python cql驱动程序 - cassandra.ReadTimeout - "操作超时 - 只收到1个响应."

Question

我正在使用Cassandra 2.0和python CQL.

我创建了一个列系列如下:

CREATE KEYSPACE IF NOT EXISTS Identification
  WITH REPLICATION = { 'class' : 'NetworkTopologyStrategy',
  'DC1' : 1 };

USE Identification;

CREATE TABLE IF NOT EXISTS entitylookup (
  name varchar,
  value varchar,
  entity_id uuid,
  PRIMARY KEY ((name, value), entity_id))
WITH
    caching=all
;

然后,我尝试计算此CF中的记录数,如下所示:

#!/usr/bin/env python
import argparse
import sys
import traceback
from cassandra import ConsistencyLevel
from cassandra.cluster import Cluster
from cassandra.query import SimpleStatement

def count(host, cf):    
    keyspace = "identification"
    cluster = Cluster([host], port=9042, control_connection_timeout=600000000)
    session = cluster.connect(keyspace)
    session.default_timeout=600000000

    st = SimpleStatement("SELECT count(*) FROM %s" % cf, consistency_level=ConsistencyLevel.ALL)
    for row in session.execute(st, timeout=600000000):
        print "count for cf %s = %s " % (cf, str(row))
    dump_pool.close()
    dump_pool.join()

if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("-cf", "--column-family", default="entitylookup", help="Column Family to query")
    parser.add_argument("-H", "--host", default="localhost", help="Cassandra host")    
    args = parser.parse_args()

    count(args.host, args.column_family)

    print "fim"

计数对我来说没有用,它只是一个需要很长时间才能完成的操作的测试.

虽然我已将超时定义为600000000秒,但在不到30秒后,我收到以下错误:

./count_entity_lookup.py  -H localhost -cf entitylookup 
    Traceback (most recent call last):
      File "./count_entity_lookup.py", line 27, in <module>
        count(args.host, args.column_family)
      File "./count_entity_lookup.py", line 16, in count
        for row in session.execute(st, timeout=None):
      File "/home/mvalle/pyenv0/local/lib/python2.7/site-packages/cassandra/cluster.py", line 1026, in execute
        result = future.result(timeout)
      File "/home/mvalle/pyenv0/local/lib/python2.7/site-packages/cassandra/cluster.py", line 2300, in result
        raise self._final_exception
    cassandra.ReadTimeout: code=1200 [Timeout during read request] message="Operation timed out - received only 1 responses." info={'received_responses': 1, 'data_retrieved': True, 'required_responses': 2, 'consistency': 5}

似乎答案只是在复制品中找到,但这对我来说真的没有意义.不管怎么说cassandra都不能查询它？

在下图中,可以看到对集群的请求数量非常低,延迟也很低.我不知道为什么会这样.

Answer 1

从回复来看：

received_responses': 1, 'data_retrieved': True, 'required_responses': 2

数据仅在一个节点上可用，而查询要求一致性==全部。Cassandra 无法满足该请求并超时。

如果需要所有节点都有数据，可以将写入一致性更改为“ALL”。

这将确保所有读取请求都可以在没有一致性==ALL的情况下得到满足，因为这将由写入请求本身来满足，尽管如果节点离线，写入可能会失败。

请参阅文档以了解每个一致性级别含义的解释。

LOCAL_QUORUM用于确保在 DC 内联系复制因子方面的大多数节点。