When using Docker, users often need to be able to view the data generated in the container, or need to back up the data in the container, or even share data among multiple containers. Therefore, it is necessary to understand the data management operation of the container.

There are two main ways to manipulate data in a container:

• Data Volume
• Data Volume Containers

The container is only in the running state, that is, all data will be reset at the end of the container's life cycle. If you want to save data in it without affecting the image, you usually use the data volume.

volume will not be packaged with commit and submitted to a new image. The official document says that

Create a data volume

When using the docker run command, use - v to create a data volume in the container, and use - v multiple times to use multiple data volumes

# Create containers using training/webapp images and mount data volumes to/webapp catalogue
$ sudo docker run -d -p --name web -v /webapp training/webapp python app.py

If you need to mount a host directory to the container, mount the host's / src/webapp to the / opt/webapp directory in the container as follows:

$ sudo docker run -d -p --name web -v /src/webapp:/opt/webapp training/webapp python app.py

The data volume can also set its read-write permission (rw). ro identifies the specified read-only, so that the data of this data volume cannot be modified after the container is mounted

$ sudo docker run -d -p --name web -v /src/webapp:/opt/webapp:ro training/webapp python app.py

Viewing the volume mounted by the container in the host machine, in fact, the principle of data volume is not so complex, and it is also implemented by the file system of the host machine.

For example, let's start a mysql container. Here we will explain the differences between two different data volume types: volume and bindmount

## Default volume
$ docker run -d --name mysql-local1 -e MYSQL_ROOT_PASSWORD=root mysql:5.7
## Such sub mounts will be stored in the volume directory of docker by default
[vagrant@docker-node3 ~]$ sudo ls /var/lib/docker/volumes
c5020bd3e7253cd1fdfce10243424074846642d6db933bf66a911f6bdd0858a8  metadata.db

## If you don't want to use a long list of names, you can also name them separately
## mysql here is the name of volume
## docker volume ls Next name
$ docker run -d -v mysql:/var/lib/mysql --name mysql-local1 -e MYSQL_ROOT_PASSWORD=root mysql:5.7

## If using bind-mount To mount, that is, the directory of the local host, not docker of volume catalogue
$ docker run -v /var/own/mysqldata:/var/lib/mysql --name mysqlnew -d mysql -e MYSQL_ROOT_PASSWORD=root mysql:5.7

View the container and its running configuration

[vagrant@docker-node3 ~]$ docker container ls
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                 NAMES
13bcca305b36        mysql:5.7           "docker-entrypoint.s..."   37 minutes ago      Up 37 minutes       3306/tcp, 33060/tcp   mysql-local1
[vagrant@docker-node3 ~]$ docker inspect 13bcca305b36
====
"Mounts": [
            {
                "Type": "volume",
                "Name": "c5020bd3e7253cd1fdfce10243424074846642d6db933bf66a911f6bdd0858a8",
                "Source": "/var/lib/docker/volumes/c5020bd3e7253cd1fdfce10243424074846642d6db933bf66a911f6bdd0858a8/_data",
                "Destination": "/var/lib/mysql",
                "Driver": "local",
                "Mode": "",
                "RW": true,
                "Propagation": ""
            }
        ],
====

View the directory of the data volume associated with the container mirrored by mysql

[vagrant@docker-node3 ~]$ sudo ls /var/lib/docker/volumes/c5020bd3e7253cd1fdfce10243424074846642d6db933bf66a911f6bdd0858a8/_data
auto.cnf    ca.pem         client-key.pem  ib_logfile0  ibdata1  performance_schema  public_key.pem    server-key.pem
ca-key.pem  client-cert.pem  ib_buffer_pool  ib_logfile1  mysql    private_key.pem     server-cert.pem    sys

You can see that the data volume of a specific container is saved in the following directory of docker: / var/lib/docker/volumes/XXX

Development environment docker+bindmount create your own development environment

In the data volume, you can mount the code in the container to develop a python web application of flash. You can mount the code in the data volume, develop it in the host computer, and then run it in the container

For example, in the current directory, the controller logic related to the route with flash:

./dockerLib/flask-skeleton/
|-- __init__.py
|-- base.py
|-- test_config.py
|-- test_main.py
`-- test_user.py

test_user.py contains routing related logic

Start a container to mount the current directory

$ docker run -d -p 80:5000 -v $(pwd):/skeleton --name flask blackbinbin/flask-skeleton

Then you can edit the files in the external host directory and debug them in the container.