I've purchased a single VPC on AWS and initiated there 6 MySql databases, and foreach one I've created a reading replica, so that I can always run queries on the reading replicas quickly.
Most of the day, my writing instances (original instances) are fully loaded and their CPUs percentage is mostly 99%. However, the reading replicas shows something ~7-10% CPU usage, but sometimes I get an error when I run a service connecting to the reading replica "TOO MANY CONNECTIONS".
I'm not that expert with AWS, but is this happening because the writing replicas are fully loaded and they're on the same VPC?
this happening because the writing replicas are fully loaded and they're on the same VPC?
No, it isn't. This is unrelated to replication. In replication, the replica counts as exactly 1 connection on the master, but replication does not consume any connections on the replica itself. There is no impact on connections related to the intensity of the total workload from replication.
This issue simply means you have more clients connecting to the replica than are allowed by the parameter group based on your RDS instance type. Use the query SELECT @@MAX_CONNECTIONS; to see what this limit is. Use SHOW STATUS LIKE 'THREADS_CONNECTED'; to see how many connections exist currently, and use SHOW PROCESSLIST; (as the administrative user, or any user holding the PROCESS privilege) in order to see what all of these connections are doing.
If many of them show Sleep and have long values in Time (seconds spent in the current state) then the problem is that your application is somehow abandoning connections, rather than properly closing them after use or when they are otherwise no longer needed.