interceptor

---

title: Intercepters

Intercepters

Guagua is designed as a microkernel-plugin mode like Eclipse, Maven. A kernel is defined to run master workers iterations in core component, while all other features are implemented as interceptors. System interceptors are provided by default like coordinator and timer, and user interceptors are also supported by setting just a parameter. All customers are considered as first-level citizens in Guagua, any internal or user-related feature can be changed by implementing replacement iterceptor.

A user interceptor example which is used to save output at the end of one application:

#!java
public class SumOutput extends
		BasicMasterInterceptor<GuaguaWritableAdapter<LongWritable>, GuaguaWritableAdapter<LongWritable>> {

	private static final Logger LOG = LoggerFactory.getLogger(SumOutput.class);

	@Override
	public void postApplication(
			final MasterContext<GuaguaWritableAdapter<LongWritable>, GuaguaWritableAdapter<LongWritable>> context) {
		LOG.info("SumOutput starts to write final sum value to file.");
		Path out = new Path(context.getProps().getProperty("guagua.sum.output"));
		LOG.info("Writing results to {}", out.toString());
		PrintWriter pw = null;
		try {
			FSDataOutputStream fos = FileSystem.get(new Configuration()).create(out);
			pw = new PrintWriter(fos);
			pw.println(context.getMasterResult().getWritable().get());
			pw.flush();
		} catch (IOException e) {
			LOG.error("Error in writing output.", e);
		} finally {
			IOUtils.closeStream(pw);
		}
	}
}

Configuration in command:

#!java
-Dguagua.master.intercepters=ml.shifu.guagua.mapreduce.example.sum.SumOutput -Dguagua.sum.output=sum-output