[infinispan-dev] Write-only commands

[Radim Vansa]

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.

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.

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.

>
> 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 :)
>
>> (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.
> 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