Hcatalog是apache开源的对于表和底层数据管理统一服务平台，目前最新release版本是0.5，不过需要Hive0.10支持，由于我们hive集群版本是0.9.0，所以只能降级使用hcatalog 0.4，由于hcatalog中所有的底层数据信息都是保存在hive metastore里，所以hive版本升级后schema变动或者api变动会对hacatalog产生影响，因此在hive 0.11中已经集成了了hcatalog，以后也会成为hive的一部分，而不是独立的项目。
HCatalog底层依赖于Hive Metastore，执行过程中会创建一个HiveMetaStoreClient，通过这个instance提供的api来获取表结构数据，如果是local metastore mode的话，会直接返回一个HiveMetaStore.HMSHandler，如果是remote mode的话（hive.metastore.local设置为false），会依据hive.metastore.uris（比如thrift://10.1.8.42:9083, thrift://10.1.8.51:9083）中设定的一串uri逐一顺序建立连接。只要有一个链接建立就可以了，同时为了避免所有client都和第一个uri建立连接，导致负载过大，我加了点小trick，对这串uris随机shuffle来做load balance
由于我们的集群开启了kerberos security，需要获取DelegationToken，但是local mode是不支持的，所以只用能remote mode
HiveMetaStoreClient.Java
```
publicString getDelegationToken(String owner, String renewerKerberosPrincipalName)throws
MetaException, TException {
if(localMetaStore) {
thrownewUnsupportedOperationException("getDelegationToken() can be "+
"called only in thrift (non local) mode");
}
returnclient.get_delegation_token(owner, renewerKerberosPrincipalName);
}
```
HCatInputFormat和HCatOutputFormat提供一些mapreduce api来读取表和写入表
HCatInputFormat API:
```
publicstaticvoidsetInput(Job job,
InputJobInfo inputJobInfo)throwsIOException;
```
先实例化一个InputJobInfo对象，该对象包含三个参数dbname，tablename，filter，然后传给setInput函数，来读取相应的数据
```
publicstaticHCatSchema getTableSchema(JobContext context)
throwsIOException;
```
在运行时(比如mapper阶段的setup函数中)，可以传进去JobContext，调用静态getTableSchema来获取先前setInput时设置的table schema信息
HCatOutputFormat API:
```
publicstaticvoidsetOutput(Job job, OutputJobInfo outputJobInfo)throwsIOException;
```
OutPutJobInfo接受三个参数databaseName, tableName, partitionValues，其中第三个参数类型是Map，partition key放在map key里，partition value放在对应map key的value中，该参数可传入null或空map，如果指定的partition存在的话，会抛org.apache.hcatalog.common.HCatException : 2002 : Partition already present with given partition key values
比如要要写入指定的partition（dt='2013-06-13',country='china' ）,可以这样写
```
Map partitionValues =newHashMap();
partitionValues.put("dt","2013-06-13");
partitionValues.put("country","china");
HCatTableInfo info = HCatTableInfo.getOutputTableInfo(dbName, tblName, partitionValues);
HCatOutputFormat.setOutput(job, info);
publicstaticHCatSchema getTableSchema(JobContext context)throwsIOException;
```
获取之前HCatOutputFormat.setOutput指定的table schema信息
```
publicstaticvoidsetSchema(finalJob job,finalHCatSchema schema)throwsIOException;
```
设置最终写入数据的schema信息，若不调用这个方法，则默认会使用table schema信息
下面提供一个完整mapreduce例子计算一天每个guid访问页面次数，map阶段从表中读取guid字段，reduce阶段统计该guid对应pageview的总数，然后写回另外一张带有guid和count字段的表中
```
importjava.io.IOException;
importjava.util.Iterator;
importorg.apache.hadoop.conf.Configuration;
importorg.apache.hadoop.conf.Configured;
importorg.apache.hadoop.io.IntWritable;
importorg.apache.hadoop.io.Text;
importorg.apache.hadoop.io.WritableComparable;
importorg.apache.hadoop.mapreduce.Job;
importorg.apache.hadoop.mapreduce.Mapper;
importorg.apache.hadoop.mapreduce.Reducer;
importorg.apache.hadoop.util.Tool;
importorg.apache.hadoop.util.ToolRunner;
importorg.apache.hcatalog.data.DefaultHCatRecord;
importorg.apache.hcatalog.data.HCatRecord;
importorg.apache.hcatalog.data.schema.HCatSchema;
importorg.apache.hcatalog.mapreduce.HCatInputFormat;
importorg.apache.hcatalog.mapreduce.HCatOutputFormat;
importorg.apache.hcatalog.mapreduce.InputJobInfo;
importorg.apache.hcatalog.mapreduce.OutputJobInfo;
publicclassGroupByGuidextendsConfiguredimplementsTool {
@SuppressWarnings("rawtypes")
publicstaticclassMapextends
Mapper {
HCatSchema schema;
Text guid;
IntWritable one;
@Override
protectedvoidsetup(org.apache.hadoop.mapreduce.Mapper.Context context)
throwsIOException, InterruptedException {
guid =newText();
one =newIntWritable(1);
schema = HCatInputFormat.getTableSchema(context);
}
@Override
protectedvoidmap(WritableComparable key, HCatRecord value,
Context context)throwsIOException, InterruptedException {
guid.set(value.getString("guid", schema));
context.write(guid, one);
}
}
@SuppressWarnings("rawtypes")
publicstaticclassReduceextends
Reducer {
HCatSchema schema;
@Override
protectedvoidsetup(org.apache.hadoop.mapreduce.Reducer.Context context)
throwsIOException, InterruptedException {
schema = HCatOutputFormat.getTableSchema(context);
}
@Override
protectedvoidreduce(Text key, Iterable values,
Context context)throwsIOException, InterruptedException {
intsum =0;
Iterator iter = values.iterator();
while(iter.hasNext()) {
sum++;
iter.next();
}
HCatRecord record =newDefaultHCatRecord(2);
record.setString("guid", schema, key.toString());
record.setInteger("count", schema, sum);
context.write(null, record);
}
}
@Override
publicintrun(String[] args)throwsException {
Configuration conf = getConf();
String dbname = args[0];
String inputTable = args[1];
String filter = args[2];
String outputTable = args[3];
intreduceNum = Integer.parseInt(args[4]);
Job job =newJob(conf,
"GroupByGuid, Calculating every guid's pageview");
HCatInputFormat.setInput(job,
InputJobInfo.create(dbname, inputTable, filter));
job.setJarByClass(GroupByGuid.class);
job.setInputFormatClass(HCatInputFormat.class);
job.setMapperClass(Map.class);
job.setReducerClass(Reduce.class);
job.setMapOutputKeyClass(Text.class);
job.setMapOutputValueClass(IntWritable.class);
job.setOutputKeyClass(WritableComparable.class);
job.setOutputValueClass(DefaultHCatRecord.class);
job.setNumReduceTasks(reduceNum);
HCatOutputFormat.setOutput(job,
OutputJobInfo.create(dbname, outputTable,null));
HCatSchema s = HCatOutputFormat.getTableSchema(job);
HCatOutputFormat.setSchema(job, s);
job.setOutputFormatClass(HCatOutputFormat.class);
return(job.waitForCompletion(true) ?0:1);
}
publicstaticvoidmain(String[] args)throwsException {
intexitCode = ToolRunner.run(newGroupByGuid(), args);
System.exit(exitCode);
}
}
```
其实hcatalog还支持动态分区dynamic partition，我们可以在OutJobInfo中指定部分partition keyvalue pair，在运行时候根据传进来的值设置HCatRecord对应的其他partition keyvalue pair，这样就能在一个job中同时写多个partition了
本文转自 http://blog.csdn.net/lalaguozhe/article/details/9083905
作者：yukangkk
