installation.

$ sudo docker run hello-world
Hello from Docker.
This message shows that your installation appears to be working correctly.
...

Magnus K Karlsson