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courses:mapreduce-tutorial:step-29 [2012/02/05 18:23] straka |
courses:mapreduce-tutorial:step-29 [2012/02/05 19:03] straka |
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====== MapReduce Tutorial : Custom sorting and grouping comparators. ====== | ====== MapReduce Tutorial : Custom sorting and grouping comparators. ====== | ||
- | ====== | + | ====== |
The keys are sorted before processed by a reducer, using a | The keys are sorted before processed by a reducer, using a | ||
- | [[http:// | + | [[http:// |
+ | |||
+ | <code java> | ||
+ | public static class IntPair implements WritableComparable< | ||
+ | private int first = 0; | ||
+ | private int second = 0; | ||
+ | |||
+ | public void set(int left, int right) { first = left; second = right; } | ||
+ | public int getFirst() { return first; } | ||
+ | public int getSecond() { return second; } | ||
+ | |||
+ | public void readFields(DataInput in) throws IOException { | ||
+ | first = in.readInt(); | ||
+ | second = in.readInt(); | ||
+ | } | ||
+ | public void write(DataOutput out) throws IOException { | ||
+ | out.writeInt(first); | ||
+ | out.writeInt(second); | ||
+ | } | ||
+ | |||
+ | public int compareTo(IntPair o) { | ||
+ | if (first != o.first) return first < o.first ? -1 : 1; | ||
+ | else return second < o.second ? -1 : second == o.second ? 0 : 1; | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | If we would like in a Hadoop job to sort the '' | ||
+ | |||
+ | <code java> | ||
+ | public static class IntPair implements WritableComparable< | ||
+ | ... | ||
+ | public static class FirstOnlyComparator implements RawComparator< | ||
+ | public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) { | ||
+ | int first1 = WritableComparator.readInt(b1, | ||
+ | int first2 = WritableComparator.readInt(b2, | ||
+ | return first1 < first2 ? -1 : first1 == first2 ? 0 : 1; | ||
+ | } | ||
+ | public int compare(IntPair x, IntPair y) { | ||
+ | return x.getFirst() < y.getFirst() ? -1 : x.getFirst() == y.getFirst() ? 0 : 1; | ||
+ | } | ||
+ | } | ||
+ | } | ||
+ | |||
+ | ... | ||
+ | |||
+ | job.setSortComparatorClass(IntPair.FirstOnlyComparator.class); | ||
+ | </ | ||
+ | Notice we used helper function '' | ||
====== Grouping comparator ====== | ====== Grouping comparator ====== | ||
In a reduce, it is guaranteed that keys are processed in ascending order. Sometimes it would be useful if the //values associated with one key// could also be processed in ascending order. | In a reduce, it is guaranteed that keys are processed in ascending order. Sometimes it would be useful if the //values associated with one key// could also be processed in ascending order. | ||
+ | |||
+ | That is possible only to some degree. The (key, value) pairs are compared //using the key only//. After the (key, value) pairs are sorted, the (key, value) pairs with the same key are grouped together. This grouping can be performed using a custom '' | ||
+ | |||
+ | As an example, consider that the input consists of ('' | ||
+ | - The mapper produces ('' | ||
+ | - These pairs are sorted by the '' | ||
+ | - The custom grouping comparator is used, which groups the '' | ||
+ | <code java> | ||
+ | </ | ||
---- | ---- |