This. With typical web applications, your request would wait until something times out and cascades back to the client (webserver, app server, db, etc.). This can mean waiting for 30 seconds or more depending on your configuration.
It actually is quite easy if you set a finite limit on connections and show an appropriate message to those connections you're actively refusing. If you're allowing as many connections as your server/network/database can allow, it might take a bit longer to determine a failure status.
