Data purging is key to ensuring that your database servers always have enough free space to store the ever-incoming humongous business data. The operational/transactional database needs to be rid of old data that is no longer necessary as per your business rules.

Cassandra being one of the most popular databases, it does offer a really simplistic approach to support your data purge needs — that is, an expiration time for each of the records that go inside. The technique is known as TTL (time to live).

After the expiration time has reached or time to live has ended, the record is automatically marked for deletion by what is known as putting a tombstone on that record.

There are a set of CQLSH commands to utilize this feature:

• Use the INSERT command to set employee details in the mytable table to expire in 86,400 seconds (one day).

  cqlsh> INSERT INTO mykeyspace.mytable (emp_id, emp_name) VALUES (200, 'Franc') USING TTL 86400;

• Extend the expiration period to three days by using the UPDATE command and change the employee's name.

  cqlsh> UPDATE mykeyspace USING TTL 259200 
    SET emp_name = 'Frank' 
    WHERE emp_id = 200 ;

• Delete a column’s existing TTL by setting its value to zero:

  cqlsh> UPDATE mykeyspace.mytable USING TTL 0 
    SET emp_name = 'somefancyname' 
    WHERE emp_id = 200 ;

• Check how much time is left for a record before it gets deleted:

  cqlsh> SELECT emp_id,TTL(emp_name)
  FROM mykeyspace.mytable;

Using TTL With Cassandra Sink Connectors

The best feature of this comes into picture when you're using a sink connector to insert data into Cassandra from a Kafka topic on-the-fly. You can use the KCQL to specify a TTL right there to ensure your records are inserted with a default expiration time.

"connect.cassandra.kcql": "INSERT INTO mytable SELECT * FROM my-topic TTL=31536000"

The value 31536000 is actually equivalent to a year in seconds.