I am developing a back-end with Node.js and MySQL. Sometimes, there are a huge number of queries to be done on the database > 50,000, and I'm using connection pooling. My question is what happens to a query after it is rejected due to the pool being exhausted? Will it be queued until a connection becomes available and then executed? Or it will simply never be executed?
There are indeed similar questions but the answers didn't highlight my point, they just recommended increasing the limit size.
Maybe.
There are options you can set to modify the behavior. See https://github.com/mysqljs/mysql#pool-options
The request may wait in a queue for a free connection, or not. This is based on the option waitForConnections. If you set this option to false, the request returns an error immediately instead of waiting.
If more than queueLimit requests are already waiting, the new request returns an error immediately. The default value is 0, which means there is no limit to the queue.
The request will wait for a maximum of acquireTimeout milliseconds, then if it still didn't get a free connection, returns an error.
P.S.: I don't use Node.js, I just read this in the documentation.