Re MySQL counts, based on your notes you’re probably using 50 conns at max whatever your settings are. 25 for sidekiq (assuming you do run a hot queue) and 25 at max for the servers. Irrespective of DB pool size, I think Rails does lazy connections, so you’re never going to go over 1.
I changed database.yml to use 10 connections. I didn't change the Sidekiq configuration, which still uses concurrency: 25.
Strangely, tonight I've seen the first exceptions like `ActiveRecord::ConnectionTimeoutError: could not obtain a connection from the pool within 5.000 seconds (waited 5.000 seconds); all pooled connections were in use` in many, many years.
All of these exceptions came from within Sidekiq Workers, which is strange because, like I pointed, I didn't change the Sidekiq concurrency, only the database.yml pool, which shouldn't affect workers.
Isn’t sidekiq working off Redis? You might be adjusting the Redis concurrency instead. And if I remember correctly even with sidekiq you’re still using normal Rails and ActiveRecord, so a process that’s running 25 sidekiq jobs simultaneously is going to need 25 active record DB conns from the database.yml. I doubt the sidekiq config is overriding it in some way.
But basically if your sidekiq workers and app servers are using the same database.yml values, you’ll need to set the greater of the two on both. I assumed you have different settings on app servers and workers.
