Jackie-Jiang commented on a change in pull request #4010: Add docs for record reader URL: https://github.com/apache/incubator-pinot/pull/4010#discussion_r268872716
########## File path: docs/record_reader.rst ########## @@ -0,0 +1,133 @@ +.. +.. Licensed to the Apache Software Foundation (ASF) under one +.. or more contributor license agreements. See the NOTICE file +.. distributed with this work for additional information +.. regarding copyright ownership. The ASF licenses this file +.. to you under the Apache License, Version 2.0 (the +.. "License"); you may not use this file except in compliance +.. with the License. You may obtain a copy of the License at +.. +.. http://www.apache.org/licenses/LICENSE-2.0 +.. +.. Unless required by applicable law or agreed to in writing, +.. software distributed under the License is distributed on an +.. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +.. KIND, either express or implied. See the License for the +.. specific language governing permissions and limitations +.. under the License. +.. + +Record Reader +============= + +Pinot supports indexing data from various file formats. To support reading from a file format, a record reader need to +be provided to read the file and convert records into the general format which the indexing engine can understand. The +record reader serves as the connector from each individual file format to Pinot record format. + +Pinot package provides the following record readers out of the box: + +- Avro record reader: record reader for Avro format files +- CSV record reader: record reader for CSV format files +- JSON record reader: record reader for JSON format files +- Thrift record reader: record reader for Thrift format files +- Pinot segment record reader: record reader for Pinot segment + +For other file formats, we provide a general interface for record reader - `RecordReader`. To index the file into Pinot +segment, implement the interface and plug it into the index engine - `SegmentCreationDriverImpl`. + +Initialize Record Reader +------------------------ + +To initialize a record reader, the data file and table schema should be provided (for Pinot segment record reader, only +need to provide the index directory because schema can be derived from the segment). The output record will follow the +table schema provided. + +For Avro/JSON/Pinot segment record reader, no extra configuration is required as column names and multi-values are +embedded in the data file. + +For CSV/Thrift record reader, extra configuration might be provided to determine the column names and multi-values for +the data. + +CSV Record Reader Config +~~~~~~~~~~~~~~~~~~~~~~~~ + +The CSV record reader config contains the following settings: + +- Header: the header for the CSV file (column names) +- Column delimiter: delimiter for each column +- Multi-value delimiter: delimiter for each value for a multi-valued column + +If no config provided, use the default setting: + +- Use the first row in the data file as the header +- Use ',' as the column delimiter +- Use ';' as the multi-value delimiter + +Thrift Record Reader Config +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The Thrift record reader config is mandatory. It contains the Thrift class name for the record reader to de-serialize +the Thrift objects. + +Implement Your Own Record Reader +-------------------------------- + +You can implement your own record reader for file formats that is not supported natively. The following methods need to +be implemented: + +.. code-block:: none + + /** + * Return true if more records remain to be read. + */ + boolean hasNext(); + + /** + * Get the next record. + */ + GenericRow next() + throws IOException; + + /** + * Get the next record. Re-use the given row if possible to reduce garbage. + * The passed in row should be returned by previous call to next(). + */ + GenericRow next(GenericRow reuse) + throws IOException; + + /** + * Rewind the reader to start reading from the first record again. + */ + void rewind() + throws IOException; + + /** + * Get the Pinot schema. + */ + Schema getSchema(); + + +Generic Row +~~~~~~~~~~~ + +Generic row is the record abstraction which the index engine can read and index with. It is a map from column name +(String) to column value (Object). For multi-valued column, the value should be an object array (Object[]). + +Contracts for Record Reader +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +There are several contracts for record readers that developers should follow when implementing their own record readers: + +- The output GenericRow should follow the table schema provided, in the sense that: + + - All the columns in the schema should be preserved (if column does not exist in the original record, put default + value instead) + - Columns not in the schema should be escaped Review comment: Done ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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