xuzifu666 commented on a change in pull request #3525: URL: https://github.com/apache/hudi/pull/3525#discussion_r697994100
########## File path: website/blog/2021-08-23-async-clustering.md ########## @@ -0,0 +1,159 @@ +--- +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 -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
