Question

Tornado advertises itself as “a relatively simple, non-blocking web server framework” and was designed to solve the C10k problem. However, looking at their database wrapper, which wraps MySQLdb, I came across the following piece of code:

def _execute(self, cursor, query, parameters):
    try:
        return cursor.execute(query, parameters)
    except OperationalError:
        logging.error("Error connecting to MySQL on %s", self.host)
        self.close()
        raise

As far as I know calls to the MySQLdb, which is built on top of libmysqlclient, are blocking.

Am I right in thinking that a long-running query would render the entire Tornado server unresponsive until it finishes or is there magic on the code?

Answer 1

Yes, absent other measures, the server will wait for the query to finish executing. That does not mean Tornado is not a non-blocking web server.

A “non-blocking web server” doesn’t block on network I/O (and may have some provision for disk I/O if it does static file serving). That does not mean you get instant, causality-violating instruction execution in your application.

Doing a database call takes time, just like reading files, formatting strings, processing templates, etc. takes time. Doing any of these things in the same thread as the server’s main event loop will prevent the loop from moving on until you’re done.