[Engine-devel] Requirement for Locking Mechanism

[Livnat Peer]

On 11/29/2011 06:32 PM, Itamar Heim wrote:
> On 11/29/2011 12:41 PM, Michael Kublin wrote:
>>
>>
>> ----- Original Message -----
>>> From: "Yaniv Kaul"<ykaul at redhat.com>
>>> To: "Michael Kublin"<mkublin at redhat.com>
>>> Cc: engine-devel at 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 at 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?


We currently have caching in oVirt core in several places, when we'll
handle distributed deployment we'll handle all of them. it can be either
distributed cache or any other suitable solution.

Livnat

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