How can I extend the postgresql connection in just one node.js instance. #971

Hi, I'm using the mapnik in my tile server, and use Express to handle the request, and for every request I will initial a mapnik instance to render the tile image, it works fine, the question is, it seems that there is a limitation for multiple request, the PostgreSQL shows that the concurrent connection count is low, how can I set up the node app to increase the concurrent connection to the DB to improve the throughout?
Thanks.