update readme

Author: thombergs

aws/aws-rds-hello-world/README.md

@@ -1,4 +1,4 @@
-# RDS Hello World
+# RDS Hello World Application
 
 This is a simple Spring Boot application which requires access to a PostgreSQL database.
 
@@ -7,3 +7,19 @@ The application has a single endpoint `/hello` which prints out if the database
 Get it in a Docker image via `docker pull reflectoring/aws-rds-hello-world`.
 
 Use the image instead of your real application to test AWS CloudFormation stacks which need access to a database.
+
+## Testing AWS RDS connectivity with this application
+
+1. Create an RDS PostgreSQL database with the AWS console.
+2. Note the endpoint of your RDS database in the AWS console.
+3. Deploy the Docker container `reflectoring/aws-rds-hello-world` into AWS instead of your real application (this could be via a CloudFormation stack, manually, or however you are deploying your app).
+4. Configure your deployment in a way that Docker will pass the coordinates to your RDS database as environment variables, equivalent to this command:
+    ``` 
+    docker run \
+      -e SPRING_DATASOURCE_URL=jdbc:postgresql://<RDS-ENDPOINT>:5432/postgres \
+      -e SPRING_DATASOURCE_USERNAME=<USERNAME> \
+      -e SPRING_DATASOURCE_PASSWORD=<PASSWORD> \
+      -p 8080:8080 reflectoring/aws-rds-hello-world
+    ```
+5. If the Spring Boot application can connect to the database, it will start up sucessfully and serve a message on the endpoint `/hello`.
+