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Date: Thu, 14 Jan 2010 17:18:54 +0000 (UTC)
From: "Tolga Oral (JIRA)" <jira@apache.org>
To: mahout-dev@lucene.apache.org
Subject: [jira] Created: (MAHOUT-247) GenericUserBasedRecommender.recommend
 causes connection leak when called for user with no preferences
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GenericUserBasedRecommender.recommend causes connection leak when called for user with no preferences
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                 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
            Priority: Minor


			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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