When developing Qt applications be it Desktop, Mobile or Web that need to talk directly with a database the use of QtSql is usually the best choice, it has many database drivers, comes ready on their installer, but has a blocking API, which means a query will block you GUI thread.
My Cutelyst Web projects also used QtSql, and for low traffic this isn’t a big issue because you are not freezing users GUI, but you are freezing the request queue instead.
One of the Cutelyst apps I developed this year has a very high traffic, and a side effect of blocking came in play, I’ve about 3k TVs connected via websockets, once a TV connects it does an authentication query, it takes ~17ms, now when there is an event PostgreSQL notifies the application which does some 20 other queries a little more expensive (~30ms) and send them to the TVs, this event also blocks the Cutelyst process.
Every TV does a websocket ping each 15 seconds to check if the connection is stalled, when that event happens plus some latency added 15s timeout and the TV disconnect, this of course happens with lots of TVs, so 1k TVs reconnecting will take around 17 seconds, this means new TVs disconnecting and the snow ball is done, system only recovers if I increase the DB hardware on AWS.
How can async fix this, you might be wondering, well the problem is that the 15s timeout happened to do a DOS on my own system, I can increase the value but when there are 10k TVs this will still be a problem, there are of course other ways to deal with this, but if the SQL query doesn’t block the Cutelyst process it will be able to timely reply the Websocket’s pongs.
ASql only supports PostgreSQL at time (PR are welcome), and depends on QtCore and libpq (PostgreSQL library), it can be used on Desktop apps and is 100% async from connection to queries, there’s even a class to help with DB versioning and migration.
Check it out: https://github.com/cutelyst/asql