This is a minimal replication of an issue we where a temporary database outage lead to a lack of available database connections, even when they shouldn't have been in use.
To reproduce:
First, update the config.yml file to specify a database. It is currently configured for a postgres docker container, but you should be able to use any database -- we have seen the issue with both postgres and oracle databases.
Then run the project:
$ mvn clean install
$ java -jar target/dbtest-1.3.5.jar server config.yml
$ curl <path-to-database>/hello-world
This should output {"id":1,"content":"Selected 1; Pool Size: 10; Active: 0; Idle: 10; Waiting: 0"}
Next -- stop the database to break the connection. Run the curl command again to confirm. Then restart the database, and wait for it to reconnect.
Run $ curl <path-to-database>/hello-world until successful again, and it will output {"id":8,"content":"Selected 1; Pool Size: 9; Active: 8; Idle: 1; Waiting: 0"}
(Note that the actual metrics might be different, though they have been very consistent).
Now, every time curl is repeated, there will be a pool size of N, and N-1 of those connections will be "active".