I’m trying to deploy a Spring Boot application with a PostgreSQL database on Fly.io, but I keep encountering the UnknownHostException: db-app.flycast error. Here’s the process I’ve followed:

1. Create PostgreSQL Database on Fly.io:
   I used the Flyctl CLI to create the PostgreSQL database named db-app:

   flyctl postgres create -n db-app
   

2. Create Spring Boot Application:
   Next, I created a Spring Boot application named example-app on Fly.io:

   flyctl apps create example-app
   

3. Attach PostgreSQL Database:
   I attached the PostgreSQL database db-app to the Spring Boot application using a configuration file fly.toml:

   flyctl attach db-app --config fly.toml -a example-app
   

fly.toml Configuration:

kill_signal = "SIGINT"
kill_timeout = 5
swap_size_mb = 1024

[build]
  dockerfile = "../docker/Dockerfile.app"

[http_service]
  internal_port = 8080
  force_https = true
  auto_stop_machines = true
  auto_start_machines = true
  min_machines_running = 0
  processes = ['app']

Error Encountered:
I consistently receive the following error:

java.net.UnknownHostException: db-app.flycast

Troubleshooting Steps Taken:

• Checked PostgreSQL Creation: Confirmed that the PostgreSQL database db-app is created successfully on Fly.io.

• DNS Resolution: Verified DNS resolution for db-app.flycast using ping and nslookup, ensuring it resolves to the correct IP address.

• Fly.io Networking Setup: Ensured that both the Spring Boot application and PostgreSQL database (db-app) are within the same Fly.io organization and region, and reviewed network policies.

• Spring Boot Application Configuration: Verified that application.properties or application.yml in the Spring Boot application correctly points to db-app.flycast with the appropriate port and credentials.

• Fly.io Service Logs: Checked Fly.io logs (flyctl logs -a example-app) for any connectivity issues or errors related to database access.

Request for Assistance:
I’m seeking guidance on resolving the UnknownHostException issue and ensuring successful deployment of my Spring Boot application with PostgreSQL on Fly.io. Any insights or suggestions would be greatly appreciated.