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cassandratransactionscql

Cassandra CAS differerence between WHERE and IF

Using the example from the article Using lightweight transactions, the example of such a transaction is given as:

UPDATE cycling.cyclist_name
  SET firstname = 'Roxane'
  WHERE id = 4647f6d3-7bd2-4085-8d6c-1229351b5498
  IF firstname = 'Roxxane';

How is this different from replacing IF with WHERE, as in:

UPDATE cycling.cyclist_name
  SET firstname = 'Roxane'
  WHERE id = 4647f6d3-7bd2-4085-8d6c-1229351b5498 
  AND firstname = 'Roxxane';

I am guessing that the second one does not do the read and write in a single step, but rather first selects based on where and then applies the update, while the first one blocks other processes from writing to the DB when the query is running.

Solution

  • In case of concurrent update from two different client, The first query will assure that only one is applied at a time (assuming NOT using NON-SERIAL consistency) however that is not true for 2nd update. Check this.