Goh Wei Wen

26 Mar 2022

Introduction

What is Docker?

  • Containerize apps
    • Isolated environment for launching processes
    • Clean separation of environments
    • Sandbox with defined resources
  • Simple interface for launching applications

Why should I use Docker?

  • Compatibility/dependency management
  • Quick dev environment set up
  • Easily convert dev environment to staging/prod environment

Is it like a Virtual Machine?

  • Use underlying kernel to run, so any Linux host OS can run any Linux distros in Docker (e.g. CentOS/Alpine on an Ubuntu host, RancherOS/k3os)
$ docker run --rm ubuntu echo "Hello world"

Unable to find image 'ubuntu:latest' locally
latest: Pulling from library/ubuntu
6a5697faee43: Pull complete
ba13d3bc422b: Pull complete
a254829d9e55: Pull complete
Digest: sha256:b709186f32a3ae7d0fa0c217b8be150ef1ce4049bf30645a9de055f54a6751fb
Status: Downloaded newer image for ubuntu:latest
Hello world

Hello World

docker run --rm ubuntu echo "Hello world"
  • Hey dockerd,
    • please start a container
    • and remove it after you're done
    • using the image ubuntu
    • and run the command echo "Hello world"
  • Docker will
    • look for a local image called ubuntu with tag latest
    • otherwise, look in the default registry (docker.io) and pull it
    • start the container using the specified command
    • remove the container after it terminates
$ docker run --rm ubuntu echo "Hello world"
Unable to find image 'ubuntu:latest' locally
latest: Pulling from library/ubuntu
6a5697faee43: Pull complete
ba13d3bc422b: Pull complete
a254829d9e55: Pull complete
Digest: sha256:b709186f32a3ae7d0fa0c217b8be150ef1ce4049bf30645a9de055f54a6751fb
Status: Downloaded newer image for ubuntu:latest
Hello world

First run:

Subsequent runs:

$ docker run --rm ubuntu echo "Hello world"
Hello world

Images and Containers

  • Docker Image
    • Immutable template for containers
    • Can be pulled and pushed to a registry
    • Fixed naming convention:
      • [registry/][namespace/]image[:tag]
    • Default tag to latest
    • Identified using an image digest (SHA256)
  • Docker Container
    • Instance of an image
    • Can be started, stopped, restarted
    • Maintains change within file system

Docker commands

$ tldr docker
Manage Docker containers and images.
 - List currently running docker containers:
   docker ps
   
 - List all docker containers (running and stopped):
   docker ps -a
   
 - Start a container from an image, with a custom name:
   docker run --name {{container_name}} {{image}}
   
 - Start or stop an existing container:
   docker {{start|stop}} {{container_name}}
   
 - Pull an image from a docker registry:
   docker pull {{image}}
   
 - Open a shell inside of an already running container:
   docker exec -it {{container_name}} {{sh}}
   
 - Remove a stopped container:
   docker rm {{container_name}}
   
 - Fetch and follow the logs of a container:
   docker logs -f {{container_name}}
$ docker run --rm postgres
  • By default, will run with STDOUT attached to your terminal
  • Use CTRL+C to stop
  • Alternatively, use -d to run in detached mode

Attach & Detach

$ docker run --rm -d postgres
a0849c33ede8c97c30d5db7ca31bcaed01212c33dd84aca9ac96402300ff005c
$ docker attach a0849
$ docker run --rm -i python
print(1 + 1)
2

Interactive Terminal

  • --interactive (-i):  Attach STDIN
  • --tty (-t): Allocate a pseudo-TTY
$ docker run --rm -it python
Python 3.10.3 (main, Mar 18 2022, 16:01:59) [GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Port mapping

docker run --rm -d -p 5432:5432 postgres
docker run --rm -d -p 3306:3306 mysql
docker run --rm -d -p 3307:3306 mariadb
  • Maps a port on the host (localhost:5432) to a port in the container
  • Can have multiple containers using the same ports, but mapped to different host ports

Port mapping

docker run --rm -d -p 8000:8000 prologic/todo
docker run --name todo -d -p 8000:8000 \
      -v /tmp/todo.db:/usr/local/go/src/todo/todo.db \
      --user root:root \
      prologic/todo
  • Docker containers are ephemeral. When removed, all data is lost
  • We can mount volumes to persist the file system. Docker will overlay the volume onto the image
  • Use absolute path for a local volume

Volume mapping

docker stop todo
docker rm todo
docker run --name todo -d -p 8000:8000 \
      --user root:root \
      -v /tmp/todo.db:/usr/local/go/src/todo/todo.db \
      -e THEME=ayu \
      prologic/todo
  • We can configure the containers using environment variables. This is useful to configure different deployments e.g. staging/prod environments
  • For example, todo supports changing the theme:

Environment Variables

docker logs -f todo
[todo] 2020/10/23 23:56:00 (172.17.0.1:35904) "GET / HTTP/1.1" 200 842 684.422µs
[todo] 2020/10/23 23:56:00 (172.17.0.1:35904) "GET /css/color-theme.css HTTP/1.1" 200 166 196.807µs
  • When running in detached mode, we lose access to STDOUT.
  • We can use docker logs to look at the logs
    • -f flag to follow the logs

Logs

Inspect

docker inspect todo | less
  • Useful for troubleshooting
  • View information
    • status (running/paused/OOMkilled/dead)
    • image
    • command and args
    • env variables
    • port binds
    • volumes mounts
    • etc

Building your own containers

Why create my own image?

  • Cannot find pre-built containers
  • Existing containers don't match your requirements
    • e.g. PHP applications that carry their own reverse proxies
  • Want to deploy your own applications with Docker

How to create my own image?

  1. Plan how to get a working environment from scratch
    • What OS or base image?
    • What dependencies to install?
    • What services does it depend on?
  2. Write a Dockerfile using that recipe
FROM ubuntu:latest

RUN apt-get update \
  && apt-get install nodejs npm

WORKDIR /app
COPY . .

CMD ["npm", "run", "start"]

Layered architecture

  • Each command creates a new layer that builds over the previous
  • Helps with incremental building
FROM node:latest

WORKDIR /app
COPY . .

CMD ["npm", "run", "start"]

Choose the right base image

  • Reduce build times
  • Save disk space if you have multiple images with similar setup
docker build . -t goweiwen/docker-workshop

Build it!

  • Build the container with a specified tag:
docker history goweiwen/docker-workshop
  • View the layers using docker history
docker run --rm goweiwen/docker-workshop
  • Run it
ENV <key>=<value>

More commands

  • Default environment variables
VOLUME ["/data"]
  • Declare volumes for mounts
EXPORT <port>
  • Declare ports to export

Docker Compose

The Problem

  • Deploying a stack of containers with commands is verbose and difficult to maintain
    • Specify all mounts and ports (if bridging to host)
    • Networking between containers
    • Specifying all environment variables
    • How to do version control?
  • One solution is to put the configuration into a yaml file
    • e.g. docker-compose.yaml, Kubernetes manifests
    • Check that yaml into a git repository maintained by DevOps team

voting-app

  • Python web app for serving front-end for voting
  • Node.js web app for showing results
  • Redis in-memory DB
  • Postgres filesystem DB
  • .NET Core worker for consuming votes
docker run -d --name=redis redis
docker run -d --name=db postgres:9.4
docker run -d --name=vote -p 5000:80 voting-app
docker run -d --name=result -p 5001:80
docker run -d --name=worker worker

Have to link the containers

  • Resolves hosts using /etc/hosts
  • If "vote" looks for a host called "redis", need to link them for it to resolve
docker run -d --name=redis redis
docker run -d --name=db postgres:9.4
docker run -d --name=vote -p 5000:80 \
  --link redis:redis \
  dockersamples/examplevotingapp_vote:before
docker run -d --name=result \
  --link db:db -p 5001:80 \
  dockersamples/examplevotingapp_result:before
docker run -d --name=worker \
  --link db:db --link redis:redis \
  dockersamples/examplevotingapp_worker
version: '3'
services:
  redis:
    image: redis
  db:
    image: postgres:9.4
    environment:
      POSTGRES_USER: "postgres"
      POSTGRES_PASSWORD: "postgres"
  vote:
    image: dockersamples/examplevotingapp_vote:before
    ports:
      - 5000:80
  result:
    image: dockersamples/examplevotingapp_result:before
    ports:
      - 5001:80
  worker:
    image: dockersamples/examplevotingapp_worker

docker-compose.yml

  • Docker compose will create a default network and link all services in it
docker-compose up

# OR

docker swarm init
docker stack deploy voting-app \
  -c docker-compose.yml
version: '3'
services:
  redis:
    image: redis
  db:
    image: postgres:9.4
    environment:
      POSTGRES_USER: "postgres"
      POSTGRES_PASSWORD: "postgres"
  vote:
    image: dockersamples/examplevotingapp_vote:before
    ports:
      - 5000:80
  result:
    image: dockersamples/examplevotingapp_result:before
    ports:
      - 5001:80
  worker:
    image: dockersamples/examplevotingapp_worker

Orchestration

  • Manage, scale containerized applications across multiple machines
  • Abstracts the concept of individual machines, now think in terms of total resource: CPU, memory
  • Containers guarantee they work the same everywhere
  • Automatically scale applications up, replace failed containers, perform rolling updates
  • e.g.
    • Kubernetes
    • Docker Swarm
    • Nomad

Kubernetes

  • a.k.a k8s
  • Most commonly used orchestration solution
  • Use manifest yamls to declare deployment requirements
  • Complicated
    • Tools such as Helm and Ksonnet to manage these yamls, allow rolling back entire deployment configs
  • Enterprise-grade, offered by cloud solutions
    • e.g. Google Kubernetes Engine, Anthos, Amazon Elastic Kubernetes Service, DigitalOcean Kubernetes
  • Not covered today

Docker Swarm

  • Docker's solution to orchestration
  • Allow using multiple Docker hosts as a single virtual host
  • Can use docker-compose.yml almost without change
  • Much lighter than k8s, quicker to deploy and lower overhead
  • Less complex

Private Registries

  • Default registry is Docker Hub (docker.io)
  • Sometimes, you don't want your containers to be public
  • e.g. GitLab's container registry (free!)
docker login registry.gitlab.com

docker pull registry.gitlab.com/kiasufoodies/kiasubot:0.10.0

Using Docker for Development

Quickly set up a database

  • Set up a disposable Postgres or MySQL database quickly
  • Restore a database dump if necessary
docker run -p 5432:5432 --name postgres \
  -e POSTGRES_USER='db' \
  -e POSTGRES_PASSWORD='password' \
  -d postgres

docker exec postgres psql -U db postgres <<EOH
  CREATE TABLE bobby (id serial PRIMARY KEY, field VARCHAR(50));
  INSERT INTO bobby (name) ('test1', 'test2');
EOH

docker exec -i postgres pg_dump -Ft -U db -d postgres > postgres.dump
docker exec -i postgres pg_restore -U db -d postgres < postgres.dump

DB_HOST=0.0.0.0 DB_USER=db DB_PASSWORD=password yarn start

Mock an API

  • MinIO is an S3-compatible block storage solution
services:
  minio:
    image: minio/minio
    command: server /data
    ports:
      - 9000:9000
    volumes:
      - minio:/data
    environment:
      MINIO_ACCESS_KEY: minio_access_key
      MINIO_SECRET_KEY: minio_secret_key
AWS_S3_ENDPOINT=http://0.0.0.0:9000
AWS_ACCESS_KEY_ID=minio_access_key
AWS_SECRET_ACCESS_KEY=minio_secret_key
yarn start

Help frontend developers

  • No need to understand how the backend is set up
  • Just keep a docker-compose.yml in the repository
  • Remember to keep secrets off the version control!
version: '3.1'
services:
  api:
    image: registry.gitlab.com/kiasufoodies/kiasubot:0.5.2
    ...
    environment:
      - TZ=Asia/Singapore
      - DATABASE_URL=${DATABASE_URL}
      - KIASU_API_KEY=${KIASU_API_KEY}
      - AWS_REGION=${AWS_REGION}
      - AWS_S3_BUCKET_NAME=${AWS_S3_BUCKET_NAME}
      - AWS_S3_KEY_PREFIX=${AWS_S3_KEY_PREFIX}
      - GOOGLE_PLACES_API_KEY=${GOOGLE_PLACES_API_KEY}

Visual Studio Code: Remote

Using Docker for Deployment

Should I Deploy using Docker?

  • Consider using container orchestration tools:
    • Kubernetes, Docker Swarm, Nomad
  • Consider NOT using Docker for databases
    • Use managed DBs instead to reduce ops workload
  • ​Do you need specific hardware interfaces?

Reverse Proxies

  • Use the request address to proxy HTTP(S) requests to containers
  • Just add labels to your containers
  • Automatically create configs as containers are added/removed
  • Automatically request for and renew LetsEncrypt SSL certs!
  • For nginx:
    • jwilder/nginx-proxy
    • jrcs/letsencrypt-nginx-proxy-companion
  • For Caddy:
    • lucaslorentz/caddy-docker-proxy

Using GitLab's CI

  • GitLab's CI is powerful
  • Use it to build your projects
  • Deploy to GitLab's container registry
  • Even automatically deploy the containers!
...
build:
  stage: build
  rules:
    - if: "$CI_COMMIT_TAG"
  script:
    - docker pull $CI_REGISTRY_IMAGE:latest
    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag
      $CI_REGISTRY_IMAGE:latest .
    - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME"
    - docker push "$CI_REGISTRY_IMAGE:latest"

Backing up databases

  • Don't reinvent the wheel
  • Remember to test your backups -- Untested backups are worthless
  • For Postgres:
    • prodrigestivill/postgres-backup-local
    • schickling/postgres-backup-s3
  • For MariaDB:
    • tiredofit/mariadb-backup

Monitoring Docker

  • cAdvisor
    • Collects and exports data about running containers
      • Resource usage
      • Network statistics
  • Prometheus
    • Polling time series database, can poll cAdvisor
  • Grafana
    • Data visualization and alert system, can use data from Prometheus

Questions

Docker

By Wei Wen Goh

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