Apache Flink：如何从 Cassandra 读取 DataStream/DataSet？答案

【问题标题】：Apache Flink: How can I read a DataStream/DataSet from Cassandra?Apache Flink：如何从 Cassandra 读取 DataStream/DataSet？
【发布时间】：2018-09-12 11:54:19
【问题描述】：

我尝试将 Cassandra 视为 Flink 中的数据源，以下链接提供了信息：

我在运行任务时遇到了AsyncWaitOperator 异常。根据第一个链接，由于网络问题而发生此异常。然而，奇怪的是，我在本地 VM 上运行 Cassandra，目标表中只有 10 行数据。

@Jicaar 在第一个链接中还提到从RichAsyncFunction 切换到RichMapFunction 可以避免AsyncWaitOperator 异常，有类似经验的人可以在RichMapFunction 中分享如何操作吗？

AsyncWaitOperator 异常跟踪 -->

02:21:00.164 [AsyncIO-Emitter-Thread (Source: Custom Source -> async wait operator -> (Flat Map, Sink: Unnamed) (1/1))] INFO  org.apache.flink.runtime.taskmanager.Task  - Source: Custom Source -> async wait operator -> (Flat Map, Sink: Unnamed) (1/1) (2809cef511194e612b2cc65510f78c64) switched from RUNNING to FAILED.
java.lang.Exception: An async function call terminated with an exception. Failing the AsyncWaitOperator.
  at org.apache.flink.streaming.api.operators.async.Emitter.output(Emitter.java:137) [flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.api.operators.async.Emitter.run(Emitter.java:85) [flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at java.lang.Thread.run(Thread.java:745) [na:1.8.0_60]
Caused by: org.apache.flink.streaming.runtime.tasks.ExceptionInChainedOperatorException: Could not forward element to next operator
  at org.apache.flink.streaming.runtime.tasks.OperatorChain$CopyingChainingOutput.pushToOperator(OperatorChain.java:566) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.runtime.tasks.OperatorChain$CopyingChainingOutput.collect(OperatorChain.java:524) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.runtime.tasks.OperatorChain$CopyingChainingOutput.collect(OperatorChain.java:504) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.runtime.tasks.OperatorChain$BroadcastingOutputCollector.collect(OperatorChain.java:611) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.runtime.tasks.OperatorChain$BroadcastingOutputCollector.collect(OperatorChain.java:572) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.api.operators.AbstractStreamOperator$CountingOutput.collect(AbstractStreamOperator.java:830) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.api.operators.AbstractStreamOperator$CountingOutput.collect(AbstractStreamOperator.java:808) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.api.operators.TimestampedCollector.collect(TimestampedCollector.java:51) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.api.operators.async.Emitter.output(Emitter.java:133) [flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  ... 2 common frames omitted
Caused by: com.esotericsoftware.kryo.KryoException: java.util.ConcurrentModificationException
Serialization trace:
classes (sun.misc.Launcher$AppClassLoader)
classloader (java.security.ProtectionDomain)
context (java.security.AccessControlContext)
acc (org.apache.flink.runtime.execution.librarycache.FlinkUserCodeClassLoaders$ChildFirstClassLoader)
contextClassLoader (java.lang.Thread)
threads (java.lang.ThreadGroup)
groups (java.lang.ThreadGroup)
threadGroup (io.netty.util.concurrent.DefaultThreadFactory)
val$backingThreadFactory (com.google.common.util.concurrent.ThreadFactoryBuilder$1)
threadFactory (java.util.concurrent.ThreadPoolExecutor)
delegate (com.google.common.util.concurrent.MoreExecutors$ListeningDecorator)
blockingExecutor (com.datastax.driver.core.Cluster$Manager)
manager (com.datastax.driver.core.Host)
triedHosts (com.datastax.driver.core.ExecutionInfo)
info (com.datastax.driver.core.ArrayBackedResultSet$SinglePage)
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:82) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:599) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:348) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:289) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObjectOrNull(Kryo.java:577) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:68) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:599) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:348) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:289) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:599) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:348) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.DefaultArraySerializers$ObjectArraySerializer.write(DefaultArraySerializers.java:289) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:599) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.CollectionSerializer.write(CollectionSerializer.java:82) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.CollectionSerializer.write(CollectionSerializer.java:22) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:505) ~[kryo-2.24.0.jar:na]
  at org.apache.flink.api.java.typeutils.runtime.kryo.KryoSerializer.copy(KryoSerializer.java:182) ~[flink-core-1.4.2.jar:1.4.2]
  at org.apache.flink.streaming.runtime.tasks.OperatorChain$CopyingChainingOutput.pushToOperator(OperatorChain.java:547) ~[flink-streaming-java_2.11-1.4.2.jar:1.4.2]
  ... 10 common frames omitted
Caused by: java.util.ConcurrentModificationException: null
  at java.util.Vector$Itr.checkForComodification(Vector.java:1184) ~[na:1.8.0_60]
  at java.util.Vector$Itr.next(Vector.java:1137) ~[na:1.8.0_60]
  at com.esotericsoftware.kryo.serializers.CollectionSerializer.write(CollectionSerializer.java:74) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.CollectionSerializer.write(CollectionSerializer.java:22) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523) ~[kryo-2.24.0.jar:na]
  at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61) ~[kryo-2.24.0.jar:na]
  ... 68 common frames omitted

【问题讨论】：

标签： cassandra apache-flink

【解决方案1】：

下面的代码应该适用于从 Cassandra 读取以在 Flink 中进行批处理。

final ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();

ClusterBuilder clusterBuilder = new ClusterBuilder() {

        @Override
        public Cluster buildCluster(Cluster.Builder builder) {

            return builder.addContactPoint(<cassandraHost>))
                  .withPort(9042)
                  .withCredentials(<cassandraUserName>,<cassandraPassword>)
                  .build();
        } 
    };  

DataSet<Tuple3<String,String,String>> inputRecords = env
            .createInput    
            (new CassandraInputFormat<Tuple3<String,String,String>>(<select query>,clusterBuilder)          
            ,TupleTypeInfo.of(new TypeHint<Tuple3<String,String,String>>() {}));

DataSet 的数据类型（示例中由三个字符串组成的 Tuple3）将根据您的选择查询返回的字段类型和数量而有所不同。

【讨论】：

我同意这个答案。 @JamesYu，使用 Cassandra 作为源是您想要使用 DataSet 功能的东西。 Cassandra 是一个表，而不是像 kafka 这样的流式源。 AsyncWaitOperator 用于根据从流中接收的数据查询数据表。因此不能用作创建数据源的一种方式。