A counter is a special column used to store an integer that is changed in increments. Counters are useful for many data models, such as:
Keeping track of the number of web page views received on a company website
Keeping track of the number of games played online or the number of players who have joined an online game.
So, let's start exploring counters. First, table creation:
CREATE TABLE WebLogs ( page_id uuid, page_name Text, insertion_time timestamp, page_count counter, PRIMARY KEY (page_id, insertion_time) );
Now, if you hit the create command, you will get the following error:
ERROR: – InvalidRequest: Error from server: code=2200 [Invalid query] message="Cannot mix counter and non counter columns in the same table"
Counter columns do not exist with non-counter columns, so every other column in the table should be a primary key/clustering keys. So, let's make our page_name column as a composite primary key with the page_id .
cqlsh:keyspace_test> CREATE TABLE WebLogs ( … page_id uuid, … page_name Text, … insertion_time timestamp, … page_count counter, … PRIMARY KEY ((page_id,page_name), insertion_time) … );
Now, let's insert some data into our table:
cqlsh:keyspace_test> insert into weblogs (page_id , page_name , insertion_time , page_count ) values(uuid(),’test.com’,dateof(now()),0) ;
And you might be astonished upon receiving another error:
ERROR: -InvalidRequest: Error from server: code=2200 [Invalid query] message=”INSERT statements are not allowed on counter tables, use UPDATE instead”
Well all right. Let's use some update statements:
cqlsh:keyspace_test> update weblogs set page_count = page_count + 1 where page_id =uuid() and page_name =’test.com’ and insertion_time =dateof(now()); cqlsh:keyspace_test> select * from weblogs; page_id | page_name | insertion_time | page_count ————————————–+———–+———————8372cee6-1d04-41f7-a70d-98fdd9036448 | test.com | 2017-01-05 05:19:31+0000 | 1
Voila! You have successfully created your counter table and performed the insert operation.
Let's do some more CURD operations, like updating and deleting on the counter table.
Updating the Data
Say one more visitor visits the test.com webpage, so we need to increment the counter column.
cqlsh:keyspace_test> update weblogs set page_count = page_count + 1 where page_id =8372cee6-1d04-41f7-a70d-98fdd9036448 and page_name =’test.com’ and insertion_time =’2017-01-05 05:19:31+0000′;
All the data in where clause should be the same.
cqlsh:keyspace_test> select * from weblogs ; page_id | page_name | insertion_time | page_count ————————————–+———–+——————- 8372cee6-1d04-41f7-a70d-98fdd9036448 | test.com | 2017-01-05 05:19:31+0000 | 2
Removing the Data
This is pretty simple, as it uses Cassandra's delete command.
cqlsh:keyspace_test> delete from weblogs where page_id =8372cee6-1d04-41f7-a70d-98fdd9036448 and page_name =’test.com’ and insertion_time =’2017-01-05 05:19:31+0000′; cqlsh:keyspace_test> select * from weblogs; page_id | page_name | insertion_time | page_count
Some Important Notes
A counter column cannot be indexed or deleted.
To load data into a counter column, or to increase or decrease the value of the counter, use the UPDATE command. Cassandra rejects USING TIMESTAMP or USING TTL when updating a counter column.
To create a table having one or more counter columns, use CREATE TABLE to define the counter and non-counter columns. Use all non-counter columns as part of the PRIMARY KEY definition.