[infinispan-dev] Read Committed Distributed Cache Concerns

[Sanne Grinovero]

On 19 September 2013 18:29, Mircea Markus <mmarkus at redhat.com> wrote:
> (Adding Jonathan who knows a thing or two about transactions.)
>
> Given that READ_COMMITTED (RC) is less performant than REPEATABLE_READ (RR)
> I don't see any value in keeping RC around. I don't think users rely on
> exact RC semantics (i.e. if an entry has been committed then an ongoing
> tx requires the most up 2 date value between reads) - that actually
> is not the case with DIST caches as you've mentioned.

I don't think you can generalize from the specific example William
made; there will still be cases in which READ_COMMITTED will be more
efficient than REPEATABLE_READ,
especially if you avoid "fixing" this, as suggested by Radim and
myself in the two previous emails [not sure you if saw them, since you
forking the conversation ignoring those mails]:
if we agree that the current semantics is acceptable, it will
consistently be faster than REPEATABLE_READ.

Sanne

> I think RC is only preferred to RR because of performance, but if the performance
> is the same (or even worse) I think we should only provide RR. Jonathan, care to comment?
>
>
> On Sep 18, 2013, at 11:03 PM, William Burns <mudokonman at gmail.com> wrote:
>
>> I was recently refactoring code dealing with isolation levels and
>> found how ReadCommitted is implemented and I have a few concerns I
>> wanted to bring up.
>>
>> ReadCommitted read operations work by storing a reference to the value
>> from the data store in its caller's context.  Thus whenever another
>> transaction is committed that updates the data store value any context
>> that has that reference now sees the latest committed value.  This
>> works well for Local and Replicated caches since all data stores are
>> updated with the latest value upon completion of the transaction.
>> However Distributed caches only the owners see the update in their
>> data store and thus any non owner will still have the old value they
>> previously read before the commit occurred.
>>
>> This seems quite inconsistent that Distributed caches run in a mix of
>> Repeatable Read/Read Committed depending on what node and what key you
>> are using.
>>
>> To operate properly we could track requests similar to how it works
>> for L1 so we can tell non owners to clear out their context values for
>> values they read remotely that they haven't updated (since Read
>> Committed writes should return the same written value).  That seems
>> like quite a bit of additional overhead though.
>>
>> I am wondering is it worth it to try to keep Read Committed isolation
>> level though?  It seems that Repeatable Read would be simpler and most
>> likely more performant as you wouldn't need all the additional remote
>> calls to get it to work properly.  Or is it okay that we have
>> different isolation levels for some keys on some nodes?  This could be
>> quite confusing if a user was using a local and remote transaction and
>> a transaction may not see the other's committed changes when they
>> expect to.
>>
>> What do you guys think?
>>
>> - Will
>>
>> P.S.
>>
>> I also found a bug with Read Committed for all caches where if you do
>> a write that changes the underlying InternalCacheEntry to a new type,
>> that reads won't see subsequent committed values.  This is caused
>> because the underlying data is changed to a new reference and a read
>> would still be holding onto a reference of the old InternalCacheEntry.
>> This can happen when using the various overridden put methods for
>> example.  We should have a good solution for it, but may not be
>> required if we find that Read Committed itself is flawed beyond
>> saving.
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>
> Cheers,
> --
> Mircea Markus
> Infinispan lead (www.infinispan.org)
>
>
>
>
>
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