[infinispan-dev] Issue with cache blocks for local read-only cache

[Manik Surtani]

On 25 Sep 2012, at 13:48, Galder Zamarreño <galder at redhat.com> wrote:

> 
> On Sep 24, 2012, at 12:27 PM, Manik Surtani <manik at jboss.org> wrote:
> 
>> 
>> On 24 Sep 2012, at 11:01, Galder Zamarreño <galder at redhat.com> wrote:
>> 
>>> 
>>> On Sep 21, 2012, at 3:43 PM, Sanne Grinovero <sanne at infinispan.org> wrote:
>>> 
>>>> On 20 September 2012 17:38, Andrig Miller <anmiller at redhat.com> wrote:
>>>>> 
>>>>> 
>>>>> ----- Original Message -----
>>>>>> From: "Galder Zamarreño" <galder at redhat.com>
>>>>>> To: "Andrig Miller" <anmiller at redhat.com>
>>>>>> Cc: "Steve Ebersole" <steve at hibernate.org>, "John O'Hara" <johara at redhat.com>, "Jeremy Whiting"
>>>>>> <jwhiting at redhat.com>, "infinispan -Dev List" <infinispan-dev at lists.jboss.org>
>>>>>> Sent: Thursday, September 20, 2012 6:48:59 AM
>>>>>> Subject: Re: [infinispan-dev] Issue with cache blocks for local read-only cache
>>>>>> 
>>>>>> 
>>>>>> On Sep 19, 2012, at 4:20 PM, Andrig Miller <anmiller at redhat.com>
>>>>>> wrote:
>>>>>> 
>>>>>>> Yes, I can see how that can happen, if the data is deleted from
>>>>>>> outside the application.
>>>>>> 
>>>>>> ^ The issue does not only happen if the data is deleted outside the
>>>>>> application. As indicated in
>>>>>> https://hibernate.onjira.com/browse/HHH-3817, this can happen with
>>>>>> two competing transactions.
>>>>>> 
>>>>>>> If you cache something as READ_ONLY, and it gets deleted, that
>>>>>>> doesn't fit the definition of READ_ONLY though.  You are using the
>>>>>>> wrong cache concurrency strategy.
>>>>>>> 
>>>>>>> Even that issue outlines the scenario where the collection is
>>>>>>> updated, which means its not a READ_ONLY.
>>>>>> 
>>>>>> I think the update is irrelevant here. The issue is related to
>>>>>> putFromLoad + remove, which both AFAIK, are allowed in READ_ONLY
>>>>>> (remember that we had the discussion on whether remove should be
>>>>>> allowed in a READ_ONLY cache:
>>>>>> https://hibernate.onjira.com/browse/HHH-7350).
>>>>>> 
>>>>> 
>>>>> Yes, remove can be done, its just update that matters to READ_ONLY.  One thing I thought about was I thought we were using MVCC for this stuff.  Any transaction that reads from the cache, while something is being added/removed, should be reading the read consistent image, and should never wait on a lock, correct?  We see all the threads in our thread pool sitting in a blocked state based on this locking.
>>> 
>>> I'm not 100% sure which locking are you talking about, but if you're refering to the lock in https://dl.dropbox.com/u/30971563/specjent_block.png, that's related to the 2LC integration, not Infinispan itself.
>> 
>> Yes, we're analysing the 2LC impl as well as Infinispan.
>> 
>>> If you're talking about threads waiting for a lock somewhere else, please provide more details.
>>> 
>>> I have some short-term ideas to improve the 2LC integration code, but I wanna check with Brian first.
>>> 
>>> Long term, I think https://issues.jboss.org/browse/ISPN-506 will be necessary to provide a lock-free solution to these edge cases in such way that 'newer' removes cannot be overridden by 'old' putFromLoad calls. However, I'm intrigued by the fact that JBoss Cache OL had the capability of being given a version externally, but the 2LC code for JBoss Cache OL still used this PutFromLoadValidator logic. Again, something I need to check with Brian.
>> 
>> ISPN-506 will only help in the clustered case.  
> 
> ^ I disagree. It might help with the edge case highlighted in https://hibernate.onjira.com/browse/HHH-3817 which happens in a local cache.

There is a much easier way to solve this: pessimistic locking + an eager cache.lock() command before retrieving the collection from the database.  This will prevent the race defined in the HHH-3817.

--
Manik Surtani
manik at jboss.org
twitter.com/maniksurtani

Platform Architect, JBoss Data Grid
http://red.ht/data-grid

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