Migrating gphdfs External Tables to PXF

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You may be using the gphdfs external table protocol in a Greenplum Database version 4 or 5 cluster to access data stored in Hadoop. Greenplum Database version 6 removes support for the gphdfs protocol. To maintain access to Hadoop data in Greenplum 6, you must migrate your gphdfs external tables to use the Greenplum Platform Extension Framework (PXF). This involves setting up PXF and creating new external tables that use the pxf external table protocol.

To migrate your gphdfs external tables to use the pxf external table protocol, you:

  1. Prepare for the migration.
  2. Map configuration properties and initialize, configure, and start PXF.
  3. Create a new pxf external table to replace each gphdfs external table.
  4. Verify access to Hadoop files with the pxf external tables.
  5. Remove the gphdfs external tables.
  6. Revoke privileges to the gphdfs protocol.
  7. Migrate data to Greenplum 6.
If you are migrating gphdfs from a Greenplum Database 5 installation, you perform the migration in the order above in your Greenplum 5 cluster before you migrate data to Greenplum 6.
If you are migrating gphdfs from a Greenplum Database 4 installation, you perform the migration in a similar order. However, since PXF is not available in Greenplum Database 4, you must perform certain actions in the Greenplum 6 installation before you migrate the data:
  1. Greenplum 4: Prepare for the migration.
  2. Greenplum 6:
    1. Install and configure the Greenplum 6 software.
    2. Map configuration properties and initialize, configure, and start PXF.
    3. Create a new pxf external table to replace each gphdfs external table.
    4. Verify access to Hadoop files with the pxf external tables.
  3. Greenplum 4:
    1. Remove the gphdfs external tables.
    2. Revoke privileges to the gphdfs protocol.
  4. Migrate Greenplum 4 data to Greenplum 6.

Limitations

Keep the following in mind as you plan for the migration:

  • PXF does not support writing to Avro files on HDFS.

Preparing for the Migration

As you prepare for migrating from gphdfs to PXF:

  1. Determine which gphdfs tables you want to migrate.

    You can run the following query to list the gphdfs external tables in a database:

     SELECT n.nspname, d.objid::regclass as tablename
       FROM pg_depend d 
       JOIN pg_exttable x ON ( d.objid = x.reloid ) 
       JOIN pg_extprotocol p ON ( p.oid = d.refobjid ) 
       JOIN pg_class c ON ( c.oid = d.objid ) 
       JOIN pg_namespace n ON ( c.relnamespace = n.oid ) 
     WHERE d.refclassid = 'pg_extprotocol'::regclass AND p.ptcname = 'gphdfs';
    
  2. For each table that you choose to migrate, identify the format of the external data and the column definitions. Also identify the options with which the gphdfs table was created. You can use the \dt+ SQL meta-command to obtain this information. For example:

    \d+ public.gphdfs_writable_parquet
         External table "public.gphdfs_writable_parquet"
     Column |  Type   | Modifiers | Storage  | Description 
    --------+---------+-----------+----------+-------------
     id     | integer |           | plain    | 
     msg    | text    |           | extended | 
    Type: writable
    Encoding: UTF8
    Format type: parquet
    Format options: formatter 'gphdfs_export' 
    External options: {}
    External location: gphdfs://hdfshost:8020/data/dir1/gphdfs_writepq?codec=GZIP
    Execute on: all segments
    
  3. Save the information that you gathered above.

Initializing, Configuring, and Starting PXF

PXF does not use the following gphdfs configuration options:

gphdfs Configuration Option Description pxf Consideration
HADOOP_HOME Environment variable that identifies the Hadoop installation directory Not applicable; PXF is bundled with the required dependent Hadoop libraries and JARs
CLASSPATH Environment variable that identifies the locations of Hadoop JAR and configuration files Not applicable, PXF automatically includes the Hadoop libraries, JARs, and configuration files that it bundles in the CLASSPATH. PXF also automatically includes user-registered dependencies found in the $PXF_CONF/lib directory in the CLASSPATH.
gp_hadoop_target_version Server configuration parameter that identifies the Hadoop distribution Not applicable, PXF works out-of-the-box with the different Hadoop distributions
gp_hadoop_home Server configuration parameter that identifies the Hadoop installation directory Not applicable, PXF works out-of-the-box with the different Hadoop distributions

Configuration properties required by PXF, and the gphdfs equivalent, if applicable, include:

Configuration Item Description gphdfs Config pxf Config
JAVA_HOME Environment variable that identifies the Java installation directory Set JAVA_HOME on each segment host Set JAVA_HOME on each segment host
JVM option settings Options with which to start the JVM Set options in the GP_JAVA_OPT environment variable in hadoop_env.sh Set options in the PXF_JVM_OPTS environment variable in $PXF_CONF/conf/pxf-env.sh
PXF Server PXF server configuration for Hadoop Not applicable Configure a PXF server for Hadoop
Privileges The Greenplum Database privileges required to create external tables in the given protocol Grant SELECT and INSERT privileges on the gphdfs protocol to appropriate users Grant SELECT and INSERT privileges on the pxf protocol to appropriate users

After you determine the equivalent PXF configuration properties, you will:

  1. Update the Java version installed on each Greenplum Database host, if necessary. PXF supports Java version 8. If your Greenplum Database cluster hosts are running Java 7, upgrade to Java version 8 as described in Installing Java for PXF.

  2. Initialize PXF as described in Initializing PXF.

    You must initialize PXF before you can access Hadoop.

  3. Configure the PXF Hadoop Connectors. This procedure creates a PXF server configuration that provides PXF the information that it requires to access Hadoop.

  4. Start PXF in your Greenplum Database cluster as described in Starting PXF.

  5. Register the PXF extension and grant users access to PXF.

Creating a PXF External Table

gphdfs and pxf are both external table protocols. Creating an external table using these protocols is similar. You specify the external table name and its column definitions. You also specify LOCATION and FORMAT clauses. gphdfs and pxf use information in these clauses to determine the location and type of the external data.

Mapping the LOCATION Clause

The LOCATION clause of an external table identifies the external table protocol, location of the external data, and protocol- and operation-specific custom options.

The format of gphdfs’s LOCATION clause is as follows:

LOCATION('gphdfs://<hdfs_host>:<hdfs_port>/<path-to-data>?[&<custom-option>=<value>[...]]')

PXF’s LOCATION clause takes the following format when you access data stored on Hadoop:

LOCATION('pxf://<path-to-data>?PROFILE=<profile_name>[&SERVER=<server_name>][&<custom-option>=<value>[...]]')

You are not required to specify the HDFS host and port number when you create a PXF external table. PXF obtains this information from the default server configuration, or from the server configuration name that you specify in <server_name>.

Refer to Creating an External Table in the PXF documentation for more information about the PXF CREATE EXTERNAL TABLE syntax and keywords.

When you create an external table specifying the gphdfs protocol, you identify the format of the external data in the FORMAT clause (discussed in the next section). PXF uses a PROFILE option in the LOCATION clause to identify the source and type of the external data.

Data Format pxf PROFILE
Avro hdfs:avro
Parquet hdfs:parquet
Text hdfs:text

Refer to Connectors, Data Formats, and Profiles in the PXF documentation for more information about the PXF profiles supported for Hadoop.

Both gphdfs and pxf utilize custom options in the LOCATION clause to identify data-format-, operation-, or profile-specific options supported by the protocol. For example, both gphdfs and pxf support parquet and compression options on INSERT operations.

Should you need to migrate a gphdfs writable external table that references an HDFS file to PXF, map gphdfs to PXF writable external table compression options as follows:

Description gphdfs LOCATION Option pxf LOCATION Option
Use of Compression compress Not applicable; depends on the profile - may be uncompressed by default or specified via COMPRESSION_CODEC
Type of compression compression_type COMPRESSION_TYPE
Compression codec codec COMPRESSION_CODEC
Level of Compression codec_level (Avro format deflate codec only) Not applicable; PXF does not support writing Avro data

If the HDFS file is a Parquet-format file, map these additional parquet options as follows:

Description gphdfs LOCATION Option pxf LOCATION Option
Parquet schema schema SCHEMA
Page size pagesize PAGE_SIZE
Row group size rowgroupsize ROWGROUP_SIZE
Parquet version parquetversion or pqversion PARQUET_VERSION
Enable a dictionary dictionaryenable The dictionary is always enabled when writing Parquet data with PXF
Dictionary page size dictionarypagesize DICTIONARY_PAGE_SIZE

Mapping the FORMAT Options

The gphdfs protocol uses the FORMAT clause to determine the format of the external data. For Avro- and Parquet-format data, the PXF FORMAT clause must identify the name of a custom formatter.

Data Format gphdfs FORMAT Option pxf FORMAT Option
Avro FORMAT ‘AVRO’ FORMAT 'CUSTOM’ (FORMATTER='pxfwritable_import’)
Parquet FORMAT 'PARQUET’ FORMAT 'CUSTOM’ (FORMATTER='pxfwritable_import’) (read)
FORMAT 'CUSTOM’ (FORMATTER='pxfwritable_export’) (write)
Text FORMAT 'TEXT’ (DELIMITER ’,’) FORMAT 'TEXT’ (DELIMITER ’,’)

For text data, the FORMAT clause may identify a delimiter or other formatting option as described on the CREATE EXTERNAL TABLE command reference page.

Example gphdfs to pxf External Table Mapping for an HDFS Text File

Example gphdfs CREATE EXTERNAL TABLE command to read a text file on HDFS:

CREATE EXTERNAL TABLE ext_expenses ( 
        name text,
        date date,
        amount float4,
        category text,
        desc1 text )
   LOCATION ('gphdfs://hdfshost-1:8081/dir/filename.txt') 
   FORMAT 'TEXT' (DELIMITER ',');

Equivalent pxf CREATE EXTERNAL TABLE command, providing that the default PXF server contains the Hadoop configuration:

CREATE EXTERNAL TABLE ext_expenses_pxf ( 
        name text,
        date date,
        amount float4,
        category text,
        desc1 text )
   LOCATION ('pxf://dir/filename.txt?PROFILE=hdfs:text') 
   FORMAT 'TEXT' (DELIMITER ',');

Verifying Access with PXF

Ensure that you can read from, or write to, each pxf external table that you have created.

Removing the gphdfs External Tables

You must remove all gphdfs external tables before you can successfuly migrate a Greenplum Database 4 or 5 database to Greenplum 6.

Drop an external table as follows:

DROP EXTERNAL TABLE <schema_name>.<external_table_name>;

Revoking Privileges to the gphdfs Protocol

Before you migrate, you must revoke privileges to the gphdfs protocol from each Greenplum Database role to which you assigned the privileges.

Revoke the privilege as follows:

ALTER ROLE <role_name> NOCREATEEXTTABLE(protocol='gphdfs',type='readable');
ALTER ROLE <role_name> NOCREATEEXTTABLE(protocol='gphdfs',type='writable');