[jira] Resolved: (MAHOUT-247) GenericUserBasedRecommender.recommend causes connection leak when called for user with no preferences

["Sean Owen (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MAHOUT-247?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Sean Owen resolved MAHOUT-247.
------------------------------

    Resolution: Fixed

Tentatively resolving this after applying a fix to the most obvious case where this issue could come up. It's not truly fixed in all cases, though at the moment, I think this is all that is practical to attempt. I'm open to more suggestions about how to address this.

> GenericUserBasedRecommender.recommend causes connection leak when called for user with no preferences
> -----------------------------------------------------------------------------------------------------
>
>                 Key: MAHOUT-247
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-247
>             Project: Mahout
>          Issue Type: Bug
>          Components: Collaborative Filtering
>    Affects Versions: 0.2
>         Environment: Reproducable on Win32 and Ubuntu
>            Reporter: Tolga Oral
>            Assignee: Sean Owen
>            Priority: Minor
>             Fix For: 0.3
>
>
> 			UserSimilarity userSimilarity = new TanimotoCoefficientSimilarity(getBooleanPrefDataModel());
> 			UserNeighborhood neighborhood = new NearestNUserNeighborhood(3, userSimilarity, getBooleanPrefDataModel());
> 			Recommender recommender = new GenericBooleanPrefUserBasedRecommender(getBooleanPrefDataModel(), neighborhood, userSimilarity);
>                         recommender.recommend(userwithnopreferencesdata, 10);
> code properly throws NoSuchUserException however one of the connections is hang on LongPrimitiveIterator backed by org.apache.mahout.cf.taste.impl.model.jdbc.AbstractJDBCDataModel$ResultSetIDIterator as Exception is thrown before TopItems.getTopUsers finishes the while loop 
> public static long[] getTopUsers(int howMany,
>                                    LongPrimitiveIterator allUserIDs,
>                                    Rescorer<Long> rescorer,
>                                    Estimator<Long> estimator) throws TasteException {
>     Queue<SimilarUser> topUsers = new PriorityQueue<SimilarUser>(howMany + 1, Collections.reverseOrder());
>     boolean full = false;
>     double lowestTopValue = Double.NEGATIVE_INFINITY;
> //HERE IS THE ITERATOR
>     while (allUserIDs.hasNext()) {
>       long userID = allUserIDs.next();
>       if (rescorer != null && rescorer.isFiltered(userID)) {
>         continue;
>       }
> //EXCEPTION THROWN HERE CAUSES THE CONNECTION LEAK
>       double similarity = estimator.estimate(userID);
>       double rescoredSimilarity = rescorer == null ? similarity : rescorer.rescore(userID, similarity);
>       if (!Double.isNaN(rescoredSimilarity) && (!full || rescoredSimilarity > lowestTopValue)) {
>         topUsers.add(new SimilarUser(userID, similarity));
>         if (full) {
>           topUsers.poll();
>         } else if (topUsers.size() > howMany) {
>           full = true;
>           topUsers.poll();
>         }
>         lowestTopValue = topUsers.peek().getSimilarity();
>       }
>     }
>     if (topUsers.isEmpty()) {
>       return NO_IDS;
>     }
>     List<SimilarUser> sorted = new ArrayList<SimilarUser>(topUsers.size());
>     sorted.addAll(topUsers);
>     Collections.sort(sorted);
>     long[] result = new long[sorted.size()];
>     int i = 0;
>     for (SimilarUser similarUser : sorted) {
>       result[i++] = similarUser.getUserID();
>     }
>     return result;
>   }
> ============================================================================================================
> I currently fixed it in our application by checking first to see if user has preferences for the given dataset (user might exists and have preferences for a different dataset).
> However this edge case does not cause issues in some other recommenders as long as we handle the NoSuchUserException.
> Easy solution is to use AbstractJDBCDataModel$ResultSetIDIterator always with try/catch/finally and release the connection.

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