hive-issues mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Mithun Radhakrishnan (JIRA)" <>
Subject [jira] [Commented] (HIVE-14792) AvroSerde reads the remote schema-file at least once per mapper, per table reference.
Date Wed, 29 Nov 2017 21:53:00 GMT


Mithun Radhakrishnan commented on HIVE-14792:

Thanks for the review, [~aihuaxu]. The failures are unrelated. I'll have to rebase this patch,
given how old it has gotten.

> AvroSerde reads the remote schema-file at least once per mapper, per table reference.
> -------------------------------------------------------------------------------------
>                 Key: HIVE-14792
>                 URL:
>             Project: Hive
>          Issue Type: Bug
>    Affects Versions: 1.2.1, 2.1.0
>            Reporter: Mithun Radhakrishnan
>            Assignee: Mithun Radhakrishnan
>         Attachments: HIVE-14792.1.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()}}:
> {|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(),
>     }
> // ...
> }
> {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.

This message was sent by Atlassian JIRA

View raw message