[infinispan-dev] Write-only commands

[Dan Berindei]

On Thu, Jun 29, 2017 at 2:19 PM, Radim Vansa <rvansa at redhat.com> wrote:
> On 06/29/2017 11:16 AM, Dan Berindei wrote:
>> On Thu, Jun 29, 2017 at 11:53 AM, Radim Vansa <rvansa at redhat.com> wrote:
>>> On 06/28/2017 04:20 PM, Dan Berindei wrote:
>>>> On Wed, Jun 28, 2017 at 2:17 PM, Radim Vansa <rvansa at redhat.com> wrote:
>>>>> On 06/28/2017 10:40 AM, Dan Berindei wrote:
>>>>>> On Wed, Jun 28, 2017 at 10:17 AM, Radim Vansa <rvansa at redhat.com> wrote:
>>>>>>> On 06/27/2017 03:54 PM, Dan Berindei wrote:
>>>>>>>> On Tue, Jun 27, 2017 at 2:43 PM, Adrian Nistor <anistor at redhat.com> wrote:
>>>>>>>>> I've said this in a previous thread on this same issue, I will repeat myself
>>>>>>>>> as many times as needed.
>>>>>>>>>
>>>>>>>>> Continuous queries require the previous value itself, not just knowledge of
>>>>>>>>> the type of the previous value. Strongly typed caches solve no problem here.
>>>>>>>>>
>>>>>>>>> So if we half-fix query but leave CQ broken I will be half-happy (ie. very
>>>>>>>>> depressed) :)
>>>>>>>>>
>>>>>>>>> I'd remove these commands completely or possibly remove them just from
>>>>>>>>> public API and keep them internal.
>>>>>>>>>
>>>>>>>> +1 to remove the flags from the public API. Most of them are not safe
>>>>>>>> for applications to use, and ignoring them when they can lead to
>>>>>>>> inconsistencies would make them useless.
>>>>>>>>
>>>>>>>> E.g. the whole point of SKIP_INDEX_CLEANUP is that the cache doesn't
>>>>>>>> know when it is safe to skip the delete statement, and it relies on
>>>>>>>> the application making a (possibly wrong) choice.
>>>>>>>>
>>>>>>>> IGNORE_RETURN_VALUES should be safe to use, and we actually recommend
>>>>>>>> that applications use it right now. If query or listeners need the
>>>>>>>> previous value, then we should load it internally, but hide it from
>>>>>>>> the user.
>>>>>>>>
>>>>>>>> But removing it opens another discussion: should we replace it in the
>>>>>>>> public API with a new method AdvancedCache.ignoreReturnValues(), or
>>>>>>>> should we make it the default and add a method
>>>>>>>> AdvancedCache.forceReturnPreviousValues()?
>>>>>>> Please don't derail the thread.
>>>>>>>
>>>>>> I don't think I'm derailing the thread: IGNORE_PREVIOUS_VALUES also
>>>>>> breaks the previous value for listeners, even if the QueryInterceptor
>>>>>> removes it from write commands. And it is public (+recommended) API,
>>>>>> in fact most if not all of our performance tests use it.
>>>>> That's just a flawed implementation. IPV is documented to be a 'safe'
>>>>> flag that should affect mostly primary -> origin replication, all the
>>>>> other is implementation. And we can fix that. Users should *not* expect
>>>>> that it e.g. skips loading from a cache store. We have already removed
>>>>> the modes that would be broken-by-design.
>>>>>
>>>> I think you're confusing IGNORE_RETURN_VALUES with SKIP_REMOTE_LOOKUP
>>>> here. The IVR javadoc doesn't say anything about remote lookups, only
>>>> SRL does.
>>> No, I am not; While IRV does not mention the replication, it's said to
>>> be 'safe'. So omitting the primary -> origin replication is basically
>>> all it can do when listeners are in place. You're right that I have
>>> missed the second part in SRL talking about put()s; I took it as a flag
>>> prohibiting any remote lookup (as the RPC operation in its whole) any
>>> time the remote value is needed. Yes, the second part seems equal to my
>>> understanding of IRV.
>>>
>>>> And I agree that the current status is far from ideal, but there is
>>>> one more valid alternative: we can decide that the previous value is
>>>> only reliable in clustered listeners, and local listeners don't always
>>>> have it. Document that, make sure continuous query uses clustered
>>>> listeners, and we're done :)
>>> Unreliable return values are worse than none; I would rather remove them
>>> if we can't guarantee that these are right. Though, clustered listeners
>>> are based on regular listeners, so you'd need some means to make them
>>> reliable.
>> We could change the clustered listeners so that they're not based on
>> the regular listeners... I've been pestering Will about this ever
>> since the clustered listeners landed!
>>
>> But I should have been clearer: I didn't mean that the listeners on
>> the backups should receive the previous value whenever we feel like
>> it, I meant we should document and enforce that the previous value is
>> only included in the event for listeners on the primary owner.
>>>>> On the other hand, write-only commands are not about *returning* the
>>>>> value but about (not) *reading* it, therefore (in my eyes) user could
>>>>> make that assumption and would like to enforce it this way. Even some
>>>>> docs explaining PersistenceMode.SKIP suggest that.
>>>>>
>>>> To me the purpose the same, there is no difference between returning
>>>> the previous value to the application or providing the previous value
>>>> via EntryView.
>>> There is a difference between what's provided locally and what's send
>>> over the network.
>>>
>>>> Applying this logic to the JCache API, it would mean
>>>> put() should never read the previous value, because some users could
>>>> assume that only getAndPut() reads it.
>>> OK, this is a valid point.
>>>
>>>> In the old times we didn't have IGNORE_RETURN_VALUES, only
>>>> SKIP_REMOTE_LOOKUP+SKIP_CACHE_LOAD, and they would sometimes be
>>>> ignored (e.g. if the write was conditional). I think that's what
>>>> Galder had in mind when he wrote the PersistenceMode api note, not the
>>>> current behaviour of SKIP_CACHE_LOAD. I'll let Galder clarify this
>>>> himself, but I'll be very disappointed if he says he designed the
>>>> write-only operations so that they'll never work with query.
>>>>
>>>>
>>>>> I don't want to talk about flags, because I see all flags but IPV as
>>>>> 'effectively internal'. Let's discuss it more high-level. Some API
>>>>> exposes non-reading operation - we can see that under some circumstances
>>>>> this is not possible so we have options to 1) break stuff 2) break API
>>>>> assumptions 3) sometimes break API assumptions 4) remove such API (to
>>>>> not allow the user to make such assumptions). There's also an option 5)
>>>>> to fail the operation if the API assumption would be broken. Though, I
>>>>> don't fancy getting exception from a WriteOnlyMap.eval just because
>>>>> someone has registered a listener.
>>>>>
>>>> I disagree with the premise: there's no good reason for the user to
>>>> assume that write-only commands are *guaranteed* to never load the
>>>> previous value from a store. We just need to add a clarification to
>>>> the write-only operations' javadoc, no need to break anything.
>>> OK then, though it diminishes the value of write-only commands a lot.
>>>
>>>>
>>>>>> For that matter, ClusteredCacheLoaderInterceptor also doesn't load the
>>>>>> previous value on backup owners for most write commands
>>>>>> (LoadType.PRIMARY), we'd need to change that as well.
>>>>> Yes, all commands will have to load current value on all owners.
>>>>>
>>>>>>>>> On 06/27/2017 01:28 PM, Sanne Grinovero wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 27 Jun 2017 10:13, "Radim Vansa" <rvansa at redhat.com> wrote:
>>>>>>>>>
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> I am working on entry version history (again). In Como we've discussed
>>>>>>>>> that previous values are needed for (continuous) query and reliable
>>>>>>>>> listeners,
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Index based queries also require the previous value on a write - unless we
>>>>>>>>> can get "strongly typed caches" giving guarantees about the class to
>>>>>>>>> represent the content of a cache to be unique.
>>>>>>>>>
>>>>>>>>> Essentially we only need to know the type of the previous object. It might
>>>>>>>>> be worth having a way to load the type metadata if the previous value only.
>>>>>>>>>
>>>>>>>>> so I wonder what should we do with functional write-only
>>>>>>>>> commands. These are different to commands with flags, because flags
>>>>>>>>> (other than ignore return value) are expected to break something.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Sorry I hope to not derail the thread but let's remind that we hope to
>>>>>>>>> evolve beyond "flags are expected to break stuff" ; we never got to it but
>>>>>>>>> search the mailing list.
>>>>>>>>>
>>>>>>>>> Since flags are exposed to the user I would rather they're not allowed to
>>>>>>>>> break things.
>>>>>>>>> Could they be treated as hints? Ignore the flag (and warn?) if the used
>>>>>>>>> configuration/integrations veto them.
>>>>>>>>>
>>>>>>>>> Alternatively, let's remove them from API. Remember "The Jokre" POC was
>>>>>>>>> intentionally designed to explore pushing the limits on performance w/o end
>>>>>>>>> users having to solve puzzles, such as learning details about these flags
>>>>>>>>> and their possible side effects.
>>>>>>>>>
>>>>>>>>> So assuming they become either "safe" or internal, maybe you can take
>>>>>>>>> advantage of them?
>>>>>>>>>
>>>>>>>>> I see
>>>>>>>>> the available options as:
>>>>>>>>>
>>>>>>>>> 1) run write-only commands 'optimized', ignoring any querying and such
>>>>>>>>> (warn user that he will break it)
>>>>>>>>>
>>>>>>>>> 2) run write-only without any optimization, rendering them useless
>>>>>>>>>
>>>>>>>>> 3) detect when querying is set up (ignoring listeners and maybe other
>>>>>>>>> stuff that could get broken)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Might be useful for making a POC work, but I believe query will be very
>>>>>>>>> likely to be often enabled.
>>>>>>>>> Having an either / or switch for different features in Infinispan will make
>>>>>>>>> it harder to use and understand, so I'd rather see work on the right design
>>>>>>>>> as taking temporary shortcuts risks baking into stone features which we
>>>>>>>>> later struggle to fix or maintain.
>>>>>>>>>
>>>>>>>> I vote for this option.
>>>>>>>>
>>>>>>>> Query, listeners, and other components that need the previous value
>>>>>>>> should not just assume that the application knows better, they should
>>>>>>>> be able to change how operations works based on their needs. Of
>>>>>>>> course, the reverse is also true: if the application uses write-only
>>>>>>>> commands (or IGNORE_RETURN_VALUES) for performance reasons, it should
>>>>>>>> be possible for the user to detect why the previous values are still
>>>>>>>> loaded.
>>>>>>> If it were just query (static configuration), I would be okay with this
>>>>>>> idea. But as per listeners - besides tainting the design (event source
>>>>>>> should not check if there's a listener) you'd need to check *before*
>>>>>> The source wouldn't check for listeners explicitly, the notifier would
>>>>>> have an isPreviousValueNeeded() method and precompute that before a
>>>>>> listener is added or after a listener is removed. I was am assuming
>>>>>> some listeners will not need the previous value, e.g. the listeners
>>>>>> installed by streams.
>>>>> You can cover your warts with a make-up but you'll still have warts :)
>>>> Cutting them off doesn't necessarily work, either :)
>>> Yep, some people tend to fix w/ hacks instead of designing :)
>>>
>>>>>>> (DistributionI, CacheLoaderI) you have to call notify (cmd.perform,
>>>>>>> EWI). So this is a space for race conditions or weird handling (if
>>>>>>> there's a listener when I am about to call notify and my flags are not
>>>>>>> cleared, skip the notification and pretend that this code was invoked
>>>>>>> before the listener was registered...). Or do you have another solution
>>>>>>> in mind (config option to disable listeners && all features using those?).
>>>>>>>
>>>>>> I was definitely going for the weird handling...
>>>>>>
>>>>>> My plan was to set a HAS_PREVIOUS_VALUE flag on the context entry when
>>>>>> it's loaded, and check that before invoking a listener that needs the
>>>>>> previous value. It is missing one edge case: if one thread starts a
>>>>>> write operation, then another thread installs a listener that requires
>>>>>> the previous value and iterates over the cache, the second thread may
>>>>>> not see the value written by the first thread.
>>>>> If the operations overlap, you could pretend that the write has finished
>>>>> before the listener was invoked and simply not notify the listener. If I
>>>>> am missing it please write it down in code. But handling this in any way
>>>>> is still clumsy.
>>>> I hope pseudo-code is fine...
>>>>
>>>> 1. cache.put(k, v1) starts, doesn't load the previous value v0 in the context
>>>> 2. cache.addListener(l) runs, doesn't block
>>>> 3. cache.entrySet().forEach() runs, finds k->v0
>>>> 4. cache.put(k, v1) commits k->v1, should notify the listener but
>>>> doesn't have the previous value
>>>> 5. cache.put(k, v0) returns, but the code that installed the listener
>>>> thinks the value of k is still v0
>>> Oh OK, I should have drawn that myself when considering the scenario.
>>> You're right, here we'll have to retry.
>>>
>>> All in all, I think this discussion is done. We'll tell users to stick
>>> their flags where the sun doesn't shine and remove any inconvenient
>>> ones. Should we issue a warning any time we're removing the flag?
>>>
>> If you mean that we should remove the flags from the public API, I
>> agree. If you mean we should just ignore them, then no, because most
>> of the flags were added for internal components that really need their
>> semantics.
>
> We can't remove them from public API before Infinspan 10, and I think
> that it will be a quite an unpopular step even after that. But until 10,
> I think that the common agreement was to not break query, that is ignore
> the flags. And make write-only reading.
>

So SKIP_INDEXING should not skip indexing because it can break query??

> R.
>
>>
>> Dan
>>
>>
>>> Radim
>>>
>>>>
>>>>>> So now I'm thinking we should retry the write commands when
>>>>>> isPreviousValueNeeded() changes... Not very appealing, but I think the
>>>>>> performance difference is worth it.
>>>>>>
>>>>>>> R.
>>>>>>>
>>>>>>>>> 4) remove write-only commands completely (and probably functional
>>>>>>>>> listeners as well because these will lose their purpose)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> +1 to remove "unconditional writes", at least an entry version check should
>>>>>>>>> be applied.
>>>>>>>>> I believe we had already pointed out this would eventually happen, pretty
>>>>>>>>> much for the reasons you're hitting now.
>>>>>>>>>
>>>>>>>> IMO version checks should be done internally, we shouldn't force the
>>>>>>>> users of the functional API to deal with versions themselves because
>>>>>>>> we know how hard making write skew checks work is for us :)
>>>>>>>>
>>>>>>>> And I wouldn't go as far as to remove the functional listeners,
>>>>>>>> instead I would change them so that read-write listeners are invoked
>>>>>>>> on write-only operations and they force the loading of the previous
>>>>>>>> value. I would also add a way for the regular listeners to say whether
>>>>>>>> they need the previous value or not.
>>>>>>>>
>>>>>>>>> Right now I am inclined towards 4). There could be some internal use
>>>>>>>>> (e.g. multimaps) that could use 1) which is ran without a fancy setup,
>>>>>>>>> though, but it's asking for trouble.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I agree!
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> WDYT?
>>>>>>>>>
>>>>>>>>> Radim
>>>>>>>>>
>>>>>>>> Cheers
>>>>>>>> Dan
>>>>>> _______________________________________________
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>>>>>> infinispan-dev at lists.jboss.org
>>>>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>>>> --
>>>>> Radim Vansa <rvansa at redhat.com>
>>>>> JBoss Performance Team
>>>>>
>>>>> _______________________________________________
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>>>>> infinispan-dev at lists.jboss.org
>>>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
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>>> --
>>> Radim Vansa <rvansa at redhat.com>
>>> JBoss Performance Team
>>>
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>
>
> --
> Radim Vansa <rvansa at redhat.com>
> JBoss Performance Team
>
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