In Seam, there are four different ways to start a conversation (or more precisely, to promote a temporary conversation to a long-running conversation, or LRC). Remember that there may be at most one active LRC per tab or window in a session (browser instance).

1) using annotations: @Begin

2) using XML: <begin-conversation/>

3) using Seam tags: <s:link propagation="begin"> or <s:button propagation="begin">

4) using code: Conversation.begin()

So this discussion focuses on why and how to do 1-4 above.

In most situations, the easiest way to start a conversation in Seam is declaratively using something like the following:

@Begin(join=true)
public void doSomething() {
   ...
}

When the method has completed processing (i.e. no RuntimeExceptions or ApplicationExceptions are thrown), then the current temporary conversation for the current thread/request is promoted to a LRC and assigned a conversationId.

From SiA book:

For many of Seam’s annotations that designate an action to be performed, such as @Begin and @End, Seam only performs the action if the method completes successfully according to Seam’s definition of success. For Seam to consider a method call successful, it must return without throwing an exception and it must return a non-null value unless it's a void method.

Sometimes you need a conversation to start a conversation prior to rendering a page:

<page view-id="/Process.xhtml">
    <navigation from-action="#{foo.addCustomer}">
        <begin-conversation/>
        <redirect view-id="/addCustomers.xhtml"/>
    </navigation>
</page>

From SiA book:

The benefit of using a page action is that it can start a long-running conversation from a bookmarked page or direct link, rather than waiting for a JSF postback.

There may be times when you need to start a conversation from a GET request, in which case either the <begin-conversation> page descriptor tag or a UI component tag is a more appropriate choice.

Example:

<s:link id="register" view="/register.xhtml" value="Register New User" propagation="begin"/>

<s:button id="register" view="/register.xhtml" value="Register New User" propagation="begin"/>

And sometimes you need fine-grained control of when the LRC begins. In these cases, you should use the Conversation API.

Example:

public void myAction(){
   if(doSomething()) {
       Conversation.instance().begin();
   }
   else{
       ...
   }
}