Docker for DevOps Engineers

Docker-Volume

• Images are a series of read-only layers

• A container is merely an instantiation of those read-only layers with a single read-write layer on top.

• Any file changes that are made within a container are reflected as a copy of modified data from the read-only layer.

• The version in the read-write layer hides the underlying file but does not remove it.

• When deleting a container, the read-write layer containing the changes is destroyed and gone forever!

• To persist these changes we use docker volumes

Advantages:

• To keep data around when a container is removed

• To share data between the host filesystem and the Docker container

Types of Docker Volume

Two types of volume mounts: Named and Bind

• Named Volume: Mounting a volume created using the ‘docker volume create command and mounting it from default volume location /var/lib/docker/volumes

Below command to create a named volume

docker volume create my-vol
docker run -d --name nginx -v myvol:/app nginx
docker run -d --name nginx --mount source=myvol2,target=/app nginx

• Bind Volume: External mounting(external hard disks etc.) Bind mounts may be stored anywhere on the host system. They usually start with ‘/’

Below command to create a bind volume

docker run –name web -v /root/html:/var/www/html/ nginx

Docker Network

Connecting containers is very important for most of the applications out there. A classic web application consists of at least a web server, an application server and a database server. The web server needs to talk to the application server and the application server would need to talk to the database server. A legacy, but popular way to do so is via passing --link flag to the docker run command.

Docker gives 3 default networks: bridge, none and host

When you start Docker, a default bridge network (also called a bridge) is created automatically, and newly started containers connect to it unless otherwise specified.

Using the below command we will get the default network list

docker network ls

Using the below command we can see in the details

docker network inspect bridge

Bridge:-

• In Bridge network, all containers get private internal IPs and they are isolated from host.

• Port forwarding forwards outside traffic to the containers.

• Containers on the default bridge network can only access each other by IP addresses, unless you use the link option, which is considered legacy.

• You can also create user defined custom bridge network

• User defined bridge networks are superior to the default bridge network.

• On a user defined bridge network, containers can resolve each other by name or alias(DNS)

Create a bridge network:-

docker network create --driver bridge my-net

Attach a container to it:-

sudo docker run -itd --name con5 --network my-net alpine ash

# Here we are using alpine beacuse lite image

When containers are run in the default bridge network they cannot find each other using their container names. Simply put, DNS resolution through container names will not work under the default bridge network

In the above screenshot, you can see if we ping through IP address then we can communicate with another container but if we ping the container name then we can not communicate with another container and we are receiving the error "Bad address"

Now a new bridge network is created and containers are attached to that network. In this case, containers find each other using their container names( DNS resolution through container names

After creating a custom bridge container we can see above screenshot that we can ping through the IP address and we can ping using the container name also

Host:-

In host network, all containers directly get connected to host. Multiple containers cannot run on same hosts because of port conflicts on host side

Need to run the below command using the host network

docker run -d --name web -net host nginx

None:-

• This offers a container-specific network stack that lacks a network interface.

• Containers run in pure isolation

• This container only has a local loopback interface (i.e., no external network interface)

Need to run the below command using the host network

docker run -d --name web --net none nginx

Overlay :-

Bridge networks apply to containers running on the same Docker daemon host. For communication among containers running on different Docker daemon hosts, we should use an overlay network which spans across the entire cluster github.

Task 1:-

• Create a multi-container docker-compose file which will bring UP and bring DOWN containers in a single shot

We have to modify the docker-compose file

version : "3.3"
services:
  web:
    image: nginx:latest
    ports:
      - "80:80"
  db:
    image: mysql
    ports:
      - "3306:3306"
    environment:
      - "MYSQL_ROOT_PASSWORD=test@123"

• Now we can start the container by using the below command

docker-compose up -d

• Use the docker-compose scale command to increase or decrease the number of replicas for a specific service.

docker-compose up -d --scale web=4 --scale db=3

• Use the docker-compose ps command to view the status of all containers

docker-compose ps

• docker-compose logs to view the logs of a specific service.

docker-compose logs

• Use the docker-compose down command to stop and remove all containers, networks, and volumes associated with the application

    docker-compose down
  

  

  Task 2:-

• Learn how to use Docker Volumes and Named Volumes to share files and directories between multiple containers.

First, we need to create a volume

 docker volume create my-vol

Now we need to create a different container using the same volume

 docker run -d --name con1 -v my-vol:/app nginx:latest
 docker run -d --name con3 -v my-vol:/app nginx:latest

• Verify that the data is the same in all containers by using the docker exec command to run commands inside each container.

Now we need to go to inside any conatiner and we have created sample file and exit

docker exec -it con3 bash
echo "Creating sample text file for docker volume" > /app/hello.txt
exit

Now if we go to the next container which is con3 and if we search the same file name. The file should be refect here

• Create two or more containers that read and write data to the same volume using the docker run --mount command.

We need to create a new volume

docker volume create mine-volume

Now we are creating conatiner using --mount and same volume

docker run -itd --name myapp --mount source=mine-volume,target=/my_data alpine ash

docker run -itd --name myphapp --mount source=mine-volume,target=/my_data alpine ash

Now we need to go to myapp conationer and need to create text file and exit

echo "Creating same text file from myapp container" > /my_data/Hello.txt

Now we need to use below command verify from second conationer

cat /my_data/Hello.txt

list down all the volumes and to remove a particular volume.

# this is use to check the volume list 
docker volume ls
# This is use for to remove volume
docker volume rm <volume_name>
# If you are getting error removing volume try the below coomand
 docker system prune

Devops#devops,#90daysofDevOps

Thank you for reading!! I hope you find this article helpful!!

if any queries or corrections to be done to this blog please let me know.

Happy Learning!!

Saikat Mukherjee