Docker container deployed via Beanstalk cannot connect to the database on RDS - amazon-web-services

Question

I'm new to both docker and AWS. I just created my very first docker image. The application is a backend microservice with rest controllers persisting data in a MySQL database. I've manually created the database in RDS and after running the container locally, the rest APIs work fine in Postman.
Here is the Dockerfile:
FROM openjdk:8-jre-alpine
MAINTAINER alireza.online
COPY ./target/Practice-1-1.0-SNAPSHOT.jar /myApplication/
COPY ./target/libs/ /myApplication/libs/
EXPOSE 8080
CMD ["java", "-jar", "./myApplication/Practice-1-1.0-SNAPSHOT.jar"]
Then I deployed the docker image via AWS Beanstalk. Here is the Dockerrun.aws.json:
{
"AWSEBDockerrunVersion": "1",
"Image": {
"Name": "aliam/backend",
"Update": "true"
},
"Ports": [
{
"ContainerPort": "8080"
}
],
"Logging": "/var/log/nginx"
}
And everything went well:
But now, I'm getting "502 Bad Gateway" in postman when trying to run "backend.us-east-2.elasticbeanstalk.com/health".
I checked the log on Beanstalk and realized that the application has problem connecting to the RDS database:
"Could not create connection to database server. Attempted reconnect 3 times. Giving up."
What I tried to do to solve the problem:
1- I tried to assign the same security group the EC2 instance is using to my RDS instance, but it didn't work.
2- I tried to make more inbound rules on the security group to add public and private IPs of the EC2 instance but I was not sure about the port and the CIDR I should define and couldn't make it.
Any comment would be highly appreciated.

Answer 1

Here are resources in your stack:
LoadBalancer -> EC2 instance(s) -> MySQL database
All of them need to have SecurityGroups assigned to them, allowing connections on the right ports to the upstream resources.
So, if you assign sg-1234 security group to your EC2 instances, and sg-5678 to your RDS database, there must be a rule existing in the sg-5678 allowing inbound connections from sg-1234 (no need for CIDRs, you can open a connection from SG to SG). The typical MySQL port is 3306.
Similarly, the LoadBalancer (which is automatically created for you by ElasticBeanstalk) must have access to your EC2 instance's 8080 port. Furthermore, if you want to access your instances with the "backend.us-east-2.elasticbeanstalk.com/health" domain name, the loadbalancer would have to listen on port 80 and have a target group of your instances on 8080 port.
Hope this helps!