xapi a.k.a XenAPI

XAPI is at the heart of XenServer. It manages everything – all the resources in your XenServer environment, 

1. reads in configs, 
2. initializes networking paths, 
3. checks licensing, 
4. maintains a Database with all this info (the XAPI database), 
5. manages Storage Repositories, 
6. manages VMs, 
7. tracks VM states, 
8. manages networking, 
9. High Availability 
10. other XenServer hosts in the pool and more……

Each XenServer request or action is done via Xapi – and only by asking the Master XenServer XAPI to do so. 

A copy of the XAPI database (or you could consider this to be the state of XAPI) is synchronized among all the hosts in the pool – so they all have a copy of the current XAPI database – giving them the knowledge of what is going on – and in the case of the master going down – a new master can be appointed from the pool using the XAPI state information or shared database.

When you collect a status report from XenServer – the XAPI Database is stored in the status report. It is a relational database – and shows us the state of the environment at that point in time (when the status report was captured).

Reference

1. http://xenme.com/688

2. XAPI Developer Guide, http://wiki.xensource.com/xenwiki/XAPI_Developer_Guide

3. http://blogs.citrix.com/2011/05/18/so-what-is-xenserver-xapi/