Host to Docker Container to Docker Container communication - docker

Question

I'm building 2 docker containers, "app" and "db", via a docker-compose file.
The app server just installs java/tomcat via a Dockerfile which is what docker-compose uses to build.
The db server uses an MS SQL image.
When I run:
docker-compose up
I follow that with a build process of software I need to load which deploys a war to the tomcat directory in the app server and builds the database in the database server.
My problem is: The build process can reference localhost:8080 to install/patch the software to the app server and reference localhost:1433 to install/patch the database portion of the software to the database server. However, when I start Tomcat the system doesn't come online because the app server can't connect to the database server via "localhost:1433" so it requires me to jump in and update the properties file after the build to the docker internal IP address and THEN it works.
My question is: How am I able to get my localhost and my app container to reference the DB in the same manner in a database url?
Dockerfile for app server:
FROM centos:centos7
COPY apache-tomcat-9.0.20.tar.gz /tmp/
WORKDIR /tmp/
RUN yum -y update
RUN yum -y install java-11-openjdk-devel
RUN tar -xf apache-tomcat-9.0.20.tar.gz
RUN mv apache-tomcat-9.0.20 /opt/tomcat/
RUN export JAVA_HOME=/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.212.b04-0.el7_6.x86_64/
RUN export PATH=$PATH:$JAVA_HOME/jre/bin
RUN export CATALINA_HOME=/opt/tomcat/
RUN export PATH=$PATH:$CATALINA_HOME/bin
WORKDIR /opt/tomcat/webapps
RUN mkdir testapp
enter code here
enter code here
Docker-Compose File:
version: '3.3'
services:
db:
image: "mcr.microsoft.com/mssql/server:2017-latest"
restart: always
volumes:
- db_data:/var/lib/mssql
environment:
- ACCEPT_EULA=Y
- SA_PASSWORD=Test123
network_mode: bridge
hostname: db
ports:
- "1433:1433"
app:
build: './testapp'
volumes:
- './system/build:/opt/tomcat/webapps/testapp/'
ports:
- "8080:8080"
- "8009:8009"
network_mode: bridge
tty: true
depends_on:
- db
volumes:
db_data:

Answer 1

Bring your service to the same network and target the service by service name. For that you need to define a docker network like below. For the following example I can access DB with http://mongo:27017.
mongo:
image: mongo:latest
ports:
- "27017:27017"
volumes:
- ./data/db:/data/db
networks:
- my-net
spring:
depends_on:
- mongo
image: docker-spring-http-alpine
ports:
- "8080:8080"
networks:
- my-net
networks:
my-net: