To activate JPA and then have it accessed by the Spring application already configured for Platform.sh, it is necessary to modify two files.

1. Add a SQL database service 

In your .platform/services.yaml file, include a SQL database service. Make sure to visit the documentation for that service to find a valid version. For PostgreSQL that would look like:

dbpostgres:
    type: postgresql:13
    disk: 256

2. Grant access to the service through a relationship 

Your .platform.app.yaml file will require a relationship in order to have access to the new service.

relationships:
    postgresdatabase: "dbpostgres:postgresql"

3. Export connection credentials to the environment 

Connection credentials for services are exposed to the application container through the PLATFORM_RELATIONSHIPS environment variable from the deploy hook onward. Since this variable is a base64 encoded JSON object of all of your project’s services, you’ll likely want a clean way to extract the information specific to the databse into it’s own environment variables that can be easily used by Spring. On Platform.sh, custom environment variables can be defined programmatically in a .environment file using jq to do just that:

export DB_PORT=`echo $PLATFORM_RELATIONSHIPS|base64 -d|jq -r ".postgresdatabase[0].port"`
export HOST=`echo $PLATFORM_RELATIONSHIPS|base64 -d|jq -r ".postgresdatabase[0].host"`
export DATABASE=`echo $PLATFORM_RELATIONSHIPS|base64 -d|jq -r ".postgresdatabase[0].path"`
export SPRING_DATASOURCE_URL="jdbc:mysql://${HOST}:${DB_PORT}/${DATABASE}"
export SPRING_DATASOURCE_USERNAME=`echo $PLATFORM_RELATIONSHIPS|base64 -d|jq -r ".postgresdatabase[0].username"`
export SPRING_DATASOURCE_PASSWORD=`echo $PLATFORM_RELATIONSHIPS|base64 -d|jq -r ".postgresdatabase[0].password"`
export JAVA_OPTS="-Xmx$(jq .info.limits.memory /run/config.json)m -XX:+ExitOnOutOfMemoryError"

4. Connect to the service 

Commit that code and push. The specified cluster will now always point to the PostgreSQL or any SQL service that you wish.