On 05/13/2014 03:58 PM, Dan Berindei wrote:
On Mon, May 12, 2014 at 1:54 PM, Radim Vansa <rvansa(a)redhat.com
<mailto:rvansa@redhat.com>> wrote:
@Dan: It's absolutely correct to do the further writes in order to
make
the cache consistent, I am not arguing against that. You've fixed the
outcome (state of cache) well. My point was that we should let the
user
know that the value he gets is not 100% correct when we already know
that - and given the API, the only option to do that seems to me as
throwing an exception.
The problem, as I see it, is that users also expect methods that throw
an exception to *not* modify the cache.
So we would break some of the users' expectations anyway.
When the response from primary owner does not arrive soon, we throw
timeout exception and the cache is modified anyway, isn't it?
If we throw ~ReturnValueUnreliableException, the user has at least some
chance to react. Currently, for code requiring 100% reliable value, you
can't do anything but ignore the return value, even for CAS operations.
@Sanne: I was not suggesting that for now - sure, value versioning
is (I
hope) on the roadmap. But that's more complicated, I though just about
making an adjustment to the current implementation.
Actually, just keeping a history of values would not fix the the
return value in all cases.
When retrying a put on the new primary owner, the primary owner would
still have to compare our value with the latest value, and return the
previous value if they are equal. So we could have something like this:
A is the originator, B is the primary owner, k = v0
A -> B: put(k, v1)
B dies before writing v, C is now primary owner
D -> C: put(k, v1) // another put operation from D, with the same value
C -> D: null
A -> C: retry_put(k, v1)
C -> A: v0 // C assumes A is overwriting its own value, so it's
returning the previous one
To fix that, we'd need a unique version generated by the originator -
kind of like a transaction id ;)
Is it such a problem to associate unique ID with each write? History
implementation seems to me like the more complicated part.
And to fix the HotRod use case, the HotRod client would have to be
the
one generating the version.
I agree.
Radim
Cheers
Dan
Radim
On 05/12/2014 12:02 PM, Sanne Grinovero wrote:
> I don't think we are in a position to decide what is a reasonable
> compromise; we can do better.
> For example - as Radim suggested - it might seem reasonable to have
> the older value around for a little while. We'll need a little
bit of
> history of values and tombstones anyway for many other reasons.
>
>
> Sanne
>
> On 12 May 2014 09:37, Dan Berindei <dan.berindei(a)gmail.com
<mailto:dan.berindei@gmail.com>> wrote:
>> Radim, I would contend that the first and foremost guarantee
that put()
>> makes is to leave the cache in a consistent state. So we can't
just throw an
>> exception and give up, leaving k=v on one owner and k=null on
another.
>>
>> Secondly, put(k, v) being atomic means that it either succeeds,
it writes
>> k=v in the cache, and it returns the previous value, or it
doesn't succeed,
>> and it doesn't write k=v in the cache. Returning the wrong
previous value is
>> bad, but leaving k=v in the cache is just as bad, even if the
all the owners
>> have the same value.
>>
>> And last, we can't have one node seeing k=null, then k=v, then
k=null again,
>> when the only write we did on the cache was a put(k, v). So
trying to undo
>> the write would not help.
>>
>> In the end, we have to make a compromise, and I think returning
the wrong
>> value in some of the cases is a reasonable compromise. Of
course, we should
>> document that :)
>>
>> I also believe ISPN-2956 could be fixed so that HotRod behaves
just like
>> embedded mode after the ISPN-3422 fix, by adding a RETRY flag
to the HotRod
>> protocol and to the cache itself.
>>
>> Incidentally, transactional caches have a similar problem when the
>> originator leaves the cluster: ISPN-3421 [1]
>> And we can't handle transactional caches any better than
non-transactional
>> caches until we expose transactions to the HotRod client.
>>
>> [1]
https://issues.jboss.org/browse/ISPN-2956
>>
>> Cheers
>> Dan
>>
>>
>>
>>
>> On Mon, May 12, 2014 at 10:21 AM, Radim Vansa
<rvansa(a)redhat.com <mailto:rvansa@redhat.com>> wrote:
>>> Hi,
>>>
>>> recently I've stumbled upon one already expected behaviour
(one instance
>>> is [1]), but which did not got much attention.
>>>
>>> In non-tx cache, when the primary owner fails after the
request has been
>>> replicated to backup owner, the request is retried in the new
topology.
>>> Then, the operation is executed on the new primary (the previous
>>> backup). The outcome has been already fixed in [2], but the
return value
>>> may be wrong. For example, when we do a put, the return value
for the
>>> second attempt will be the currently inserted value (although
the entry
>>> was just created). Same situation may happen for other operations.
>>>
>>> Currently, it's not possible to return the correct value
(because it has
>>> already been overwritten and we don't keep a history of
values), but
>>> shouldn't we rather throw an exception if we were not able to
fulfil the
>>> API contract?
>>>
>>> Radim
>>>
>>> [1]
https://issues.jboss.org/browse/ISPN-2956
>>> [2]
https://issues.jboss.org/browse/ISPN-3422
>>>
>>> --
>>> Radim Vansa <rvansa(a)redhat.com <mailto:rvansa@redhat.com>>
>>> JBoss DataGrid QA
>>>
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JBoss DataGrid QA
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