kafka git commit: MINOR: Add comment for round robin partitioner with different subscriptions

[ew...@apache.org]

Repository: kafka
Updated Branches:
  refs/heads/trunk 06a57cf19 -> 4b28d38ca


MINOR: Add comment for round robin partitioner with different subscriptions

Author: Liquan Pei <li...@gmail.com>

Reviewers: Ewen Cheslack-Postava <ew...@confluent.io>

Closes #1475 from Ishiihara/roundrobin-comment


Project: http://git-wip-us.apache.org/repos/asf/kafka/repo
Commit: http://git-wip-us.apache.org/repos/asf/kafka/commit/4b28d38c
Tree: http://git-wip-us.apache.org/repos/asf/kafka/tree/4b28d38c
Diff: http://git-wip-us.apache.org/repos/asf/kafka/diff/4b28d38c

Branch: refs/heads/trunk
Commit: 4b28d38ca463ff3788885a904591a5699127158a
Parents: 06a57cf
Author: Liquan Pei <li...@gmail.com>
Authored: Tue Jun 7 19:38:39 2016 -0700
Committer: Ewen Cheslack-Postava <me...@ewencp.org>
Committed: Tue Jun 7 19:38:39 2016 -0700

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 .../kafka/clients/consumer/RoundRobinAssignor.java  | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)
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http://git-wip-us.apache.org/repos/asf/kafka/blob/4b28d38c/clients/src/main/java/org/apache/kafka/clients/consumer/RoundRobinAssignor.java
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diff --git a/clients/src/main/java/org/apache/kafka/clients/consumer/RoundRobinAssignor.java b/clients/src/main/java/org/apache/kafka/clients/consumer/RoundRobinAssignor.java
index a5de595..a598258 100644
--- a/clients/src/main/java/org/apache/kafka/clients/consumer/RoundRobinAssignor.java
+++ b/clients/src/main/java/org/apache/kafka/clients/consumer/RoundRobinAssignor.java
@@ -25,8 +25,8 @@ import java.util.SortedSet;
 import java.util.TreeSet;
 
 /**
- * The roundrobin assignor lays out all the available partitions and all the available consumers. It
- * then proceeds to do a roundrobin assignment from partition to consumer. If the subscriptions of all consumer
+ * The round robin assignor lays out all the available partitions and all the available consumers. It
+ * then proceeds to do a round robin assignment from partition to consumer. If the subscriptions of all consumer
  * instances are identical, then the partitions will be uniformly distributed. (i.e., the partition ownership counts
  * will be within a delta of exactly one across all consumers.)
  *
@@ -36,6 +36,18 @@ import java.util.TreeSet;
  * The assignment will be:
  * C0: [t0p0, t0p2, t1p1]
  * C1: [t0p1, t1p0, t1p2]
+ *
+ * When subscriptions differ across consumer instances, the assignment process still considers each
+ * consumer instance in round robin fashion but skips over an instance if it is not subscribed to
+ * the topic. Unlike the case when subscriptions are identical, this can result in imbalanced
+ * assignments. For example, we have three consumers C0, C1, C2, and three topics t0, t1, t2,
+ * with 1, 2, and 3 partitions, respectively. Therefore, the partitions are t0p0, t1p0, t1p1, t2p0,
+ * t2p1, t2p2. C0 is subscribed to t0; C1 is subscribed to t0, t1; and C2 is subscribed to t0, t1, t2.
+ *
+ * Tha assignment will be:
+ * C0: [t0p0]
+ * C1: [t1p0]
+ * C2: [t1p1, t2p0, t2p1, t2p2]
  */
 public class RoundRobinAssignor extends AbstractPartitionAssignor {