I have 3 server that same server application is running on each of them. each installation of server app has its own configurations, data and settings. other client users or client applications can connect to this servers and communicate with them.
this servers are in different places.
this application are created by PHP and servers are ubuntu servers, each server has its own static IP.
but now i need to share some data between this server applications. for example server A needs to access to information of server B's customers. or main admin of system want to see some information of server C. other scenario may be back up/sync. each server with 4th server with special application.
what is the best and more secure way to share some data between server applications? for example application A on server A needs some data get from application B on server B, or send some data to it.
There are several ways to do this, and it depends on if your goal is to send data synchronously or asynchronously.
If you want to send data synchronously (that is, send data, and wait for a response before proceeding to whatever the next step), use HTTPS.
If you want to send data asynchronously (that is, send data, then go off and do something else while the response can come back at any time), use XMPPS.
Both run over SSL, so that will handle the security side of things. Both HTTP and XMPP services are plentiful, so building the scripts to use these services for communication would be relatively straight-forward.