You can modify parameter values in a custom DB parameter group, but you can't change the parameter values in a default DB parameter group.
It is recommended to create a new parameter group, and change the parameter_group_name
to that.
resource "aws_db_instance" "mysql" {
identifier = "app"
allocated_storage = 50
storage_type = "gp2"
engine = "mysql"
engine_version = "5.7.11"
instance_class = "db.m4.large"
name = "app_db"
port = 3306
publicly_accessible = false
vpc_security_group_ids = ["${aws_security_group.mysql.id}"]
db_subnet_group_name = "app-subnet-group"
parameter_group_name = "default.mysql5.7" // default DB parameter group!
multi_az = true
}