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https://issues.apache.org/jira/browse/BEAM-7700?focusedWorklogId=282496&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-282496
]
ASF GitHub Bot logged work on BEAM-7700:
----------------------------------------
Author: ASF GitHub Bot
Created on: 25/Jul/19 06:51
Start Date: 25/Jul/19 06:51
Worklog Time Spent: 10m
Work Description: robinyqiu commented on pull request #9129: [BEAM-7700]
Java transform catalog
URL: https://github.com/apache/beam/pull/9129#discussion_r307109547
##########
File path: website/src/documentation/transforms/java/aggregation/combine.md
##########
@@ -0,0 +1,90 @@
+---
+layout: section
+title: "Combine"
+permalink: /documentation/transforms/java/aggregation/combine/
+section_menu: section-menu/documentation.html
+---
+<!--
+Licensed 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.
+-->
+# Combine
+<table align="left">
+ <a target="_blank" class="button"
+
href="https://beam.apache.org/releases/javadoc/current/index.html?org/apache/beam/sdk/transforms/Combine.html";>
+ <img src="https://beam.apache.org/images/logos/sdks/java.png";
width="20px" height="20px"
+ alt="Javadoc" />
+ Javadoc
+ </a>
+</table>
+<br>
+A user-defined `CombineFn` may be applied to combine all elements in a
+`PCollection` (global combine) or to combine all elements associated
+with each key.
+
+While the result is similar to applying a `GroupByKey` followed by
+counting the number of values in each `Iterable`, there is an impact
+on the code you must write as well as the performance of the pipeline.
+Writing a `ParDo` that counts the number of elements in each value
+would be very straightforward. However, as described in the execution
+model, it would also require all values associated with each key to be
+processed by a single worker. This introduces a lot of communication overhead.
+Using a `CombineFn` requires the code be structured as an associative and
+commumative operation. But, it allows the use of partial sums to be
precomputed.
+
+See more information in the [Beam Programming Guide]({{ site.baseurl
}}/documentation/programming-guide/#combine).
+
+## Examples
+**Example 1**: Global combine
+Use the global combine to combine all of the elements in a given `PCollection`
+into a single value, represented in your pipeline as a new `PCollection`
containing
+one element. The following example code shows how to apply the Beam-provided
+sum combine function to produce a single sum value for a `PCollection` of
integers.
+
+```java
+// Sum.SumIntegerFn() combines the elements in the input PCollection. The
resulting PCollection, called sum,
+// contains one value: the sum of all the elements in the input PCollection.
+PCollection<Integer> pc = ...;
+PCollection<Integer> sum = pc.apply(
+ Combine.globally(new Sum.SumIntegerFn()));
+```
+
+**Example 2**: Keyed combine
+Use a keyed combine to to combine all of the values associated with each key
+into a single output value for each key. As with the global combine, the
+function passed to a keyed combine must be associative and commutative.
+
+```java
+// PCollection is grouped by key and the Double values associated with each
key are combined into a Double.
+PCollection<KV<String, Double>> salesRecords = ...;
+PCollection<KV<String, Double>> totalSalesPerPerson =
+ salesRecords.apply(Combine.<String, Double, Double>perKey(
+ new Sum.SumDoubleFn()));
+// The combined value is of a different type than the original collection of
values per key. PCollection has
+// keys of type String and values of type Integer, and the combined value is a
Double.
+PCollection<KV<String, Integer>> playerAccuracy = ...;
+PCollection<KV<String, Double>> avgAccuracyPerPlayer =
+ playerAccuracy.apply(Combine.<String, Integer, Double>perKey(
+ new MeanInts())));
+```
+
+```java
Review comment:
Why adding this Python code here?
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Issue Time Tracking
-------------------
Worklog Id: (was: 282496)
Time Spent: 4h 20m (was: 4h 10m)
> Java transform catalog
> ----------------------
>
> Key: BEAM-7700
> URL: https://issues.apache.org/jira/browse/BEAM-7700
> Project: Beam
> Issue Type: Improvement
> Components: website
> Reporter: Rose Nguyen
> Assignee: Rose Nguyen
> Priority: Minor
> Time Spent: 4h 20m
> Remaining Estimate: 0h
>
> Create catalog of core transforms (Java)
> -Java transforms overview
> -Links to Javadocs
> -Brief description
> -Related transforms
> -Links to programming guide
> -Examples section to integrate Colab notebooks
>
> See BEAM-7464 for Python.
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