Question

我正在使用Apache Hadoop，MapReduce和Cassandra来运行从Cassandra表读入的MapReduce作业，并输出到另一个Cassandra表。

我有几个作业输出到一个主键的表。例如，这个用于计算每种单词类型数量的表只有一个键。

    CREATE TABLE word_count(
        word text,
        count int,
        PRIMARY KEY(text)
    ) WITH COMPACT STORAGE;

关联的reduce类看起来有点像这样：

public static class ReducerToCassandra 
    extends Reducer<Text, IntWritable, ByteBuffer, List<Mutation>>
{
    public void reduce(Text word, Iterable<IntWritable> values, Context context) 
        throws IOException, InterruptedException
    {
        int sum = 0;
        for (IntWritable val : values){
            sum += val.get();
        }

        org.apache.cassandra.thrift.Column c 
                = new org.apache.cassandra.thrift.Column();
        c.setName(ByteBufferUtil.bytes("count");
        c.setValue(ByteBufferUtil.bytes(sum));
        c.setTimestamp(System.currentTimeMillis());

        Mutation mutation = new Mutation();
        mutation.setColumn_or_supercolumn(new ColumnOrSuperColumn());
        mutation.column_or_supercolumn.setColumn(c);

        ByteBuffer keyByteBuffer = ByteBufferUtil.bytes(word.toString());
        context.write(keyByteBuffer, Collections.singletonList(mutation));
    }
}

如果我想添加一个额外的列，那么我只需要为List<Mutation>已经输出的reduce添加另一个突变，但我无法确定如何输出到表在复合主键中包含新列。例如，此表与上面的表相同，但也会将单词与其发布时间一起索引。

    CREATE TABLE word_count(
        word text,
        publication_hour bigint,
        count int,
        PRIMARY KEY(word, publication_hour)
    ) WITH COMPACT STORAGE;

我尝试了一些不同的方法，例如尝试输出自定义WritableComparable（同时包含一个单词和一小时）并更新class和method签名以及{相应地{1}}配置，但是当job尝试将自定义reduce转换为ClassCastException时，WritableComparable会抛出ByteBuffer。

我尝试使用Builder构建适当的列名。

public static class ReducerToCassandra 
    //              MappedKey     MappedValue  ReducedKey  ReducedValues
    extends Reducer<WordHourPair, IntWritable, ByteBuffer, List<Mutation>>
{
    //                 MappedKey                  Values with the key wordHourPair
    public void reduce(WordHourPair wordHourPair, Iterable<IntWritable> values, 
    Context context) 
        throws IOException, InterruptedException
    {
        int sum = 0;
        for (IntWritable val : values){
        sum += val.get();
        }
        long hour = wordHourPair.getHourLong();

        org.apache.cassandra.thrift.Column c 
            = new org.apache.cassandra.thrift.Column();
        c.setName(ByteBufferUtil.bytes("count");
        c.setValue(ByteBufferUtil.bytes(sum));
        c.setTimestamp(System.currentTimeMillis());

        Mutation mutation = new Mutation();
        mutation.setColumn_or_supercolumn(new ColumnOrSuperColumn());
        mutation.column_or_supercolumn.setColumn(c);

        //New Code
        List<AbstractType<?>> keyTypes = new ArrayList<AbstractType<?>>(); 
        keyTypes.add(UTF8Type.instance);
        keyTypes.add(LongType.instance);
        CompositeType compositeKey = CompositeType.getInstance(keyTypes);

        Builder builder = new Builder(compositeKey);
        builder.add(ByteBufferUtil.bytes(word.toString());
        builder.add(ByteBufferUtil.bytes(hour));

        ByteBuffer keyByteBuffer = builder.build();
        context.write(keyByteBuffer, Collections.singletonList(mutation));
    }
}

但是会引发IOException

java.io.IOException: InvalidRequestException(why:String didn't validate.)
    at org.apache.cassandra.hadoop.ColumnFamilyRecordWriter$RangeClient.run(ColumnFamilyRecordWriter.java:204)
Caused by: InvalidRequestException(why:String didn't validate.)
    at org.apache.cassandra.thrift.Cassandra$batch_mutate_result$batch_mutate_resultStandardScheme.read(Cassandra.java:28232)
    at org.apache.cassandra.thrift.Cassandra$batch_mutate_result$batch_mutate_resultStandardScheme.read(Cassandra.java:28218)
    at org.apache.cassandra.thrift.Cassandra$batch_mutate_result.read(Cassandra.java:28152)
    at org.apache.thrift.TServiceClient.receiveBase(TServiceClient.java:78)
    at org.apache.cassandra.thrift.Cassandra$Client.recv_batch_mutate(Cassandra.java:1069)
    at org.apache.cassandra.thrift.Cassandra$Client.batch_mutate(Cassandra.java:1055)
    at org.apache.cassandra.hadoop.ColumnFamilyRecordWriter$RangeClient.run(ColumnFamilyRecordWriter.java:196)

这个问题：Cassandra CQL3 composite key not written by Hadoop reducer似乎展示了我正在寻找的那种代码，但它调用的context.write参数类型为HashMap, ByteBuffer，我不知道我是怎么做的d使context.write接受这些参数。

如何将我想要的数据（字时键，整数值）放入我的表格中？

Answer 1

答案是使用Cassandra的CQL接口，而不是Thrift API。

现在我可以通过将我的reduce类的输出键/值类声明为＆＃34; Map，List＆＃34;然后为复合键创建一个Map，其中Key可以写入具有复合键的表。（类型为字符串）是列名，Value（类型为ByteBuffer）是使用ByteBufferUtil转换为ByteBuffer的列值。

例如，要写入如此定义的表：

CREATE TABLE foo (
    customer_id uuid,
    time timestamp,
    my_value int,
    PRIMARY KEY (customer_id, time)
)

我可以写：

String customerID = "the customer's id";
long time = DateTime.now().getMillis();
int myValue = 1;

Map<String, ByteBuffer> key = new Map<String, ByteBuffer>();
key.put("customer_id",ByteBufferUtil.bytes(customerID));
key.put("time",ByteBufferUtil.bytes(time));

List<ByteBuffer> values = Collections.singletonList(ByteBufferUtil.bytes(myValue));

context.write(key, values);

使用hadoop中的复合主键插入cassandra表

1 个答案: