HCatalog包含用于并行输入和输出的数据传输API，而不使用MapReduce.此API使用表和行的基本存储抽象来从Hadoop集群读取数据并将数据写入其中.
数据传输API主要包含三个类;那些是 :
HCatReader : 从Hadoop集群读取数据.
HCatWriter : 将数据写入Hadoop集群.
DataTransferFactory : 生成读取器和编写器实例.
此API适用于主从节点设置.让我们更多地讨论 HCatReader 和 HCatWriter .
HCatReader是一个摘要HCatalog内部的类，抽象出要从中检索记录的底层系统的复杂性.
| Sr.No. | 方法名称&描述 |
|---|---|
| 1 | Public ReaderContext prepareRead( )throws HCatException 这应该在主节点调用以获取ReaderContext，然后应该序列化并发送从属节点. |
| 2 | Public abstract Iterator <HCatRecorder> read() throws HCaException 这应该在从属节点处调用以读取HCatRecords. |
| 3 | Public Configuration getConf() 它将返回配置类对象. |
HCatReader类用于从HDFS读取数据.读取是一个两步过程，其中第一步发生在外部系统的主节点上.第二步是在多个从节点上并行执行.
读取在 ReadEntity 上完成.在开始阅读之前，您需要定义一个ReadEntity来读取.这可以通过 ReadEntity.Builder 来完成.您可以指定数据库名称，表名称，分区和筛选字符串.例如 :
ReadEntity.Builder builder = new ReadEntity.Builder();
ReadEntity entity = builder.withDatabase("mydb").withTable("mytbl").build(); 10.上面的代码片段定义了一个ReadEntity对象("entity")，它包含一个名为 mytbl 的表，名为 mydb ，可用于读取此表的所有行.请注意，此表必须在此操作开始之前存在于HCatalog中.
在定义ReadEntity之后，您将使用ReadEntity和集群配置 : 获取HCatReader的实例;
HCatReader reader = DataTransferFactory.getHCatReader(entity，config);
下一步是从读者那里获得一个ReaderContext，如下所示;
ReaderContext cntxt = reader.prepareRead();
此抽象是HCatalog内部的.这是为了便于从外部系统写入HCatalog.不要试图直接实例化它.相反，使用DataTransferFactory.
| Sr.No. | 方法名称&描述 |
|---|---|
| 1 | Public abstract WriterContext prepareRead() throws HCatException 外部系统应该从主节点调用此方法一次.它返回 WriterContext .这应序列化并发送到从节点以构建 HCatWriter . |
| 2 | Public abstract void write(Iterator<HCatRecord> recordItr) throws HCaException 此方法应该在从节点处用于执行写入. recordItr是一个迭代器对象，它包含要写入HCatalog的记录集合. |
| 3 | Public abstract void abort(WriterContext cntxt) throws HCatException 应该在主节点调用此方法.这种方法的主要目的是在发生故障时进行清理. |
| 4 | public abstract void commit(WriterContext cntxt) throws HCatException 应该在主节点调用此方法.此方法的目的是进行元数据提交. |
类似于阅读，写作也是两步过程，其中第一步发生在主节点上.随后，第二步在从节点上并行发生.
写入是在 WriteEntity 上完成的，它可以类似于读取和减去的方式构建;
WriteEntity.Builder builder = new WriteEntity.Builder();
WriteEntity entity = builder.withDatabase("mydb").withTable("mytbl").build();上面的代码创建了一个WriteEntity对象 entity ，可用于写入名为
创建WriteEntity后，下一步是获取WriterContext :
HCatWriter writer = DataTransferFactory.getHCatWriter(entity, config); WriterContext info = writer.prepareWrite();
以上所有步骤都在主节点上进行.然后主节点序列化WriterContext对象并使其可供所有从属使用.
在从属节点上，您需要使用WriterContext获取HCatWriter，如下所示 :
HCatWriter writer = DataTransferFactory.getHCatWriter(context);
然后，编写器将迭代器作为写入 method :
writer.write(hCatRecordItr);
writer 然后在循环中对此迭代器调用 getNext()并写出所有附加到迭代器的记录.
TestReaderWriter.java 文件用于测试HCatreader和HCatWriter类.以下程序演示了如何使用HCatReader和HCatWriter API从源文件中读取数据，然后将其写入目标文件.
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hive.metastore.api.MetaException;
import org.apache.hadoop.hive.ql.CommandNeedRetryException;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hive.HCatalog.common.HCatException;
import org.apache.hive.HCatalog.data.transfer.DataTransferFactory;
import org.apache.hive.HCatalog.data.transfer.HCatReader;
import org.apache.hive.HCatalog.data.transfer.HCatWriter;
import org.apache.hive.HCatalog.data.transfer.ReadEntity;
import org.apache.hive.HCatalog.data.transfer.ReaderContext;
import org.apache.hive.HCatalog.data.transfer.WriteEntity;
import org.apache.hive.HCatalog.data.transfer.WriterContext;
import org.apache.hive.HCatalog.mapreduce.HCatBaseTest;
import org.junit.Assert;
import org.junit.Test;
public class TestReaderWriter extends HCatBaseTest {
@Test
public void test() throws MetaException, CommandNeedRetryException,
IOException, ClassNotFoundException {
driver.run("drop table mytbl");
driver.run("create table mytbl (a string, b int)");
Iterator<Entry<String, String>> itr = hiveConf.iterator();
Map<String, String> map = new HashMap<String, String>();
while (itr.hasNext()) {
Entry<String, String> kv = itr.next();
map.put(kv.getKey(), kv.getValue());
}
WriterContext cntxt = runsInMaster(map);
File writeCntxtFile = File.createTempFile("hcat-write", "temp");
writeCntxtFile.deleteOnExit();
// Serialize context.
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(writeCntxtFile));
oos.writeObject(cntxt);
oos.flush();
oos.close();
// Now, deserialize it.
ObjectInputStream ois = new ObjectInputStream(new FileInputStream(writeCntxtFile));
cntxt = (WriterContext) ois.readObject();
ois.close();
runsInSlave(cntxt);
commit(map, true, cntxt);
ReaderContext readCntxt = runsInMaster(map, false);
File readCntxtFile = File.createTempFile("hcat-read", "temp");
readCntxtFile.deleteOnExit();
oos = new ObjectOutputStream(new FileOutputStream(readCntxtFile));
oos.writeObject(readCntxt);
oos.flush();
oos.close();
ois = new ObjectInputStream(new FileInputStream(readCntxtFile));
readCntxt = (ReaderContext) ois.readObject();
ois.close();
for (int i = 0; i < readCntxt.numSplits(); i++) {
runsInSlave(readCntxt, i);
}
}
private WriterContext runsInMaster(Map<String, String> config) throws HCatException {
WriteEntity.Builder builder = new WriteEntity.Builder();
WriteEntity entity = builder.withTable("mytbl").build();
HCatWriter writer = DataTransferFactory.getHCatWriter(entity, config);
WriterContext info = writer.prepareWrite();
return info;
}
private ReaderContext runsInMaster(Map<String, String> config,
boolean bogus) throws HCatException {
ReadEntity entity = new ReadEntity.Builder().withTable("mytbl").build();
HCatReader reader = DataTransferFactory.getHCatReader(entity, config);
ReaderContext cntxt = reader.prepareRead();
return cntxt;
}
private void runsInSlave(ReaderContext cntxt, int slaveNum) throws HCatException {
HCatReader reader = DataTransferFactory.getHCatReader(cntxt, slaveNum);
Iterator<HCatRecord> itr = reader.read();
int i = 1;
while (itr.hasNext()) {
HCatRecord read = itr.next();
HCatRecord written = getRecord(i++);
// Argh, HCatRecord doesnt implement equals()
Assert.assertTrue("Read: " + read.get(0) + "Written: " + written.get(0),
written.get(0).equals(read.get(0)));
Assert.assertTrue("Read: " + read.get(1) + "Written: " + written.get(1),
written.get(1).equals(read.get(1)));
Assert.assertEquals(2, read.size());
}
//Assert.assertFalse(itr.hasNext());
}
private void runsInSlave(WriterContext context) throws HCatException {
HCatWriter writer = DataTransferFactory.getHCatWriter(context);
writer.write(new HCatRecordItr());
}
private void commit(Map<String, String> config, boolean status,
WriterContext context) throws IOException {
WriteEntity.Builder builder = new WriteEntity.Builder();
WriteEntity entity = builder.withTable("mytbl").build();
HCatWriter writer = DataTransferFactory.getHCatWriter(entity, config);
if (status) {
writer.commit(context);
} else {
writer.abort(context);
}
}
private static HCatRecord getRecord(int i) {
List<Object> list = new ArrayList<Object>(2);
list.add("Row #: " + i);
list.add(i);
return new DefaultHCatRecord(list);
}
private static class HCatRecordItr implements Iterator<HCatRecord> {
int i = 0;
@Override
public boolean hasNext() {
return i++ < 100 ? true : false;
}
@Override
public HCatRecord next() {
return getRecord(i);
}
@Override
public void remove() {
throw new RuntimeException();
}
}
}上述程序以记录的形式从HDFS读取数据并写入将数据记录到 mytable
中
