[jira] [Created] (CASSANDRA-15413) Missing results on reading large frozen text map

["Tyler Codispoti (Jira)" <ji...@apache.org>]

Tyler Codispoti created CASSANDRA-15413:
-------------------------------------------

             Summary: Missing results on reading large frozen text map
                 Key: CASSANDRA-15413
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-15413
             Project: Cassandra
          Issue Type: Bug
            Reporter: Tyler Codispoti


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)
{code:sql}
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);
{code}
2. Insert data (python3)
{code:python}
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)})
    )
{code}
 

3. Flush

 
{code:java}
nodetools flush{code}
 

 

4. Fetch data (python3)
{code:python}
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)
{code}
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.



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