[CASSANDRA-18988] Updating the column of a non-existent row in an Accord transaction results in Atomicity violation

• Type: Bug
• Status: Resolved
• Priority: Normal
• Resolution: Not A Problem
• Fix Version/s: 5.x
• Component/s: Accord
• Epic Link: CEP-15: General Purpose Transactions
• Bug Category: Correctness - Consistency
• Severity: Normal
• Complexity: Normal
• Discovered By: User Report
• Platform: All
• Assignee: Caleb Rackliffe
• Reporter: Luis E Fernandez

Authors

Caleb Rackliffe

• Created: 31/Oct/23 18:29
• Updated: 25/Jun/24 16:48
• Resolved: 25/Jun/24 16:48

Description

System configuration and information:

Single node Cassandra with Accord transactions enabled running on docker

Built from commit: a7cd114435704b988c81f47ef53d0bfd6441f38b

CQLSH: [cqlsh 6.2.0 | Cassandra 5.0-alpha2-SNAPSHOT | CQL spec 3.4.7 | Native protocol v5]

 

Steps to reproduce in CQLSH:

CREATE KEYSPACE accord WITH replication = {'class': 'SimpleStrategy', 'replication_factor': '1'} AND durable_writes = true;

CREATE TABLE accord.accounts (
    partition text,
    account_id int,
    balance int,
    PRIMARY KEY (partition, account_id)
) WITH CLUSTERING ORDER BY (account_id ASC);

BEGIN TRANSACTION
    INSERT INTO accord.accounts (partition, account_id, balance) VALUES ('default', 0, 100);
    INSERT INTO accord.accounts (partition, account_id, balance) VALUES ('default', 1, 100);
COMMIT TRANSACTION;

atomicity bug happens after executing the following statement:

Based on Cassandra documentation regarding the use of UPDATE statements, I expect the result of this transaction to be the insertion of a new account ({ account_id: 3, balance: 10 }). The total balance across the three (3) accounts should be maintained (200). After executing the below transaction, the total number of accounts remains at two (2) and the total balance drops to 190. Basically, it appears as if only one half of the transaction proceeds.

BEGIN TRANSACTION
    UPDATE accord.accounts
    SET balance -= 10
    WHERE
      partition = 'default'
      AND account_id = 1;
    UPDATE accord.accounts
    SET balance += 10
    WHERE
      partition = 'default'
      AND account_id = 3;
COMMIT TRANSACTION;

Bug / Error:
======================================================================

The result of performing a table read after executing the buggy transaction is:

 partition | account_id | balance
-----------+------------+---------
   default |          0 |     100
   default |          1 |      90

Note that the above transactions are not possible without a transaction block because only counter type columns can be updated with += or -= syntax in normal (non-transactional) cql statements. Using counter type columns also results in a separate, related bug: CASSANDRA-18987

This was found while testing Accord transactions with henrik.ingo and team.

Issue Links

• is part of: CASSANDRA-18107 CEP-15: Multi-Partition Transaction CQL Support (Alpha -> v1)