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https://issues.apache.org/jira/browse/SPARK-14409?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15902639#comment-15902639
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Nick Pentreath commented on SPARK-14409:
----------------------------------------
I commented on the [PR for
SPARK-19535|https://github.com/apache/spark/pull/17090#issuecomment-284648012]
and am copying that comment here for future reference as it contains further
detail of the discussion:
=====
{noformat}
Sorry if my other comments here and on JIRA were unclear. But the proposed
schema for input to RankingEvaluator is:
Schema 1
+------+-------+------+----------+
|userId|movieId|rating|prediction|
+------+-------+------+----------+
| 230| 318| 5.0| 4.2403245|
| 230| 3424| 4.0| null|
| 230| 81191| null| 4.317455|
+------+-------+------+----------+
You will notice that rating and prediction columns can be null. This is by
design. There are three cases shown above:
1st row indicates a (user-item) pair that occurs in both the ground-truth set
and the top-k predictions;
2nd row indicates a (user-item) pair that occurs in the ground-truth set, but
not in the top-k predictions;
3rd row indicates a (user-item) pair that occurs in the top-k predictions, but
not in the ground-truth set.
Note for reference, the input to the current mllib RankingMetrics is:
Schema 2
RDD[(true labels array, predicted labels array)],
i.e.
RDD of ([318, 3424, 7139,...], [81191, 93040, 31...])
(So actually neither of the above schemas are easily compatible with the return
schema here - but I think it is not really necessary to match the
mllib.RankingMetrics format)
ALS cross-validation
My proposal for fitting ALS into cross-validation is the ALSModel.transform
will output a DF of Schema 1 - only when the parameters k and recommendFor are
appropriately set, and the input DF contains both user and item columns. In
practice, this scenario will occur during cross-validation only.
So what I am saying is that ALS itself (not the evaluator) must know how to
return the correct DataFrame output from transform such that it can be used in
a cross-validation as input to the RankingEvaluator.
Concretely:
val als = new ALS().setRecommendFor("user").setK(10)
val validator = new TrainValidationSplit()
.setEvaluator(new RankingEvaluator().setK(10))
.setEstimator(als)
.setEstimatorParamMaps(...)
val bestModel = validator.fit(ratings)
So while it is complex under the hood - to users it's simply a case of setting
2 params and the rest is as normal.
Now, we have the best model selected by cross-validation. We can make
recommendations using these convenience methods (I think it will need a cast):
val recommendations =
bestModel.asInstanceOf[ALSModel].recommendItemsforUsers(10)
Alternatively, the transform version looks like this:
val usersDF = ...
+------+
|userId|
+------+
| 1|
| 2|
| 3|
+------+
val recommendations = bestModel.transform(usersDF)
So the questions:
should we support the above transform-based recommendations? Or only support it
for cross-validation purposes as a special case?
if we do, what should the output schema of the above transform version look
like? It must certainly match the output of recommendX methods.
The options are:
(1) The schema in this PR:
Pros: as you mention above - also more "compact"
Cons: doesn't match up so closely with the transform "cross-validation" schema
above
(2) The schema below. It is basically an "exploded" version of option (1)
+------+-------+----------+
|userId|movieId|prediction|
+------+-------+----------+
| 1| 1| 4.3|
| 1| 5| 3.2|
| 1| 9| 2.1|
+------+-------+----------+
Pros*: matches more closely with the cross-validation / evaluator input format.
Perhaps slightly more "dataframe-like".
Cons: less compact; lose ordering?; may require more munging to save to
external data stores etc.
Anyway sorry for hijacking this PR discussion - but as I think you can see, the
evaluator / ALS transform interplay is a bit subtle and requires some thought
to get the right approach.
{noformat}
> Investigate adding a RankingEvaluator to ML
> -------------------------------------------
>
> Key: SPARK-14409
> URL: https://issues.apache.org/jira/browse/SPARK-14409
> Project: Spark
> Issue Type: New Feature
> Components: ML
> Reporter: Nick Pentreath
> Priority: Minor
>
> {{mllib.evaluation}} contains a {{RankingMetrics}} class, while there is no
> {{RankingEvaluator}} in {{ml.evaluation}}. Such an evaluator can be useful
> for recommendation evaluation (and can be useful in other settings
> potentially).
> Should be thought about in conjunction with adding the "recommendAll" methods
> in SPARK-13857, so that top-k ranking metrics can be used in cross-validators.
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