[Libvir] Re: Third version of virDomainMigrate API
Daniel P. Berrange wrote:
In terms of the set of data we need for a basic impl, I think these
are resonable. That said I've been thinking about this in relation to
the earlier points in this thread about cancellation, and progress info,
etc. I'm wondering if we would be well served by introducing a new object
to co-ordinate the whole thing.
/* Prepare for migration to dconn */
virDomainMigratePtr mig = virDomainMigratePrepare(dom, dconn)
/* Optionally specify a custom transport */
virDomainMigrateTransport(mig, "ssh://root@dest/");
/* Optionally throttle */
virDomainMigrateBandwidth(mig, 10);
/* Perform the migration */
virDomainMigrateRun(mig, flags);
/* Release resources */
virDomainMigrateFree(mig);
This would make it easier for us to extend the capabilities in the
future. eg adding more properties, or add APIs to run async, or
getting progress info, etc, etc.
eg, if flags request ASYNC, then one could imagine cancellation via
virDomainMigrateAbort(mig);
Or to poll for completion...
virDomainMigrateStatus(mig);
Finally we could have a convenience API
virDomainMigrate(dom, dconn);
For apps which don't care about custom transports, etc, etc
Totally off on a tangent here, but a trick from functional programming
is to return a "suspension". The call still looks like this:
ddom = Domain.migrate dom dconn;
The trick is that the call returns immediately, and 'ddom' isn't
necessarily a domain object, at least not until you try to use it. For
example, if the next statement was:
printf "new domain id = %d\n" (Domain.id ddom);
then the call to Domain.id ddom would (in the jargon) "force the
suspension" -- basically cause the program to wait until the domain has
migrated before returning the ID.
With suspensions you can examine their state _without_ forcing them.
For example:
while Domain.is_migrating ddom; do
printf "Domain still migrating ... %d percent done.\n"
(Domain.migration_percent ddom);
sleep 1;
done;
printf "Domain migrated, ID = %d\n" (Domain.id ddom)
(Of course error handling is omitted here, but in a functional language
it would just use exceptions. The C equivalent is more involved because
you have to explicitly check for errors at every call).
The advantage of suspensions is that in the simple case where you don't
care about fancy progress bars, the code looks exactly the same as normal.
Rich.