Quick Docker notes in preparation for beginning to make use of this technology.
The point of container technologies is to wrap up a software implementation in a complete filesystem and
operating environment that contains everything it needs to run: the application itself, runtime, system
tools and libraries—anything and everything you install on a server. This guarantees that the
implementation will always run the same regardless of the environment it is running in. Um, well, in theory.
Docker is a tool that is designed to benefit both developers and system administrators. This makes it the
essential quiver in DevOps' weaponry or toolstack. For developers especially it means not having to focus
on writing code that must take into account the greater environment (operating system, tools and frameworks
present, etc.). Another benefit is the many, prebuilt, open-source containers published that already do
something a developer wants to do.
Elasticsearch, Logstash, Kibana (ELK) Docker image documentation comes to mind as an excellent
example of this.
Docker links
Docker for Developers docker-machine that isn't directly part of Docker:
# BASE=https://github.com/docker/machine/releases/download/v0.15.0*
# curl -L $BASE/docker-machine-$(uname -s)-$(uname -m) > ./docker-machine
# install ./docker-machine /usr/local/bin/docker-machine
# rm -rf ./docker-machine
* Check path https://github.com/docker/machine/releases/ for the
latest version.
How to Install and Use Docker on Ubuntu 18.04
How to Install and Use Docker on CentOS 7
Get Started, Part 1: Orientation , where to start officially.
Covers installation (generally), testing Docker as it lands on your host,
and a hello-world exercise.
Get Started, Part 2: Containers , works nicely. Don't forget to
adopt your username into group docker (then reboot),
type "Flask" (not "Flash") into requirements.txt ,
tag your image to see how that works,
review what you've learned:
[email protected] :~/dev/docker-dev/exercise-1$ docker image ls
REPOSITORY TAG IMAGE ID CREATED SIZE
friendlyhello latest 023c3e07e25c 10 minutes ago 132MB
windofkeltia/get-started part2 023c3e07e25c 10 minutes ago 132MB
python 2.7-slim 14dad3ead5f4 18 hours ago 120MB
hello-world latest 4ab4c602aa5e 4 weeks ago 1.84kB
Get Started, Part 3: Services , continues the joy.
Docker for beginners Get Started documents above.
My own Docker super-simple tutorial .
Glossary
...as it occurs to me to add to this:
Term Definition bind mount
the mounting of a small piece of the host filesytem into a container;
more limited than (mounting) a volume. This is the recommended method of
sharing configuration files between the host machine and the container.
(Configuration file content commonly varies between instantions of running
containers.) For example, /etc/resolv.conf could be a useful bind
mount in a container.
If your current working directory were ~/Downloads and you had
filebeat.yml there, assuming the running executable in the
Docker container was looking for this file, the following command line
would provide (expose) it (and only this one file of the host's filesystem):
$ docker run --mount type=bind,source="$(pwd)"/filebeat.yml ,\
target=/usr/share/filebeat/filebeat.yml \
docker.elastic.co/beats/filebeat:6.4.2
volume mount
the mounting of an entire volume from the host filesysem into a container.
This is the recommended method of sharing data between different containers
when that is necessary. Volumes mounted are usually filesystems off path
/var/lib/docker/volumes/ . In the example below, (read-oinly) HTML
content is set up for use by nginx running in a container:
$ docker run --volume nginx:/usr/share/nginx/html:ro nginx:latest
When you run docker inspect , you see (in the Mounts section):
"Mounts": [
{
"Type": "volume",
"Name": "nginx",
"Source": "/var/lib/docker/volumes/nginx/_data ",
"Destination": "/usr/share/nginx/html ",
"Driver": "local",
"Mode": "",
"RW": false,
"Propagation": ""
}
],
Install Docker on Ubuntu 18.04.1 Server
Docker isn't available from the usual repositories.
# apt-get update
# apt-get install apt-transport-https ca-certificates curl software-properties-common
# curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
# add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu bionic stable"
# apt-get update
# apt-cache policy docker-ce
# apt-get install docker-ce
This enabled the Docker dæmon to start on boot, but you can check its status:
# systemctl status docker
Add your username to Docker's group. This avoids having to resort to root
in order to run any docker command:
# usermod -aG docker username
# su - username # to set the group
$ id -nG username
Adopt user into group docker
When you try the Docker hello-world example, it fails:
[email protected] :~$ docker run hello-world
docker: Got permission denied while trying to connect to the Docker daemon socket at
unix /var/run/docker.sock: connect: permission denied.
This is because:
[email protected] :~$ ll /var/run/docker.sock
srw-rw---- 1 root docker 0 Oct 10 08:48 /var/run/docker.sock
You can use sudo to run the example or you can do what you should which
is to adopt yourself into group docker :
[email protected] :~$ sudo usermod -aG docker username
Then log out and back in (to instantiate the group membership change).
The Dockerfile
Here's a Docker (configuration) file example and some comments on its content.
dockerfile FROM debian:jessie
MAINTAINER Daniel Alan Miller [email protected]
RUN apt-key adv --keyserver pgp.mit.edu --recv-keys 1614552E5765227AEC39EFCFA7E00EF33A8F2399
RUN echo "deb http://download.rethinkdb.com/apt jessie main" > /etc/apt/sources.list.d/rethinkdb.list
ENV RETHINKDBPACKAGEVERSION 2.0.4~0jessie
RUN apt-get update \ && apt-get install -y rethinkdb=$RETHINKDBPACKAGEVERSION \ && rm -rf /var/lib/apt/lists/*
VOLUME ["/data"]
WORKDIR /data
CMD ["rethinkdb", "--bind", "all"]
EXPOSE 28015 29015 8080
FROM —pulls (includes) dependencies from other Docker image
configurations.
MAINTAINER —comment on whose Docker configuration this is.
RUN —defines command to run from within the Docker container
once it's created and begun to run. If your image comes with Python, this
could be in Python.
ENV —sets an environment variable and exports it in the
container.
VOLUME —defines a path in the container that Docker exposes
to the host system (the host running Docker) and mapped using the
-v argument when launching that container.
WORKDIR —changes the current working directory of the
container in case more commands are to be run (in that location).
CMD —runs commands in the format:CMD [ [ [ "executable" ] "argument" ] "more arguments" ]
Ideally, there should only be one instance of CMD . Otherwise, see
ENTRYPOINT :dockerfile ENTRYPOINT [ "/swarm" ] CMD [ "--help" ]
EXPOSE —expose the listed ports for mapping to the host via
the -p option when launching a container.
Docker best practices
Automate everything. Don't assume that the host server will around for
a long time. Build in the proper automation for recovering a Docker
instance including data on the local filesytem. In a proper Docker
environment, hosting hardware/software must be replaced with new
instances upon failure and Docker-hosted services must not skip a
beat.
Orchestrate containers. Application code must not assume that the executing
container will live forever. It must assume that an application will
always exist, because that's the purpose anyway, but not rely on
(especially a) hardware instance. For example, a database must mount an
external—never a local—volume for data files.
Docker and Snap versus package-manager installation
Do not use Snap. It's cool and snappy, but it makes things more difficult when
dealing with everyday write-ups, tutorials, questions on stackoverflow.com,
etc.
The sample, extended installation done here is because I'm interested in the
containerization of ELK. This shows you that in the interest of practice
application.
If you did use Snap, for example, when offered the option to install
Docker as part of the Ubuntu 18.04.1 Bionic Beaver Server installation,
do this:
# snap remove docker
Install docker
Now do the proper, Debian package installation:
# apt-get update
# apt-get install apt-transport-https ca-certificates software-properties-common
# curl -fsSL http://download.docker.com/linux/ubuntu/gpg | apt-key add -
It may be important to you to verify that you have the key with the
fingerprint thus (and you'll see):
# apt-key fingerprint 0EBFCD88
pub rsa4096 2017-02-22 [SCEA]
9DC8 5822 9FC7 DD38 854A E2D8 8D81 803C 0EBF CD88 *
uid [ unknown] Docker Release (CE deb) <[email protected] >
sub rsa4096 2017-02-22 [S]
* As I understand it, this is what you should see when you
test the advanced package tools key.
Now set up the stable repository using the key:
# add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu bionic stable"
# apt-get update
# apt-cache policy docker-ce
# apt-get install docker-ce
# systemctl status docker.service
Docker is now running.
Add your username to Docker's group. This avoids having to resort to
root in order to run Docker commands (and it's the right thing
to do).
# usermod -aG docker username
# su - username
# id -nG username
Now fix vm.max_map_count :
# sysctl vm.max_map_count=262144
Now pull Sebastien's docker stuff:
$ docker pull sebp/elk
Install docker-compose
Install this Docker aid that makes life easier in terms of Docker command
lines. You can see this in the YAML file which obviates the starting of
Docker with gazillions of rather lengthy options.
$ wget https://github.com/docker/compose/releases/download/1.22.0/docker-compose-Linux-x86_64
$ chmod a+x docker-compose-Linux-x86_64
# cp docker-compose-Linux-x86_64 /usr/local/bin/docker-compose
$ vim ./docker-compose.yml
elk:
image: sebp/elk
ports:
- "5601:5601"
- "9200:9200"
- "9300:9300"
- "5044:5044"
ulimits:
nofile:
soft: "65536"
hard: "65536"
Now try to run Docker via docker-compose :
$ docker-compose up elk
This doesn't work complaining that there was no known port. I googled the
fool out of this problem (and wondered why I had not encountered it last
week when I did this). The solution is ugly and I don't feel super
confident that this is even the best way to proceed; however, it works.
I created /etc/systemd/system/docker.service.d/hosts.conf into
which I put:
# vim /etc/systemd/system/docker.service.d/hosts.conf
[Service]
ExecStart=
ExecStart=/usr/bin/dockerd -H fd:// -H tcp://127.0.0.1:2736
Then, I bounced Docker:
# systemctl daemon-reload
# systemctl restart docker.service
# systemctl status docker.service
Dropping back down to user russ :
$ DOCKER_HOST=127.0.0.1:2736
$ docker-compose up elk
And it was off to the races!
Getting started exercises
I got started (on a recently configured installation of Linux Mint 19) by
installing Docker for Part 1:
# apt-get install docker.io
# docker --version
Docker version 17.12.1-ce, build 7390fc6
# usermod -aG docker russ
# reboot
Then I began the exercises:
[email protected] :~$ mkdir -p dev/docker-dev
[email protected] :~$ cd dev/docker-dev
[email protected] :~/dev/docker-dev$ docker run hello-world
Hello from Docker!
This message shows that your installation appears to be working correctly.
To generate this message, Docker took the following steps:
1. The Docker client contacted the Docker daemon.
2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
(amd64)
3. The Docker daemon created a new container from that image which runs the
executable that produces the output you are currently reading.
4. The Docker daemon streamed that output to the Docker client, which sent it
to your terminal.
To try something more ambitious, you can run an Ubuntu container with:
$ docker run -it ubuntu bash
Share images, automate workflows, and more with a free Docker ID:
https://hub.docker.com/
For more examples and ideas, visit:
https://docs.docker.com/get-started/
This next bit is covered in Part 2.
[email protected] :~/dev/docker-dev/exercise-1$ docker build -t friendlyhello .
Sending build context to Docker daemon 5.12kB
Step 1/7 : FROM python:2.7-slim
---> 14dad3ead5f4
Step 2/7 : WORKDIR /app
---> Using cache
---> 6623958da619
Step 3/7 : COPY . /app
---> a6eda813106a
Step 4/7 : RUN pip install --trusted-host pypi.python.org -r requirements.txt
---> Running in b6e6626dfaac
Collecting Flask (from -r requirements.txt (line 1))
Downloading https://files.pythonhosted.org/packages/7f/e7/08578...ed4/Flask-1.0.2-py2.py3-none-any.whl (91kB)
Collecting Redis (from -r requirements.txt (line 2))
Downloading https://files.pythonhosted.org/packages/3b/f6/7a763...f0b/redis-2.10.6-py2.py3-none-any.whl (64kB)
Collecting itsdangerous>=0.24 (from Flask->-r requirements.txt (line 1))
Downloading https://files.pythonhosted.org/packages/dc/b4/a60bc...945/itsdangerous-0.24.tar.gz (46kB)
Collecting Jinja2>=2.10 (from Flask->-r requirements.txt (line 1))
Downloading https://files.pythonhosted.org/packages/7f/ff/ae64b...fc9/Jinja2-2.10-py2.py3-none-any.whl (126kB)
Collecting Werkzeug>=0.14 (from Flask->-r requirements.txt (line 1))
Downloading https://files.pythonhosted.org/packages/20/c4/12e3e...73e/Werkzeug-0.14.1-py2.py3-none-any.whl (322kB)
Collecting click>=5.1 (from Flask->-r requirements.txt (line 1))
Downloading https://files.pythonhosted.org/packages/fa/37/45185...abb/Click-7.0-py2.py3-none-any.whl (81kB)
Collecting MarkupSafe>=0.23 (from Jinja2>=2.10->Flask->-r requirements.txt (line 1))
Downloading https://files.pythonhosted.org/packages/4d/de/32d74...316/MarkupSafe-1.0.tar.gz
Building wheels for collected packages: itsdangerous, MarkupSafe
Running setup.py bdist_wheel for itsdangerous: started
Running setup.py bdist_wheel for itsdangerous: finished with status 'done'
Stored in directory: /root/.cache/pip/wheels/2c/4a/61/5599631c1...768
Running setup.py bdist_wheel for MarkupSafe: started
Running setup.py bdist_wheel for MarkupSafe: finished with status 'done'
Stored in directory: /root/.cache/pip/wheels/33/56/20/ebe49a5c6...ffe
Successfully built itsdangerous MarkupSafe
Installing collected packages: itsdangerous, MarkupSafe, Jinja2, Werkzeug, click, Flask, Redis
Successfully installed Flask-1.0.2 Jinja2-2.10 MarkupSafe-1.0 Redis-2.10.6 Werkzeug-0.14.1 click-7.0 itsdangerous-0.24
Removing intermediate container b6e6626dfaac
---> 051aa1146d1c
Step 5/7 : EXPOSE 80
---> Running in f9d60f536bf4
Removing intermediate container f9d60f536bf4
---> e2e49ccb7202
Step 6/7 : ENV NAME World
---> Running in 3103174b4a63
Removing intermediate container 3103174b4a63
---> f6835ec41a96
Step 7/7 : CMD ["python", "app.py"]
---> Running in 42c927ad63ae
Removing intermediate container 42c927ad63ae
---> 023c3e07e25c
Successfully built 023c3e07e25c
Successfully tagged friendlyhello:latest
[email protected] :~/dev/docker-dev/exercise-1$ ll
total 20
drwxr-xr-x 2 russ russ 4096 Oct 10 10:10 .
drwxr-xr-x 3 russ russ 4096 Oct 10 09:26 ..
-rw-r--r-- 1 russ russ 679 Oct 10 09:28 app.py
-rw-r--r-- 1 russ russ 514 Oct 10 09:26 dockerfile
-rw-r--r-- 1 russ russ 12 Oct 10 10:10 requirements.txt
[email protected] :~/dev/docker-dev/exercise-1$ docker image ls
REPOSITORY TAG IMAGE ID CREATED SIZE
friendlyhello latest 023c3e07e25c 2 minutes ago 132MB
26bada283754 42 minutes ago 120MB
python 2.7-slim 14dad3ead5f4 18 hours ago 120MB
hello-world latest 4ab4c602aa5e 4 weeks ago 1.84kB
[email protected] :~/dev/docker-dev/exercise-1$ docker run -p 4000:80 friendlyhello
* Serving Flask app "app" (lazy loading)
* Environment: production
WARNING: Do not use the development server in a production environment.
Use a production WSGI server instead.
* Debug mode: off
* Running on http://0.0.0.0:80/ (Press CTRL+C to quit)
172.17.0.1 - - [10/Oct/2018 16:14:48] "GET / HTTP/1.1" 200 -
172.17.0.1 - - [10/Oct/2018 16:14:48] "GET /favicon.ico HTTP/1.1" 404 -
^C
[email protected] :~/dev/docker-dev/exercise-1$ docker login
Login with your Docker ID to push and pull images from Docker Hub. If you don't have a Docker ID, head over
to https://hub.docker.com to create one.
Username: windofkeltia
Password:
Login Succeeded
[email protected] :~/dev/docker-dev/exercise-1$ docker tag friendlyhello windofkeltia/get-started:part2
[email protected] :~/dev/docker-dev/exercise-1$ docker image ls
REPOSITORY TAG IMAGE ID CREATED SIZE
friendlyhello latest 023c3e07e25c 10 minutes ago 132MB
windofkeltia/get-started part2 023c3e07e25c 10 minutes ago 132MB
26bada283754 About an hour ago 120MB
python 2.7-slim 14dad3ead5f4 18 hours ago 120MB
hello-world latest 4ab4c602aa5e 4 weeks ago 1.84kB
[email protected] :~/dev/docker-dev/exercise-1$ docker push windofkeltia/get-started:part2
The push refers to repository [docker.io/windofkeltia/get-started]
f5d5f8e0d82e: Pushed
669e5a5551be: Pushed
c0075ee77d65: Pushed
47c126cf49af: Mounted from library/python
18cc3d97f405: Mounted from library/python
80db77e224a0: Mounted from library/python
8b15606a9e3e: Mounted from library/python
part2: digest: sha256:3603d5196dd1b60df07487f3ce0833a50e830ce5a6065936e057a20025dedd19 size: 1788
[email protected] :~/dev/docker-dev/exercise-1$ docker run -p 4000:80 windofkeltia/get-started:part2
* Serving Flask app "app" (lazy loading)
* Environment: production
WARNING: Do not use the development server in a production environment.
Use a production WSGI server instead.
* Debug mode: off
* Running on http://0.0.0.0:80/ (Press CTRL+C to quit)
172.17.0.1 - - [10/Oct/2018 16:35:18] "GET / HTTP/1.1" 200 -
^C
Here's the output (albeit in a browser):
Hello World!
Hostname: c98cf05759f8
Visits: cannot connect to Redis, counter disabled
Docker-compose YAML for exercise above
At this point, we're looking at Part 3 of the Get Started Docker
introductory exercises.
First, install docker-compose :
[email protected] :~/Downloads$ wget https://github.com/docker/compose/releases/download/1.22.0/docker-compose-Linux-x86_64
[email protected] :~/Downloads$ chmod a+x docker-compose-Linux-x86_64
[email protected] :~/Downloads$ sudo cp docker-compose-Linux-x86_64 /usr/local/bin/docker-compose
[email protected] :~/Downloads$ which docker-compose
/usr/local/bin/docker-compose
[email protected] :~/Downloads$ docker-compose --version
docker-compose version 1.22.0, build f46880fe
[email protected] :~/Downloads$ rm docker-compose-Linux-x86_64
Here's the docker-compose YAML file for the contained application we
created earlier (see note above).
docker-compose.yml :
version: "3"
services:
web:
image: windofkeltia/get-started:part2
deploy:
replicas: 5
resources:
limits:
cpus: "0.1"
memory: 50M
restart_policy:
condition: on-failure
ports:
- "4000:80"
networks:
- webnet
networks:
webnet:
Repeating the useful, guiding explanations, this is what the YAML file
accomplishes:
Pull the image, the one we uploaded ourselves to the registry, from that
registry.
Run 5 instances of that image as a service called web , limiting
each one to use, at most, 10% of the CPU (across all cores), and 50MB of
RAM.
Immediately restart containers if one fails.
Map port 4000 on the host to web 's port 80 .
Instruct web 's containers to share port 80 via a
load-balanced network called webnet . (Internally, the containers
themselves publish to web 's port 80 at an ephemeral
port.)
Define the webnet network with the default settings (which is a
load-balanced overlay network).
Docker machine
Here's installing Docker machine:
[email protected] :~/Downloads$ BASE=https://github.com/docker/machine/releases/download/v0.15.0
[email protected] :~/Downloads$ curl -L $BASE/docker-machine-$(uname -s)-$(uname -m) > ./docker-machine
...
[email protected] :~/Downloads$ sudo install ./docker-machine /usr/local/bin/docker-machine
[email protected] :~/Downloads$ rm ./docker-machine
* Check path https://github.com/docker/machine/releases/ for
the latest version.
Must Dockerfile be capitalized?
I have created Dockerfile /dockerfile in lower case on Linux.
It still works. Remember that Windows treats filenames insensitively with regard
to case making both spellings identical. Allowing lowercase on Linux is probably
how Docker gets around any ambiguity.
10 things to avoid in Docker containers
But first, something positive:
Containers are immutable: the same image tests by QA is what reaches production.
Containers are lightweight: the memory footprint is very small since only memory
for the main process is ever allocated.
Containers are fast: they usually load as quickly as a typical Linux process,
that is, in seconds or less.
However, containers are completely and utterly disposable! They are ephemeral. This
fact must condition the mindset of those who develop them. And now for the negative:
Don't store data in containers because they can be stopped, destroyed or
replaced.
Don't ship applications in pieces because containers are immutable.
Don't produce a single-layer image , but make effective use of the layered
filesystem and base images for the OS you choose. Use another layer for the
definition of the username, another for the runtime, and still another for the
application. (There's an art to this.)
Don't create images from running containers using docker commit .
(This is how most tutorials do it for convenience, but it's misleading.)
Use Dockerfile and track changes to that file using git .
Don't use only the :latest tag. It's like using SNAPSHOT
in Maven.
Don't run more than one process in a single container because you can't
manage or update the processes individually.
Don't store credentials in the image. Duh. Use environment variables.
Don't run processes as root . .
Don't rely on IP addresses because each container will have its own,
internal address that could change if you start and stop the container (unless
you've set up a static one, but that creates other problems of how data works).
Use DNS and names that, along with ports, can be communicated via environment
variables or other, good methods between containers.
