Docker -link containers

March 29, 2021

Docker link is actually labeled dec, but there are still some traditional applications using it

The most typical is linking web applications and databases

For example, you can start a WordPress

docker run --name wp_mysql -p 3306:3306 -v /data/mysql:/var/lib/mysql -e MYSQL_ROOT_PASSWORD=123 -d mysql:5.7

docker run --name wp_web --link wp_mysql:mysql -v /data/wordpress:/var/www/html -p 80:80 -p 443:443 -d wordpress:5.6.0

WordPress docker deployment using – link can directly connect to the database without configuring MySQL database connection.

Here I want to discuss what link has done

First of all, we need to clarify two concepts: MySQL is the source container and the connected container, WordPress is the target container and the receiving container

For the next test, we start two busybox containers and connect them together

docker run -i -t --name source -p 3001:3001 -p 3002:3002 -e source_env_1=123 -e source_env_2=456 busybox sleep infinity
docker run -i -t --name target -p 3003:3003 -p 3004:3004 -e target_env_1=123 -e target_env_2=456 --link source:mysource busybox sleep infinity

alias

–link wp_mysql:mysql

For WordPress containers, WP_ MySQL and MySQL are a container with the same containerid

–link source:mysource

For the target container, source and mysource are one container

2. DNS

3. environment variable

The reason why WordPress can directly connect to the database above is that it actually uses environment variables

The image of WordPress will read the database configuration from the alias MySQL by default, so if it is written as –link wp_mysql:test_mysql and WordPress can’t recognize the database, which can be understood as a convention

The target container gets the following environment variables