proxy error


I recently add 2 server to my cs-cart all server are running centos 6.4, apache, and mysql.

I divide the server to become like this 1 is acting as a load balancer and also store the database, and the second and third acting as a processing server.

so every time there is a request the server a will direct the request to server b, then the next request is go to server c, then server b again, then c again. and so on.

everything is fine until if a person is creating an order. it seems to become double. so if the customer click order, the cart is creating 2 exact order but different ID. and there is a message error

Proxy Error

The proxy server received an invalid response from an upstream server.

The proxy server could not handle the request POST/

Reason: Error reading from remote server

Anyone can help with this matter?

Thank you