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From "Mithun Radhakrishnan (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-14792) AvroSerde reads the remote schema-file at least once per mapper, per table reference.
Date Mon, 19 Sep 2016 22:51:20 GMT
Mithun Radhakrishnan created HIVE-14792:
-------------------------------------------

             Summary: 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: 2.1.0, 1.2.1
            Reporter: Mithun Radhakrishnan
            Assignee: Mithun Radhakrishnan


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 files 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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