使用python UDF和CqlStorage处理Cassandra - pass列表对象

Question

我正在处理一个数据流，包括Pig中的一些聚合步骤和将步骤存储到Cassandra中。我已经能够传递相对简单的数据类型，例如integer、long或dates，但无法找到如何使用CqlStorage将某种列表、集合或元组从Pig传递到Cassandra。

我使用 Pig 0.9.2，所以我不能使用 FLATTEN 方法。

如何从Pig 0.9.2中填充带有复杂数据类型(如集合或列表)的Cassandra表？

> < li>

我根据描述创建了相应的Cassandra表:

CREATE TABLE mycassandracf (
my_id int,
date timestamp,
my_count bigint,
grouped_ids list<bigint>,
PRIMARY KEY (my_id, date)); 

以及携带准备好的声明的STORE指令：

STORE CassandraAggregate
INTO 'cql://test/mycassandracf?output_query=UPDATE+test.mycassandracf+set+my_count+%3D+%3F%2C+grouped_ids+%3D+%3F'
USING CqlStorage;

从“GROUP BY”关系中，我以cql友好的格式（例如在元组中）“生成”一个关系，我想将其存储到Cassandra中。

CassandraAggregate = FOREACH GroupedRelation
    GENERATE TOTUPLE(TOTUPLE('my_id', $0.my_id),
    TOTUPLE('date', ISOToUnix($0.createdAt))),
    TOTUPLE(COUNT($1), $1.grouped_id);

DUMP CassandraAggregate;

(((my_id,30021),(date,1357084800000)),(2,{(60128490006325819),(62726281032786005)}))
(((my_id,30165),(date,1357084800000)),(1,{(60128411174143024)}))
(((my_id,30376),(date,1357084800000)),(4,{(60128411146211875),(63645100121476995),(60128411146211875),(63645100121476995)}))

不出所料，在此关系上使用STORE指令会引发异常：

java.lang.ClassCastException： org.apache.pig.data.DefaultDataBag 不能被投射到 org.apache.pig.data.data.DataByteArray

因此，我添加了一个用python编写的UDF，以在grouped_id袋上应用一些扁平化：

@outputSchema("flat_bag:bag{}")
def flattenBag(bag):
    return tuple([long(item) for tup in bag for item in tup])

我使用tuple是因为使用python集和python列表最终会导致转换错误。

将其添加到我的管道中，我有：

CassandraAggregate = FOREACH GroupedRelation
    GENERATE TOTUPLE(TOTUPLE('my_id', $0.my_id),
    TOTUPLE('date', ISOToUnix($0.createdAt))),
    TOTUPLE(COUNT($1), py_f.flattenBag($1.grouped_id));

DUMP CassandraAggregate;

(((my_id,30021),(date,1357084800000)),(2,(60128490006325819,62726281032786005)))
(((my_id,31120),(date,1357084800000)),(1,(60128411174143024)))
(((my_id,31120),(date,1357084800000)),(1,(60128411146211875,63645100121476995,6012841114621187563645100121476995)))

对最后一个关系使用 STORE 指令会引发错误堆栈的异常：

java.io.IOException: java.io.IOException: org.apache.thrift.transport.TTransportException
at     org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.runPipeline(PigGenericMapReduce.java:465)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.processOnePackageOutput(PigGenericMapReduce.java:428)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.reduce(PigGenericMapReduce.java:408)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigGenericMapReduce$Reduce.reduce(PigGenericMapReduce.java:262)
at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:176)
at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:652)
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:420)
at org.apache.hadoop.mapred.Child$4.run(Child.java:266)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1121)
at org.apache.hadoop.mapred.Child.main(Child.java:260)
Caused by: java.io.IOException: org.apache.thrift.transport.TTransportException
at org.apache.cassandra.hadoop.cql3.CqlRecordWriter$RangeClient.run(CqlRecordWriter.java:248)
Caused by: org.apache.thrift.transport.TTransportException
at org.apache.thrift.transport.TIOStreamTransport.read(TIOStreamTransport.java:132)
at org.apache.thrift.transport.TTransport.readAll(TTransport.java:84)
at org.apache.thrift.transport.TFramedTransport.readFrame(TFramedTransport.java:129)
at org.apache.thrift.transport.TFramedTransport.read(TFramedTransport.java:101)
at org.apache.thrift.transport.TTransport.readAll(TTransport.java:84)
at org.apache.thrift.protocol.TBinaryProtocol.readAll(TBinaryProtocol.java:378)
at org.apache.thrift.protocol.TBinaryProtocol.readI32(TBinaryProtocol.java:297)
at org.apache.thrift.protocol.TBinaryProtocol.readMessageBegin(TBinaryProtocol.java:204)
at org.apache.thrift.TServiceClient.receiveBase(TServiceClient.java:69)
at org.apache.cassandra.thrift.Cassandra$Client.recv_execute_prepared_cql3_query(Cassandra.java:1724)
at org.apache.cassandra.thrift.Cassandra$Client.execute_prepared_cql3_query(Cassandra.java:1709)
at org.apache.cassandra.hadoop.cql3.CqlRecordWriter$RangeClient.run(CqlRecordWriter.java:232)

我用简单的数据类型测试了完全相同的工作流程，并且工作正常。我真正想要的是用复杂的类型（例如Pig的集合或列表）填充卡桑德拉表的方法。

非常感谢

Answer 1

经过进一步调查，我在这里找到了解决方案：

https://issues.apache.org/jira/browse/CASSANDRA-5867

基本上，CqlStorage支持复杂类型。为此，类型应该由元组中的元组来表示，将数据类型作为字符串作为第一个元素。对于list来说，这是一种方法:

# python
@outputSchema("flat_bag:bag{}")
def flattenBag(bag):
    return ('list',) + tuple([long(item) for tup in bag for item in tup])

因此，在咕噜声中：

# pig
CassandraAggregate = FOREACH GroupedRelation
    GENERATE TOTUPLE(TOTUPLE('my_id', $0.my_id),
    TOTUPLE('date', ISOToUnix($0.createdAt))),
    TOTUPLE(COUNT($1), py_f.flattenBag($1.grouped_id));

DUMP CassandraAggregate;

(((my_id,30021),(date,1357084800000)),(2,(list, 60128490006325819,62726281032786005)))
(((my_id,31120),(date,1357084800000)),(1,(list, 60128411174143024)))
(((my_id,31120),(date,1357084800000)),(1,(list, 60128411146211875,63645100121476995,6012841114621187563645100121476995)))

然后使用经典的编码准备语句将其存储到cassandra中。

希望这将有所帮助。