[infinispan-dev] Write-only commands

[Radim Vansa]

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.

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
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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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-- 
Radim Vansa <rvansa at redhat.com>
JBoss Performance Team