| Hi Chonduy,
Host liveness is controlled in the Db_gc module (xen-api.hg/ocaml/xapi/db_gc.ml). The way it works is that each pool slave is heartbeating to the master, calling every 30 seconds (This is defined in xapi_globs.ml - 'host_heartbeat_interval'). If the master hasn't heard from a slave for 200 seconds (Xapi_globs.host_assumed_dead_interval), then the host is marked as dead by setting to false the database field 'live' on the Host_metrics object.
This 'live' field is checked when a message is forwarded to the host in the Message_forwarding module (xen-api.hg/ocaml/xapi/message_forwarding.ml). If the host is thought to be dead, it immediately returns the error HOST_OFFLINE. If it's not thought to be dead, then we usually re-use an existing stunnel session that had previously been set up to talk to the host. This might take a long time to fail, but when it does it causes a 'CANNOT_CONTACT_HOST' error. Both of these errors are thrown from the Message_forwarding module.
There are only a couple of small parts of the codebase that haven't been open-sourced, like the HA and licensing code. The Db module is actually not present in the repository as it is generated from the code living in the ocaml/idl subdirectory of xen-api.hg. The file 'ocaml/idl/datamodel.ml' contains the descriptions of both the Xen API itself and also the database backing the objects. The database is not actually a complex beast - the code that is generated is essentially an interface between the strongly typed world that most of xapi inhabits and the underlying representation of the database (currently hashtables in memory, serialised to an xml file on disk). The database is currently only in memory on the pool master, and calls to the Db module from slaves are translated into requests sent across the network. If you manage to build xapi (not an easy task!), the source of the database will be in the ocaml/autogen dir (IIRC it's called db_actions.ml).
Hope this helps!
Jon
On 20 Mar 2010, at 01:07, Chonduy Nguyen wrote: Hi Dave
I am trying to look into XCP to see if I can make change on the code to improve of some scnerio - but it looks like
not all codes are open - May I confirm this?
I observed that when a node is down - and if my software tried to do shutdown on VMs - it took 20 minutes
for the api to return Exception - error CANNOT_CONTACT_HOST
for another call like disable_host - it takes 3 minutes to return error : CANNOT_CONTACT_HOST
after the first error - it looks like the second calls - will faster with error -HOST_OFFLINE
May I ask info of Host down etc... stored on storage or memory of XCP ?
I look at the code and saw object call Db. --> what is this DB ? - is a local file or pool storage ?
Thank you,
Chonduy/TNguyen
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