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https://issues.apache.org/jira/browse/FLINK-8516?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16350045#comment-16350045
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ASF GitHub Bot commented on FLINK-8516:
---------------------------------------
Github user tzulitai commented on a diff in the pull request:
https://github.com/apache/flink/pull/5393#discussion_r165590209
--- Diff:
flink-connectors/flink-connector-kinesis/src/main/java/org/apache/flink/streaming/connectors/kinesis/util/KinesisShardAssigner.java
---
@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.streaming.connectors.kinesis.util;
+
+import
org.apache.flink.streaming.connectors.kinesis.model.StreamShardHandle;
+
+import java.io.Serializable;
+
+/**
+ * Utility to map Kinesis shards to Flink subtask indices.
+ */
+public interface KinesisShardAssigner extends Serializable {
+ /**
+ * Returns the index of the target subtask that a specific Kafka
partition should be
+ * assigned to. For return values outside the subtask range, modulus
operation will
+ * be applied automatically, hence it is also valid to just return a
hash code.
+ *
+ * <p>The resulting distribution of shards has the following contract:
+ * <ul>
+ * <li>1. Uniform distribution across subtasks</li>
+ * <li>2. Deterministic, calls for a given shard always return same
index.</li>
+ * </ul>
+ *
+ * <p>The above contract is crucial and cannot be broken. Consumer
subtasks rely on this
+ * contract to filter out partitions that they should not subscribe to,
guaranteeing
+ * that all partitions of a single topic will always be assigned to
some subtask in a
+ * uniformly distributed manner.
+ *
+ * <p>Kinesis and the consumer support dynamic re-sharding and shard
IDs, while sequential,
+ * cannot be assumed to be consecutive. There is no perfect generic
default assignment function.
+ * Default subtask index assignment, which is based on hash code, may
result in skew,
+ * with some subtasks having many shards assigned and others none.
--- End diff --
I feel like this section of the Javadoc should be part of the Javadoc for
the original consumer constructors, and should guide them to use the
`setShardAssigner` method if the do encounter the case of serious shard skew.
> FlinkKinesisConsumer does not balance shards over subtasks
> ----------------------------------------------------------
>
> Key: FLINK-8516
> URL: https://issues.apache.org/jira/browse/FLINK-8516
> Project: Flink
> Issue Type: Bug
> Components: Kinesis Connector
> Affects Versions: 1.4.0, 1.3.2, 1.5.0
> Reporter: Thomas Weise
> Assignee: Thomas Weise
> Priority: Major
>
> The hash code of the shard is used to distribute discovered shards over
> subtasks round robin. This works as long as shard identifiers are sequential.
> After shards are rebalanced in Kinesis, that may no longer be the case and
> the distribution become skewed.
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