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From ShaoFeng Shi <shaofeng...@apache.org>
Subject [DISCUSS] Columnar storage engine for Apache Kylin
Date Fri, 28 Sep 2018 06:14:25 GMT
Hi Kylin developers.

HBase has been Kylin’s storage engine since the first day; Kylin on HBase
has been verified as a success which can support low latency & high
concurrency queries on a very large data scale. Thanks to HBase, most Kylin
users can get on average less than 1-second query response.

But we also see some limitations when putting Cubes into HBase; I shared
some of them in the HBaseConf Asia 2018[1] this August. The typical
limitations include:

   - Rowkey is the primary index, no secondary index so far;

Filtering by row key’s prefix and suffix can get very different performance
result. So the user needs to do a good design about the row key; otherwise,
the query would be slow. This is difficult sometimes because the user might
not predict the filtering patterns ahead of cube design.

   - HBase is a key-value instead of a columnar storage

Kylin combines multiple measures (columns) into fewer column families for
smaller data size (row key size is remarkable). This causes HBase often
needing to read more data than requested.

   - HBase couldn't run on YARN

This makes the deployment and auto-scaling a little complicated, especially
in the cloud.

In one word, HBase is complicated to be Kylin’s storage. The maintenance,
debugging is also hard for normal developers. Now we’re planning to seek a
simple, light-weighted, read-only storage engine for Kylin. The new
solution should have the following characteristics:

   - Columnar layout with compression for efficient I/O;
   - Index by each column for quick filtering and seeking;
   - MapReduce / Spark API for parallel processing;
   - HDFS compliant for scalability and availability;
   - Mature, stable and extensible;

With the plugin architecture[2] introduced in Kylin 1.5, adding multiple
storages to Kylin is possible. Some companies like Kyligence Inc and
Meituan.com, have developed their customized storage engine for Kylin in
their product or platform. In their experience, columnar storage is a good
supplement for the HBase engine. Kaisen Kang from Meituan.com has shared
their KOD (Kylin on Druid) solution[3] in this August’s Kylin meetup in
Beijing.

We plan to do a PoC with Apache Parquet + Apache Spark in the next phase.
Parquet is a standard columnar file format and has been widely supported by
many projects like Hive, Impala, Drill, etc. Parquet is adding the page
level column index to support fine-grained filtering.  Apache Spark can
provide the parallel computing over Parquet and can be deployed on
YARN/Mesos and Kubernetes. With this combination, the data persistence and
computation are separated, which makes the scaling in/out much easier than
before. Benefiting from Spark's flexibility, we can not only push down more
computation from Kylin to the Hadoop cluster. Except for Parquet, Apache
ORC is also a candidate.

Now I raise this discussion to get your ideas about Kylin’s next-generation
storage engine. If you have good ideas or any related data, welcome discuss in
the community.

Thank you!

[1] Apache Kylin on HBase
https://www.slideshare.net/ShiShaoFeng1/apache-kylin-on-hbase-extreme-olap-engine-for-big-data
[2] Apache Kylin Plugin Architecture
https://kylin.apache.org/development/plugin_arch.html
[3] 基于Druid的Kylin存储引擎实践 https://blog.bcmeng.com/post/kylin-on-druid.html--
Best regards,

Shaofeng Shi 史少锋

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