JSON data ListGrid live updates

[dao]

Hello,

I want to display a grid that is updated say each 5s on on user action with data that comes from a servlet. The servlet answers a JSON array that I get thru a dataSource. The JSON array contains messages that are added slowly.

I want my grid to be updated according to the content of the json. I have filters in my page to drive the access to the servlet.

example: I want all the messages of this kind of today. The filtering is performed in the serverside.
The url to acces is like "my-servlet?fromDate=20101021&kind=WARN" according to the filter.

What is the best practice to do that? below, I have put the piece of code I tried (method query is called). My grid does not change. The callback returns a com.google.gwt.core.client.JavaScriptObject, I don't know what to do with it.

Thank's for helping me start with smartGWT

package com.airbus.mcctls.client;

import java.util.HashMap;
import java.util.Map;

import org.apache.tools.ant.types.CommandlineJava.SysProperties;

import com.smartgwt.client.data.DSCallback;
import com.smartgwt.client.data.DSRequest;
import com.smartgwt.client.data.DSResponse;
import com.smartgwt.client.data.DataSource;
import com.smartgwt.client.data.DataSourceField;
import com.smartgwt.client.data.fields.DataSourceDateTimeField;
import com.smartgwt.client.data.fields.DataSourceTextField;
import com.smartgwt.client.types.Alignment;
import com.smartgwt.client.types.DSDataFormat;
import com.smartgwt.client.types.ListGridFieldType;
import com.smartgwt.client.widgets.grid.ListGrid;
import com.smartgwt.client.widgets.grid.ListGridField;

public class MessageGrid extends ListGrid{
	

	private DataSource dataSource = null;

	public MessageGrid() {
		dataSource  = new DataSource();
		dataSource.setDataFormat(DSDataFormat.JSON);
		dataSource.setDataURL("my-servlet?fromDate=20101021");

		DataSourceTextField msnSrc = new DataSourceTextField(
				"EXT_AIRCRAFT_MSN", "MSN");
		DataSourceTextField airlineSrc = new DataSourceTextField(
				"EXT_AIRLINE_IDENTIFIER", "Airline");
		DataSourceTextField acTypeSrc = new DataSourceTextField(
				"EXT_AIRCRAFT_FAMILY", "AcType");
		DataSourceTextField msgTypeSrc = new DataSourceTextField(
				"TYPE_IDENTIFIER", "Type");
		DataSourceTextField tailNumberSrc = new DataSourceTextField(
				"MSG_AIRCRAFT_TAILNUMBER", "Id");
		DataSourceDateTimeField timeSrc = new DataSourceDateTimeField(
				"MSG_UTC_TRANSMISSION_DATE_TIME", "time");
		
		dataSource .setFields(timeSrc, airlineSrc, tailNumberSrc, msnSrc, 
				acTypeSrc, msgTypeSrc);

		
		//airline column
		ListGridField airlineField = defaultField(airlineSrc);
		airlineField.setAlign(Alignment.CENTER);
		airlineField.setWidth(40);
		airlineField.setType(ListGridFieldType.IMAGE);
		airlineField.setImageURLPrefix("airline/icon/");
		airlineField.setImageURLSuffix(".gif");
		
		ListGridField timeField = defaultField (timeSrc);
		timeField.setWidth(125);
		timeField.setAlign(Alignment.CENTER);
		
		ListGridField acTypeField = defaultField(acTypeSrc);
		acTypeField.setWidth(50);
		ListGridField msnField = defaultField(msnSrc);
		msnField.setWidth(50);
		ListGridField tailNumberField = defaultField(tailNumberSrc);
		tailNumberField.setWidth(50);
		

		ListGridField msgTypeField = defaultField(msgTypeSrc);
		msgTypeField.setWidth(50);
		setFields(timeField, airlineField, tailNumberField, msnField, acTypeField, msgTypeField);
		
		setDataSource(dataSource );
		setAutoFetchData(true);

	}
	
	private ListGridField defaultField(DataSourceField src) {
		return new ListGridField(src.getName(), src.getTitle());
	}

	public void query() {
		DSCallback callback = new DSCallback() {
			
			@Override
			public void execute(DSResponse response, Object rawData, DSRequest request) {
				System.out.println(rawData);
				//what have I to do?
			}
		};
		DSRequest request = new DSRequest();
		Map<String, String> params = new HashMap<String, String>();
		params.put("kind",//getTheKind);
		params.put("fromDate",//getTheFromDate);
		request.setParams(params);
		dataSource.fetchData(null, callback, request);
	}



}

version:
SmartGWT 2.2 / Firefox 3.6