Understanding Cassandra static field - cassandra

Question

I learn Cassandra through its documentation. Now I'm learning about batch and static fields.
In their example at the end of the page, they somehow managed to make balance have two different values (-200, -208) even though it's a static field.
Could someone explain to me how this is possible? I've read the whole page but I did not catch on.

Answer 1

In Cassandra static field is static under a partition key.
Example : Let's define a table
CREATE TABLE static_test (
pk int,
ck int,
d int,
s int static,
PRIMARY KEY (pk, ck)
);
Here pk is the partition key and ck is the clustering key.
Let's insert some data :
INSERT INTO static_test (pk , ck , d , s ) VALUES ( 1, 10, 100, 1000);
INSERT INTO static_test (pk , ck , d , s ) VALUES ( 2, 20, 200, 2000);
If we select the data
pk | ck | s | d
----+----+------+-----
1 | 10 | 1000 | 100
2 | 20 | 2000 | 200
here for partition key pk = 1 static field s value is 1000 and for partition key pk = 2 static field s value is 2000
If we insert/update static field s value of partition key pk = 1
INSERT INTO static_test (pk , ck , d , s ) VALUES ( 1, 11, 101, 1001);
Then static field s value will change for all the rows of the partition key pk = 1
pk | ck | s | d
----+----+------+-----
1 | 10 | 1001 | 100
1 | 11 | 1001 | 101
2 | 20 | 2000 | 200

Answer 2

In a table that uses clustering columns, non-clustering columns can be declared static in the table definition. Static columns are only static within a given partition.
Example:
CREATE TABLE test (
partition_column text,
static_column text STATIC,
clustering_column int,
PRIMARY KEY (partition_column , clustering_column)
);
INSERT INTO test (partition_column, static_column, clustering_column) VALUES ('key1', 'A', 0);
INSERT INTO test (partition_column, clustering_column) VALUES ('key1', 1);
SELECT * FROM test;
Results:
primary_column | clustering_column | static_column
----------------+-------------------+--------------
key1 | 0 | A
key1 | 1 | A
Observation:
Once declared static, the column inherits the value from given partition key
Now, lets insert another record
INSERT INTO test (partition_column, static_column, clustering_column) VALUES ('key1', 'C', 2);
SELECT * FROM test;
Results:
primary_column | clustering_column | static_column
----------------+-------------------+--------------
key1 | 0 | C
key1 | 1 | C
key1 | 2 | C
Observation:
If you update the static key, or insert another record with updated static column value, the value is reflected across all the columns ==> static column values are static (constant) across given partition column
Restriction (from the DataStax reference documentation below):
A table that does not define any clustering columns cannot have a static column. The table having no clustering columns has a one-row partition in which every column is inherently static.
A table defined with the COMPACT STORAGE directive cannot have a static column.
A column designated to be the partition key cannot be static.
Reference : DataStax Reference

Answer 3

In the example on the page you've linked they don't have different values at the same point in time.
They first have the static balance field set to -208 for the whole user1 partition:
user | expense_id | balance | amount | description | paid
-------+------------+---------+--------+-------------+-------
user1 | 1 | -208 | 8 | burrito | False
user1 | 2 | -208 | 200 | hotel room | False
Then they apply a batch update statement that sets the balance value to -200:
BEGIN BATCH
UPDATE purchases SET balance=-200 WHERE user='user1' IF balance=-208;
UPDATE purchases SET paid=true WHERE user='user1' AND expense_id=1 IF paid=false;
APPLY BATCH;
This updates the balance field for the whole user1 partition to -200:
user | expense_id | balance | amount | description | paid
-------+------------+---------+--------+-------------+-------
user1 | 1 | -200 | 8 | burrito | True
user1 | 2 | -200 | 200 | hotel room | False
The point of a static fields is that you can update/change its value for the whole partition at once. So if I would execute the following statement:
UPDATE purchases SET balance=42 WHERE user='user1'
I would get the following result:
user | expense_id | balance | amount | description | paid
-------+------------+---------+--------+-------------+-------
user1 | 1 | 42 | 8 | burrito | True
user1 | 2 | 42 | 200 | hotel room | False