[jira] [Commented] (HIVE-14792) AvroSerde reads the remote schema-file at least once per mapper, per table reference.

["Aihua Xu (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-14792?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16390199#comment-16390199 ] 

Aihua Xu commented on HIVE-14792:
---------------------------------

[~mithun] Can we get this fixed or do you need help on this? If so, I can take it. Let me know if you have time to work on this. Thanks.

> AvroSerde reads the remote schema-file at least once per mapper, per table reference.
> -------------------------------------------------------------------------------------
>
>                 Key: HIVE-14792
>                 URL: https://issues.apache.org/jira/browse/HIVE-14792
>             Project: Hive
>          Issue Type: Bug
>    Affects Versions: 1.2.1, 2.1.0
>            Reporter: Mithun Radhakrishnan
>            Assignee: Mithun Radhakrishnan
>            Priority: Major
>              Labels: TODOC2.2, TODOC2.4
>             Fix For: 3.0.0, 2.4.0, 2.2.1
>
>         Attachments: HIVE-14792.1.patch, HIVE-14792.3.patch
>
>
> Avro tables that use "external" schema files stored on HDFS can cause excessive calls to {{FileSystem::open()}}, especially for queries that spawn large numbers of mappers.
> This is because of the following code in {{AvroSerDe::initialize()}}:
> {code:title=AvroSerDe.java|borderStyle=solid}
> public void initialize(Configuration configuration, Properties properties) throws SerDeException {
> // ...
>     if (hasExternalSchema(properties)
>         || columnNameProperty == null || columnNameProperty.isEmpty()
>         || columnTypeProperty == null || columnTypeProperty.isEmpty()) {
>       schema = determineSchemaOrReturnErrorSchema(configuration, properties);
>     } else {
>       // Get column names and sort order
>       columnNames = Arrays.asList(columnNameProperty.split(","));
>       columnTypes = TypeInfoUtils.getTypeInfosFromTypeString(columnTypeProperty);
>       schema = getSchemaFromCols(properties, columnNames, columnTypes, columnCommentProperty);
>          properties.setProperty(AvroSerdeUtils.AvroTableProperties.SCHEMA_LITERAL.getPropName(), schema.toString());
>     }
> // ...
> }
> {code}
> For tables using {{avro.schema.url}}, every time the SerDe is initialized (i.e. at least once per mapper), the schema file is read remotely. For queries with thousands of mappers, this leads to a stampede to the handful (3?) datanodes that host the schema-file. In the best case, this causes slowdowns.
> It would be preferable to distribute the Avro-schema to all mappers as part of the job-conf. The alternatives aren't exactly appealing:
> # One can't rely solely on the {{column.list.types}} stored in the Hive metastore. (HIVE-14789).
> # {{avro.schema.literal}} might not always be usable, because of the size-limit on table-parameters. The typical size of the Avro-schema file is between 0.5-3MB, in my limited experience. Bumping the max table-parameter size isn't a great solution.
> If the {{avro.schema.file}} were read during query-planning, and made available as part of table-properties (but not serialized into the metastore), the downstream logic will remain largely intact. I have a patch that does this.



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