= Computer identification =

== How computers are identified ==

We haven't found a universal hardware-level mechanism (CPU chip ID, MAC address)
for uniquely identifying computers.
So we do it in software as follows:
When a computer first contacts a project's scheduling server,
the server creates a database record for the computer ,
which includes a host ID and an RPC sequence number.
The computer ID and the RPC sequence number are also stored in the client's `client_state.xml` file.
The client increments the RPC sequence number on each scheduler request.

If the scheduling server receives an RPC with a sequence number less than the expected sequence number
(usually indicating that the user has copied the `client_state.xml` file between computers)
it creates a new database record and returns a new ID.

== Merging duplicate computer records ==

This mechanism can lead to situations where a project's server has multiple database records for a single computer.
For example, this will occur if the user deletes the `client_state.xml` file.
The user can merge these duplicates into a single record via a web interface.

You may only merge two computer records if
 * They have the same processor type (Intel, AMD etc.) and operating system.
 * They don't overlap in time; i.e. computer 1's last RPC happened before computer 2's first RPC, or vice-versa.

There are two ways of merging computer records:

 * To merge a single computer, open its Summary page, and click on "Merge this computer".  You will see a list of computers eligible to be merged with this one, and you can select any or all of them.
 * The "Your computers" page has a link '''Merge computers by name'''.  This feature lets you automatically merge all eligible computers having the same domain name.  This is handy if you run a "computer farm" and periodically reformat all the drives.

== Alternate identification method at World Community Grid ==

In absence of a merge feature at this project, WCG introduced an alternate method called ''host matching'' to recognise previously existing devices to prevent new / duplicate registrations. For that the server compares standard information exchanged during server-client communications being:

 * user name
 * domain_name
 * ip_addr (the ip of the client on the local network)
 * operating system name
 * processor vendor
 * memory

The most recent record that matches this critiera (if found) will be re-used. It will cancel any results currently assigned to the client, and then issue new work. This is because a user might be trying to clear out some work that was causing some form of trouble. If any of this information is hidden through for instance setting the <suppress_net_info> flag in the cc_config.xml file suppressing the IP address or domain_name, the method fails and will create a new device registration.