Re: [Engine-devel] Requirement for Locking Mechanism

On 11/29/2011 12:41 PM, Michael Kublin wrote: > > > ----- Original Message ----- >> From: "Yaniv Kaul"<ykaul(a)redhat.com> >> To: "Michael Kublin"<mkublin(a)redhat.com> >> Cc: engine-devel(a)ovirt.org >> Sent: Tuesday, November 29, 2011 12:18:15 PM >> Subject: Re: [Engine-devel] Requirement for Locking Mechanism >> >> On 11/29/2011 12:13 PM, Michael Kublin wrote: >>> Hi All, >>> >>> We are introducing a new short term locking mechanism at engine. >>> The motivation is races which are occurring between different >>> flows, the main problem is : >>> we did not update status of some entity and other flow was started >>> , which is a main cause for different collisions and situation >>> which >>> can not be fixed in appropriate way by engine or vdsm. >>> The current status is a workaround which is trying to reduce a race >>> : in many places in the code, there is additional query to DB in >>> order to check the appropriate status of entity. >>> The proposed solution is internal locking mechanism - which will >>> lock an appropriate entity until its status will not be updated in >>> DB, after that >>> an internal short term lock will be released. >>> >>> The design for locking mechanism can be found here: >>> http://www.ovirt.org/wiki/Features/DetailedLockMechanism >> >> The page still has many remains of the template. >> >>> The patch which is introduce an implementation for mechanism can be >>> found here: http://gerrit.ovirt.org/#change,403 >>> (Using of new mechanism all around a code and integration with bll >>> will be sent in the future, including cleaning of workaround) >>> >>> Regards Michael >>> >>> _______________________________________________ >>> Engine-devel mailing list >>> Engine-devel(a)ovirt.org >>> http://lists.ovirt.org/mailman/listinfo/engine-devel >> >> There's an implicit assumption that there's only going to be a single >> active backend working against the database? >> Y. >> > Yes, the backend will lock some entity, until its status will not be > updated in db, after that the lock will be released. > So, it will solve a race between canDoAction and beginning of execute > of the command so how will distributed engines for scale out later on will look like with this mechanism?

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