HBase Endpoint编程示例

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Endpoint类似于关系型数据库中的存储过程，只不过这个存储过程会运行在多个Region上。
终端是动态RPC插件的接口，它的实现代码被安装在服务器端，从而能够通过HBase RPC唤醒。客户端类库提供了非常方便的方法来调用这些动态接口，它们可以在任意时候调用一个终端，它们的实现代码会被目标Region远程执行，结果会返回到终端。用户可以结合使用这些强大的插件接口，为HBase添加全新的特性。

Endpoint的使用需要经过如下步骤（示例为计算某列的sum值）：

1.定义一个新的protocol接口，并继承CoprocessorProtocol

public interface ColumnAggregationProtocol extends CoprocessorProtocol {    public long sum(byte[] family, byte[] qualifier) throws IOException;}

2.继承抽象类BaseEndpointCoprocessor，并实现已定义的接口

public class ColumnAggregationEndpoint extends BaseEndpointCoprocessor implements ColumnAggregationProtocol {    @Override    public long sum(byte[] family, byte[] qualifier) throws IOException {        Scan scan = new Scan();        scan.addColumn(family, qualifier);        long sumResult = 0;        InternalScanner scanner = ((RegionCoprocessorEnvironment) getEnvironment()).getRegion().getScanner(scan);        try {            List<KeyValue> curVals = new ArrayList<KeyValue>();            boolean hasMore = false;            do {                curVals.clear();                hasMore = scanner.next(curVals);                KeyValue kv = curVals.get(0);                sumResult += Bytes.toLong(kv.getValue());            } while (hasMore);        } finally {            scanner.close();        }        return sumResult;    }}

3.客户端调用定义好的方法

public class EndpointTest {    private static final byte[] TABLE_NAME = Bytes.toBytes("testtable");    private static final byte[] CF = Bytes.toBytes("cf");    private static final byte[] QUALIFIER = Bytes.toBytes("counts");    private static final byte[] STRAT_KEY = Bytes.toBytes("000");    private static final byte[] END_KEY = Bytes.toBytes("999");    public static void main(String[] args) throws Throwable {        Configuration conf = HBaseConfiguration.create();        conf.set("hbase.zookeeper.quorum", "172.16.0.126");        conf.set("hbase.zookeeper.property.clientPort", "2181");        conf.set("mapred.task.timeout", "0");        HTableTnterface table = new HTable(conf, TABLE_NAME);        Map<byte[], Long> results;        results = table.coprocessorExec(ColumnAggregationProtocol.class,                START_KEY, END_KEY, new Batch.Call<ColumnAggregationProtocol, Long>() {                    @Override                    public Long call(ColumnAggregationProtocol instance) throws IOException {                        return instance.sum(CF, QUALIFIER);                    }                });        long sumResult = 0;        for (Map.Entry<byte[], Long> e : results.entrySet()) {            sumResult += e.getValue();        }        System.out.println(sumResult);    }}

4.客户端调用Endpoint的方法有三种方式：

//适用于单个region
public <T extends CoprocessorProtocol> T coprocessorProtocol(Class<T> protocol, Row row);
//适用于多个region
public <T extends CoprocessorProtocol, R> void coprocessorExec(Class<T> protocol, byte[] startkey,
byte[] endkey, Batch.Call<T, R> call, Batch.Callback<R> callback);
//适用于多个region
public <T extends CoprocessorProtocol, R> Map<byte[], R> coprocessorExec(
Class<T> protocol, byte[] startkey, byte[] endkey,
Batch.Call<T, R> call);

5.整体的Endpoint加载过程

原文：http://blog.selfup.cn/466.html

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