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<chapter id="conversations">
    <title>Conversations and workspace management</title>
    
    <para>
        It's time to understand Seam's conversation model in more detail.
    </para>
    
    <para>
        Historically, the notion of a Seam "conversation" came about as
        a merger of three different ideas:
    </para>
    
    <itemizedlist>
        <listitem>
        <para>
            The idea of a <emphasis>workspace</emphasis>, which I 
            encountered in a project for the Victorian government in 
            2002. In this project I was forced to implement workspace 
            management on top of Struts, an experience I pray never
            to repeat.
        </para>
        </listitem>
        <listitem>
        <para>
            The idea of an <emphasis>application transaction</emphasis>
            with optimistic semantics, and the realization that existing
            frameworks based around a stateless architecture could not
            provide effective management of extended persistence contexts.
            (The Hibernate team is truly fed up with copping the blame for
            <literal>LazyInitializationException</literal>s, which are
            not really Hibernate's fault, but rather the fault of the 
            extremely limiting persistence context model supported by 
            stateless architectures such as the Spring framework or the 
            traditional <emphasis>stateless session facade</emphasis> 
            (anti)pattern in J2EE.)
        </para>
        </listitem>
        <listitem>
        <para>
            The idea of a workflow <emphasis>task</emphasis>.
        </para>
        </listitem>
    </itemizedlist>
    
    <para>
        By unifying these ideas and providing deep support in the framework, 
        we have a powerful construct that lets us build richer and more efficient 
        applications with less code than before.
    </para>
    
    <section>
        <title>Seam's conversation model</title>
    
    <para>
        The examples we have seen so far make use of a very simple
        conversation model that follows these rules:
    </para>
    
    <itemizedlist>
        <listitem>
        <para>
            There is always a conversation context active during the
            apply request values, process validations, update model values, 
            invoke application and render response phases of the JSF request 
            lifecycle.
        </para>
        </listitem>
        <listitem>
        <para>
            At the end of the restore view phase of the JSF request 
            lifecycle, Seam attempts to restore any previous long-running
            conversation context. If none exists, Seam creates a new
            temporary conversation context.
        </para>
        </listitem>
        <listitem>
        <para>
            When an <literal>@Begin</literal> method is encountered, 
            the temporary conversation context is promoted to a long
            running conversation.
        </para>
        </listitem>
        <listitem>
        <para>
            When an <literal>@End</literal> method is encountered, 
            any long-running conversation context is demoted to a 
            temporary conversation.
        </para>
        </listitem>
        <listitem>
        <para>
            At the end of the render response phase of the JSF request
            lifecycle, Seam stores the contents of a long running
            conversation context or destroys the contents of a temporary
            conversation context.
        </para>
        </listitem>
        <listitem>
        <para>
            Any faces request (a JSF postback) will propagate the 
            conversation context. By default, non-faces requests (GET 
            requests, for example) do not propagate the conversation 
            context, but see below for more information on this.
        </para>
        </listitem>
        <listitem>
        <para>
            If the JSF request lifecycle is foreshortened by a redirect,
            Seam transparently stores and restores the current conversation 
            context&#8212;unless the conversation was already ended via
            <literal>@End(beforeRedirect=true)</literal>.
        </para>
        </listitem>
    </itemizedlist>
    
        <para>
            Seam transparently propagates the conversation context (including
            the temporary conversation context) across JSF postbacks and 
            redirects. If you don't do anything special, a <emphasis>non-faces request</emphasis>
            (a GET request for example) will not propagate the conversation context and
            will be processed in a new temporary conversation. This is usually - but not
            always - the desired behavior.
        </para>
        
        <para>
            If you want to propagate a Seam conversation across a non-faces request, you
            need to explicitly code the Seam <emphasis>conversation id</emphasis> as a 
            request parameter:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<a href="main.jsf?conversationId=#{conversation.id}">Continue</a>]]></programlisting>
        
        <para>
            Or, the more JSF-ish:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<h:outputLink value="main.jsf">
    <f:param name="conversationId" value="#{conversation.id}"/>
    <h:outputText value="Continue"/>
</h:outputLink>]]></programlisting>

        <para>
            If you use the Seam tag library, this is equivalent:
        </para>

        <programlisting role="XHTML"><![CDATA[<h:outputLink value="main.jsf">
    <s:conversationId/>
    <h:outputText value="Continue"/>
</h:outputLink>]]></programlisting>

        <para>
            If you wish to disable propagation of the conversation context for a
            postback, a similar trick is used:
        </para>

        <programlisting role="XHTML"><![CDATA[<h:commandLink action="main" value="Exit">
    <f:param name="conversationPropagation" value="none"/>
</h:commandLink>]]></programlisting>

        <para>
            If you use the Seam tag library, this is equivalent:
        </para>

        <programlisting role="XHTML"><![CDATA[<h:commandLink action="main" value="Exit">
    <s:conversationPropagation type="none"/>
</h:commandLink>]]></programlisting>

        <para>
            Note that disabling conversation context propagation is absolutely not the
            same thing as ending the conversation.
        </para>
        
        <para>
            The <literal>conversationPropagation</literal> request parameter, or 
            the <literal>&lt;s:conversationPropagation&gt;</literal> tag may even
            be used to begin and end conversation, or begin a nested 
            conversation.
        </para>

        <programlisting role="XHTML"><![CDATA[<h:commandLink action="main" value="Exit">
    <s:conversationPropagation type="end"/>
</h:commandLink>]]></programlisting>

        <programlisting role="XHTML"><![CDATA[<h:commandLink action="main" value="Select Child">
    <s:conversationPropagation type="nested"/>
</h:commandLink>]]></programlisting>

        <programlisting role="XHTML"><![CDATA[<h:commandLink action="main" value="Select Hotel">
    <s:conversationPropagation type="begin"/>
</h:commandLink>]]></programlisting>

        <programlisting role="XHTML"><![CDATA[<h:commandLink action="main" value="Select Hotel">
    <s:conversationPropagation type="join"/>
</h:commandLink>]]></programlisting>

    <para>
        This conversation model makes it easy to build applications which
        behave correctly with respect to multi-window operation. For many
        applications, this is all that is needed. Some complex applications
        have either or both of the following additional requirements:
    </para>
    
    <itemizedlist>
        <listitem>
        <para>
            A conversation spans many smaller units of user interaction,
            which execute serially or even concurrently. The smaller
            <emphasis>nested conversations</emphasis> have their own
            isolated set of conversation state, and also have access to
            the state of the outer conversation.
        </para>
        </listitem>
        <listitem>
        <para>
            The user is able to switch between many conversations
            within the same browser window. This feature is called
            <emphasis>workspace management</emphasis>.
        </para>
        </listitem>
    </itemizedlist>
    
    </section>
    
    <section>
        <title>Nested conversations</title>
        
        <para>
            A nested conversation is created by invoking a method marked
            <literal>@Begin(nested=true)</literal> inside the scope of an
            existing conversation. A nested conversation has its own
            conversation context, and also has read-only access to the
            context of the outer conversation. (It can read the outer
            conversation's context variables, but not write to them.)
            When an <literal>@End</literal> is subsequently encountered,
            the nested conversation will be destroyed, and the outer
            conversation will resume, by "popping" the conversation stack.
            Conversations may be nested to any arbitrary depth.
        </para>
        
        <para>
            Certain user activity (workspace management, or the back button)
            can cause the outer conversation to be resumed before the inner
            conversation is ended. In this case it is possible to have 
            multiple concurrent nested conversations belonging to the
            same outer conversation. If the outer conversation ends before
            a nested conversation ends, Seam destroys all nested conversation
            contexts along with the outer context.
        </para>
        
        <para>
            A conversation may be thought of as a <emphasis>continuable state</emphasis>.
            Nested conversations allow the application to capture a consistent
            continuable state at various points in a user interaction, thus
            insuring truly correct behavior in the face of backbuttoning and
            workspace management.
        </para>
        
        <para>
            TODO: an example to show how a nested conversation prevents bad 
            stuff happening when you backbutton.
        </para>
        
        <para>
            Usually, if a component exists in a parent conversation of the 
            current nested conversation, the nested conversation will use
            the same instance. Occasionally, it is useful to have a different
            instance in each nested conversation, so that the component 
            instance that exists in the parent conversation is invisible to
            its child conversations. You can achieve this behavior by 
            annotating the component <literal>@PerNestedConversation</literal>.
        </para>
        
    </section>
    
    <section>
        <title>Starting conversations with GET requests</title>
        <para>
            JSF does not define any kind of action listener that is triggered
            when a page is accessed via a non-faces request (for example, a
            HTTP GET request). This can occur if the user bookmarks the page,
            or if we navigate to the page via an <literal>&lt;h:outputLink&gt;</literal>.
        </para>
        
        <para>
            Sometimes we want to begin a conversation immediately the page is
            accessed. Since there is no JSF action method, we can't solve the problem
            in the usual way, by annotating the action with <literal>@Begin</literal>.
        </para>
        
        <para>
            A further problem arises if the page needs some state to be fetched
            into a context variable. We've already seen two ways to solve this 
            problem. If that state is held in a Seam component, we can fetch the 
            state in a <literal>@Create</literal> method. If not, we can define a
            <literal>@Factory</literal> method for the context variable.
        </para>
        
        <para>
            If none of these options works for you, Seam lets you define a
            <emphasis>page action</emphasis> in the <literal>pages.xml</literal>
            file.
        </para>
        
        <programlisting role="XML"><![CDATA[<pages>
    <page view-id="/messageList.jsp" action="#{messageManager.list}"/>
    ...
</pages>]]></programlisting>

        <para>
            This action method is called at the beginning of the render response
            phase, any time the page is about to be rendered. If a page action
            returns a non-null outcome, Seam will process any appropriate JSF and
            Seam navigation rules, possibly resulting in a completely different page 
            being rendered.
        </para>
        
        <para>
            If <emphasis>all</emphasis> you want to do before rendering the page
            is begin a conversation, you could use a built-in action method that
            does just that:
        </para>
        
        <programlisting role="XML"><![CDATA[<pages>
    <page view-id="/messageList.jsp" action="#{conversation.begin}"/>
    ...
</pages>]]></programlisting>

        <para>
            Note that you can also call this built-in action from a JSF
            control, and, similarly, you can use 
            <literal>#{conversation.end}</literal> to end conversations.
        </para>
        
        <para>
            If you want more control, to join existing conversations or
            begin a nested conversion, to begin a pageflow or an atomic
            conversation, you should use the 
            <literal>&lt;begin-conversation&gt;</literal> element.
        </para>

        <programlisting role="XML"><![CDATA[<pages>
    <page view-id="/messageList.jsp">
       <begin-conversation nested="true" pageflow="AddItem"/>
    <page>
    ...
</pages>]]></programlisting>

        <para>
            There is also an <literal>&lt;end-conversation&gt;</literal> 
            element.
        </para>

        <programlisting role="XML"><![CDATA[<pages>
    <page view-id="/home.jsp">
       <end-conversation/>
    <page>
    ...
</pages>]]></programlisting>

        <para>
            To solve the first problem, we now have five options:
        </para>
        
        <itemizedlist>
            <listitem>
            <para>
                Annotate the <literal>@Create</literal> method with
                <literal>@Begin</literal>
            </para>
            </listitem>
            <listitem>
            <para>
                Annotate the <literal>@Factory</literal> method with
                <literal>@Begin</literal>
            </para>
            </listitem>
            <listitem>
            <para>
                Annotate the Seam page action method with
                <literal>@Begin</literal>
            </para>
            </listitem>
            <listitem>
            <para>
                Use <literal>&lt;begin-conversation&gt;</literal> in
                <literal>pages.xml</literal>.
            </para>
            </listitem>
            <listitem>
            <para>
                Use <literal>#{conversation.begin}</literal> as
                the Seam page action method
            </para>
            </listitem>
        </itemizedlist>
        
    </section>
    
    <section>
        <title>Using <literal>&lt;s:link&gt;</literal> and <literal>&lt;s:button&gt;</literal></title>
        
        <para>
            JSF command links always perform a form submission via JavaScript, 
            which breaks the web browser's "open in new window" or "open in new tab"
            feature. In plain JSF, you need to use an <literal>&lt;h:outputLink&gt;</literal>
            if you need this functionality. But there are two major limitations to 
            <literal>&lt;h:outputLink&gt;</literal>.
        </para>
        
    <itemizedlist>
        <listitem>
        <para>
            JSF provides no way to attach an action listener to an 
            <literal>&lt;h:outputLink&gt;</literal>.
        </para>
        </listitem>
        <listitem>
        <para>
            JSF does not propagate the selected row of a <literal>DataModel</literal> 
            since there is no actual form submission.
        </para>
        </listitem>
    </itemizedlist>
    
        <para>
            Seam provides the notion of a <emphasis>page action</emphasis> to help 
            solve the first problem, but this does nothing to help us with the second 
            problem. We <emphasis>could</emphasis> work around this by using the
            RESTful approach of passing a request parameter and requerying
            for the selected object on the server side. In some cases&#8212;such as the 
            Seam blog example application&#8212;this is indeed the best approach. The 
            RESTful style supports bookmarking, since it does not require server-side state. 
            In other cases, where we don't care about bookmarks, the use of 
            <literal>@DataModel</literal> and <literal>@DataModelSelection</literal> is 
            just so convenient and transparent!
        </para>
        
        <para>
            To fill in this missing functionality, and to make conversation propagation
            even simpler to manage, Seam provides the <literal>&lt;s:link&gt;</literal>
            JSF tag.
        </para>
        
        <para>
            The link may specify just the JSF view id:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link view="/login.xhtml" value="Login"/>]]></programlisting>
        
        <para>
            Or, it may specify an action method (in which case the action outcome determines
            the page that results):
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link action="#{login.logout}" value="Logout"/>]]></programlisting>
        
        <para>
            If you specify <emphasis>both</emphasis> a JSF view id and an action method, the
            'view' will be used <emphasis>unless</emphasis> the action method returns a
            non-null outcome:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link view="/loggedOut.xhtml"  action="#{login.logout}" value="Logout"/>]]></programlisting>
        
        <para>
            The link automatically propagates the selected row of a <literal>DataModel</literal>
            using inside <literal>&lt;h:dataTable&gt;</literal>:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link view="/hotel.xhtml" action="#{hotelSearch.selectHotel}" value="#{hotel.name}"/>]]></programlisting>
        
        <para>
            You can leave the scope of an existing conversation:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link view="/main.xhtml" propagation="none"/>]]></programlisting>
        
        <para>
            You can begin, end, or nest conversations:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link action="#{issueEditor.viewComment}" propagation="nest"/>]]></programlisting>
        
        <para>
            If the link begins a conversation, you can even specify a pageflow to be used:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link action="#{documentEditor.getDocument}" propagation="begin"
        pageflow="EditDocument"/>]]></programlisting>
        
        <para>
            The <literal>taskInstance</literal> attribute if for use in jBPM task lists:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:link action="#{documentApproval.approveOrReject}" taskInstance="#{task}"/>]]></programlisting>
        
        <para>
            (See the DVD Store demo application for examples of this.)
        </para>
        
        <para>
            Finally, if you need the "link" to be rendered as a button, use <literal>&lt;s:button&gt;</literal>:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<s:button action="#{login.logout}" value="Logout"/>]]></programlisting>
        
    </section>
    
    <section>
        <title>Success messages</title>
        <para>
            It is quite common to display a message to the user indicating 
            success or failure of an action. It is convenient to use a JSF
            <literal>FacesMessage</literal> for this. Unfortunately, a
            successful action often requires a browser redirect, and JSF 
            does not propagate faces messages across redirects. This makes
            it quite difficult to display success messages in plain JSF.
        </para>
        
        <para>
            The built in conversation-scoped Seam component named 
            <literal>facesMessages</literal> solves this problem.
            (You must have the Seam redirect filter installed.)
        </para>
        
        <programlisting role="JAVA"><![CDATA[@Name("editDocumentAction")
@Stateless
public class EditDocumentBean implements EditDocument {
    @In EntityManager em;
    @In Document document;
    @In FacesMessages facesMessages;
    
    public String update() {
        em.merge(document);
        facesMessages.add("Document updated");
    }
}]]></programlisting>
        
        <para>
            Any message added to <literal>facesMessages</literal> is
            used in the very next render response phase for the current
            conversation. This even works when there is no long-running
            conversation since Seam preserves even temporary conversation
            contexts across redirects.
        </para>
        
        <para>
            You can even include JSF EL expressions in a faces message summary:
        </para>
        
        <programlisting role="JAVA"><![CDATA[facesMessages.add("Document #{document.title} was updated");]]></programlisting>
                
        <para>
            You may display the messages in the usual way, for example:
        </para>
        
        <programlisting role="XHTML"><![CDATA[<h:messages globalOnly="true"/>]]></programlisting>

    </section>
    
    <section>
        <title>Natural conversation ids</title>
        <para>
            When working with conversations that deal with persistent objects, it may be 
            desirable to use the natural business key of the object instead of the standard,
            "surrogate" conversation id:
        </para>
        
               <para>
                  <emphasis>Easy redirect to existing conversation</emphasis>
               </para>
               <para>
                  It can be useful to redirect to an existing conversation if 
                  the user requests the same operation twice. Take this example:
               
               <quote>
                  You are on ebay, half way through paying for an item you just 
                  won as a Christmas present for your parents. Lets say you're
                  sending it straight to them - you enter your payment details 
                  but you can't remember their address. You accidentally reuse 
                  the same browser window finding out their address. Now you 
                  need to return to the payment for the item.
               </quote>
       		</para>
               <para>
                  With a natural conversation its really easy to have the user
                  rejoin the existing conversation, and pick up where they left 
                  off - just have them to rejoin the payForItem conversation 
                  with the itemId as the conversation id.
               </para>
           
               <para>
                  <emphasis>User friendly URLs</emphasis>
               </para>
               
               <para>
                  For me this consists of a navigable 
                  hierarchy (I can navigate by editing the url) and a meaningful
                  URL (like this Wiki uses - so don't identify things by random 
                  ids). For some applications user friendly URLs are less 
                  important, of course.
               </para>

               <para>
                  With a natural conversations, when you are building your hotel 
                  booking system (or, of course, whatever your app is) you can 
                  generate a URL like 
                  <literal>http://seam-hotels/book.seam?hotel=BestWesternAntwerpen</literal> 
                  (of course, whatever parameter <literal>hotel</literal> maps 
                  to on your domain model must be unique) and with URLRewrite 
                  easily transform this to 
                  http://seam-hotels/book/BestWesternAntwerpen.
               </para>
               
               <para>
                  Much better!
               </para> 
           
       </section>
        <section>
            <title>Creating a natural conversation</title>        
            <para>
                Natural conversations are defined in <literal>pages.xml</literal>: 
            </para>
            
            <programlisting role="XML"><![CDATA[  <conversation name="PlaceBid"
                  parameter-name="auctionId"
                  parameter-value="#{auction.auctionId}"/>]]></programlisting>
                  
            <para>
                The first thing to note from the above definition is that the conversation
                has a name, in this case <literal>PlaceBid</literal>.  This name uniquely
                identifies this particular named conversation, and is used by the 
                <literal>page</literal> definition to identify a named conversation to participate
                in.
            </para>
                
            <para>
                The next attribute, <literal>parameter-name</literal> defines the request parameter 
                that will contain the natural conversation id, in place of the default conversation 
                id parameter. In this example, the <literal>parameter-name</literal> is <literal>auctionId</literal>.  
                This means that instead of a conversation parameter like <literal>cid=123</literal> 
                appearing in the URL for your page, it will contain <literal>auctionId=765432</literal> 
                instead.
            </para>
            
            <para>
                The last attribute in the above configuration, <literal>parameter-value</literal>,
                defines an EL expression used to evaluate the value of the natural business key to
                use as the conversation id.  In this example, the conversation id will be the primary
                key value of the <literal>auction</literal> instance currently in scope.
            </para>
            
            <para>
                Next, we define which pages will participate in the named conversation.
                This is done by specifying the <literal>conversation</literal> attribute for a
                <literal>page</literal> definition:
            </para>
            
            <programlisting role="XML"><![CDATA[  <page view-id="/bid.xhtml" conversation="PlaceBid" login-required="true">
      <navigation from-action="#{bidAction.confirmBid}">        
          <rule if-outcome="success">
              <redirect view-id="/auction.xhtml">
                  <param name="id" value="#{bidAction.bid.auction.auctionId}"/>
              </redirect>
          </rule>        
      </navigation>
  </page>]]></programlisting>
       
        </section>
        
        <section>
            <title>Redirecting to a natural conversation</title>
            
            <para>
                When starting, or redirecting to, a natural conversation there are a number
                of options for specifying the natural conversation name.  Let's start by looking at
                the following page definition:
            </para>
            
            <programlisting role="XML"><![CDATA[  <page view-id="/auction.xhtml">
    <param name="id" value="#{auctionDetail.selectedAuctionId}"/>
       
    <navigation from-action="#{bidAction.placeBid}">
      <redirect view-id="/bid.xhtml"/>
    </navigation>
  </page>]]></programlisting>
    
            <para>
                From here, we can see that invoking the action <literal>#{bidAction.placeBid}</literal>
                from our auction view (by the way, all these examples are taken from the seamBay example in Seam),
                that we will be redirected to <literal>/bid.xhtml</literal>, which, as we saw previously,
                is configured with the natural conversation <literal>PlaceBid</literal>.  The declaration for
                our action method looks like this:                
            </para>
            
            <programlisting role="JAVA"><![CDATA[   @Begin(join = true)
   public void placeBid()]]></programlisting>
   
            <para>
                When named conversations are specified in the <literal>&lt;page/&gt;</literal> element,
                redirection to the named conversation occurs as part of navigation rules, after the 
                action method has already been invoked.  This is a problem when redirecting to an
                existing conversation, as redirection needs to be occur before the action method is 
                invoked.  Therefore it is necessary to specify the conversation name when
                the action is invoked.  One way of doing this is by using the <literal>s:conversationName</literal>
                tag:
            </para>
            
            <programlisting role="XHTML"><![CDATA[  <h:commandButton id="placeBidWithAmount" styleClass="placeBid" action="#{bidAction.placeBid}">
    <s:conversationName value="PlaceBid"/>
  </h:commandButton>]]></programlisting>
  
            <para>
              Another alternative is to specify the <literal>conversationName</literal> attribute when
              using either <literal>s:link</literal> or <literal>s:button</literal>:
            </para>
            
            <programlisting role="XHTML"><![CDATA[  <s:link value="Place Bid" action="#{bidAction.placeBid}" conversationName="PlaceBid"/>]]></programlisting>
        
        </section>
    
    
    <section>
        <title>Workspace management</title>
        
        <para>
            Workspace management is the ability to "switch" conversations in
            a single window. Seam makes workspace management completely 
            transparent at the level of the Java code. To enable workspace
            management, all you need to do is:
        </para>
        
        <itemizedlist>
            <listitem>
            <para>
                Provide <emphasis>description</emphasis> text for each view id 
                (when using JSF or Seam navigation rules) or page node (when using 
                jPDL pageflows). This description text is displayed to the user 
                by the workspace switchers.
            </para>
            </listitem>
            <listitem>
                <para>
                Include one or more of the standard workspace switcher JSP
                or facelets fragments in your pages. The standard fragments
                support workspace management via a drop down menu, a list
                of conversations, or breadcrumbs.
                </para>
            </listitem>
        </itemizedlist>
        
        <section>
            <title>Workspace management and JSF navigation</title>
            <para>
                When you use JSF or Seam navigation rules, Seam switches to a
                conversation by restoring the current <literal>view-id</literal>
                for that conversation. The descriptive text for the
                workspace is defined in a file called <literal>pages.xml</literal>
                that Seam expects to find in the <literal>WEB-INF</literal>
                directory, right next to <literal>faces-config.xml</literal>:
            </para>
            
            <programlisting role="XML"><![CDATA[<pages>
    <page view-id="/main.xhtml">
        <description>Search hotels: #{hotelBooking.searchString}</description>
    </page>
    <page view-id="/hotel.xhtml">
        <description>View hotel: #{hotel.name}</description>
    </page>
    <page view-id="/book.xhtml">
        <description>Book hotel: #{hotel.name}</description>
    </page>
    <page view-id="/confirm.xhtml">
        <description>Confirm: #{booking.description}</description>
    </page>
    </pages>]]></programlisting>
            
           <para>
               Note that if this file is missing, the Seam application will
               continue to work perfectly! The only missing functionality
               will be the ability to switch workspaces.
           </para>

        </section>
        
        <section>
            <title>Workspace management and jPDL pageflow</title>
            <para>
                When you use a jPDL pageflow definition, Seam switches
                to a conversation by restoring the current jBPM process
                state. This is a more flexible model since it allows the
                same <literal>view-id</literal> to have different
                descriptions depending upon the current 
                <literal>&lt;page&gt;</literal> node. The description
                text is defined by the <literal>&lt;page&gt;</literal> 
                node:
            </para>
            
<programlisting role="XML"><![CDATA[<pageflow-definition name="shopping">

   <start-state name="start">
      <transition to="browse"/>
   </start-state>
   
   <page name="browse" view-id="/browse.xhtml">
      <description>DVD Search: #{search.searchPattern}</description>
      <transition to="browse"/>
      <transition name="checkout" to="checkout"/>
   </page>
   
   <page name="checkout" view-id="/checkout.xhtml">
      <description>Purchase: $#{cart.total}</description>
      <transition to="checkout"/>
      <transition name="complete" to="complete"/>
   </page>
   
   <page name="complete" view-id="/complete.xhtml">
      <end-conversation />
   </page>
   
</pageflow-definition>]]></programlisting>

        </section>
        
        <section>
            <title>The conversation switcher</title>
            
            <para>
                Include the following fragment in your JSP or facelets page
                to get a drop-down menu that lets you switch to any
                current conversation, or to any other page of the application:
            </para>
            
            <programlisting role="XHTML"><![CDATA[<h:selectOneMenu value="#{switcher.conversationIdOrOutcome}">
    <f:selectItem itemLabel="Find Issues" itemValue="findIssue"/>
    <f:selectItem itemLabel="Create Issue" itemValue="editIssue"/>
    <f:selectItems value="#{switcher.selectItems}"/>
</h:selectOneMenu>
<h:commandButton action="#{switcher.select}" value="Switch"/>]]></programlisting>
       
            <para>
                In this example, we have a menu that includes an item for each
                conversation, together with two additional items that let the
                user begin a new conversation.
            </para>
            
            <para>
                Only conversations with a description (specified in 
                <literal>pages.xml</literal>) will be included in the drop-down 
                menu.
            </para>
       
        <mediaobject>
          <imageobject role="fo">
            <imagedata fileref="images/switcher.png" align="center"/>
          </imageobject>
          <imageobject role="html">
            <imagedata fileref="images/switcher.png" align="center"/>
          </imageobject>
        </mediaobject>

        </section>
        
        <section>
            <title>The conversation list</title>
            
            <para>
                The conversation list is very similar to the conversation switcher,
                except that it is displayed as a table:
            </para>
            
            <programlisting role="XHTML"><![CDATA[<h:dataTable value="#{conversationList}" var="entry"
        rendered="#{not empty conversationList}">
    <h:column>
        <f:facet name="header">Workspace</f:facet>
        <h:commandLink action="#{entry.select}" value="#{entry.description}"/>
        <h:outputText value="[current]" rendered="#{entry.current}"/>
    </h:column>
    <h:column>
        <f:facet name="header">Activity</f:facet>
        <h:outputText value="#{entry.startDatetime}">
            <f:convertDateTime type="time" pattern="hh:mm a"/>
        </h:outputText>
        <h:outputText value=" - "/>
        <h:outputText value="#{entry.lastDatetime}">
            <f:convertDateTime type="time" pattern="hh:mm a"/>
        </h:outputText>
    </h:column>
    <h:column>
        <f:facet name="header">Action</f:facet>
        <h:commandButton action="#{entry.select}" value="#{msg.Switch}"/>
        <h:commandButton action="#{entry.destroy}" value="#{msg.Destroy}"/>
    </h:column>
</h:dataTable>]]></programlisting>

            <para>
                We imagine that you will want to customize this for your own application.
            </para>
            
        <mediaobject>
          <imageobject role="fo">
            <imagedata fileref="images/list.png" align="center" scalefit="1"/>
          </imageobject>
          <imageobject role="html">
            <imagedata fileref="images/list.png" align="center"/>
          </imageobject>
        </mediaobject>
        
            <para>
                Only conversations with a description will be included in the
                list.
            </para>
            
            <para>
                Notice that the conversation list lets the user destroy workspaces.
            </para>

        </section>
        
        <section>
            <title>Breadcrumbs</title>
            
            <para>
                Breadcrumbs are useful in applications which use a nested conversation
                model. The breadcrumbs are a list of links to conversations in the
                current conversation stack:
            </para>
            
            <programlisting role="XHTML"><![CDATA[<ui:repeat value="#{conversationStack}" var="entry">
    <h:outputText value=" | "/> 
    <h:commandLink value="#{entry.description}" action="#{entry.select}"/>
</ui:repeat]]></programlisting>
            
            <mediaobject>
              <imageobject role="fo">
                <imagedata fileref="images/breadcrumbs.png" align="center" scalefit="1"/>
              </imageobject>
              <imageobject role="html">
                <imagedata fileref="images/breadcrumbs.png" align="center"/>
              </imageobject>
            </mediaobject>
            
        </section>
    </section>
    
    <section>
        <title>Conversational components and JSF component bindings</title>

        <para>
            Conversational components have one minor limitation: they cannot be used to hold bindings 
            to JSF components. (We generally prefer not to use this feature of JSF unless absolutely
            necessary, since it creates a hard dependency from application logic to the view.) On a 
            postback request, component bindings are updated during the Restore View phase, 
            before the Seam conversation context has been restored.
        </para>
        
        <para>
            To work around this use an event scoped component to store the component bindings and inject 
            it into the conversation scoped component that requires it.
        </para>

        <programlisting role="JAVA"><![CDATA[@Name("grid")
@Scope(ScopeType.EVENT)
public class Grid
{
    private HtmlPanelGrid htmlPanelGrid;

    // getters and setters
    ...
}]]></programlisting>


        <programlisting role="JAVA"><![CDATA[@Name("gridEditor")
@Scope(ScopeType.CONVERSATION)
public class GridEditor
{
    @In(required=false)
    private Grid grid;
    
    ...
}]]></programlisting>

       <para>
         Also, you can't inject a conversation scoped component into an event
         scoped component which you bind a JSF control to. This includes Seam
         built in components like <literal>facesMessages</literal>. 
       </para>

       <para>
          Alternatively, you can access the JSF component tree through the implicit <literal>uiComponent</literal>
          handle. The following example accesses <literal>getRowIndex()</literal>of the
          <literal>UIData</literal> component which backs the data table during iteration, it prints
          the current row number:
       </para>

        <programlisting role="XHTML"><![CDATA[
<h:dataTable id="lineItemTable" var="lineItem" value="#{orderHome.lineItems}">
   <h:column>
      Row: #{uiComponent['lineItemTable'].rowIndex}
   </h:column>
   ...
</h:dataTable>]]></programlisting>

       <para>
          JSF UI components are available with their client identifier in this map.
       </para>

    </section>
    
    <section>
      <title>Concurrent calls to conversational components</title>
      
      <para>
        A general discussion of concurrent calls to Seam components can be 
        found in <xref linkend="concurrency" />. Here we will discuss 
        the most common situation in which you will encounter concurrency 
        &#8212; accessing conversational components from AJAX requests.
        We're going to discuss the options that a Ajax client library should
        provide to control events originating at the client &#8212; and we'll
        look at the options RichFaces gives you.
      </para>
    
      <para>
        Conversational components don't allow real concurrent access therefore
        Seam queues each request to process them serially.  This allows each
        request to be executed in a deterministic fashion. However, a simple
        queue isn't that great &#8212; firstly, if a method is, for some
        reason, taking a very long time to complete, running it over and over
        again whenever the client generates a request is bad idea (potential
        for Denial of Service attacks), and, secondly, AJAX is often to used
        to provide a quick status update to the user, so continuing to run the
        action after a long time isn't useful. 
      </para>
      
      <para>
        Therefore, when you are working inside a long running conversation,
        Seam queues the action event for a period of time (the concurrent 
        request timeout); if it can't process the event in time, it creates a 
        temporary conversation and prints out a message to the user to let them
        know what's going on.  It's therefore very important not to flood the 
        server with AJAX events!
      </para>
      
      <para>
        We can set a sensible default for the concurrent request timeout (in 
        ms) in components.xml:
      </para>
      
      <programlisting role="XML"><![CDATA[<core:manager concurrent-request-timeout="500" />]]></programlisting>
     
      <para>
        We can also fine tune the concurrent request timeout on a page-by-page
        basis:
      </para>
       
      <programlisting role="XML"><![CDATA[<page view-id="/book.xhtml" 
         conversation-required="true" 
         login-required="true"
         concurrent-request-timeout="2000" />]]></programlisting>

    <para>
      So far we've discussed serial AJAX requests - the client tells the
      server that an event has occur, and then rerenders part of the page based
      on the result.  This approach is great when the AJAX request is
      lightweight (the methods called are simple e.g. calculating the sum of a
      column of numbers).  But what if we need to do a complex computation? 
    </para>
    <para>
      For heavy computation we should use a truly asynchronous (poll based) 
      approach &#8212; the client sends an AJAX request to the server, which
      causes action to be executed asynchronously on the server (so the the
      response to the client is immediate); the client then polls the server 
      for updates.  This is useful when you have a long-running action for
      which it is important that every action executes (you don't want some to
      be dropped as duplicates, or to timeout).
    </para>
    
    <section>
      <title>How should we design our conversational AJAX application?</title>
    
    <para>
      Well first, you need to decide whether you want to use the simpler 
      "synchronous" request or whether you want to add using a poll-style
      approach.
    </para>
    
    <para>
      If you go for a "synchronous" approach, then you need to make an
      estimate of how long your AJAX request will take to complete - is it much
      shorter than the concurrent request timeout? If not, you probably want to
      alter the concurrent request timeout for this method (as discussed 
      above). Next you probably want a queue on the client side to prevent 
      flooding the server with requests.  If the event occurs often (e.g. a 
      keypress, onblur of input fields) and immediate update of the client is
      not a priority you should set a request delay on the client side. When
      working out your request delay, factor in that the event may also be
      queued on the server side.
    </para>
    
    <para>
      Finally, the client library may provide an option to abort unfinished
      duplicate requests in favor of the most recent.  You need to be careful
      with this option as it can lead to flooding of the server with requests
      if the server is not able to abort the unfinished request.
    </para>
    
    <para>
      Using a poll-style design requires less fine-tuning.  You just mark your
      action method <literal>@Asynchronous</literal> and decide on a polling
      interval:
    </para>
    
    <programlisting role="JAVA"><![CDATA[int total;

// This method is called when an event occurs on the client
// It takes a really long time to execute
@Asynchronous      
public void calculateTotal() {
   total = someReallyComplicatedCalculation();
}

// This method is called as the result of the poll
// It's very quick to execute
public int getTotal() {
   return total;
}]]></programlisting>
      </section>
      
      <section>
        <title>Dealing with errors</title>
        
        <para>
          However carefully you design your application to queue concurrent
          requests to your conversational component, there is a risk that the
          server will become overloaded and be unable to process all the
          requests before the request will have to wait longer than the 
          <literal>concurrent-request-timeout</literal>. In this case Seam will
          throw a <literal>ConcurrentRequestTimeoutException</literal> which can
          be handled in <literal>pages.xml</literal>. We recommend sending an
          HTTP 503 error: 
        </para>
        
        <programlisting role="XML"><![CDATA[   <exception class="org.jboss.seam.ConcurrentRequestTimeoutException" logLevel="trace">
      <http-error error-code="503" />
   </exception>]]></programlisting>
        
        <note>
          <title>503 Service Unavailable (HTTP/1.1 RFC)</title>
          
          <para>
            The server is currently unable to handle the request due to a 
            temporary overloading or maintenance of the server. The implication 
            is that this is a temporary condition which will be alleviated after 
            some delay.
          </para>
        </note>
        
        <para>
          Alternatively you could redirect to an error page:
        </para>
        
        <programlisting role="XML"><![CDATA[<exception class="org.jboss.seam.ConcurrentRequestTimeoutException" logLevel="trace">
   <end-conversation/>
   <redirect view-id="/error.xhtml">
      <message>The server is too busy to process your request, please try again later</message>
   </redirect>
</exception>]]></programlisting>
        
        <para>
           ICEfaces, RichFaces Ajax and Seam Remoting can all handle HTTP error
           codes. Seam Remoting will pop up a dialog box showing the HTTP error
           and ICEfaces will indicate the error in it's connection status 
           component. RichFaces Ajax provides the most complete support for
           handling HTTP errors by providing a user definable callback. For
           example, to show the error message to the user: 
        </para>
        
        <programlisting><![CDATA[<script type="text/javascript">
   A4J.AJAX.onError = function(req,status,message) { 
      alert("message");    
   };
</script>]]></programlisting>
      </section>
      
      <section>
        <title>RichFaces Ajax</title>
    
        <para>
           RichFaces Ajax is the AJAX library most commonly used with Seam, and
          provides all the controls discussed above:
        </para>
      
        <itemizedlist>
          <listitem>
            <para>
              <literal>eventsQueue</literal> &#8212; provide a queue in which 
              events are placed.  All events are queued and requests are sent to
              the server serially.  This is useful if the request can to the
              server can take some time to execute (e.g. heavy computation,
              retrieving information from a slow source) as the server isn't
              flooded.
            </para>
          </listitem>
          <listitem>
            <para>
              <literal>ignoreDupResponses</literal> &#8212; ignore the response 
              produced by the request if a more recent 'similar' request is
              already in the queue. ignoreDupResponses="true" does <emphasis>not 
              cancel</emphasis> the the processing of the request on the server
              side &#8212; just prevents unnecessary updates on the client side.
            </para>
            <para>
              This option should be used with care with Seam's conversations as
              it allows multiple concurrent requests to be made.
            </para>
          </listitem>
          <listitem>
            <para>
              <literal>requestDelay</literal> &#8212; defines the time (in ms.)
              that the request will be remain on the queue. If the request has
              not been processed by after this time the request will be sent 
              (regardless of whether a response has been received) or discarded
              (if there is a more recent similar event on the queue).
            </para>
            <para>
              This option should be used with care with Seam's conversations as
              it allows multiple concurrent requests to be made.  You need to be
              sure that the delay you set (in combination with the concurrent
              request timeout) is longer than the action will take to execute.
            </para>
          </listitem>
          <listitem>
            <para>
              <literal>&lt;a:poll reRender="total" interval="1000" /&gt;</literal> &#8212;
              Polls the server, and rerenders an area as needed
            </para>
          </listitem>
        </itemizedlist>
      </section>
  </section>
</chapter>