How to establish Docker multi-container connection with Mysql+Tomcat

This article mainly introduces Docker multi-container connection (taking Tomcat+Mysql as an example). Docker provides a method for direct access by multiple containers, which allows multiple containers to directly access Access through network ports

Docker provides a method for direct access by multiple containers. The simplest way is to directly use the port mapping -p parameter to specify the mapped port or -P to map all ports. Multiple containers can directly access through network port for access.

But the network port mapping method is not the only way to connect multiple containers in Docker. A safer method is to use Docker's connection system (--link) to connect multiple containers. When the containers are connected together , the recipient container can see the information of the source container.

Take Tomcat + Mysql as an example to establish a connection between containers

To establish a connection directly between containers, use the --link option

--link b55b5a51e06b65c594b87dda7b4aa1dc:alias

Here we create a Tomcat + Mysql service to illustrate how to establish a connection between two or more containers.

To establish a container connection, you must rely on the name of the container. Use --name to specify the name of the source container as mysql

docker run --name mysql -d gsoft/mysql:5.6

Next create the tomcat container and connect to the mysql container

The code is as follows:

docker run --name tomcat -d -p 80:8080 --link mysql:mysql gsoft/tomcat:7.0

The --link option specifies that the container to be connected is mysql.

Container intercommunication information

After establishing a connection between two containers, the receiving container (Recipient) will inevitably need to access the resources of the source container (Source). When we establish a connection for the container, the source container does not use -p/-P to specify the port to be exposed when it is created, so how to access the source container information?

In order to allow the receiving container to access the information of the source container, Docker provides two methods:

1. Environment Variables

2. /etc/hosts file

Environment variables

When Docker connects to the container, it will use --link The provided parameters automatically create some environment variables in the recipient container, including the environment variables set using the ENV command in the source container's Dockerfile and when the source container starts (docker run), use -e or - -env, the environment variable specified by the --env-file parameter.

Mainly includes the following environment variables, assuming alias=mysql.

<alias>_PORT
<alias>_PORT_<port>_<protocol>
<alias>_PORT_<port>_<protocol>_ADDR
<alias>_PORT_<port>_<protocol>_PORT
<alias>_PORT_<port>_<protocol>_PROTO
<alias>_NAME

For example:

#docker run -i -t --rm --link mysql:mysql ubuntu:14.04 env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=9c74aa611463
TERM=xterm
MYSQL_PORT=tcp://172.17.0.3:3306
MYSQL_PORT_3306_TCP=tcp://172.17.0.3:3306
MYSQL_PORT_3306_TCP_ADDR=172.17.0.3
MYSQL_PORT_3306_TCP_PORT=3306
MYSQL_PORT_3306_TCP_PROTO=tcp
MYSQL_NAME=/desperate_ritchie/mysql
HOME=/root

In the above example, the container alias is specified as msyql, so all environment variables start with MYSQL_.

Note that if the source container is restarted, the environment variable information in the receiving container will not be automatically updated. Therefore, if you want to use the IP address of the source container, please use /etc/ Host information configured in hosts.

/etc/hosts file

In addition to environment variables, Docker also updates the hosts information in the /etc/hosts file of the receiving container.

# docker run -i -t --rm --link mysql:mysql ubuntu:14.04 /bin/bash
# cat /etc/hosts
127.0.0.1  localhost
::1  localhost ip6-localhost ip6-loopback
fe00::0  ip6-localnet
ff00::0  ip6-mcastprefix
ff02::1  ip6-allnodes
ff02::2  ip6-allrouters
172.17.0.3  mysql 115346bdb403
172.17.0.5  09bdf7805133

As can be seen from the above, two additional pieces of information are added to the hosts file of the receiving container, the local IP and alias and the IP and alias of the source container (mysql).

Different from environment variables, if the source container is restarted, the information in /etc/hosts in the receiving container will be automatically updated.

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