Scaling USSD Application
Manning high traffic USSD applications is a tricky business because they should maintain a session for each user, other than SMS where there is no need to remember a state. Making it more hard is that each USSD request has a session timeout of avg 1s.
Most of the time is network traffic and fetching users state and responding to the new input.
PHP USSD Application Scaling
LB - load balancer without sticky sessions.
PHP Application - Are the PHP application servers.
Memcache - quick access memory to retrieve session information.
MySQL Master - Is write biased it help to keep a record about users activities which can be analysed later.
MySQL Slave - Is DB is read biased and feeds the dashboard where we track many variables like number of action’s per second, unique number of users etc.