[infinispan-dev] Design change in Infinispan Query

[Emmanuel Bernard]

On 28 Feb 2014, at 22:14, Mircea Markus <mmarkus at redhat.com> wrote:

> 
> On Feb 26, 2014, at 5:14 PM, Emmanuel Bernard <emmanuel at hibernate.org> wrote:
> 
>> 
>> On 25 Feb 2014, at 16:08, Mircea Markus <mmarkus at redhat.com> wrote:
>> 
>>> 
>>> On Feb 25, 2014, at 9:28 AM, Emmanuel Bernard <emmanuel at hibernate.org> wrote:
>>> 
>>>>> On 24 févr. 2014, at 17:39, Mircea Markus <mmarkus at redhat.com> wrote:
>>>>> 
>>>>> 
>>>>>> On Feb 17, 2014, at 10:13 PM, Emmanuel Bernard <emmanuel at hibernate.org> wrote:
>>>>>> 
>>>>>> By the way, Mircea, Sanne and I had quite a long discussion about this one and the idea of one cache per entity. It turns out that the right (as in easy) solution does involve a higher level programming model like OGM provides. You can simulate it yourself using the Infinispan APIs but it is just cumbersome.
>>>>> 
>>>>> Curious to hear the whole story :-)
>>>>> We cannot mandate all the suers to use OGM though, one of the reasons being OGM is not platform independent (hotrod). 
>>>> 
>>>> Then solve all the issues I have raised with a magic wand and come back to me when you have done it, I'm interested.
>>> 
>>> People are going to use infinispan with one cache per entity, because it makes sense:
>>> - different config (repl/dist | persistent/non-persistent) for different data types
>>> - have map/reduce tasks running only the Person entires not on Dog as well, when you want to select (Person) where age > 18
>>> I don't see a reason to forbid this, on the contrary. The way I see it the relation between (OGM, ISPN) <=> (Hibernate, JDBC). Indeed OGM would be a better abstraction and should be recommended as such for the Java clients, but ultimately we're a general purpose storage engine that is available to different platforms as well.
>>> 
>> 
>> I do disagree on your assessment.
>> I did write a whole essay on why I think your view is problematic - I was getting tired of repeating myself ;P
>> https://github.com/infinispan/infinispan/wiki/A-continuum-of-data-structure-and-query-complexity
> 
> Thanks for writing this up, it is a good taxonomy of data storage schemes and querying.
> 
>> 
>> To anecdotally answer your specific example, yes different configs for different entities is an interesting benefit but it has to outweigh the drawbacks.
> 
> Using a single cache for all the types is practical at all :-) Just to expand my idea, people prefer using different caches for many reasons:
> - security: Account cache has a different security requirements than the News cache
> - data consistency: News is a non-transactional cache, Account require pessimistic XA transactions
> - expiry: expire last year's news from the system. Not the same for Accounts
> - availability: I want the Accounts cache to be backed up to another site. I don't want that for the News cache
> - logical data grouping: mixing Accounts with News doesn't make sense. I might want to know which account appeared in the news, though.

This kind of reasons reminds me in the RDBMS world of why people use different databases.
In fact, I have had experience where literally News was a different database than Accounts.

But again in this model, in one database, you have many tables.

> 
>> If you have to do a map reduce for tasks so simple as age > 18, I think you system better have to be prepared to run gazillions of M/R jobs.
> 
> I want to run a simple M/R job in the evening to determine who turns 18 tomorrow, to congratulate them. Once a day, not gazzilions of times, and I don't need to index the age filed just for that. Also when it comes to Map/Reduce, the drawback of holding all the data in a single cache is two-folded:
> - performance: you iterate over the data that is not related to your query. 

If the data are never related (query wise), then we are in the database split category. Which is fine. But if some of your queries are related, what do you do? Deny the user the ability to do them?

> - programming model: the Map/Reduce implementation has a dependency on both Dog and Person. If I add Cats to the cache, I'll need to update the M/R code to be aware of that as well. Same if I rename/remove Dog. Not nice.

Well it’s called type safety, some people find it good ;)
By the way, OGM does abstract a class from it’s representation in the datastore (including its name). But that’s another story ;)

> 
>> I think that Dogs and any domestic animal is fundamentally related to humans - Person in your case. So queries involving both will be required - a cross cache M/R is not doable today AFAIK and even if it was, it’s still M/R and all its drawbacks.
>> To me, the Cache API and Hot Rod are well suited for what I call self contained object graph (i.e. where Dog would be an embedded object of Person and not a separate Entity). In that situation, there is a single cache.
> 
> I see where you come from but I don't think requiring people to use a single cache for all the entities is an option. Besides a natural logical separation, different data has different storage requirements: security, access patterns, consistency, durability, availability etc. For most of the non-trivial use cases, using a single cache just wont do. 

Let me rephrase and sum up my position.
If you are storing unrelated data, use different caches if you want, that’s fine.
If you are storing related data, store it as one root entity and embeddable objects (ie one cache entry for the whole graph)
    you can have one root entity per cache, that’s fine.
If you are storing related entities and want to do queries on it: you are more or less screwed today with Infinispan and need a higher level abstraction.

So _recommending_ one entity = one cache to me is wrong. It’s more one entity graph = one cache which is vastly different and has deep consequences (see my wiki page).

> 
>> One cache per entity does make sense for API that do support what I call connected entities. Hibernate OGM specifically.
> 
> OGM does a great job covering this, but it is very specific: java only and OOP - our C/S mode, hotrod specifically, is language independent and not OOP.

Sure. Outside of Java consider that connected objects are not a supported feature of Infinispan. When I mean not supported, I mean:
- no inter object query
- no automatically handled relation between objects

> Also I would like to comment on the following statements:
> "I believe a cache API and Hot Rod are well suited to address up to the self contained object graph use case with a couple of relations maintained manually by the application but that cannot be queried. For the connected entities use case, only a high level paradigm is suited like JPA."
> 
> I don't think storing object graphs should be under scrutiny here: Infinispan C/S mode (and there's where most of the client focus is BTW) has a schema (prtobuf) that does not support object graphs. I also think expecting people to use multiple caches for multiple data types is a solid assumption to start from. And here's me speculating: these data types have logical relations between them so people will ask for querying. In order to queries on multiple data types, you can either merge them together (your suggestion) or support some sort of new cross-cache indexing/querying/api. x-cache querying is more flexible and less restraining than merging data, but from what I understand from you has certain implementation challenges. There's no pressure to take a decision now around supporting queries spreading multiple caches - just something to keep an eye on when dealing with use cases/users. ATM merging data is the only solution available, let's wait and see if people ask for more.

That’s fine I guess. My problem is that nowhere in your documentations do you guys list what can and cannot be done in that regard. And when you call a data structure an entity, it comes with an implicit bagage: relation between entities, polymorphism etc.
So this needs to be clarified.

Now circling back to the main topic. Doing cross-cache query on non related data (as they are in different caches) might be useful in some situations but is generally not interesting as we specifically don’t support joins (they are not related).
So back to the root of that discussion now, what was the use case(s) that lead to believe we need cross-cache query support?

Emmanuel
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