[GitHub] [hudi] xuzifu666 commented on a change in pull request #3525: [HUDI-2346] Async clustering usage blog

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xuzifu666 commented on a change in pull request #3525:
URL: https://github.com/apache/hudi/pull/3525#discussion_r697994100



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File path: website/blog/2021-08-23-async-clustering.md
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+---
+title: "Asynchronous Clustering using Hudi"
+excerpt: "How to setup Hudi for asynchronous clustering"
+author: codope
+category: blog
+---
+
+In one of the [previous blog](https://hudi.apache.org/blog/2021/01/27/hudi-clustering-intro) posts, we introduced a new
+kind of table service called clustering to reorganize data for improved query performance without compromising on
+ingestion speed. We learnt how to setup inline clustering. In this post, we will discuss what has changed since then and
+see how asynchronous clustering can be setup using HoodieClusteringJob as well as DeltaStreamer utility.
+
+<!--truncate-->
+
+## Introduction
+
+On a high level, clustering creates a plan based on a configurable strategy, groups eligible files based on specific
+criteria and then executes the plan. Hudi's [MVCC model](https://hudi.apache.org/docs/concurrency_control) provides
+snapshot isolation between multiple table services, which allows writers to continue with ingestion while clustering
+runs in the background. For a more detailed overview of the clustering architecture please check out the previous blog
+post.
+
+## Clustering Strategies
+
+As mentioned before, clustering plan as well as execution depends on configurable strategy. These strategies can be
+broadly classified into three types: clustering plan strategy, execution strategy and update strategy.
+
+### Plan Strategy
+
+This strategy comes into play while creating clustering plan. It helps to decide what file groups should be clustered.
+Let's look at different plan strategies that are available with Hudi. Note that these strategies are easily pluggable
+using this [config](https://hudi.apache.org/docs/next/configurations#hoodieclusteringplanstrategyclass).
+
+1. `SparkSizeBasedClusteringPlanStrategy`: It selects file slices based on
+   the [small file limit](https://hudi.apache.org/docs/next/configurations/#hoodieclusteringplanstrategysmallfilelimit)
+   of base files and creates clustering groups upto max file size allowed per group. The max size can be specified using
+   this [config](https://hudi.apache.org/docs/next/configurations/#hoodieclusteringplanstrategymaxbytespergroup). This
+   strategy is useful for stitching together medium-sized files into larger ones to reduce lot of files spread across
+   cold partitions.
+2. `SparkRecentDaysClusteringPlanStrategy`: It looks back previous 'N' days partitions and creates a plan that will
+   cluster the 'small' file slices within those partitions. This is the default strategy. It could be useful when the
+   workload is predictable and data is partitioned by timestamp.
+3. `SparkSelectedPartitionsClusteringPlanStrategy`: In case you want to cluster only specific partitions within a range,
+   no matter how old or new are those partitions, then this strategy could be useful. To use this partition, one needs
+   to set below two configs additionally (both begin and end partitions are inclusive):
+
+```
+hoodie.clustering.plan.strategy.cluster.begin.partition
+hoodie.clustering.plan.strategy.cluster.end.partition
+```
+
+:::note
+All the strategies are partition-aware and the latter two are still bound by the size limits of the first strategy.
+:::
+
+### Execution Strategy
+
+After building the clustering groups in the planning phase, Hudi applies execution strategy, for each group, primarily
+based on sort columns and size. The strategy can be specified using
+this [config](https://hudi.apache.org/docs/next/configurations/#hoodieclusteringexecutionstrategyclass).
+
+`SparkSortAndSizeExecutionStrategy` is the default strategy. Users can specify the columns to sort the data by, when
+clustering using
+this [config](https://hudi.apache.org/docs/next/configurations/#hoodieclusteringplanstrategysortcolumns). Apart from
+that, we can also set [max file size](https://hudi.apache.org/docs/next/configurations/#hoodieparquetmaxfilesize)
+for the parquet files produced due to clustering. The strategy uses bulk insert to write data into new files, in which
+case, Hudi implicitly uses a partitioner that does sorting based on specified columns. In this way, the strategy changes
+the data layout in a way that not only improves query performance but also balance rewrite overhead automatically.
+
+Now this strategy can be executed either as a single spark job or multiple jobs depending on number of clustering groups
+created in the planning phase. By default, Hudi will submit multiple spark jobs and union the results. In case you want
+to force Hudi to use single spark job, set the execution strategy
+class [config](https://hudi.apache.org/docs/next/configurations/#hoodieclusteringexecutionstrategyclass)
+to `SingleSparkJobExecutionStrategy`.
+
+### Update Strategy
+
+Currently, clustering can only be scheduled for tables/partitions not receiving any concurrent updates. By default,
+the [config for update strategy](https://hudi.apache.org/docs/next/configurations/#hoodieclusteringupdatesstrategy) is
+set to ***SparkRejectUpdateStrategy***. If some file group has updates during clustering then it will reject updates and
+throw an exception. However, in some use-cases updates are very sparse and do not touch most file groups. The default
+strategy to simply reject updates does not seem fair. In such use-cases, users can set the config to ***SparkAllowUpdateStrategy***.
+
+We discussed the critical strategy configurations. All other configurations related to clustering are
+listed [here](https://hudi.apache.org/docs/next/configurations/#Clustering-Configs). Out of this list, a few
+configurations that will be very useful are:
+
+|  Config key  | Remarks | Default |
+|  -----------  | -------  | ------- |
+| `hoodie.clustering.async.enabled` | Enable running of clustering service, asynchronously as writes happen on the table. | False |
+| `hoodie.clustering.async.max.commits` | Control frequency of async clustering by specifying after how many commits clustering should be triggered. | 4 |
+| `hoodie.clustering.preserve.commit.metadata` | When rewriting data, preserves existing _hoodie_commit_time. This means users can run incremental queries on clustered data without any side-effects. | False |
+
+## Setup Asynchronous Clustering
+
+Previously, we have seen how users
+can [setup inline clustering](https://hudi.apache.org/blog/2021/01/27/hudi-clustering-intro#setting-up-clustering).
+Additionally, users can
+leverage [HoodieClusteringJob](https://cwiki.apache.org/confluence/display/HUDI/RFC+-+19+Clustering+data+for+freshness+and+query+performance#RFC19Clusteringdataforfreshnessandqueryperformance-SetupforAsyncclusteringJob)
+to setup 2-step asynchronous clustering.
+
+### HoodieClusteringJob
+
+With the release of Hudi version 0.9.0, we can schedule as well as execute clustering in the same step. We just need to
+specify the `—mode` or `-m` option. There are three modes:
+
+1. `schedule`: Make a clustering plan. This gives an instant which can be passed in execute mode.
+2. `execute`: Execute a clustering plan at given instant which means --instant-time is required here.
+3. `scheduleAndExecute`: Make a clustering plan first and execute that plan immediately.
+
+A sample spark-submit command to setup HoodieClusteringJob is as below:
+
+```bash
+spark-submit \
+--class org.apache.hudi.utilities.HoodieClusteringJob \
+/path/to/hudi-utilities-bundle/target/hudi-utilities-bundle_2.12-0.9.0-SNAPSHOT.jar \
+--props /path/to/config/clusteringjob.properties \
+--mode scheduleAndExecute \
+--base-path /path/to/hudi_table/basePath \
+--table-name hudi_table_schedule_clustering \
+--spark-memory 1g
+```
+
+### HoodieDeltaStreamer

Review comment:
       the content of clusteringjob.properties need to show for reader, otherwise hard to use for newer




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