[CASSANDRA-15413] Missing results on reading large frozen text map

• Type: Bug
• Status: Open
• Priority: Normal
• Resolution: Unresolved
• Component/s: Local/SSTable
• Bug Category: Correctness - Recoverable Corruption / Loss
• Severity: Critical
• Complexity: Challenging
• Discovered By: User Report
• Platform: All
• Assignee: Alex Petrov
• Reporter: Tyler Codispoti

Authors

Alex Petrov

• Created: 12/Nov/19 19:56
• Updated: 25/Nov/19 11:45

Description

Cassandra version: 2.2.15

I have been running into a case where, when fetching the results from a table with a frozen<map<text, text>>, if the number of results is greater than the fetch size (default 5000), we can end up with missing data.

Side note: The table schema comes from using KairosDB, but we've isolated this issue to Cassandra itself. But it looks like this can cause problems for users of KairosDB as well.

Repro case. Tested against fresh install of Cassandra 2.2.15.

1. Create table (csqlsh)

CREATE KEYSPACE test
  WITH REPLICATION = { 
   'class' : 'SimpleStrategy', 
   'replication_factor' : 1 
  };

  CREATE TABLE test.test (
    name text,
    tags frozen<map<text, text>>,
    PRIMARY KEY (name, tags)
  ) WITH CLUSTERING ORDER BY (tags ASC);

2. Insert data (python3)

import time
from cassandra.cluster import Cluster

cluster = Cluster(['127.0.0.1'])
session = cluster.connect('test')

for i in range(0, 20000):
    session.execute(
        """
        INSERT INTO test (name, tags)  
        VALUES (%s, %s)
        """,
        ("test_name", {'id':str(i)})
    )

 

3. Flush

 

nodetools flush

 

 

4. Fetch data (python3)

import time
from cassandra.cluster import Cluster

cluster = Cluster(['127.0.0.1'], control_connection_timeout=5000)
session = cluster.connect('test')
session.default_fetch_size = 5000
session.default_timeout = 120

count = 0
rows = session.execute("select tags from test where name='test_name'")
for row in rows:
    count += 1

print(count)

Result: 10111 (expected 20000)

 

Changing the page size changes the result count. Some quick samples:

 

default_fetch_size	count
5000	10111
1000	1830
999	1840
998	1850
20000	20000
100000	20000

 

 

In short, I cannot guarantee I'll get all the results back unless the page size > number of rows.

This seems to get worse with multiple SSTables (eg nodetool flush between some of the insert batches). When using replication, the issue can get disgustingly bad - potentially giving a different result on each query.

Interesting, if we pad the values on the tag map ("id" in this repro case) so that the insertion is in lexicographical order, there is no issue. I believe the issue also does not repro if I do not call "nodetools flush" before querying.