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| <h1 class="entry-title">Default Partitioning Changes Coming in Kudu 0.9</h1> |
| <p class="meta">Posted 02 Jun 2016 by Dan Burkert</p> |
| </header> |
| <div class="entry-content"> |
| <p>The upcoming Apache Kudu (incubating) 0.9 release is changing the default |
| partitioning configuration for new tables. This post will introduce the change, |
| explain the motivations, and show examples of how code can be updated to work |
| with the new release.</p> |
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| <p>The most common source of frustration with new Kudu users is the default |
| partitioning behavior when creating new tables. If partitioning is not |
| specified, the Kudu client prior to 0.9 creates tables with a <em>single tablet</em>. |
| Single tablet tables are a Kudu anti-pattern, since they are unable to get the |
| scalability benefit of distributing data across the cluster, and instead keep |
| all data on a single machine.</p> |
| |
| <p>Unfortunately, automatically choosing a better default partitioning |
| configuration for new tables is not simple. In most cases, hash partitioning on |
| the primary key is a better default, but this approach can have its own |
| drawbacks. In particular, it is not clear how many buckets should be used for |
| the new table.</p> |
| |
| <p>Since there is no bullet-proof default and changing the partitioning |
| configuration after table creation is impossible, <a href="https://lists.apache.org/thread.html/ca8972620839109334493424a1022fc08c77c315d9d623f5caaa815f@1463699013@%3Cuser.kudu.apache.org%3E">we |
| decided</a> |
| to remove the default altogether. Removing the default is a backwards |
| incompatible change, so it must be done before the 1.0 release. If we later find |
| a better way to create a default partitioning configuration, it should be |
| possible to adopt it in a backwards compatible way. The result of removing the |
| default is that new tables created with the 0.9 client must specify a |
| partitioning configuration, or table creation will fail. You can still create a |
| table with a single tablet, but it must be configured explicitly. These changes |
| only affect new table creation; existing tables, including tables created with |
| default partitioning before the 0.9 release, will continue to work.</p> |
| |
| <p>In most cases updating existing code to explicitly set a partitioning |
| configuration should be simple. The examples below add hash partitioning, but |
| you can also specify range partitioning or a combination of range and hash |
| partitioning. See the <a href="http://getkudu.io/docs/schema_design.html#data-distribution">schema design |
| guide</a> for more |
| advanced configurations.</p> |
| |
| <h1 id="c-client">C++ Client</h1> |
| |
| <p>With the C++ client, creating a new table with hash partitions is as simple as |
| calling <code>KuduTableCreator:add_hash_partitions</code> with the columns to hash and the |
| number of buckets to use:</p> |
| |
| <p><code>cpp |
| unique_ptr<KuduTableCreator> table_creator(my_client->NewTableCreator()); |
| Status create_status = table_creator->table_name("my-table") |
| .schema(my_schema) |
| .add_hash_partitions({ "key_column_a", "key_column_b" }, 16) |
| .Create(); |
| if (!create_status.ok() { /* handle error */ } |
| </code></p> |
| |
| <h1 id="java-client">Java Client</h1> |
| |
| <p>And similarly, in Java:</p> |
| |
| <p><code>java |
| List<String> hashColumns = new ArrayList<>(); |
| hashColumns.add("key_column_a"); |
| hashColumn.add("key_column_b"); |
| CreateTableOptions options = new CreateTableOptions().addHashPartitions(hashColumns, 16); |
| myClient.createTable("my-table", my_schema, options); |
| </code></p> |
| |
| <p>In the examples above, if the hash partition configuration is omitted the create |
| table operation will fail with the error <code>Table partitioning must be specified |
| using setRangePartitionColumns or addHashPartitions</code>. In the Java client this |
| manifests as a thrown <code>IllegalArgumentException</code>, while in the C++ client it is |
| returned as a <code>Status::InvalidArgument</code>.</p> |
| |
| <h1 id="impala">Impala</h1> |
| |
| <p>When creating Kudu tables with Impala, the formerly optional <code>DISTRIBUTE BY</code> |
| clause is now required:</p> |
| |
| <p><code>SQL |
| CREATE TABLE my_table (key_column_a STRING, key_column_b STRING, other_column STRING) |
| DISTRIBUTE BY HASH (key_column_a, key_column_b) INTO 16 BUCKETS |
| TBLPROPERTIES( |
| 'storage_handler' = 'com.cloudera.kudu.hive.KuduStorageHandler', |
| 'kudu.table_name' = 'my_table', |
| 'kudu.master_addresses' = 'kudu-master.example.com:7051', |
| 'kudu.key_columns' = 'key_column_a,key_column_b' |
| ); |
| </code></p> |
| |
| </div> |
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| <div class="col-lg-3 recent-posts"> |
| <h3>Recent posts</h3> |
| <ul> |
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| <li> <a href="/2016/06/21/weekly-update.html">Apache Kudu (incubating) Weekly Update June 21, 2016</a> </li> |
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| <li> <a href="/2016/06/02/no-default-partitioning.html">Default Partitioning Changes Coming in Kudu 0.9</a> </li> |
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