How to manage Struts web application unavailbility ?

Question

Hello !
I have a web application (portal) based on Struts framework. And I am wondering how is the best way to manage the portal unavailbility (for maintenance reasons for example) ?
We can deploy a new web application/page saying that "the portal is unavailable for the moment" but it is a heavy solution and not proper for people that are currently using the portal. So not good.
Maybe I should check in the init Action of the portal in a database if the portal is available or not... but how to manage people who are currently logged in ? should I check this state in each action of the portal ? it is quite repetitive...
Do you have any idea ?
Thanks for advice.

Answer 1

The simplest, lowest impact, lowest risk solution would be to use a filter:
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class AvailabilityFilter
implements Filter
   // private member variables for connection url/user/pass or JNDI
   // resource name
   // private getters and setters
   public void init( FilterConfig config )
      // Read necessary filter init-params here, like connection parameters
      // or the JNDI resource name of a connection pool used to determine
      // availability.
   public void destroy( )
   public void doFilter( ServletRequest request, ServletResponse response, FilterChain filterChain )
      if( this.isAvailable( ) )
         filterChain.doFilter( request, response );
      else if( response instanceof HttpServletResponse )
         HttpServletResponse httpResponse = (HttpServletResponse)response;
         response.sendError( 503, "Application temporarily unavailable" );
      else
         response.getWriter( ).println( "Application temporarily unavailable" );
         response.getWriter( ).close( );
   private boolean isAvailable( )
      // Attempt to connection to the database and/or check availability status
      // Return true if available, false otherwise
}Now, as far as managing existing user sessions goes, you can also implement an HttpSessionListener to track sessions:
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class SessionTracker
implements HttpSessionListener
    private static Map activeSessions = new HashMap( );
    public void sessionCreated( HttpSessionEvent event )
        HttpSession session = event.getSession( );
        synchronized( activeSessions )
           activeSessions.put( session.getId( ), session );
    public void sessionDestroyed( HttpSessionEvent event )
        HttpSession session = event.getSession( );
        synchronized( activeSessions )
           activeSessions.remove( session.getId( ) );
    public static void closeAllSessions( )
        synchronized( activeSessions )
            for( Iterator i = activeSessions.entrySet( ).iterator( ); i.hasNext( ); )
                Map.Entry entry = (Map.Entry)i.next( );
                String sessionId = (String)entry.getKey( );
                HttpSession session = (HttpSession)entry.getValue( );
                session.invalidate( );
                i.remove( );
}Some user event could call a servlet which would set a flag in the DB marking it as down for maintenance and then call the static method SessionTracker.closeAllSessions( ), which effectively logs everyone out. Any attempts to access the app after that period would fail; the filter would try to connect and/or check the flag in the DB and would throw an HTTP 503 instead of executing the rest of the filter chain. No reconfiguration would ever be necessary, and you wouldn't be changing code, just adding these two new classes and a couple of entries in web.xml.
Hope that helps,
- Jesse