Does Facelets rebuild the whole page if Ajax is trigger from a form and updates another?

Question

Setup: I have 2 forms A & B I have a commandLink in form A:

Answer 1

Upon initial request the view is created, upon postbacks the view is restored. To recite some points of JSF 2.2 specification for clarity (emphasis mine):

P. 2.2.1:

If the request is not a postback ... call createView() on the ViewHandler. If the request is a postback, ... call ViewHandler.restoreView(), passing the FacesContext instance for the current request and the view identifier, and returning a UIViewRoot for the restored view.

P. 2.5.8:

Selected components in a JSF view can be priocessed (known as partial processing) and selected components can be rendered to the client (known as partial rendering).

P. 13.4:

The JavaServer Faces lifecycle, can be viewed as consisting of an execute phase and a render phase. Partial traversal is the technique that can be used to “visit” one or more components in the view, potentially to have them pass through the “execute” and/or “render” phases of the request processing lifecycle.

When you use AJAX, PartialViewContext class will contain all the information that's needed to traverse the restored view.

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So, to get back to your question, under <f:ajax render=":B" execute="@this" /> setup, only the form with id="B" will be rerendered, which implies <h:form id="B">, no form nestings, etc.

Regarding your 'doesn't work' comment the simple test case with a plain view scoped managed bean gave me the expected results:

<h:form id="A" >
    <h:outputText value="#{twoFormsBean.a}"/>
    <h:commandLink actionListener="#{twoFormsBean.actionA}">
        Update B!
        <f:ajax execute="@this" render=":B"/>
    </h:commandLink>
</h:form>
<h:form id="B" >
    <h:outputText value="#{twoFormsBean.b}"/>
    <h:commandLink>
        Update Both!
        <f:ajax execute="@this" render=":A :B"/>
    </h:commandLink>
</h:form>

with

@ManagedBean
@ViewScoped
public class TwoFormsBean implements Serializable {

    private String a = "A";//getter
    private String b = "B";//getter

    public void actionA(ActionEvent ae) {
        a = "newA";
        b = "newB";
    }

}