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我在顶级 cassandra 上使用 Titan 1.0.0。我想在 Titan/cassandra 图上使用 SparkGraphComputer 使用 OLAP 服务。我有两个问题

1）怎么做？

配置： https ://github.com/thinkaurelius/titan/blob/titan10/titan-dist/src/assembly/static/conf/hadoop-graph/read-cassandra.properties

gremlin 代码：

graph = GraphFactory.open('conf/hadoop-graph/read-cassandra.properties')
g = graph.traversal(computer(SparkGraphComputer))
g.V().count() //Here is the error

错误：

11:20:33 ERROR org.apache.spark.executor.Executor  - Exception in task 3.0 in stage 0.0 (TID 3)
java.lang.RuntimeException: error communicating via Thrift
    at org.apache.cassandra.hadoop.ColumnFamilyRecordReader$RowIterator.<init>(ColumnFamilyRecordReader.java:267)
    at org.apache.cassandra.hadoop.ColumnFamilyRecordReader$RowIterator.<init>(ColumnFamilyRecordReader.java:215)
    at org.apache.cassandra.hadoop.ColumnFamilyRecordReader$StaticRowIterator.<init>(ColumnFamilyRecordReader.java:331)
    at org.apache.cassandra.hadoop.ColumnFamilyRecordReader$StaticRowIterator.<init>(ColumnFamilyRecordReader.java:331)
    at org.apache.cassandra.hadoop.ColumnFamilyRecordReader.initialize(ColumnFamilyRecordReader.java:171)
    at com.thinkaurelius.titan.hadoop.formats.cassandra.CassandraBinaryRecordReader.initialize(CassandraBinaryRecordReader.java:39)
    at com.thinkaurelius.titan.hadoop.formats.util.GiraphRecordReader.initialize(GiraphRecordReader.java:38)
    at org.apache.spark.rdd.NewHadoopRDD$$anon$1.<init>(NewHadoopRDD.scala:135)
    at org.apache.spark.rdd.NewHadoopRDD.compute(NewHadoopRDD.scala:107)
    at org.apache.spark.rdd.NewHadoopRDD.compute(NewHadoopRDD.scala:69)
    at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:280)
    at org.apache.spark.rdd.RDD.iterator(RDD.scala:247)
    at org.apache.spark.rdd.MappedRDD.compute(MappedRDD.scala:31)
    at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:280)
    at org.apache.spark.rdd.RDD.iterator(RDD.scala:247)
    at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:68)
    at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:41)
    at org.apache.spark.scheduler.Task.run(Task.scala:56)
    at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:200)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

这是完整的跟踪： http: //pastebin.com/CiuXjFB2

2）当数据已经存储在titan/cassandra上时，为什么要转换为hadoopGraph？参考资料： https ://groups.google.com/forum/#!topic/gremlin-users/fVijONCxvSI

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