I have a really simple bookshop webapplication written in Spring framework, just to test its scalability.
I deployed this bookshop on one EC2 instance (t1.micro), and database on Amazon RDS (t1.micro) with master/slave replication of one master instance and 3 slave instances (There's really a lot more reads than writes). One t1.micro RDS instance can have at maximum of 32 concurrent connections
Then I did stress testing with JMeter, figured out that the bottleneck is in the database, since you can have at maximum 32 concurrent connections to t1.micro RDS instance.
Should I auto scale RDS database instances, since creating new replica modifies master and it really takes long time to make it available?
Instead of using RDS should I create EC2 instances with MySQL master/replica and then auto scale these instances?
Should I shard my database instead of replication?
Application also uses com.mysql.jdbc.ReplicationDriver to load balance between master and slave instances. Should I use something different like HAProxy?