Package org.apache.hadoop.hbase.client.coprocessor

Provides client classes for invoking Coprocessor RPC protocols

See: Description

Package org.apache.hadoop.hbase.client.coprocessor Description

Provides client classes for invoking Coprocessor RPC protocols

Overview

The coprocessor framework provides a way for custom code to run in place on the HBase region servers with each of a table's regions. These client classes enable applications to communicate with coprocessor instances via custom RPC protocols.

In order to provide a custom RPC protocol to clients, a coprocessor implementation must:

Clients may then call the defined service methods on coprocessor instances via the Table.coprocessorService(byte[]), Table.coprocessorService(Class, byte[], byte[], org.apache.hadoop.hbase.client.coprocessor.Batch.Call), and Table.coprocessorService(Class, byte[], byte[], org.apache.hadoop.hbase.client.coprocessor.Batch.Call, org.apache.hadoop.hbase.client.coprocessor.Batch.Callback) methods.

Since coprocessor Service instances are associated with individual regions within the table, the client RPC calls must ultimately identify which regions should be used in the Service method invocations. Since regions are seldom handled directly in client code and the region names may change over time, the coprocessor RPC calls use row keys to identify which regions should be used for the method invocations. Clients can call coprocessor Service methods against either:

Note that the row keys passed as parameters to the Table methods are not passed directly to the coprocessor Service implementations. They are only used to identify the regions for endpoints of the remote calls.

The Batch class defines two interfaces used for coprocessor Service invocations against multiple regions. Clients implement Batch.Call to call methods of the actual coprocessor Service instance. The interface's call() method will be called once per selected region, passing the Service instance for the region as a parameter. Clients can optionally implement Batch.Callback to be notified of the results from each region invocation as they complete. The instance's Batch.Callback.update(byte[], byte[], Object) method will be called with the Batch.Call.call(Object) return value from each region.

Example usage

To start with, let's use a fictitious coprocessor, RowCountEndpoint that counts the number of rows and key-values in each region where it is running. For clients to query this information, the coprocessor defines the following protocol buffer service:

message CountRequest {
}

message CountResponse {
  required int64 count = 1 [default = 0];
}

service RowCountService {
  rpc getRowCount(CountRequest)
    returns (CountResponse);
  rpc getKeyValueCount(CountRequest)
    returns (CountResponse);
}

Next run the protoc compiler on the .proto file to generate Java code for the Service interface. The generated RowCountService interface should look something like:

public static abstract class RowCountService
  implements com.google.protobuf.Service {
  ...
  public interface Interface {
    public abstract void getRowCount(
        com.google.protobuf.RpcController controller,
        org.apache.hadoop.hbase.coprocessor.example.generated.ExampleProtos.CountRequest request,
        com.google.protobuf.RpcCallback done);

    public abstract void getKeyValueCount(
        com.google.protobuf.RpcController controller,
        org.apache.hadoop.hbase.coprocessor.example.generated.ExampleProtos.CountRequest request,
        com.google.protobuf.RpcCallback done);
  }
}

Our coprocessor Service will need to implement this interface and the CoprocessorService in order to be registered correctly as an endpoint. For the sake of simplicity the server-side implementation is omitted. To see the implementing code, please see the RowCountEndpoint class in the HBase source code.

Now we need a way to access the results that RowCountService is making available. If we want to find the row count for all regions, we could use:

Connection connection = ConnectionFactory.createConnection(conf);
Table table = connection.getTable(TableName.valueOf("mytable"));
final ExampleProtos.CountRequest request = ExampleProtos.CountRequest.getDefaultInstance();
Map results = table.coprocessorService(
    ExampleProtos.RowCountService.class, // the protocol interface we're invoking
    null, null,                          // start and end row keys
    new Batch.Call() {
        public Long call(ExampleProtos.RowCountService counter) throws IOException {
          BlockingRpcCallback rpcCallback =
              new BlockingRpcCallback();
          counter.getRowCount(null, request, rpcCallback);
          ExampleProtos.CountResponse response = rpcCallback.get();
          return response.hasCount() ? response.getCount() : 0;
        }
    });

This will return a java.util.Map of the counter.getRowCount() result for the RowCountService instance running in each region of mytable, keyed by the region name.

By implementing Batch.Call as an anonymous class, we can invoke RowCountService methods directly against the Batch.Call.call(Object) method's argument. Calling Table.coprocessorService(Class, byte[], byte[], org.apache.hadoop.hbase.client.coprocessor.Batch.Call) will take care of invoking Batch.Call.call() against our anonymous class with the RowCountService instance for each table region.

Implementing Batch.Call also allows you to perform additional processing against each region's Service instance. For example, if you would like to combine row count and key-value count for each region:

Connection connection = ConnectionFactory.createConnection(conf);
Table table = connection.getTable(TableName.valueOf("mytable"));
// combine row count and kv count for region
final ExampleProtos.CountRequest request = ExampleProtos.CountRequest.getDefaultInstance();
Map results = table.coprocessorService(
    ExampleProtos.RowCountService.class, // the protocol interface we're invoking
    null, null,                          // start and end row keys
    new Batch.Call>() {
       public Long call(ExampleProtos.RowCountService counter) throws IOException {
         BlockingRpcCallback rowCallback =
             new BlockingRpcCallback();
         counter.getRowCount(null, request, rowCallback);

         BlockingRpcCallback kvCallback =
             new BlockingRpcCallback();
         counter.getKeyValueCount(null, request, kvCallback);

         ExampleProtos.CountResponse rowResponse = rowCallback.get();
         ExampleProtos.CountResponse kvResponse = kvCallback.get();
         return new Pair(rowResponse.hasCount() ? rowResponse.getCount() : 0,
             kvResponse.hasCount() ? kvResponse.getCount() : 0);
    }
});

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