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6	HDFS

HDFS

This engine provides integration with Apache Hadoop ecosystem by allowing to manage data on HDFS via ClickHouse. This engine is similar to the File and URL engines, but provides Hadoop-specific features.

Usage

ENGINE = HDFS(URI, format)

The URI parameter is the whole file URI in HDFS. The format parameter specifies one of the available file formats. To perform SELECT queries, the format must be supported for input, and to perform INSERT queries – for output. The available formats are listed in the Formats section. The path part of URI may contain globs. In this case the table would be readonly.

Example:

1. Set up the hdfs_engine_table table:

CREATE TABLE hdfs_engine_table (name String, value UInt32) ENGINE=HDFS('hdfs://hdfs1:9000/other_storage', 'TSV')

2. Fill file:

INSERT INTO hdfs_engine_table VALUES ('one', 1), ('two', 2), ('three', 3)

3. Query the data:

SELECT * FROM hdfs_engine_table LIMIT 2

┌─name─┬─value─┐
│ one  │     1 │
│ two  │     2 │
└──────┴───────┘

Implementation Details

Reads and writes can be parallel.
Zero-copy replication is supported.
Not supported:
- ALTER and SELECT...SAMPLE operations.
- Indexes.

Globs in path

Multiple path components can have globs. For being processed file should exists and matches to the whole path pattern. Listing of files determines during SELECT (not at CREATE moment).

* — Substitutes any number of any characters except / including empty string.
? — Substitutes any single character.
{some_string,another_string,yet_another_one} — Substitutes any of strings 'some_string', 'another_string', 'yet_another_one'.
{N..M} — Substitutes any number in range from N to M including both borders.

Constructions with {} are similar to the remote table function.

Example

Suppose we have several files in TSV format with the following URIs on HDFS:

'hdfs://hdfs1:9000/some_dir/some_file_1'
'hdfs://hdfs1:9000/some_dir/some_file_2'
'hdfs://hdfs1:9000/some_dir/some_file_3'
'hdfs://hdfs1:9000/another_dir/some_file_1'
'hdfs://hdfs1:9000/another_dir/some_file_2'
'hdfs://hdfs1:9000/another_dir/some_file_3'

There are several ways to make a table consisting of all six files:

CREATE TABLE table_with_range (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/some_file_{1..3}', 'TSV')

Another way:

CREATE TABLE table_with_question_mark (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/some_file_?', 'TSV')

Table consists of all the files in both directories (all files should satisfy format and schema described in query):

CREATE TABLE table_with_asterisk (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/{some,another}_dir/*', 'TSV')

!!! warning "Warning" If the listing of files contains number ranges with leading zeros, use the construction with braces for each digit separately or use ?.

Example

Create table with files named file000, file001, … , file999:

CREATE TABLE big_table (name String, value UInt32) ENGINE = HDFS('hdfs://hdfs1:9000/big_dir/file{0..9}{0..9}{0..9}', 'CSV')

Configuration

Similar to GraphiteMergeTree, the HDFS engine supports extended configuration using the ClickHouse config file. There are two configuration keys that you can use: global (hdfs) and user-level (hdfs_*). The global configuration is applied first, and then the user-level configuration is applied (if it exists).

  <!-- Global configuration options for HDFS engine type -->
  <hdfs>
	<hadoop_kerberos_keytab>/tmp/keytab/clickhouse.keytab</hadoop_kerberos_keytab>
	<hadoop_kerberos_principal>clickuser@TEST.CLICKHOUSE.TECH</hadoop_kerberos_principal>
	<hadoop_security_authentication>kerberos</hadoop_security_authentication>
  </hdfs>

  <!-- Configuration specific for user "root" -->
  <hdfs_root>
	<hadoop_kerberos_principal>root@TEST.CLICKHOUSE.TECH</hadoop_kerberos_principal>
  </hdfs_root>

Configuration Options

Supported by libhdfs3

HDFS Configuration Reference might explain some parameters.

ClickHouse extras

Limitations

hadoop_security_kerberos_ticket_cache_path can be global only, not user specific

Kerberos support

If hadoop_security_authentication parameter has value 'kerberos', ClickHouse authentifies via Kerberos facility. Parameters here and hadoop_security_kerberos_ticket_cache_path may be of help. Note that due to libhdfs3 limitations only old-fashioned approach is supported, datanode communications are not secured by SASL (HADOOP_SECURE_DN_USER is a reliable indicator of such security approach). Use tests/integration/test_storage_kerberized_hdfs/hdfs_configs/bootstrap.sh for reference.

If hadoop_kerberos_keytab, hadoop_kerberos_principal or hadoop_kerberos_kinit_command is specified, kinit will be invoked. hadoop_kerberos_keytab and hadoop_kerberos_principal are mandatory in this case. kinit tool and krb5 configuration files are required.

Virtual Columns

_path — Path to the file.
_file — Name of the file.

See Also

Virtual columns

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