42 Using the Active Data Service

This chapter provides information for using the ADF Model layer and ADF Faces components with Active Data Service (ADS) in a Fusion web application.

This chapter includes the following sections:

Section 42.1, "Introduction to the Active Data Service"
Section 42.2, "Configuring the Active Data Service"
Section 42.3, "Configuring Components to Use the Active Data Service"
Section 42.4, "Using the Active Data Proxy"
Section 42.5, "Using the Active Data with a Scalar Model"

42.1 Introduction to the Active Data Service

The Fusion technology stack includes Active Data Service (ADS), which is a server-side push framework that allows you to provide real-time data updates for ADF Faces components. You bind ADF Faces components to a data source and ADS pushes the data updates to the browser client without requiring the browser client to explicitly request it. For example, you may have a table bound to attributes of an ADF data control whose values change on the server periodically, and you want the updated values to display in the table. You can configure your application and the component so that whenever the data changes on the server, the ADF Model layer notifies the component and the component rerenders the changed data with the new value highlighted, as shown in Figure 42-1.

Figure 42-1 Table Displays Updated Data as Highlighted

Using ADS is an alternative to using automatic partial page rendering (PPR) to rerender data that changes on the backend as a result of business logic associated with the ADF data control bound to the ADF Faces component. Whereas automatic PPR requires sending a request to the server (typically initiated by the user), ADS enables changed data to be pushed from the data store as the data arrives on the server. Also, in contrast to PPR, ADS makes it possible for the component to rerender only the changed data instead of the entire component. This makes ADS ideal for situations where the application needs to react to data that changes periodically.

To use this functionality, you must configure the application to use ADS. If your application services do not support ADS, then you also need to create a proxy of the service so that the components can display the data as it updates in the source.

Any ADF Faces page can use ADS. However, you can configure only the following ADF Faces components to work with active data:

activeCommandToolbarButton
activeImage
activeOutputText
table
tree
treeTable
DVT graph, gauge, and geographical map components

42.1.1 Limitations of the Active Data Service Framework

The framework for ADS has the following limitations.

ADS does not support active data for ADF Faces components added dynamically, using PPR.
ADS does not support active data for ADF Faces components that appear in a web page or region that gets invoked from a task flow call activity. Do not use task flow call activities with bounded task flows when active data is required.
ADS does not support active data in web browsers that have popup blockers enabled.

Before users can run the ADF Faces page with ADS configured for the application, they must disable the popup blocker for their web browser.
ADS does not support active data on ADF Faces table components with filtering enabled. Once a table is filtered at runtime, active data cannot be displayed.

42.1.2 Active Data Service Framework

The framework for ADS contains a number of components that work together to send the active data from the source to the UI component. When a data event occurs, if the associated ADF Model layer binding is configured for active data, the Active Data model delivers the data to the Event Manager. The Event Manager then retrieves the data and invokes the Push Service, which delivers the data to the correct component, based on how the service is configured (for more information, see Section 42.1.3, "Data Transport Modes"). The component then applies the new data pushed from the server. Figure 42-2 shows the ADS framework.

Figure 42-2 Active Data Service Framework

In order to use the Active Data Service, you need to have a data store that publishes events when data is changed, and you need to create business services that react to those events. By default, ADF Business Components does not react to data events. The Active Data Proxy framework allows all types of data sources, including ADF Business Components, to work with ADS. It combines the ActiveDataModel with the JSF model, so that you need to override functionality only on this proxy rather than on both the ActiveDataModel and the JSF model.

The following comprise the ADS framework:

ActiveDataModel interface: Abstraction of the active data model. Its responsibilities include:
- Starting and stopping active data
- Keeping track of the current active data event ID
- Letting the renderer know whether the model needs active data or not.
Event Manager: A server-side component that works with the ADF Model layer. It is responsible for the following:
- Listening to binding events
- Retrieving active data
- Managing active data encoding
- Invoking the Push service to send the encoded active data
Push service: A delivery channel that interacts with the Event Manager on the server side and with the Active Data Manager on the client side. It provides the following:
- Establishing and maintaining the connection between the server and the client
- Transmitting the active data over this connection from the server to the client
- Ensuring that active data gets delivered within desired parameters and forcing component update if not
Active Data Manager: A client-side component that distributes the active data to the correct component. Specifically, it is responsible for the following;
- Delivering events from the server side that are coming through the channel, using an event delivery service
- Handling multiple browser windows through a shared channel
- Dispatching active data events to rich client components, so that the components can render the change accordingly
Active Data Proxy: A proxy that allows all types of data sources to enable push functionality. Specifically, the proxy is responsible for the following:
- Implementing and delegating ActiveDataModel functionality
- Delegating to JSF models
- Listening to data change events from the data source
- Generating active data events based on the data change events

42.1.3 Data Transport Modes

Active data is sent to the client using data streaming (push) or one of two types of data polling. With data streaming, there is only one request, which stays open. When a data change event occurs, a partial response is sent (the response is not closed), the client is notified, and the associated component is updated to show the new data, as shown in Figure 42-3.

Figure 42-3 Streaming Mode

With data polling, the application is configured to poll the data source at specified intervals, as shown in Figure 42-4. With each request, a response is sent and closed, whether or not a data change event has occurred. If the data has changed, then the client is notified and the component is updated.

Figure 42-4 Poll Mode

Long polling is similar to streaming. When the page is rendered, a request is sent to the active channel. However, a response is not returned until there is a data change event. At that point, the connection is closed. As soon as the connection is closed, a new connection is initiated, which results in the connection being active most of the time: there are no specific intervals. Long polling results in the majority of data change events being received when they occur, because the connection is already established and ready to send a response to the client, as shown in Figure 42-5.

Figure 42-5 Long Polling Mode

For more information, see Section 42.2.2, "What You May Need to Know About Transport Modes."

To use ADS, you need to configure your application to determine the method of data transport, as well as other performance options.You also need to configure the bindings for your components so that they can use ADS. If you are using ADF Business Components, you need to modify your model to implement the ActiveModel interface to register itself as the listener for active data events using the Active Data Proxy.

Tip:

Oracle offers Oracle Business Activity Monitoring (BAM), which is a complete solution that provides the ability to create an active data store. For more information, see the Oracle Fusion Middleware Developer's Guide for Oracle SOA Suite.

42.2 Configuring the Active Data Service

You need to configure ADS to determine the data transport mode, as well as to set other configurations, such as a latency threshold and reconnect information.

Note:

If you enable ADS but do not configure it, the ADS framework will use the default values shown in Table 42-1.

42.2.1 How to Configure the Active Data Service

Configuration for running the Fusion web application with ADS in JDeveloper and Integrated WebLogic Server is done in the adf-config.xml file. For more information about the adf-config.xml file, including how to create one if you need to, see the "Configuration in adf-config.xml" section of the Oracle Fusion Middleware Web User Interface Developer's Guide for Oracle Application Development Framework.

To configure the Active Data Service:

In the Application Resources panel, double-click the adf-config.xml file.

Click the Source tab to open the file in the source editor, and create an entry for each of the elements shown in Table 42-1.

Table 42-1 ADS Configuration Elements in adf-config.xml

Element	Description	Default Value (in milliseconds)	Minimum Value (in milliseconds)
`<transport>`	The method by which data will be delivered to the client. Value values are: `streaming` (default) `polling` `long-polling` For more information, see Section 42.2.2, "What You May Need to Know About Transport Modes."
`<latency threshold>`	Latency threshold in milliseconds. Active data messages with network delays greater than this threshold will be treated as late.	10000	1000
`<keep-alive-interval>`	Frequency in milliseconds for sending keep-alive messages when no events are generated.	10000	5000
`<polling-interval>`	When `<transport>` set to `polling`, frequency in milliseconds of the poll request.	5000	1000
`<max-reconnect-attempt-time>`	Maximum period of time in milliseconds a client will attempt to reconnect the push channel to the server upon getting disconnected.	1800000 (30 minutes)	0
`<reconnect-wait-time>`	Time interval in milliseconds to wait between reconnect attempts.	10000	1000

Performance Tip:

Keep the following in mind when configuring the ADS.

Set the latency threshold to more than 1000 to avoid frequent component refreshing.
Set the keep-alive interval and reconnect wait time to be less than the browser's keep-alive timeout.
Set the max reconnect time to be less than your web server's session timeout.

Example 42-1 shows a sample configuration that has content pushed to the client.

Example 42-1 Sample Configuration for ADS in adf-config.xml

<?xml version="1.0" encoding="utf-8" ?>
<adf-config xmlns="http://xmlns.oracle.com/adf/config"
            xmlns:ads="http://xmlns.oracle.com/adf/activedata/config">
  <ads:adf-activedata-config xmlns=
                            "http://xmlns.oracle.com/adf/activedata/config">
    <transport>streaming</transport>
    <latency-threshold>5000</latency-threshold>
    <keep-alive-interval>10000</keep-alive-interval>
    <max-reconnect-attempt-time>90000</max-reconnect-attempt-time>
    <reconnect-wait-time>8000</reconnect-wait-time>
  </ads:adf-activedata-config>
</adf-config>

Create a properties file with an entry for the ADS configuration.
1. From the Main menu, choose File > New.
2. In the New Gallery, select General Category and then File, and click OK.
3. Name the file adf-config.properties and save it to the META-INF\services directory.
4. Add the following line:
```
http\://xmlns.oracle.com/adf/activedata/config=oracle.adfinternal.view.
faces.activedata.ActiveDataConfiguration$ActiveDataConfigCallback
```
5. Save and close the file.
Synchronize the clocks on the data server and on the application server. If these are not synchronized, then events may appear to ADS to have occurred in the future.

42.2.2 What You May Need to Know About Transport Modes

ADS can use one of three transport modes to deliver active data to the component: streaming, polling, or long polling.

When you configure ADS to use the streaming mode, data is pushed to the client whenever a change event is raised by the data. On the client side, after the push channel is established, if there is no activity within the time of the value for the latency-threshold element plus the keep-alive-interval, an establish-channel-request will be sent out repeatedly based on the value of the reconnect-wait-time element, until the amount of time passed reaches the value of the max-reconnect-attempt-time element. After that, the connection will be considered disconnected. For example, given the values in Example 42-1, if there is no activity in 15,000 milliseconds, an establish channel request will be sent out every 8,000 milliseconds for up to 90,000 milliseconds. After that, the connection will be considered disconnected.

On the server side, the server disconnects the push channel and starts a timer to clean up with a cleanup-delay-time when there is an empty active data message or when it fails to send out an active data message. The cleanup-delay-time is calculated as max-reconnect-attempt-time + 30 * 1000 ms. Its default value is 30 minutes.

When you configure ADS to use the polling mode, on the client side the polling request is scheduled to be sent out repeatedly after the value of the polling-interval element has been reached. If no response is received after the value of the max-reconnect-attempt-time has elapsed, the connection is treated as disconnected and no more requests will be sent. After receiving a polling response, if the time the response has taken is greater than the polling-interval element, the service sends the next request out right away. If it is less, the next request will be sent as scheduled.

For the server side, the session ends after the polling response is returned. At that point, a timer with a cleanup-delay-time is set up to trigger cleanup. If a new request comes in before the timer fires, the old timer is canceled, and new timer is created.

When you configure ADS to use the long polling mode, requests are made as they are in streaming mode; however, as soon as the connection is treated as disconnected, a new connection is initiated. The result is a significant reduction in latency.

Table 42-2 compares the three different modes.

Table 42-2 Comparison of Streaming, Polling, and Long-Polling Modes

	Streaming	Polling	Long-Polling
Latency	Very good. Directly after an event occurs on the server side, a partial response is sent to the client. If there is another event, immediately, it is also sent as a partial response. There is almost no latency with this approach.	Poor, depending on the polling interval. If the polling interval is short (for example, 0.5 seconds), it will slow down the network because the connections are repeatedly opened. It is also expensive on the client- and server-side resources.	Good. However, when there is a new event immediately after a response has been closed, there is some latency until the new data appears on the client side. On average, this is not a problem.
HTTP Proxy	Poor. For some older servers, because the response is never released, when a proxy is sitting between client and server, it is possible that the proxy will buffer responses. This is an unfortunate optimization that prevents real-time data from flowing into the browser. Long polling should be used if a proxy is used.	Good.	Good.
Number of live connections	Poor. Many concurrent connections, as the stream is always live.	Good. Connections are live only during the actual poll. Note however that if there is a high polling rate then the number of concurrent connections will also be high.	Poor. Many concurrent connections, as the stream is almost always live.
Communication channel	`HTTP GET` This can result in the display of a "busy" cursor or the animation of a browser's "throbber" icon.	`XMLHttpRequest` (XHR) `HTTP GET`	`XMLHttpRequest` (XHR) `HTTP GET`

42.3 Configuring Components to Use the Active Data Service

How you configure components to use ADS depends on whether or not you must use the Active Data proxy. If your application uses a data store that publishes events when data is changed, and your business services react to those events (for example, if your application uses BAM), then you do not need to use the Active Data proxy.

If your business services do not react to those events (for example, if your application uses ADF Business Components), then you must use the Active Data proxy and follow different procedures for configuring your components.

Note:

If your business service requires the use of the Active Data proxy, then you can only use the following components with active data:

table
tree
treeTable
DVT graphs, gauges, and geographical map components

42.3.1 How to Configure Components to Use the Active Data Service Without the Active Data Proxy

To use ADS without the proxy, you need to set a value on the binding element in the corresponding page definition file.

To configure a component to display active data without the Active Data proxy:

Drop a component onto a JSF page.
If you are using an ADF bound tree or tree table, you need to ensure the following:
- The binding represents homogeneous data (that is, only one rule), although an accessor can still access a like accessor.
- The binding rule contains a single attribute.
- The table does not use filtering.
- The tree component's nodeStamp facet contains a single outputText tag and contains no other tags, as described in Section 42.3.3, "What You May Need to Know About Displaying Active Data in ADF Trees."
Open the page's associated page definition file.
In the Structure window for the page definition file, select the node that represents the attribute binding for the component. In the Property Inspector, set the ChangeEventPolicy attribute to Push.

Tip:

You can use the statusIndicator component to indicate the server state. For more information, see the "Displaying Application Status Using Icons" section of the Oracle Fusion Middleware Web User Interface Developer's Guide for Oracle Application Development Framework.

42.3.2 How to Configure Components to Use the Active Data Service with the Active Data Proxy

To use ADS with the proxy, you need bind the value of your component to decorator class that will use the proxy. For more information about this class, see Section 42.4, "Using the Active Data Proxy." to set a value on the binding element in the corresponding page definition file.

To configure a component to display active data with the Active Data proxy:

Drop a component onto a JSF page.
Change the value attribute to be bound to a decorator class that you will create for use with the proxy. For more information, see Section 42.4, "Using the Active Data Proxy."

If you are using an ADF bound tree or tree table, you need to ensure the following:
- The binding represents homogeneous data (that is, only one rule), although an accessor can still access a like accessor.
- The binding rule contains a single attribute.
- The table does not use filtering.
- The tree component's nodeStamp facet contains a single outputText tag and contains no other tags, as described in Section 42.3.3, "What You May Need to Know About Displaying Active Data in ADF Trees."

42.3.3 What You May Need to Know About Displaying Active Data in ADF Trees

When you create an ADF Faces tree (or tree table) component, you configure a nodeStamp facet, which is a holder for the component used to display the data for each node of the tree. Each node is rendered (stamped) once, repeatedly for all nodes.

Because of this stamping behavior, only certain types of components are supported as children inside an ADF Faces tree component. When the tree component is not bound to an active data source, all components that have no behavior are supported. However, when you configure the tree to use ADS, only the outputText component is supported inside of the nodeStamp facet, as shown in Example 42-2. The nodeStamp facet must not contain any other tags. So, for example, active data will not work if you add panelGroupLayout tags to the nodeStamp facets of a tree configured to use ADS.

Example 42-2 Tree Configured to Use ADS Must Contain outputText Component

<f:facet name="nodeStamp">
  <af:outputText value="#{row.str2}"/> 
</f:facet>

42.3.4 What Happens at Runtime: How Components Render When Bound to Active Data

After you configure your application and the component for ADS, whenever the data changes on the server, the component is notified and rerenders with only the changed data. In contrast, a component not configured for either active data or automatic PPR will need to be explicitly refreshed after a data change occurs. The explicit refresh will refetch all data that is visible on the client, including data that has not changed. Consequently, this will force the entire component to rerender. Additionally, when the component is bound to an active data source (with active data policy "Push"), the component rerenders with the new value highlighted.

42.3.5 What You May Need to Know About ADS and Google Chrome

When the Fusion web application runs in the Google Chrome web browser and a user presses Ctrl+N (or Ctrl+T) to open a new window (or tab) and then copies the URL from the original window into the new window, active data in the original window will stop streaming. According to the Google Chrome process model, the new browser window will be created in a separate process and both windows will share the same HTTP session. However, because browser windows that are created in two separate processes cannot communicate with each other, ADS will become out of sync between the client and server and will stop streaming. As a workaround, to allow active data in multiple Google Chrome windows, before copying the URL from the original window into the new window users must press Ctrl-Shift-N to open the browser window in incognito mode (private browsing). Because Ctrl-Shift-N opens the window in a separate process and a separate HTTP session, ADS will not attempt to synchronize between the windows and streaming will be unaffected.

42.4 Using the Active Data Proxy

You use the active data proxy when your business services do not react to data events. If you want your components to update based on events passed into ADF Business Components, then you need to use the Active Data Proxy.

Tip:

If your application uses BAM for the business service, then you do not need to use the Active Data Proxy.

Note:

If your business service requires the use of the Active Data Proxy, then you can only use the following components with active data:

table
tree
treeTable
Graph, geographical map, and gauge DVT components

You need to create a Java class that subclasses one of the following decorator classes:

ActiveCollectionModelDecorator class
ActiveDataModelDecorator class (for use with graphs)
ActiveGeoMapDataModelDecorator class
ActiveGaugeDataModelDecorator class

These classes are wrapper classes that delegate the active data functionality to a default implementation of ActiveDataModel. The ActiveDataModel class listens for data change events and interacts with the Event Manager. Specifically, it does the following:

Starts and stops the active data and the ActiveDataModel object, and registers and unregisters listeners to the data source.
Wraps the JSF model interface. For example, the ActiveCollectionModelDecorator class wraps the CollectionModel class.
Generates active data events based on data change events from the data source.
Manages listeners from the Event Manager and pushes active data events to the Event Manager.

You need to implement methods on this Java class that registers itself as the listener of the active data source and gets the model to which the data is being sent.

Note:

The Active Data framework does not support complicated business logic or transformations that require the ADF runtime context, such as a user profile or security. For example, the framework cannot convert an ADF context locale-dependent value and return a locale-specific value.

As an example of complicated business logic, say you have logic that allows a user to move an order from open status to pending. This change results in an update event, which should cause the order to be removed from a data object called "Open Orders." The framework cannot handle this event type transformation based on business logic. Instead, you need to have your data source handle this before publishing the data change event.

Before you begin:

Implement the logic to fire the active data events asynchronously from the data source. For example, this logic might be a business process that updates the database, or a JMS client that gets notified from JMS.

To use the active data service:

Create a Java class that extends the decorator class appropriate for your component. Example 42-3 shows a class created for a table.

Example 42-3 Extend the Decorator Class

package view;
 
import oracle.adf.view.rich.model.ActiveCollectionModelDecorator;
import oracle.adf.view.rich.activedata.ActiveDataEventUtil;
import oracle.adf.view.rich.activedata.JboActiveDataEventUtil;
 
/**
 * This code wraps the existing collection model in the page and implements the
   ActiveDataModel interface to enable ADS for the existing page.
 */
public class DeptModel
  extends ActiveCollectionModelDecorator
{
}

Implement the method that returns the model. Example 42-4 shows an implementation of the getCollectionModel() method that relies on expression language (EL) to avoid needing to typecast to an internal class. The method returns the DepartmentsView1 collection from the binding container.

Example 42-4 Return the Model

public CollectionModel getCollectionModel()
{
   if (_model == null)
   {
       FacesContext fc = FacesContext.getCurrentInstance();
       Application app = fc.getApplication();
       ExpressionFactory elFactory = app.getExpressionFactory();
       ELContext el = fc.getELContext();

       // This is EL to avoid typecasting to an Internal class.
       ValueExpression ve = elFactory.createValueExpression(el,
                    "#{bindings.DepartmentsView1.collectionModel}", Object.class);

       // Now GET the collectionModel
       _model = (CollectionModel)ve.getValue(el);
   }
   return _model;
}

Create an inner class that is your own implementation of an ActiveDataModel class, which the decorator can use to start and stop the active data and connect and disconnect from the data source. This class should also use the changeCount API to maintain data read consistency, as shown in Example 42-5. For more information, see Section 42.4.1, "What You May Need to Know About Read Consistency."

Example 42-5 Connect to the Data Source

public class MyActiveDataModel extends BaseActiveDataModel
  {
    protected void startActiveData(Collection<Object> rowKeys,
                                   int startChangeCount)
    {
      _listenerCount.incrementAndGet();
      if (_listenerCount.get() == 1)
      {
        System.out.println("start up");
 
        Runnable dataChanger = new Runnable()
        {
          public void run()
          {
            System.out.println("MyThread starting.");
            try
            {
              Thread.sleep(2000);
              System.out.println("thread running");
              triggerDataChange(MyActiveDataModel.this);
            }
            catch (Exception exc)
            {
              System.out.println("MyThread exceptioned out.");
            }
            System.out.println("MyThread terminating.");
          }
        };
        Thread newThrd = new Thread(dataChanger);
        newThrd.start();
      }
    }
 
    protected void stopActiveData(Collection<Object> rowKeys)
    {
      _listenerCount.decrementAndGet();
      if (_listenerCount.get() == 0)
      {
        System.out.println("tear down");
      }
    }
 
    public int getCurrentChangeCount()
    {
      return _currEventId.get();
    }
 
    public void bumpChangeCount()
    {
      _currEventId.incrementAndGet();
    }
 
    public void dataChanged(ActiveDataUpdateEvent event)
    {
      fireActiveDataUpdate(event);
    }
 
    private final AtomicInteger _listenerCount = new AtomicInteger(0);
    private final AtomicInteger _currEventId = new AtomicInteger();
}

Implement the method that will return the ActiveDataModel class, as shown in Example 42-6.
Example 42-6 Return the ActiveDataModel
```
public ActiveDataModel getActiveDataModel()
  {
    return _activeDataModel;
  }
```

Create a method that creates application-specific events that can be used to insert or update data on the active model.

Example 42-7 shows the triggerDataChange() method, which uses the active model (an instance of MyActiveDataModel) to create ActiveDataUpdateEvent objects to insert and update data. You will need to import oracle.adf.view.rich.activedata.ActiveDataEventUtil and oracle.adf.view.rich.activedata.JboActiveDataEventUtil to implement this method.

Example 42-7 Create Event Objects to Update Model

public void triggerDataChange(MyActiveDataModel l)
  throws Exception
  {
    
    
    // do an update on dept 1
      l.bumpChangeCount();
      ActiveDataUpdateEvent event =
        ActiveDataEventUtil.buildActiveDataUpdateEvent(ActiveDataEntry.ChangeType.UPDATE,
                                     l.getCurrentChangeCount(),
                                     JboActiveDataEventUtil.convertKeyPath(new Key
                                            (new Object[]{ new Long(1), new Integer(0) })),
                                     null,
                                     new String[] { "name" },
                                     new Object[] { "Name Pushed" });

      l.dataChanged(event);

 
    try
    {
      Thread.sleep(2000);
    }
    catch (InterruptedException ie)
    {
      ie.printStackTrace();
    }
    // insert dept 99
 
      l.bumpChangeCount();
       event =
        ActiveDataEventUtil.buildActiveDataUpdateEvent(ActiveDataEntry.ChangeType.INSERT_AFTER,
                                     l.getCurrentChangeCount(),
                                     JboActiveDataEventUtil.convertKeyPath(new Key
                                           (new Object[]{ new Long(99), new Integer(0) })),
                                     JboActiveDataEventUtil.convertKeyPath(null),
                                     new String[]{ "addr1", "name" },
                                     new Object[]{ "Addr Inserted", "Name Inserted" });

      l.dataChanged(event);


    try
    {
      Thread.sleep(2000);
    }
    catch (InterruptedException ie)
    {
      ie.printStackTrace();
    }
 
    // delete dept 10
 
      l.bumpChangeCount();
       event =
        ActiveDataEventUtil.buildActiveDataUpdateEvent(ActiveDataEntry.ChangeType.REMOVE,
                                     l.getCurrentChangeCount(),
                                     JboActiveDataEventUtil.convertKeyPath(new Key 
                                           (new Object[]{ new Long(9), new Integer(0) })),
                                     null, null, null);

      l.dataChanged(event);


    try
    {
      Thread.sleep(2000);
    }
    catch (InterruptedException ie)
    {
      ie.printStackTrace();
    }
 
    // refresh the entire table
 
      l.bumpChangeCount();
       event =
        ActiveDataEventUtil.buildActiveDataUpdateEvent(ActiveDataEntry.ChangeType.REFRESH,
                                     l.getCurrentChangeCount(),
                                     null, null, null, null);

      l.dataChanged(event);
    }
 
 
  private MyActiveDataModel _activeDataModel = new MyActiveDataModel();
 
  private CollectionModel _model = null;
 
}

42.4.1 What You May Need to Know About Read Consistency

Using active data means that your component has two sources of data: the active data feed and the standard data fetch. Because of this, you must make sure your application maintains read consistency.

For example, say your page contains a table and that table has active data enabled. The table has two methods of delivery from which it updates its data: normal table data fetch and active data push. Say the back end data changes from foo to bar to fred. For each of these changes, an active data event is fired. If the table is refreshed before those events hit the browser, the table will display fred because standard data fetch will always get the latest data. But then, because the active data event might take longer, some time after the refresh the data change event would cause foo to arrive at the browser, and so the table would update to display foo instead of fred for a period of time. Therefore, you must implement a way to maintain the read consistency.

To achieve read consistency, the ActiveDataModel has the concept of a change count, which effectively timestamps the data. Both data fetch and active data push need to maintain this changeCount object by monotonically increasing the count, so that if any data returned has a lower changeCount, the active data event can throw it away. Example 42-7 shows how you can use your implementation of the ActiveDataModel class to maintain read consistency.

42.5 Using the Active Data with a Scalar Model

ADF components that display collection-based data can be configured to work with ADS and require no extra setup in the view layer. However, imagine that your JSPX page uses an activeOutputText component to display new counts based on a Java timer. In this case, you will replace the model layer with scalar or "flat" data that you display from a Java Bean.

To implement the scalar model, follow these basic steps (as illustrated in the Example 42-8):

Use the ActiveModelContext API to register the bean with ADS so the bean (a scalar model) imitates an actual model.
Implement a custom mechanism to push the data to the view layer.

Example 42-8 Implement the Scalar Model in the Java Bean

package oracle.adfdemo.view.feature.rich;

import java.util.Collection;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.atomic.AtomicInteger;

import oracle.adf.view.rich.activedata.ActiveModelContext;
import oracle.adf.view.rich.activedata.BaseActiveDataModel;
import oracle.adf.view.rich.activedata.ActiveDataEventUtil;
import oracle.adf.view.rich.event.ActiveDataEntry;
import oracle.adf.view.rich.event.ActiveDataUpdateEvent;


public class CounterBean extends BaseActiveDataModel
// Example using a Java timer to create new counts
{
  public CounterBean()
  // 1. Use the ActiveModelContext API to register the scalar model key path for
  //    the "state" attribute.
  {
    ActiveModelContext context = ActiveModelContext.getActiveModelContext();
    Object[] keyPath = new String[0];
    context.addActiveModelInfo(this, keyPath, "state");

    timer.schedule(new UpdateTask(), 2000, 2000);
  }

  public String getState()
  {
    return String.valueOf(counter);
  }

  // Not needed. We do not need to connect to a (real) active data scource.
  protected void startActiveData(Collection<Object> rowKeys, int
          startChangeCount) {}

  // Not needed. We do not need to connect to a (real) active data scource.

  protected void stopActiveData(Collection<Object> rowKeys) {}

  public int getCurrentChangeCount() 
  { 
    return counter.get(); 
  } 

  protected class UpdateTask extends TimerTask 
  { 
    public void run() 
    { 
      counter.incrementAndGet(); 

      // 2. Use ActiveDataEventUtil to create an event object to update the model.
      ActiveDataUpdateEvent event = 
        ActiveDataEventUtil.buildActiveDataUpdateEvent( 
          ActiveDataEntry.ChangeType.UPDATE, 
          counter.get(), 
          new String[0], 
          null, 
          new String[] { "state" }, 
          new Object[] { counter.get() }); 
      fireActiveDataUpdate(event); 
    } 
  } 

  private static final Timer timer = new Timer(); 
  private final AtomicInteger counter = new AtomicInteger(0); 
}

After you create the bean, register the bean as a managed bean in the faces-config.xml file (as illustrated for counterBean in Example 42-9):

Example 42-9 Register the Managed Bean

...
<managed-bean>
  <managed-bean-name>counterBean</managed-bean-name>
  <managed-bean-class>
      oracle.adfdemo.view.feature.rich.CounterBean
  </managed-bean-class>
  <managed-bean-scope>session</managed-bean-scope>
</managed-bean>

Once the bean is registered, you can use ADS to stream the data to the view layer. Your ADF Faces component use expression language to receive the pushed data (as illustrated by the activeOutputText component value attribute in Example 42-10):

Example 42-10 Display the Active Data

...
<f:view>
   <af:document title="Active Data Visual Design Demo"
                  binding="#{templateBindings.documentComponent}"
                 smallIconSource="#{aboutBean.smallIconSource}"
                 largeIconSource="#{aboutBean.largeIconSource}" theme="dark"
                  id="d1">
      <af:pageTemplate id="dmoTpl" viewId="#{templates.componentTemplate}">
        <f:attribute name="tagName" value="Active Data Visual Design"/>
        <f:attribute name="demoKind" value="visualDesign"/>
        <f:attribute name="customEditorPresent" value="true"/>
        <f:facet name="center">
          <af:panelGroupLayout layout="scroll">
             <af:activeOutputText 
                value="#{counterBean.state}"
                inlineStyle=
                   "color:brown;
                    font-size:100px;
                    font-weight:bold;
                    text-align:center;"
              />
          </af:panelGroupLayout>
        </f:facet>
      </af:pageTemplate>
   </af:document>
</f:view>