CODEWITHSUNDEEP
cassandra
user12331
user12331

Reputation: 518

Cassandra light weight transaction confusion

I am a bit confused here in terms of terminology for light weight transactions. I am not sure why in most of the cassandra literature it says that it works only for a single partition.

Like when I use IF NOT EXISTS, IF EXISTS, it should apply to the whole primary key not just partition key as it says in this post as well How the LWT- Light Weight Transaction is working when we use IF NOT EXIST?

However, in the book Cassandra, the Definition Guide, I see this example 

INSERT INTO reservation.reservations_by_confirmation 
(confirm_number,
hotel_id, start_date, end_date, room_number, guest_id) VALUES (
'RS2G0Z', 'NY456', '2020-06-08', '2020-06-10', 111, 1b4d86f4-ccff- 
4256-a63d-45c905df2677) IF NOT EXISTS;

This command checks to see if there is a record with the partition key, which for this table consists of the confirm_number. So let’s find out what happens when you execute this command a second time:

INSERT INTO reservation.reservations_by_confirmation 
(confirm_number,
hotel_id, start_date, end_date, room_number, guest_id) VALUES (
'RS2G0Z', 'NY456', '2020-06-08', '2020-06-10', 111, 1b4d86f4-ccff- 
4256-a63d-45c905df2677) IF NOT EXISTS;

In this case, the transaction fails, because there is already a reservation with the number “RS2G0Z,” and cqlsh helpfully echoes back a row containing a failure indication and the values you tried to enter.

enter image description here

Now my question is if I run, another query 

INSERT INTO reservation.reservations_by_confirmation 
(confirm_number,
hotel_id, start_date, end_date, room_number, guest_id) VALUES (
'RS2G0Z', 'NY466', '2020-06-08', '2020-06-10', 111, 1b4d86f4-ccff- 
4256-a63d-45c905df2677) IF NOT EXISTS;

which is a different primary key but with the same partition key, it should succeed

So when the book says 

This command checks to see if there is a record with the partition key

Isn't this a wrong statement? Please let me know if I am misinterpreting something

Upvotes: 0

Views: 180

Answers (1)

Alex Ott
Alex Ott

Reputation: 87154

It's a error in the book, although diagram shows complex primary key, the table reservation.reservations_by_confirmation has very simple primary key - confirm_number, so in this case queries work as described in the text, and it doesn't allow to insert duplicate primary key.

When you see mentioning of the partition key in context of the LWT, this usually means that coordination happens between nodes that have replica of given partition...

Upvotes: 1

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