Why should I use docker ?
- for testers/hackers :
- install a complex stack in no time to test your favorite new system. Nothing will be installed directly into your host system.
- for developers :
- maintain your dev environment isolated
- share your work easily
- archived versions
- deploy a postgresql, mariadb, redis, rabbitmq, ... to test something quickly
- continuous integration : push -> test -> release
- for devops :
- package once, deploy everywhere
- deploy complexes apps in no time
- easy scalabily, load-balancing
- easier and standardized deployment processes (continous deployment)
- built-in orchestrator (swarm)
- for system admins:
- simplified / standardised deploy pipelines
- orchestration
- security
Nowaday entreprise needs IT teams to make:
- scalable applications
- well architectured systems (microservices)
- built-in security (process isolation, networks management)
from docker documentation
Docker is a platform for developers and sysadmins to build, share, and run applications with containers. The use of containers to deploy applications is called containerization. Containers are not new, but their use for easily deploying applications is.
Containerization is increasingly popular because containers are:
- Flexible: Even the most complex applications can be containerized.
- Lightweight: Containers leverage and share the host kernel, making them much more efficient in terms of system resources than virtual machines.
- Portable: You can build locally, deploy to the cloud, and run anywhere.
- Loosely coupled: Containers are highly self sufficient and encapsulated, allowing you to replace or upgrade one without disrupting others.
- Scalable: You can increase and automatically distribute container replicas across a datacenter.
- Secure: Containers apply aggressive constraints and isolations to processes without any configuration required on the part of the user.
Fundamentally, a container is nothing but a running process, with some added encapsulation features applied to it in order to keep it isolated from the host and from other containers. One of the most important aspects of container isolation is that each container interacts with its own, private filesystem; this filesystem is provided by a Docker image.
An image is a lightweight, stand-alone, executable package that includes everything needed to run a piece of software, including the code, a runtime, libraries, environment variables, and config files.
A container is a runtime instance of an image—what the image becomes in memory when actually executed. It runs completely isolated from the host environment by default, only accessing host files and ports if configured to do so.
Containers run apps natively on the host machine’s kernel.
Virtual machines run guest operating systems—note the OS layer in each box. This is resource intensive, and the resulting disk image and application state is an entanglement of OS settings, system-installed dependencies, OS security patches, and other easy-to-lose, hard-to-replicate ephemera.
Containers can share a single kernel, and the only information that needs to be in a container image is the executable and its package dependencies, which never need to be installed on the host system. These processes run like native processes, and you can manage them individually by running commands like docker ps—just like you would run ps on Linux to see active processes. Finally, because they contain all their dependencies, there is no configuration entanglement; a containerized app “runs anywhere.”
$ docker --help
...
Management Commands:
config Manage Docker configs
container Manage containers
image Manage images
network Manage networks
node Manage Swarm nodes
plugin Manage plugins
secret Manage Docker secrets
service Manage services
stack Manage Docker stacks
swarm Manage Swarm
system Manage Docker
trust Manage trust on Docker images (experimental)
volume Manage volumes
Commands:
...
docker run hello-world
Dockerfile reference Best practices for writing Dockerfiles
docker image push <image>
docker image pull <image>
docker image ls
Dockerfile will define what goes on in the environment inside your container. Access to resources like networking interfaces and disk drives is virtualized inside this environment, which is isolated from the rest of your system, so you have to map ports to the outside world, and be specific about what files you want to “copy in” to that environment. However, after doing that, you can expect that the build of your app defined in this Dockerfile will behave exactly the same wherever it runs.
This instruction is used to set the base image for subsequent instructions. It is mandatory to set this in the first line of a Dockerfile.
This instruction lets you execute a command on top of an existing layer and create a new layer with the results of command execution.
FROM ubuntu
RUN apt-get update && aupt-get install -y htop
Specifies the intended command for the image. Whereas RUN actually executes the command during build time.
CMD echo "Hello World!"
Expose a port
EXPOSE 80
This instruction can be used to set the environment variables in the container.
ENV UID=1000
This instruction is used to copy files and directories from a specified source (from build context) to a destination (in the file system of the container).
COPY requirements.txt /app/requirements.txt
This instruction is similar to the COPY instruction with few added features like remote URL support in the source field and local-only tar extraction. But if you don’t need a extra features, it is suggested to use COPY as it is more readable.
ADD http://www.site.com/downloads/sample.tar.xz /usr/src
###Â ENTRYPOINT
You can use this instruction to set the primary command for the image.
CMD "Hello World!"
ENTRYPOINT echo
You can use the VOLUME instruction to enable access to a location on the host system from a container. Just pass the path of the location to be accessed.
VOLUME /data
This is used to set the UID (or username) to use when running the image.
USER www-data
Public vs Private
- hello-world: test image
- os images:
- application images:
- dev images:
- ...
$ docker network ls
NETWORK ID NAME DRIVER
7fca4eb8c647 bridge bridge
9f904ee27bf5 none null
cf03ee007fb4 host host
--publish 80:80
--publish 8888:80 # publish port 80 of container to port 8888 ofx
--publish-all # randomly publish container exposed ports
Embedded DNS server in user-defined networks
Containers in same networks will be able to communicate using their ip and name.
$ docker network create test-network
90b16f25d2d0f55e4a9bcf6942b0b3ef069c136a35f007e4d4c55ea1c1f01ecadocker network create test-network
$ docker run -it --network test-network --name ubuntu1 -d ubuntu:16.04
1665aa117ef08ed8b3d93e8e8ff037a8e64e1048bed705bef9d88a32ff87e6fc
$ docker run --rm -it --network test-network willfarrell/ping ping ubuntu1
PING ubuntu1 (172.28.0.2): 56 data bytes
64 bytes from 172.28.0.2: seq=0 ttl=64 time=0.082 ms
64 bytes from 172.28.0.2: seq=1 ttl=64 time=0.139 ms
64 bytes from 172.28.0.2: seq=2 ttl=64 time=0.155 ms
It is possible to store data within the writable layer of a container, but there are some downsides:
- The data won’t persist when that container is no longer running, and it can be difficult to get the data out of the container if another process needs it.
- You can’t easily move the data somewhere else.
- ...
- Volumes are stored in a part of the host filesystem which is managed by Docker (/var/lib/docker/volumes/ on Linux). Non-Docker processes should not modify this part of the filesystem. Volumes are the best way to persist data in Docker.
- Bind mounts may be stored anywhere on the host system. They may even be important system files or directories. Non-Docker processes on the Docker host or a Docker container can modify them at any time.
- tmpfs mounts are stored in the host system’s memory only, and are never written to the host system’s filesystem.
More details about mount types
- Sharing data among multiple running containers. Volumes are only removed when you explicitly remove them.
- When you want to store your container’s data on a remote host or a cloud provider, rather than locally.
-
Sharing configuration files from the host machine to containers. This is how Docker provides DNS resolution to containers by default, by mounting /etc/resolv.conf from the host machine into each container.
-
Sharing source code or build artifacts between a development environment on the Docker host and a container.
Create a dedicated directory and put the following files:
- Dockerfile
# Use an official Python runtime as a parent image
FROM python:2.7-slim
# Set the working directory to /app
WORKDIR /app
# Copy the current directory contents into the container at /app
ADD . /app
# Install any needed packages specified in requirements.txt
RUN pip install --trusted-host pypi.python.org -r requirements.txt
# Make port 80 available to the world outside this container
EXPOSE 80
# Define environment variable
ENV NAME World
# Run app.py when the container launches
CMD ["python", "app.py"]
- requirements.txt
Flask
Redis
- app.py
from flask import Flask
from redis import Redis, RedisError
import os
import socket
# Connect to Redis
redis = Redis(host="redis", db=0, socket_connect_timeout=2, socket_timeout=2)
app = Flask(__name__)
@app.route("/")
def hello():
try:
visits = redis.incr("counter")
except RedisError:
visits = "<i>cannot connect to Redis, counter disabled</i>"
html = "<h3>Hello {name}!</h3>" \
"<b>Hostname:</b> {hostname}<br/>" \
"<b>Visits:</b> {visits}"
return html.format(name=os.getenv("NAME", "world"), hostname=socket.gethostname(), visits=visits)
if __name__ == "__main__":
app.run(host='0.0.0.0', port=80)$ ls
Dockerfile app.py requirements.txt
$ docker build -t friendlyhello .
$ docker image ls
$ docker run -p 4000:80 friendlyhello
* Running on http://0.0.0.0:80/ (Press CTRL+C to quit)
Then visit http://localhost:4000
Or use curl :
$ curl http://localhost:4000
The output should be similar to :
Hello World!
Hostname: e64a15d1d468
Visits: cannot connect to Redis, counter disabled
Using embedded docker DNS, we can easily create a redis container and use it in our project.
$ docker network create my-project-network
$ docker run -d --network my-project-network --name redis redis
$ docker run --network my-project-network -p 4000:80 friendlyhello
You should see something like :
Hello World!
Hostname: 9794fad77d45
Visits: 1
The app.py include a special feature, it can say hello to someone else using an environment variable.
$ docker run -p 4000:80 -e NAME=Robin friendlyhello
Hello Robin!
Hostname: 98772c42077a
Visits: cannot connect to Redis, counter disabled
$ docker run -p 4000:80 --name testlog -d friendlyhello
$ docker logs -f testlog
* Running on http://0.0.0.0:80/ (Press CTRL+C to quit)
172.17.0.1 - - [24/Jan/2018 12:10:32] "GET / HTTP/1.1" 200 -
172.17.0.1 - - [24/Jan/2018 12:10:33] "GET / HTTP/1.1" 200 -
172.17.0.1 - - [24/Jan/2018 12:10:34] "GET / HTTP/1.1" 200 -
172.17.0.1 - - [24/Jan/2018 12:10:34] "GET / HTTP/1.1" 200 -
docker-compose is a python utility that permit to describe/launch/manage several containers at once on a single host.
$ docker-compose -h
Define and run multi-container applications with Docker.
Usage:
docker-compose [-f <arg>...] [options] [COMMAND] [ARGS...]
docker-compose -h|--help
...
Commands:
build Build or rebuild services
bundle Generate a Docker bundle from the Compose file
config Validate and view the compose file
create Create services
down Stop and remove containers, networks, images, and volumes
events Receive real time events from containers
exec Execute a command in a running container
help Get help on a command
kill Kill containers
logs View output from containers
pause Pause services
port Print the public port for a port binding
ps List containers
pull Pull service images
push Push service images
restart Restart services
rm Remove stopped containers
run Run a one-off command
scale Set number of containers for a service
start Start services
stop Stop services
top Display the running processes
unpause Unpause services
up Create and start containers
version Show the Docker-Compose version information
Create a docker-compose.yml file
version: '2'
services:
web:
image: friendlyhello
ports:
- "4000:80"
redis:
image: redis$ docker-compose up
version: '2'
services:
web:
image: friendlyhello
volumes:
- ./app.py:/app/app.py
ports:
- "4000:80"
redis:
image: redisYou can now edit your app.py file and get result in your browser without rebuilding the image.
Definition: An application consisting of one or more processes running on one or more nodes.
- Bandwidth and latency
- Ephemeral components
- Stateless versus stateful
- Service discovery
- Load balancing
- Health checking
- Logging and monitoring
- Circuit breakers
To deploy an application image when Docker Engine is in swarm mode, you create a service. Frequently a service will be the image for a microservice within the context of some larger application. Examples of services might include an HTTP server, a database, or any other type of executable program that you wish to run in a distributed environment.
When you create a service, you specify which container image to use and which commands to execute inside running containers. You also define options for the service including:
- the port where the swarm will make the service available outside the swarm
- an overlay network for the service to connect to other services in the swarm
- CPU and memory limits and reservations
- a rolling update policy
- the number of replicas of the image to run in the swarm
Create the docker-stack.yml file.
version: "3"
services:
web:
image: friendlyhello
deploy:
replicas: 2
resources:
limits:
cpus: "0.1"
memory: 50M
restart_policy:
condition: on-failure
ports:
- "4000:80"
networks:
- webnet
networks:
webnet:docker swarm init
$ docker stack deploy -c docker-stack.yml friendlyhello
Creating network friendlyhello_webnet
Creating service friendlyhello_web
http://localhost:4000 should output as usual :
Hello World!
Hostname: e64a15d1d468
Visits: cannot connect to Redis, counter disabled
Refresh the browser several times and you should see that your service is actually running in several containers
$ docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
31o20ib5hhaa friendlyhello_web replicated 2/2 friendlyhello:latest *:4000->80/tcp
$ docker service ps friendlyhello_web
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
dj7rt17busr3 friendlyhello_web.1 friendlyhello:latest robin-XPS Running Running 6 minutes
f31r54oyzzs5 friendlyhello_web.2 friendlyhello:latest robin-XPS Running Running 6 minutes ago
Edit the docker-stack.yml file to run 4 replicas of your web service, and
$ docker stack deploy -c docker-stack.yml friendlyhello
Updating service friendlyhello_web (id: 31o20ib5hhaa010g22lczquz0)
$ docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
31o20ib5hhaa friendlyhello_web replicated **4/4** friendlyhello:latest *:4000->80/tcp
Edit your docker-stack.yml file :
version: "3"
services:
web:
image: friendlyhello
deploy:
replicas: 2
resources:
limits:
cpus: "0.1"
memory: 50M
restart_policy:
condition: on-failure
ports:
- "4000:80"
networks:
- webnet
visualizer:
image: dockersamples/visualizer:stable
ports:
- "8080:8080"
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
deploy:
placement:
constraints: [node.role == manager]
networks:
- webnet
networks:
webnet:Then, deploy and browse http://localhost:8080
$ docker stack deploy -c docker-stack.yml friendlyhello
- docker overlay network