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Optimistic Lock vs Pessimistic Lock – Application Module in Oracle ADF

ADF BC (Application module) manage the state of data. It have two locking mechanism -Optimistic Lock and Pessimistic Lock.Locking is Database specific feature which prevents users from different transactions to modify the same data concurrently.
In ADF , we will specify the locking mode either as optimistic/Pessimistic in adf-config.xml file.

We can the locking mode in Application Module -> Configuration -> AppModuleLocal/Shared -> Properties tab.

For the property, jbo.locking.mode it will show either optimistic /pessimistic. It will take the value overridden in adf-config.xml file.. It have same concept of database. Database also have same two locking mechanism.
Now understand each other –

Pessimistic Locking is when you lock the record for your exclusive use until you have finished with it. It has much better integrity than optimistic locking but requires you to be careful with your application design to avoid Deadlocks. To use pessimistic locking you need either a direct connection to the database (as would typically be the case in a two tier client server application) or an externally available transaction ID that can be used independently of the connection.

Example- user A (travel agent) try to book last flight ticket to London.He started the transaction but before clicking book the ticket, he got a phone call and talking. He got an lock of that data.
User B (Another travel agent) tries to book same ticket and wanted to make an lock to that row and he will get

oracle.jbo.AlreadyLockedException. . So its a situation of deadlock.

The disadvantage of pessimistic locking is that a resource is locked from the time it is first accessed in a transaction until the transaction is finished, making it inaccessible to other transactions during that time.

Optimistic Locking is a strategy where you read a record, take note of a version number (other methods to do this involve dates, timestamps or checksums/hashes) and check that the version hasn’t changed before you write the record back. When you write the record back you filter the update on the version to make sure it’s atomic. (i.e. hasn’t been updated between when you check the version and write the record to the disk) and update the version in one hit.
If the record is dirty (i.e. different version to yours) you abort the transaction and the user can re-start it.

This strategy is most applicable to high-volume systems and three-tier architectures where you do not necessarily maintain a connection to the database for your session. In this situation the client cannot actually maintain database locks as the connections are taken from a pool and you may not be using the same connection from one access to the next.

example-
user A (travel agent) try to book last flight ticket to London.He started the transaction but before clicking book the ticket, he got a phone call and talking. Here it will not try to acquire lock.

User B (Another travel agent) tries to book same ticket and he click commit. Ticket booked.Changes will happen in Database. But now user A will try to book the ticket , he will get oracle.jbo.RowInconsistentException.

which one to use
Oracle recommends using optimistic locking for web applications. Pessimistic locking, which is the default, should not be used for web applications as it creates pending transnational state in the database in the form of row-level locks.

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JSF Component state Save process

JSF can save state of component tree in either session or on clients machine inform of serialized data .
While creating component tree JSF creates a key and keep all objects of component tree as UIViewRoot in session with against it.For next JSF request it will use same key to restore that view 🙂
For client side saving has performance issues like it has to serialize whole tree and write it in response ship in form of gzip to client.Next time when same client make JSF request from same page it de-serialize it and inflate all objects in corresponding state .

Setting for state saving is done in web.xml.

Server side state saving is where the component tree and all component state are stored within the user’s session. This entry within the session is tracked by writing a key in the response that is used to lookup the entry on subsequent post-backs.

Client side state saving doesn’t leverage the server side session mechanism at all, instead, the component tree and state will be serialized using Java Serialization, GZIP compressed (at least that is the default), Base64 encoded, and written to the response. When a post-back occurs, the encoding process will be reversed which will result in the tree and state we started with.

Setting in web.xml

  <context-param>
<description>State saving method: 'client' or 'server' (=default). See JSF Specification 2.5.2</description>
<param-name>javax.faces.STATE_SAVING_METHOD</param-name>
<param-value>server</param-value>
</context-param>

Why does JSF need to save the state of UI components on the server side ?

Because HTTP is stateless and JSF is stateful. The JSF component tree is subject to dynamic (programmatic) changes. JSF simply needs to know the exact state as it was when the form had been displayed to the enduser, so that it can successfully process the whole JSF lifecycle based on the information provided by the original JSF component tree when the form has been submitted back to the server. The component tree provides information about the request parameter names, the necessary converters/validators, the bound managed bean properties and action methods.

As a logged-in user on the application navigates though pages, will the state of components keep on accumulating on the server?

Technically, that depends on the implementation. If you’re talking about page-to-page navigation (just GET requests) then Mojarra won’t save anything in session. If they are however POST requests (forms with commandlinks/buttons), then Mojarra will save state of each form in session until the max limit. This enables the enduser to open multiple forms in different browser tabs in the same session.

Or, when the state saving is set to client, then JSF won’t store anything in session. You can do that by the following context param in web.xml:

<context-param>
    <param-name>javax.faces.STATE_SAVING_METHOD</param-name>
    <param-value>client</param-value>
</context-param>

It will then be serialized to an encrypted string in a hidden input field with the name javax.faces.ViewState of the form.

Happy learning with Vinay in techartifact

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Get all the children components recursively for all UI Components

Problem- Need to reset all the input Components in the page on the click of a button.On first thought developer think of do this in the bean by getting the binding of the outermost layout component and then using the getChildren() method for that UIComponent.Then for all the UI components in the list,set the reset property to true.But problem here is that page contain multiple layout components. And getChildren() method would not recursively search for all the UI components (including inside the other layout components such as panelgrouplayout.


Implementation – Its plain java.Call the method recursively.get each input Component and reset it.

// reset all the child uicomponents
private void resetValueInputItems(AdfFacesContext adfFacesContext,
                                 UIComponent component){
   List<UIComponent> items = component.getChildren();
   for ( UIComponent item : items ) {
      
       resetValueInputItems(adfFacesContext,item);
      
       if ( item instanceof RichInputText  ) {
           RichInputText input = (RichInputText)item;
           if ( !input.isDisabled() ) {
               input.resetValue() ;
               adfFacesContext.addPartialTarget(input);
           };
       } else if ( item instanceof RichInputDate ) {
           RichInputDate input = (RichInputDate)item;
           if ( !input.isDisabled() ) {
               input.resetValue() ;
               adfFacesContext.addPartialTarget(input);
           };
       }
   }
}

Happy learning with Vinay Kumar in techartifact…