Using an External Data Store

This section introduces CsvDataSourceServlet. CsvDataSourceServlet is an example implementation that uses a CSV file as an external data store. This section also provides step-by-step instructions on how to run and test CsvDataSourceServlet.

Note: You should complete the Getting Started with Data Sources section before you begin this section.

Introducing CsvDataSourceServlet

The CsvDataSourceServlet class is located in the examples package. This class provides an example implementation that uses a CSV file as an external data store. CsvDataSourceServlet inherits from DataSourceServlet, implements generateDataTable(), and must be run within a servlet container.

A snippet of CsvDataSourceServlet is provided below. The generateDataTable function exposes data to the library. This function creates a data table description, defines the data table columns, and populates the data table with data obtained from a CSV file. The CSV file is read from a URL specified in a requesting visualization's query. The library handles all other actions required to return the data table to the querying visualization. 

/**
 * A demo servlet for serving a simple, constant data table.
 * This servlet extends DataSourceServlet, but does not override the default
 * empty implementation of method getCapabilities(). This servlet therefore ignores the
 * user query (as passed in the 'tq' url parameter), leaving the
 * query engine to apply it to the data table created here.
 *
 * @author Nimrod T.
 */
public class CsvDataSourceServlet extends DataSourceServlet {

  /**
   * Log.
   */
  private static final Log log = LogFactory.getLog(CsvDataSourceServlet.class.getName());

  /**
   * The name of the parameter that contains the url of the CSV to load.
   */
  private static final String URL_PARAM_NAME = "url";

  /**
   * Generates the data table.
   * This servlet assumes a special parameter that contains the CSV URL from which to load
   * the data.
   */
  @Override
  public DataTable generateDataTable(Query query, HttpServletRequest request)
      throws DataSourceException {
    String url = request.getParameter(URL_PARAM_NAME);
    if (StringUtils.isEmpty(url)) {
      log.error("url parameter not provided.");
      throw new DataSourceException(ReasonType.INVALID_REQUEST, "url parameter not provided");
    }

    Reader reader;
    try {
      reader = new BufferedReader(new InputStreamReader(new URL(url).openStream()));
    } catch (MalformedURLException e) {
      log.error("url is malformed: " + url);
      throw new DataSourceException(ReasonType.INVALID_REQUEST, "url is malformed: " + url);
    } catch (IOException e) {
      log.error("Couldn't read from url: " + url, e);
      throw new DataSourceException(ReasonType.INVALID_REQUEST, "Couldn't read from url: " + url);
    }
    DataTable dataTable = null;
    ULocale requestLocale = DataSourceHelper.getLocaleFromRequest(request);
    try {
      // Note: We assume that all the columns in the CSV file are text columns. In cases where the
      // column types are known in advance, this behavior can be overridden by passing a list of
      // ColumnDescription objects specifying the column types. See CsvDataSourceHelper.read() for
      // more details.
      dataTable = CsvDataSourceHelper.read(reader, null, true, requestLocale);
    } catch (IOException e) {
      log.error("Couldn't read from url: " + url, e);
      throw new DataSourceException(ReasonType.INVALID_REQUEST, "Couldn't read from url: " + url);
    }
    return dataTable;
  }
}

Running and testing CsvDataSourceServlet

This section provides instructions on how to run and test CsvDataSourceServlet.

To run and test CsvDataSourceServlet, create a CSV file, update your web application, and set up a visualization that queries the data source, as described in the following sections:

Creating a CSV File

The file csv_example.csv is provided in the <data_source_library_install>/examples/src/html directory. It contains the following values:

Employee,Manager
Roger,John
Robert,John
Jane,Roger
Jack,Jane
Bob,Jane

Copy this file to the <tomcat_home>/webapps/myWebApp directory you created in the Getting Started section.

Updating Your Web Application on Apache Tomcat

Follow or adapt the instructions below to update your web application on Apache Tomcat. These instructions are specific to Apache Tomcat on a Windows system:

  1. The web.xml file you previously copied to the WEB-INF directory already contains the definition and mapping required for this example. The lines that define this are:
    <servlet>
  <servlet-name>CSV Example</servlet-name>
  <description>
  CsvDataSourceServlet
  </description>
  <servlet-class>CsvDataSourceServlet</servlet-class>
</servlet>

<servlet-mapping>
  <servlet-name>CSV Example</servlet-name>
  <url-pattern>/csv</url-pattern>
</servlet-mapping>
  2. Start Tomcat, or restart Tomcat if it is already running.
     
  3. Click the following link: http://localhost:8080/myWebApp/csv?url=http://localhost:8080/myWebApp/csv_example.csv

    The screen displays 6-7 lines of text, depending on your screen width.
    The text begins with google.visualization.Query.setResponse
    and ends with {c:[{v:'Bob'},{v:'Jane'}]}]}});

    This is the response that the example CSV data source sends to a visualization.

Using a Visualization to View the Data

The all_examples.html file in the <data_source_library_install>/examples/src/html directory can be used to view a visualization of the data.

If you view the source of the all_examples.html file, you will see there are three visualizations included in the file. The following snippets reproduce the specification of these visualizations.

  • The following line specifies the csv example covered in this section:
    query = new google.visualization.Query('csv?url=http://localhost:8080/myWebApp/csv_example.csv');
    The following line specifies an organization chart visualization: 
    var chart = new google.visualization.OrgChart(document.getElementById('csv_div'));
  • The following line specifies the simpleexample covered in the Getting Started section:
    var query = new google.visualization.Query('simpleexample?tq=select name,population');
    The following line specifies a pie chart visualization: 
    var chart = new google.visualization.PieChart(document.getElementById('simple_div'));
  • The following line specifies the advanced example that is covered in the Defining Capabilities and the Flow of Events section:
    query = new google.visualization.Query('advanced?tableId=planets&tq=select planet,mass');
    The following line specifies a bar chart visualization: 
    var chart = new google.visualization.BarChart(document.getElementById('advanced_div'));

For more information on how to specify a chart and use the query language, see Introduction to Using Chart Tools and the Query Language Reference.

Follow, or adapt, the instructions below to view a visualization of the data served by CsvDataSourceServlet:

  1. Copy the all_examples.html file from the <data_source_library_install>/examples/src/html directory to the <tomcat_home>/webapps/myWebApp/ directory.
     
  2. Click the following link: http://localhost:8080/myWebApp/all_examples.html, you should see the following visualization.


The Advanced Data Source example is discussed in Defining Capabilities and the Flow of Events.

Next Steps

The next example is described in the Defining Capabilities and the Flow of Events section. Alternatively, explore the following links: