ZK Developer's Reference/Clustering/Programming Tips

Objects Referenced by UI Must be Serializable

Objects that are referenced by a UI object, such as components and pages, have to be serializable. Otherwise, they might have no value after de-serialized, or cause an exception (depending on how it is used).

Attributes of UI Objects

If the value of an attribute is not serializable, it will be ignored. Thus, it will become null after de-serialized. So, it is better to make them all serializable (such as implementing java.io.Serializable), or handle the serialization manually (refer to the Clustering Listeners section below) .

zscript

It is OK, though not recommended, to use zscript in a clustering environment, but there are some limitations.

• BeanShell's function is not serializable. For example, the following won't work:

void foo() {
}

• The value of variables must be serializable

Notice that it is not recommended to use zscript in the clustering environment. After all, the performance of BeanShell is not good.

Event Listeners

Event listeners have to be serializable. Otherwise, it will be ignored after serialization.

The simplest way to make an event listener serializable is to implement SerializableEventListener (available since 5.0.6), instead of EventListener.

For example,

button.addEventListener(Events.ON_CLICK,
  new SerializableEventListener() {
    public void onEvent(Event event) {
        ....
    }
  });

Data Models

The data models, such as ListModel and ChartModel, have to be serializable. Otherwise, the UI object (such as grid) won't behave correctly. The implementations provided by ZK are serializable. However, the items to be stored in the data models have to be serializable too.

Composers

If you extend from ZK built-in composer like GenericAutowireComposer, or SelectorComposer you have to make sure all of its members are serializable (or transient), since the implementation will keep a reference in the applied component.

When implementing from Composer directly, the composer could be non-serializable if you don't keep a reference in any UI object. In other words, the composer will be dropped after Composer.doAfterCompose(Component)

ViewModels

If you are using ZK MVVM then your ViewModel classes must be serializable.

Clustering Listeners

If there are non-serializable objects, you could implement one of the clustering listeners to handle them manually as described below. Basically, there are two kinds of clustering listeners for different purposes:

• Serialization Listeners: they are called when an object is about to be serialized, and after it has been de-serialized. Example: ComponentSerializationListener and PageSerializationListener
• Activation Listeners: they are called when a session is about to be passivated, and after it has been activated. Examples: ComponentActivationListener and PageActivationListener.

To register a listener is straightforward: just implement the corresponding listener interface. For example, you could implement ComponentActivationListener if an object is stored in a component and wants to be called on activation and passivation.

Passivation Flow

When a session is about to be passivated (such as moving to another machine), the activation listeners will be called first to notify the passivation, and then the serialization listeners will be called before the object is serialized.

Sequence	Description
1	Invokes SessionActivationListener.willPassivate(Session) for each object referenced by the Session that will be passivated
2	Invokes DesktopActivationListener.willPassivate(Desktop) for each object referenced by each Desktop that will be passivated
3	Invokes PageActivationListener.willPassivate(Page) for each object referenced by each Page that will be passivated
4	Invokes ComponentActivationListener.willPassivate(Component) for each object referenced by each Component that will be passivated
5	Invokes SessionSerializationListener.willSerialize(Session) for each object referenced by the Session that will be passivated
6	Serializes the session
7	Invokes DesktopSerializationListener.willSerialize(Desktop) for each object referenced by each Desktop that will be passivated
8	Serializes desktops of the session
9	Invokes PageSerializationListener.willSerialize(Page) for each object referenced by each Page that will be passivated
10	Serializes pages of each desktop
11	Invokes ComponentSerializationListener.willSerialize(Component) for each object referenced by each Component that will be passivated
12	Serializes components of each page

Activation Flow

When a session is about to be activated (such as moving from another machine), the serialization listener is called after the object has been deserialized. After all objects are deserialized, the activation listener will be called to notify a session has been activated.

Sequence	Description
1	Deserializes the session
2	Deserializes desktops of the session
3	Deserializes pages of each desktop
4	Deserializes components of each page
5	Invokes ComponentSerializationListener.didDeserialize(Component) for each object referenced by each Component that will be passivated
6	Invokes PageSerializationListener.didDeserialize(Page) for each object referenced by each Page that will be passivated
7	Invokes DesktopSerializationListener.didDeserialize(Desktop) for each object referenced by each Desktop that will be passivated
8	Invokes SessionSerializationListener.didDeserialize(Session) for each object referenced by the Session that will be passivated
9	Invokes SessionActivationListener.didActivate(Session) for each object referenced by the Session that will be passivated
10	Invokes DesktopActivationListener.didActivate(Desktop) for each object referenced by each Desktop that will be passivated
11	Invokes PageActivationListener.didActivate(Page) for each object referenced by each Page that will be passivated
12	Invokes ComponentActivationListener.didActivate(Component) for each object referenced by each Component that will be passivated

Working Thread Cannot Last Two or More Requests

Since the thread cannot be migrated from one machine to another, you couldn't use a working thread that works across multiple requests. For example, you cannot start a working thread in one request, and then invoke it in another request, since the session might be passivated between the requests.

It also implies you cannot use a working thread to handle a long operation. Rather, you have to use the so-called Echo Event.

Users of ZK 5.0.5 or prior cannot deploy the event queues for the session and application scope. However, users of ZK 5.0.6 or later have no such limitation.

Debugging Tips

System.setProperty("sun.io.serialization.extendedDebugInfo", "true") can print detailed debugging information about which member field is not serializable when a java.io.NotSerializableException happens.