Microservices, Docker, and Kubernetes can get quickly overwhelming for anyone. We can make it easier for ourselves by understanding the basics. It would be an understatement to say that understanding the fundamentals is critical to performing root cause analysis on production problems.
Any application is ultimately a process that is running on an operating system (OS). The process requires the following to be usable:
- Compute (CPU)
- Memory (RAM)
- Storage (disk)
- Network (NIC)
Launch the online Linux Terminal emulator at https://bellard.org/jslinux/vm.html?url=https://bellard.org/jslinux/buildroot-x86.cfg. Then, type the ls command, as follows:
This command lists the files in the current directory (which happens to be the root user's home directory).
Congratulations, you have launched your first container!
Well, obviously, this is not really the case. In principle, there is no difference between the command you ran versus launching a container.
So, what is the difference between the two? The clue lies in the word, contain. The ls process has very limited containment (it is limited only by the rights that the user has). ls can potentially see all files, has access to all the memory, network, and the CPU that's available to the OS.
A container is contained by the OS by controlling access to computing, memory, storage, and network. Each container runs in its own namespace(https://medium.com/@teddyking/linux-namespaces-850489d3ccf). The rights of the namespace processes are controlled by control groups (cgroups).
Every container process has contained access via cgroups and namespaces, which makes it look (from the container process perspective) as if it is running as a complete instance of an OS. This means that it appears to have its own root filesystem, init process (PID 1), memory, compute, and network.
Since running containers is just a set of processes, it makes it extremely lightweight and fast, and all the tools that is used to debug and monitor processes can be used out of the box.
You can play with Docker by creating a free Docker Hub account at Docker Hub (https://hub.docker.com/) and using that login at play with Docker (https://labs.play-with-docker.com/).
First, type docker run -it ubuntu. After a short period of time, you will get a prompt such as root@<randomhexnumber>:/#. Next, type exit, and run the docker run -it ubuntu command again. You will notice that it is super fast! Even though you have launched a completely new instance of Ubuntu (on a host that is probably running alpine OS), it is available instantly. This magic is, of course, due to the fact that containers are nothing but regular processes on the OS. Finally, type exit to complete this exercise. The full interaction of the session on play with Docker (https://labs.play-with-docker.com/) is shown in the following script for your reference. It demonstrates the commands and their output:
docker run -it ubuntu # runs the standard ubuntu linux distribution as a container
exit # the above command after pulling it from dockerhub will put you into the shell of the container. exit command gets you out of the container
docker run -it ubuntu # running it again shows you how fast launching a container is. (Compare it to launching a full blown Virtual Machine (VM), booting a computer)
exit # same as above, gets you out of the container
The following content displays the output that is produced after implementing the preceding commands:
###############################################################
# WARNING!!!! #
# This is a sandbox environment. Using personal credentials #
# is HIGHLY! discouraged. Any consequences of doing so are #
# completely the user's responsibilites. #
# #
# The PWD team. #
###############################################################
[node1] (local) [email protected] ~
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
[node1] (local) [email protected] ~
$ date
Mon Oct 29 05:58:25 UTC 2018
[node1] (local) [email protected] ~
$ docker run -it ubuntu
Unable to find image 'ubuntu:latest' locally
latest: Pulling from library/ubuntu
473ede7ed136: Pull complete
c46b5fa4d940: Pull complete
93ae3df89c92: Pull complete
6b1eed27cade: Pull complete
Digest: sha256:29934af957c53004d7fb6340139880d23fb1952505a15d69a03af0d1418878cb
Status: Downloaded newer image for ubuntu:latest
root@03c373cb2eb8:/# exit
exit
[node1] (local) [email protected] ~
$ date
Mon Oct 29 05:58:41 UTC 2018
[node1] (local) [email protected] ~
$ docker run -it ubuntu
root@4774cbe26ad7:/# exit
exit
[node1] (local) [email protected] ~
$ date
Mon Oct 29 05:58:52 UTC 2018
[node1] (local) [email protected] ~