将数据作为 RDD 保存回 Cassandra

Question

我正在尝试从 Kafka 读取消息，处理数据，然后将数据添加到 cassandra，就好像它是一个 RDD。

我的麻烦是将数据保存回 cassandra。

from __future__ import print_function

from pyspark.streaming import StreamingContext
from pyspark.streaming.kafka import KafkaUtils
from pyspark import SparkConf, SparkContext

appName = 'Kafka_Cassandra_Test'
kafkaBrokers = '1.2.3.4:9092'
topic = 'test'
cassandraHosts = '1,2,3'
sparkMaster = 'spark://mysparkmaster:7077'


if __name__ == "__main__":
    conf = SparkConf()
    conf.set('spark.cassandra.connection.host', cassandraHosts)

    sc = SparkContext(sparkMaster, appName, conf=conf)

    ssc = StreamingContext(sc, 1)

    kvs = KafkaUtils.createDirectStream(ssc, [topic], {"metadata.broker.list": kafkaBrokers})
    lines = kvs.map(lambda x: x[1])
    counts = lines.flatMap(lambda line: line.split(" ")) \
        .map(lambda word: (word, 1)) \
        .reduceByKey(lambda a, b: a+b)
    counts.saveToCassandra('coreglead_v2', 'wordcount')

    ssc.start()
    ssc.awaitTermination()

还有错误：

[root@gasweb2 ~]# spark-submit --jars /var/spark/lib/spark-streaming-kafka-assembly_2.10-1.6.0.jar --packages datastax:spark-cassandra-connector:1.5.0-RC1-s_2.11 /var/spark/scripts/kafka_cassandra.py
Ivy Default Cache set to: /root/.ivy2/cache
The jars for the packages stored in: /root/.ivy2/jars
:: loading settings :: url = jar:file:/var/spark/lib/spark-assembly-1.6.0-hadoop2.6.0.jar!/org/apache/ivy/core/settings/ivysettings.xml
datastax#spark-cassandra-connector added as a dependency
:: resolving dependencies :: org.apache.spark#spark-submit-parent;1.0
    confs: [default]
    found datastax#spark-cassandra-connector;1.5.0-RC1-s_2.11 in spark-packages
    found org.apache.cassandra#cassandra-clientutil;2.2.2 in central
    found com.datastax.cassandra#cassandra-driver-core;3.0.0-rc1 in central
    found io.netty#netty-handler;4.0.33.Final in central
    found io.netty#netty-buffer;4.0.33.Final in central
    found io.netty#netty-common;4.0.33.Final in central
    found io.netty#netty-transport;4.0.33.Final in central
    found io.netty#netty-codec;4.0.33.Final in central
    found io.dropwizard.metrics#metrics-core;3.1.2 in central
    found org.slf4j#slf4j-api;1.7.7 in central
    found org.apache.commons#commons-lang3;3.3.2 in central
    found com.google.guava#guava;16.0.1 in central
    found org.joda#joda-convert;1.2 in central
    found joda-time#joda-time;2.3 in central
    found com.twitter#jsr166e;1.1.0 in central
    found org.scala-lang#scala-reflect;2.11.7 in central
:: resolution report :: resolve 647ms :: artifacts dl 15ms
    :: modules in use:
    com.datastax.cassandra#cassandra-driver-core;3.0.0-rc1 from central in [default]
    com.google.guava#guava;16.0.1 from central in [default]
    com.twitter#jsr166e;1.1.0 from central in [default]
    datastax#spark-cassandra-connector;1.5.0-RC1-s_2.11 from spark-packages in [default]
    io.dropwizard.metrics#metrics-core;3.1.2 from central in [default]
    io.netty#netty-buffer;4.0.33.Final from central in [default]
    io.netty#netty-codec;4.0.33.Final from central in [default]
    io.netty#netty-common;4.0.33.Final from central in [default]
    io.netty#netty-handler;4.0.33.Final from central in [default]
    io.netty#netty-transport;4.0.33.Final from central in [default]
    joda-time#joda-time;2.3 from central in [default]
    org.apache.cassandra#cassandra-clientutil;2.2.2 from central in [default]
    org.apache.commons#commons-lang3;3.3.2 from central in [default]
    org.joda#joda-convert;1.2 from central in [default]
    org.scala-lang#scala-reflect;2.11.7 from central in [default]
    org.slf4j#slf4j-api;1.7.7 from central in [default]
    ---------------------------------------------------------------------
    |                  |            modules            ||   artifacts   |
    |       conf       | number| search|dwnlded|evicted|| number|dwnlded|
    ---------------------------------------------------------------------
    |      default     |   16  |   0   |   0   |   0   ||   16  |   0   |
    ---------------------------------------------------------------------
:: retrieving :: org.apache.spark#spark-submit-parent
    confs: [default]
    0 artifacts copied, 16 already retrieved (0kB/14ms)
16/02/15 16:26:14 WARN NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
Traceback (most recent call last):
  File "/var/spark/scripts/kafka_cassandra.py", line 27, in <module>
    counts.saveToCassandra('coreglead_v2', 'wordcount')
AttributeError: 'TransformedDStream' object has no attribute 'saveToCassandra'

---

通过四处搜索，我找到了this GitHub issue，但这似乎与另一个库有关（我无法使用该库，因为我使用的是 Cassandra 3.0，并且尚不支持）。

目标是从单个消息创建聚合数据（字数仅用于测试）并将其插入到多个表中。

我几乎只使用Datastax Python Driver 并自己编写语句，但有没有更好的方法来实现这一点？

Answer 1

您正在使用 Datastax 的 Spark Cassandra 连接器，它在 RDD / DStream 级别不支持 python。仅支持数据帧。请参阅docs 了解更多信息。

我为上述连接器编写了一个包装器：PySpark Cassandra。 Datastax 的连接器功能并不完整，但有很多东西。此外，如果性能很重要，那么调查性能影响可能是值得的。

最后，Spark 附带了 python example 使用来自 hadoop mapreduce 的 CqlInput/OutputFormat。在我看来，这不是一个对开发人员非常友好的选项，但它就在那里。

Answer 2

查看您的代码并阅读您的问题描述：您似乎没有使用任何 Cassandra 连接器。 Spark 没有开箱即用的 Cassandra 支持，因为 RDD 和 DStream 数据类型没有 saveToCassandra 方法。您需要导入一个扩展 RDD 和 DStream 类型以支持 Cassandra 集成的外部 Spark-Cassandra 连接器。

这就是您收到错误消息的原因：Python 在 DStream 类型上找不到任何函数 saveToCassandra，因为当前不存在。

您需要获取 DataStax 连接器或其他连接器以使用 saveToCassandra 扩展 DStream 类型。