CODEWITHSUNDEEP
hive
NJ_315
NJ_315

Reputation: 1883

What does the hive metastore and name node do in a cluster?

In a cluster having Hive installed, What does the metastore and namenode have? i understand that the Metastore has all the table schema and partition details and metadata. Now what is this metadata? then what does the namenode have? and where is this metastore present in a cluster?

Upvotes: 13

Views: 27144

Answers (4)

Arjuna Das
Arjuna Das

Reputation: 1

Above diagram is excellent one to understand Hive and hadoop communication.

Regarding Hive-Metastore (not hadoop - metastore):

  1. It is not necessary/compulsory to have metastore in your hadoop environment as it is only required if you are using HIVE on top of your HDFS cluster.

  2. Metastore is the metadata repository for HIVE only and used by HIVE to store created database object's meta information only(not actual data, which is already in HDFS because HIVE do not store data. Hive uses already stored datain File system)

  3. Hive implementation required a metastore service using any RDBMS.

Regarding Namenode (hadoop -namenode):

  1. core part of Hadoop, which behaves like metastore for cluster.

  2. Not a RDBMS . Stores file system meta info in File System only.

Upvotes: -1

Prashant_M
Prashant_M

Reputation: 3134

Metastore - Its a database which stores metadata a.k.a all the details about the tables you create in HIVE. By default, HIVE comes with and uses Derby database. But you can use any other database like MySQL or Oracle.

Use of Metastore: Whenever you fire a query from your Hive CLI, the Execution engine gathers all the details regarding the table and creates an Execution plan(Job). These details comes from Metastore. Finally the Execution engine sends the Job to Hadoop. From here, the common Hadoop Map Reduce Job is executed and the result is send back to Hive. The Name node communicates with Execution engine to successfully execute the MR Job.

enter image description here

Upvotes: 3

Jeff Taylor
Jeff Taylor

Reputation: 471

Hive data (not metadata) is spread across Hadoop HDFS DataNode servers. Typically, each block of data is stored on 3 different DataNodes. The NameNode keeps track of which DataNodes have which blocks of actual data.

For a Hive production environment, the metastore service should run in an isolated JVM. Hive processes can communicate with the metastore service using Thrift. The Hive metastore data is persisted in an ACID database such as Oracle DB or MySQL. You can use SQL to find out what is in the Hive metastore:

Here are the tables in the Hive metastore:

SQL> select table_name from user_tables;

DBS
DATABASE_PARAMS
SEQUENCE_TABLE
SERDES
TBLS
SDS
CDS
BUCKETING_COLS
TABLE_PARAMS
PARTITION_KEYS
SORT_COLS
SD_PARAMS
COLUMNS_V2
SERDE_PARAMS

You can describe the structure of each table:

SQL> describe partition_keys;

TBL_ID                       NUMBER
PKEY_COMMENT                 VARCHAR2(4000)
PKEY_NAME                    VARCHAR2(128)
PKEY_TYPE                    VARCHAR2(767)
INTEGER_IDX                  NUMBER(10)

And find the contents of each table:

SQL> select * from partition_keys;

So if in Hive you "CREATE TABLE xxx (...) PARTITIONED BY (...)" the Hive partitioning data is stored into the metastore (Oracle, MySQL...) database.

For example, in Hive if you create a table like this:

hive> create table employee_table (id bigint, name string) partitioned by (region string);

You will find this in the metastore:

SQL> select tbl_id,pkey_name from partition_keys;

TBL_ID PKEY_NAME
------ ---------
     8    region

SQL> select tbl_name from tbls where tbl_id=8;

TBL_NAME
--------
employee_table

When you insert data into employee_table, the data will be stored in HDFS on Hadoop DataNodes and the NameNode will keep track of which DataNodes have the data.

Upvotes: 14

Tariq
Tariq

Reputation: 34184

The NameNode keeps the directory tree of all files in the file system, and tracks where across the cluster the file data is kept. It also keeps track of all the DataNode(Dead+Live) through heartbeat mechanism. It also helps client for reads/writes by receiving their requests and redirecting them to the appropriate DataNode.

The metadata which metastore stores contains things like :

IDs of Database

IDs of Tables

IDs of Index

The time of creation of an Index

The time of creation of a Table

IDs of roles assigned to a particular user

InputFormat used for a Table

OutputFormat used for a Table etc etc.

Is this what you wanted to know?

And it is not mandatory to have metastore in the cluster itself. Any machine(inside or outside the cluster) having a JDBC-compliant database can be used for the metastore.

HTH

P.S : You might find the E/R diagram of metastore useful.

Upvotes: 25

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