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From: bugzilla@apache.org
To: tomcat-dev@jakarta.apache.org
Subject: DO NOT REPLY [Bug 34615]  New:  -
    Unecessary cluster replication of DeltaRequest AttributeInfo objects
Message-Id: <20050426085327.847942DE@ajax.apache.org>
Date: Tue, 26 Apr 2005 10:53:27 +0200 (CEST)

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http://issues.apache.org/bugzilla/show_bug.cgi?id=34615

           Summary: Unecessary cluster replication of DeltaRequest
                    AttributeInfo objects
           Product: Tomcat 5
           Version: 5.0.30
          Platform: PC
        OS/Version: Windows 2000
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Catalina:Cluster
        AssignedTo: tomcat-dev@jakarta.apache.org
        ReportedBy: chris@skipoles.co.uk


Setup:
Two Tomcat 5.0.30 servers arranged in a cluster with synchronous replication
using multicast.
Apache server using mod_jk2 to load balance between the two servers in a
round-robin manner (no sticky sessions)

Application:
Initially discovered the problem while testing clustering of the wicket
framework. However, problem can be replicated with small servlet (see bottom of
description). This servlet handles two requests:

1) Request arrives to servlet (on serverA), which sets a session attribute and
then outputs an HTML page. The HTML page contains a style sheet link with an
href back to the same servlet.
2) Request for dynamically generated stylesheet arrives (on serverB). The
session is not accessed and the css content is output.

I added debug to org.apache.catalina.cluster.session.DeltaRequest and what I see
happening is as follows:

1) Request to servlet arrives on serverA. Servlet sets a session attribute and
returns the HTML content.
2) ServerA sends the delta request containing one AttributeInfo object to
serverB and then resets its actions list.
3) ServerB receives the delta request containing the one AttributeInfo object
4) DeltaRequest.execute(DeltaSession) sets the attribute on the session.
However, it DOESN'T remove the AttributeInfo object from the actions list.

5) Request to servlet arrives on serverB (to retrieve the css content). Servlet
returns the css content but does not update the session.
6) DeltaManager.requestCompleted(String) is called on serverB. This checks if
the actions list of the delta request is non-empty.
7) The actions list is non-empty because it contains the AttributeInfo object
from the previous synchronization (4). Thus, the AttributeInfo object is sent
back to serverA (which starts again at 3). 

The net result is that the same AttributeInfo object bounces back and forth
between the two servers for each request that come in (regardless of whether the
request alters the session or not). When the original page contains many
dynamically generated images (retrieved concurrently by the browser) the
externalization code finally falls in a heap with EOF and NPE problems due to
the two machines trying to synchronize different copies of the same
AttributeInfo object.

The solution to the problem is to change the DeltaRequest.execute(DeltaSession)
method to remove the AttributeInfo objects from the actions list after it has
finished processing them:

public synchronized void execute(DeltaSession session) {
    .... 
        }//switch
    }//for
>>  reset();  
    session.endAccess();
}

This appears to solve the problem.

Example servlet (if anyone wants to test this out):
public class MyAppServlet extends HttpServlet {

    protected void doGet(final HttpServletRequest request, final
HttpServletResponse response)
            throws ServletException, IOException {
        final String action = request.getParameter("action");
        if ( "css".equals(action) ) {
            response.setContentType("text/css");
            PrintWriter out = response.getWriter();
            out.println(".someStyle {\n    padding: 5px;\n}");
        }
        else {
            request.getSession().setAttribute("test", "value");
            response.setContentType("text/html");
            PrintWriter out = response.getWriter();
            out.println("<html>\n<head>\n<title>Test Page</title>");
            out.println("    <link rel=\"stylesheet\"
href=\"/myapp/myapp?action=css\"></script>");
            out.println("</head>\n<body>\n    <h2>Test
Page</h2>\n</body>\n</html>");
        }
    }

    protected void doPost(final HttpServletRequest request, final
HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

}

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