Good use of BATCH statement

How to use a BATCH statement.

Batch operations can be beneficial. See these examples below to see a good use of BATCH. The examples below use the following table cyclist_expenses:

cqlsh> CREATE TABLE cycling.cyclist_expenses ( 
  cyclist_name text, 
  balance float STATIC, 
  expense_id int, 
  amount float, 
  description text, 
  paid boolean, 
  PRIMARY KEY (cyclist_name, expense_id) 
);

Note that balance is STATIC.

Note: If there are two different tables in the same keyspace and the two tables have the same partition key, this scenario is considered a single partition batch. There will be a single mutation for each table. This happens because the two tables could have different columns, even though the keyspace and partition are the same. Batches allow a caller to bundle multiple operations into a single batch request. All the operations are performed by the same coordinator. The best use of a batch request is for a single partition in multiple tables in the same keyspace. Also, batches provide a guarantee that mutations will be applied in a particular order.

Procedure

Single partition batch

The first INSERT in the BATCH statement sets the balance to zero. The next two statements insert an expense_id and change the balance value. All the INSERT and UPDATE statements in this batch write to the same partition, keeping the latency of the write operation low.
```
cqlsh> BEGIN BATCH
  INSERT INTO cycling.cyclist_expenses (cyclist_name, balance) VALUES ('Vera ADRIAN', 0) IF NOT EXISTS;
  INSERT INTO cycling.cyclist_expenses (cyclist_name, expense_id, amount, description, paid) VALUES ('Vera ADRIAN', 1, 7.95, 'Breakfast', false);
  APPLY BATCH;
```
This batching example includes conditional updates combined with using static columns. Recall that single partition batches are not logged.
Note: It would be reasonable to expect that an UPDATE to the balance could be included in this BATCH statement:
```
cqlsh> UPDATE cycling.cyclist_expenses SET balance = -7.95 WHERE cyclist_name = 'Vera ADRIAN' IF balance = 0;
```
However, it is important to understand that all the statements processed in a BATCH statement timestamp the records with the same value. The operations may not perform in the order listed in the BATCH statement. The UPDATE might be processed BEFORE the first INSERT that sets the balance value to zero, allowing the conditional to be met.
An acknowledgement of a batch statement is returned if the batch operation is successful.

The resulting table will only have one record so far.
The balance can be adjusted separately with an UPDATE statement. Now the balance will reflect that breakfast was unpaid.
```
cqlsh> UPDATE cycling.cyclist_expenses SET balance = -7.95 WHERE cyclist_name = 'Vera ADRIAN' IF balance = 0;
```
The table cyclist_expenses stores records about each purchase by a cyclist and includes the running balance of all the cyclist's purchases. Because the balance is static, all purchase records for a cyclist have the same running balance. This BATCH statement inserts expenses for two more meals changes the balance to reflect that breakfast and dinner were unpaid.
```
cqlsh> BEGIN BATCH
INSERT INTO cycling.cyclist_expenses (cyclist_name, expense_id, amount, description, paid) VALUES ('Vera ADRIAN', 2, 13.44, 'Lunch', true);
INSERT INTO cycling.cyclist_expenses (cyclist_name, expense_id, amount, description, paid) VALUES ('Vera ADRIAN', 3, 25.00, 'Dinner', false);
UPDATE cycling.cyclist_expenses SET balance = -32.95 WHERE cyclist_name = 'Vera ADRIAN' IF balance = -7.95;
APPLY BATCH;
```
Finally, the cyclist pays off all outstanding bills and the balance of the account goes to zero.
```
cqlsh> BEGIN BATCH
UPDATE cycling.cyclist_expenses SET balance = 0 WHERE cyclist_name = 'Vera ADRIAN' IF balance = -32.95;
UPDATE cycling.cyclist_expenses SET paid = true WHERE cyclist_name = 'Vera ADRIAN' AND expense_id = 1 IF paid = false;
UPDATE cycling.cyclist_expenses SET paid = true WHERE cyclist_name = 'Vera ADRIAN' AND expense_id = 3 IF paid = false;
APPLY BATCH;
```
Because the column is static, you can provide only the partition key when updating the data. To update a non-static column, you would also have to provide a clustering key. Using batched conditional updates, you can maintain a running balance. If the balance were stored in a separate table, maintaining a running balance would not be possible because a batch having conditional updates cannot span multiple partitions.

Multiple partition logged batch

A classic example for using BATCH for a multiple partition insert involves writing the same data to two related tables:
```
cqlsh> BEGIN LOGGED BATCH
INSERT INTO cycling.cyclist_names (cyclist_name, race_id) VALUES ('Vera ADRIAN', 100);
INSERT INTO cycling.cyclist_by_id (race_id, cyclist_name) VALUES (100, 'Vera ADRIAN');
APPLY BATCH;
```
Here, it is important that the same data is written to both tables to keep them in synchronization. Another common use for this batch operation is updating usernames and passwords.