Does the ConnectionPool from SqlJocky require a close - mysql

Question

I'm creating a back-end server application in Dart which is using a MySQL database to store data. To make the SQL call I'm using the ConnectionPool from SqlJocky.
What I do when the app starts:
Create a singleton which store the ConnectionPool
Execute multiple queries with prepareExecute and query
Locally this approach is working fine. Now I pushed a development version to Heroku and I'm getting connection issues after a few minutes.
So I wonder, do I need to close/release a single connection from the pool I use to execute a query? Or is the connection after the query placed again in the pool and free for use?
The abstract base class for all the MySQL stores:
abstract class MySQLStore {
MySQLStore(ConnectionPool connectionPool) {
this._connectionPool = connectionPool;
}
ConnectionPool get connectionPool => this._connectionPool;
ConnectionPool _connectionPool;
}
A concrete implementation for the method getAll:
Future<List<T>> getAll() async {
Completer completer = new Completer();
connectionPool.query("SELECT id, name, description FROM role").then((result) {
return result.toList();
}).then((rows) {
completer.complete(this._processRows(rows));
}).catchError((error) {
// TODO: Better error handling.
print(error);
completer.complete(null);
});
return completer.future;
}
The error I get:
SocketException: OS Error: Connection timed out, errno = 110, address = ...

Answer 1

This doesn't fully answer your question but I think you could simplify your code like:
Future<List<T>> getAll() async {
try {
var result = await connectionPool.query(
"SELECT id, name, description FROM role");
return this._processRows(await result.toList());
} catch(error) {
// TODO: Better error handling.
print(error);
return null;
}
}
I'm sure here is no need to close a connection with query. I don't know about prepareExecute though.
According to a comment in the SqlJocky code it can take quite some time for a connection to be released by the database server.
Maybe you need to increase the connection pool size (default 5) so you don't run out of connections while ConnectionPool is waiting for connections to be released.

Answer 2

After some feedback from Heroku I managed to resolve this problem by implementing a timer task that does every 50 seconds a basic MySQL call.
The response from Heroku:
Heroku's networking enforces an idle timeout of 60-90 seconds to prevent runaway processes. If you're using persistent connections in your application, make sure that you're sending a keep-alive at, say, 55 seconds to prevent your open connection from being dropped by the server.
The work around code:
const duration = const Duration(seconds: 50);
new Timer.periodic(duration, (Timer t) {
// Do a simple MySQL call with on the connection pool.
this.connectionPool.execute('SELECT id from role');
print('*** Keep alive triggered for MySQL heroku ***');
});