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From Wenchen Fan <cloud0...@gmail.com>
Subject Re: [VOTE] [SPIP] SPARK-15689: Data Source API V2
Date Wed, 30 Aug 2017 01:59:12 GMT
Hi James,

Thanks for your feedback! I think your concerns are all valid, but we need
to make a tradeoff here.

> Explicitly here, what I'm looking for is a convenient mechanism to accept
a fully specified set of arguments

The problem with this approach is: 1) if we wanna add more arguments in the
future, it's really hard to do without changing the existing interface. 2)
if a user wants to implement a very simple data source, he has to look at
all the arguments and understand them, which may be a burden for him.
I don't have a solution to solve these 2 problems, comments are welcome.


> There are loads of cases like this - you can imagine someone being able
to push down a sort before a filter is applied, but not afterwards.
However, maybe the filter is so selective that it's better to push down the
filter and not handle the sort. I don't get to make this decision, Spark
does (but doesn't have good enough information to do it properly, whilst I
do). I want to be able to choose the parts I push down given knowledge of
my datasource - as defined the APIs don't let me do that, they're strictly
more restrictive than the V1 APIs in this way.

This is true, the current framework applies push downs one by one,
incrementally. If a data source wanna go back to accept a sort push down
after it accepts a filter push down, it's impossible with the current data
source V2.
Fortunately, we have a solution for this problem. At Spark side, actually
we do have a fully specified set of arguments waiting to be pushed down,
but Spark doesn't know which is the best order to push them into data
source. Spark can try every combination and ask the data source to report a
cost, then Spark can pick the best combination with the lowest cost. This
can also be implemented as a cost report interface, so that advanced data
source can implement it for optimal performance, and simple data source
doesn't need to care about it and keep simple.


The current design is very friendly to simple data source, and has the
potential to support complex data source, I prefer the current design over
the plan push down one. What do you think?


On Wed, Aug 30, 2017 at 5:53 AM, James Baker <j.baker@outlook.com> wrote:

> Yeah, for sure.
>
> With the stable representation - agree that in the general case this is
> pretty intractable, it restricts the modifications that you can do in the
> future too much. That said, it shouldn't be as hard if you restrict
> yourself to the parts of the plan which are supported by the datasources V2
> API (which after all, need to be translateable properly into the future to
> support the mixins proposed). This should have a pretty small scope in
> comparison. As long as the user can bail out of nodes they don't
> understand, they should be ok, right?
>
> That said, what would also be fine for us is a place to plug into an
> unstable query plan.
>
> Explicitly here, what I'm looking for is a convenient mechanism to accept
> a fully specified set of arguments (of which I can choose to ignore some),
> and return the information as to which of them I'm ignoring. Taking a query
> plan of sorts is a way of doing this which IMO is intuitive to the user. It
> also provides a convenient location to plug in things like stats. Not at
> all married to the idea of using a query plan here; it just seemed
> convenient.
>
> Regarding the users who just want to be able to pump data into Spark, my
> understanding is that replacing isolated nodes in a query plan is easy.
> That said, our goal here is to be able to push down as much as possible
> into the underlying datastore.
>
> To your second question:
>
> The issue is that if you build up pushdowns incrementally and not all at
> once, you end up having to reject pushdowns and filters that you actually
> can do, which unnecessarily increases overheads.
>
> For example, the dataset
>
> a b c
> 1 2 3
> 1 3 3
> 1 3 4
> 2 1 1
> 2 0 1
>
> can efficiently push down sort(b, c) if I have already applied the filter
> a = 1, but otherwise will force a sort in Spark. On the PR I detail a case
> I see where I can push down two equality filters iff I am given them at the
> same time, whilst not being able to one at a time.
>
> There are loads of cases like this - you can imagine someone being able to
> push down a sort before a filter is applied, but not afterwards. However,
> maybe the filter is so selective that it's better to push down the filter
> and not handle the sort. I don't get to make this decision, Spark does (but
> doesn't have good enough information to do it properly, whilst I do). I
> want to be able to choose the parts I push down given knowledge of my
> datasource - as defined the APIs don't let me do that, they're strictly
> more restrictive than the V1 APIs in this way.
>
> The pattern of not considering things that can be done in bulk bites us in
> other ways. The retrieval methods end up being trickier to implement than
> is necessary because frequently a single operation provides the result of
> many of the getters, but the state is mutable, so you end up with odd
> caches.
>
> For example, the work I need to do to answer unhandledFilters in V1 is
> roughly the same as the work I need to do to buildScan, so I want to cache
> it. This means that I end up with code that looks like:
>
> public final class CachingFoo implements Foo {
>     private final Foo delegate;
>
>     private List<Filter> currentFilters = emptyList();
>     private Supplier<Bar> barSupplier = newSupplier(currentFilters);
>
>     public CachingFoo(Foo delegate) {
>         this.delegate = delegate;
>     }
>
>     private Supplier<Bar> newSupplier(List<Filter> filters) {
>         return Suppliers.memoize(() -> delegate.computeBar(filters));
>     }
>
>     @Override
>     public Bar computeBar(List<Filter> filters) {
>         if (!filters.equals(currentFilters)) {
>             currentFilters = filters;
>             barSupplier = newSupplier(filters);
>         }
>
>         return barSupplier.get();
>     }
> }
>
> which caches the result required in unhandledFilters on the expectation
> that Spark will call buildScan afterwards and get to use the result..
>
> This kind of cache becomes more prominent, but harder to deal with in the
> new APIs. As one example here, the state I will need in order to compute
> accurate column stats internally will likely be a subset of the work
> required in order to get the read tasks, tell you if I can handle filters,
> etc, so I'll want to cache them for reuse. However, the cached information
> needs to be appropriately invalidated when I add a new filter or sort order
> or limit, and this makes implementing the APIs harder and more error-prone.
>
> One thing that'd be great is a defined contract of the order in which
> Spark calls the methods on your datasource (ideally this contract could be
> implied by the way the Java class structure works, but otherwise I can just
> throw).
>
> James
>
> On Tue, 29 Aug 2017 at 02:56 Reynold Xin <rxin@databricks.com> wrote:
>
>> James,
>>
>> Thanks for the comment. I think you just pointed out a trade-off between
>> expressiveness and API simplicity, compatibility and evolvability. For the
>> max expressiveness, we'd want the ability to expose full query plans, and
>> let the data source decide which part of the query plan can be pushed down.
>>
>> The downside to that (full query plan push down) are:
>>
>> 1. It is extremely difficult to design a stable representation for
>> logical / physical plan. It is doable, but we'd be the first to do it. I'm
>> not sure of any mainstream databases being able to do that in the past. The
>> design of that API itself, to make sure we have a good story for backward
>> and forward compatibility, would probably take months if not years. It
>> might still be good to do, or offer an experimental trait without
>> compatibility guarantee that uses the current Catalyst internal logical
>> plan.
>>
>> 2. Most data source developers simply want a way to offer some data,
>> without any pushdown. Having to understand query plans is a burden rather
>> than a gift.
>>
>>
>> Re: your point about the proposed v2 being worse than v1 for your use
>> case.
>>
>> Can you say more? You used the argument that in v2 there are more support
>> for broader pushdown and as a result it is harder to implement. That's how
>> it is supposed to be. If a data source simply implements one of the trait,
>> it'd be logically identical to v1. I don't see why it would be worse or
>> better, other than v2 provides much stronger forward compatibility
>> guarantees than v1.
>>
>>
>> On Tue, Aug 29, 2017 at 4:54 AM, James Baker <j.baker@outlook.com> wrote:
>>
>>> Copying from the code review comments I just submitted on the draft API (
>>> https://github.com/cloud-fan/spark/pull/10#pullrequestreview-59088745):
>>>
>>> Context here is that I've spent some time implementing a Spark
>>> datasource and have had some issues with the current API which are made
>>> worse in V2.
>>>
>>> The general conclusion I’ve come to here is that this is very hard to
>>> actually implement (in a similar but more aggressive way than DataSource
>>> V1, because of the extra methods and dimensions we get in V2).
>>>
>>> In DataSources V1 PrunedFilteredScan, the issue is that you are passed
>>> in the filters with the buildScan method, and then passed in again with the
>>> unhandledFilters method.
>>>
>>> However, the filters that you can’t handle might be data dependent,
>>> which the current API does not handle well. Suppose I can handle filter A
>>> some of the time, and filter B some of the time. If I’m passed in both,
>>> then either A and B are unhandled, or A, or B, or neither. The work I have
>>> to do to work this out is essentially the same as I have to do while
>>> actually generating my RDD (essentially I have to generate my partitions),
>>> so I end up doing some weird caching work.
>>>
>>> This V2 API proposal has the same issues, but perhaps moreso. In
>>> PrunedFilteredScan, there is essentially one degree of freedom for pruning
>>> (filters), so you just have to implement caching between unhandledFilters
>>> and buildScan. However, here we have many degrees of freedom; sorts,
>>> individual filters, clustering, sampling, maybe aggregations eventually -
>>> and these operations are not all commutative, and computing my support
>>> one-by-one can easily end up being more expensive than computing all in one
>>> go.
>>>
>>> For some trivial examples:
>>>
>>> - After filtering, I might be sorted, whilst before filtering I might
>>> not be.
>>>
>>> - Filtering with certain filters might affect my ability to push down
>>> others.
>>>
>>> - Filtering with aggregations (as mooted) might not be possible to push
>>> down.
>>>
>>> And with the API as currently mooted, I need to be able to go back and
>>> change my results because they might change later.
>>>
>>> Really what would be good here is to pass all of the filters and sorts
>>> etc all at once, and then I return the parts I can’t handle.
>>>
>>> I’d prefer in general that this be implemented by passing some kind of
>>> query plan to the datasource which enables this kind of replacement.
>>> Explicitly don’t want to give the whole query plan - that sounds painful -
>>> would prefer we push down only the parts of the query plan we deem to be
>>> stable. With the mix-in approach, I don’t think we can guarantee the
>>> properties we want without a two-phase thing - I’d really love to be able
>>> to just define a straightforward union type which is our supported pushdown
>>> stuff, and then the user can transform and return it.
>>>
>>> I think this ends up being a more elegant API for consumers, and also
>>> far more intuitive.
>>>
>>> James
>>>
>>> On Mon, 28 Aug 2017 at 18:00 蒋星博 <jiangxb1987@gmail.com> wrote:
>>>
>>>> +1 (Non-binding)
>>>>
>>>> Xiao Li <gatorsmile@gmail.com>于2017年8月28日 周一下午5:38写道:
>>>>
>>>>> +1
>>>>>
>>>>> 2017-08-28 12:45 GMT-07:00 Cody Koeninger <cody@koeninger.org>:
>>>>>
>>>>>> Just wanted to point out that because the jira isn't labeled SPIP,
it
>>>>>> won't have shown up linked from
>>>>>>
>>>>>> http://spark.apache.org/improvement-proposals.html
>>>>>>
>>>>>> On Mon, Aug 28, 2017 at 2:20 PM, Wenchen Fan <cloud0fan@gmail.com>
>>>>>> wrote:
>>>>>> > Hi all,
>>>>>> >
>>>>>> > It has been almost 2 weeks since I proposed the data source
V2 for
>>>>>> > discussion, and we already got some feedbacks on the JIRA ticket
>>>>>> and the
>>>>>> > prototype PR, so I'd like to call for a vote.
>>>>>> >
>>>>>> > The full document of the Data Source API V2 is:
>>>>>> > https://docs.google.com/document/d/1n_vUVbF4KD3gxTmkNEon5qdQ-
>>>>>> Z8qU5Frf6WMQZ6jJVM/edit
>>>>>> >
>>>>>> > Note that, this vote should focus on high-level design/framework,
>>>>>> not
>>>>>> > specified APIs, as we can always change/improve specified APIs
>>>>>> during
>>>>>> > development.
>>>>>> >
>>>>>> > The vote will be up for the next 72 hours. Please reply with
your
>>>>>> vote:
>>>>>> >
>>>>>> > +1: Yeah, let's go forward and implement the SPIP.
>>>>>> > +0: Don't really care.
>>>>>> > -1: I don't think this is a good idea because of the following
>>>>>> technical
>>>>>> > reasons.
>>>>>> >
>>>>>> > Thanks!
>>>>>>
>>>>>> ---------------------------------------------------------------------
>>>>>> To unsubscribe e-mail: dev-unsubscribe@spark.apache.org
>>>>>>
>>>>>>
>>>>>
>>

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