[infinispan-dev] Write-only commands

[Dan Berindei]

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.

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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>>>
>>> --
>>> 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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