DataStax Enterprise 4.0

Example: Save Pig relations from/to Cassandra

For Pig to access data in Cassandra, the target keyspace and table must already exist. Pig can save data from a Pig relation to a table in Cassandra and from a Cassandra table to a pig relation, but it cannot create the table. This example shows how to merge the data from two CQL tables having simple primary keys using Pig.



A subsequent example shows how to merge data from CQL tables having compound primary keys into one CQL table using Pig.

Procedure

  1. Start cqlsh.
  2. Using cqlsh, create and use a keyspace named, for example, cql3ks.
    cqlsh> CREATE KEYSPACE cql3ks WITH replication =
             {'class': 'SimpleStrategy', 'replication_factor': 1 };
    
    cqlsh> USE cql3ks;
  3. Create a two-column (a and b) Cassandra table named simple_table1 and another two-column (x and y) table named simple_table2. Insert data into the tables.
    cqlsh:cql3ks> CREATE TABLE test (a int PRIMARY KEY, b int);
    cqlsh:cql3ks> CREATE TABLE moredata (x int PRIMARY KEY, y int);
    cqlsh:cql3ks> INSERT INTO test (a,b) VALUES (1,1);
    cqlsh:cql3ks> INSERT INTO test (a,b) VALUES (2,2);
    cqlsh:cql3ks> INSERT INTO test (a,b) VALUES (3,3);
    cqlsh:cql3ks> INSERT INTO moredata (x, y) VALUES (4,4);
    cqlsh:cql3ks> INSERT INTO moredata (x, y) VALUES (5,5);
    cqlsh:cql3ks> INSERT INTO moredata (x, y) VALUES (6,6);
  4. Using Pig, add logic to load the data (4, 5, 6) from the Cassandra simple_table2 table into a Pig relation.
    grunt> moretestvalues= LOAD 'cql://cql3ks/simple_table2/' USING CqlStorage;
  5. Convert the simple_table2 table data to a tuple. The key column is a chararray, 'a'.
    grunt> insertformat= FOREACH moretestvalues GENERATE
             TOTUPLE(TOTUPLE('a',x)),TOTUPLE(y);
  6. Save the relation to the Cassandra simple_table1 table.
    grunt> STORE insertformat INTO
           'cql://cql3ks/simple_table1?output_query=UPDATE+cql3ks.simple_table1+set+b+%3D+%3F'
           USING CqlStorage;

    Pig uses a URL-encoded prepared statement to store the relation to Cassandra. The cql:// URL is followed by an output_query, which specifies which key should be used in the command. The rest of the arguments, the "?"s, for the prepared statement are filled in by the values related to that key in Pig.

  7. On the cqlsh command line, check that the simple_table1 table now contains its original values plus the values from the simple_table2 table:
    cqlsh:cql3ks> SELECT * FROM simple_table1;
    
    a | b
    --+--
    5 | 5
    1 | 1
    2 | 2
    4 | 4
    6 | 6
    3 | 3
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