Well, never having tried it, I don't know for sure :)
But, since we're only talking about a pretty small static object in memory, I assume that would be better-performing than writing out to a database (although obviously you have no persistence, but I don't think that's a problem when talking about sessions). And although there is a critical section involved, since it's only during logon and not every request, I don't imagine it's a significant problem. If your just recording the session ID and not the extra info I do later on, I suspect it would scale just fine.
The one problem I can see is if your talking about a distributed environment. Maybe in that case writing out to a database is a better idea. Certainly it's trivial to do that from the listener. You avoid the critical section then, so maybe the trade-off between that and the extra hit of the database access makes it a wash. If you kept it in-memory, it might even be OK because the AppConfig class should be serializable, so theoretically it should be able to get replicated, but I certainly wouldn't go that route, I'd write it to a database and not deal with any potential synchronization issues at all. That means you probably want to write a Struts plug-in, or something else to run at startup, to clear out the database, but that's also not a big deal.
So, while I don't know for sure that it will scale to 250 users, I can't see any real problem with it doing so, certainly if it's a single server I'd see it's probably fine as-is, a cluster might require going to a database instead.
Frank
From: Alex <[EMAIL PROTECTED]> Reply-To: "Tomcat Users List" <[EMAIL PROTECTED]> To: Tomcat Users List <[EMAIL PROTECTED]> Subject: RE: list active sessions. Date: Mon, 21 Jun 2004 09:31:33 -0400 (EDT)
How would your application cope if it were required to scale up to 250 concurrent users ?
It's an interesting approach. Currently I record limited information to a db. All I really want is to be able to list all the active session id's...nothing more. If it's being done already, why do it again...
On Mon, 21 Jun 2004, Frank Zammetti wrote:
> Date: Mon, 21 Jun 2004 09:45:05 -0400
> From: Frank Zammetti <[EMAIL PROTECTED]>
> Reply-To: Tomcat Users List <[EMAIL PROTECTED]>
> To: [EMAIL PROTECTED]
> Subject: RE: list active sessions.
>
> I spent a couple of days last week implementing just such a thing, so I feel
> qualified to answer :)
>
> There is no easy way to do it. There USED to be a SessionContext object
> available in the servlet spec that would allow you to do a lot of cool
> things with sessions, but it was removed as of spec 2.1 I think because Sun
> believed it to be a security risk. Unfortunately there was nothing added to
> take it's place.
>
> The way you have to do this, or at least one way (the only way I found) is
> to track it yourself.
>
> First, I already had an AppConfig object that contains a static HashMap.
> This has a bunch of config values for my app loaded from a config file at
> startup. I then added an activeSessions HashMap to that class. Create a
> similar class for yourself, along the lines of the following:
>
> import java.util.HashMap;
> public class AppConfig {
> private static HashMap activeUsers = null;
> public static HashMap activeUsers() {
> return activeUsers;
> }
> public static void setActiveUsers(HashMap inActiveUsers) {
> activeUsers = inActiveUsers;
> }
> }
>
> Then, create a SessionListener something like the following:
>
> package com.mycompany.myapp;
> import java.util.HashMap;
> import javax.servlet.http.HttpSession;
> import javax.servlet.http.HttpSessionEvent;
> import javax.servlet.http.HttpSessionListener;
> public class SessionListener implements HttpSessionListener {
> public void sessionCreated(HttpSessionEvent se) {
> HashMap activeUsers = (HashMap)AppConfig.getActiveUsers();
> synchronized (activeUsers) {
> activeUsers.put(se.getSession().getId(), new HashMap());
> }
> }
> public void sessionDestroyed(HttpSessionEvent se) {
> HashMap activeUsers = (HashMap)AppConfig.getActiveUsers();
> synchronized (appConfig) {
> activeUsers.remove(se.getSession().getId());
> }
> }
> }
>
> Then just add the following to web.xml after your <servlet> section:
>
> <listener>
> <listener-class>com.mycompany.myapp.SessionListener</listener-class>
> </listener>
>
> Basically, every time a session is created, you'll get an entry in the
> activeUsers HashMap keyed by sessionID, and the record will be removed when
> the session is destroyed. Further, what I do is that when my logon Action
> is called, when the user is validated I add some information for that user
> into the HashMap (first name, last name, logon time, etc). This allows me
> to have a pretty nice little tracking display in my app.
>
> The one problem you run into with this is that if the user just closes the
> browser window rather than using your nice logout function, the session
> lingers until the timeout period elapses. I didn't like that! My app is
> frames-based, and I already had one hidden frame, so I added the following
> to the <body> tag of that frame's source document:
>
> onUnload="openLogoffPopup();"
>
> ... and then the openLogoffPopup() function:
>
> function openLogoffPopup() {
> windowHandle = window.open("", "", "width=200,height=1,top=1,left=1");
> str = "<" + "html><" + "head><" + "title><" + "/title><" + "/head>";
> str += "<" + "body onLoad=\"window.location='<%=request.getContextPath()
> + "/app/logoff.app"%>';\">";
> str += "<" + "table width=\"100%\" height=\"100%\" border=\"0\">";
> str += "<" + "tr><" + "td align=\"center\" valign=\"middle\">";
> str += "<" + "span
> style=\"color:#000000;font-family:arial;font-size:11pt;font-weight:bold;\">";
> str += "Logging out of application...<" + "/span";
> str += "<" + "/td><" + "/tr>";
> str += "<" + "/table>";
> str += "<" + "/body><" + "/html>";
> windowHandle.document.write(str);
> windowHandle.document.close();
> }
>
> That calls the logoff Action whcih does not much more than
> session.invalidate(). This works well in IE, I do not know if it is
> cross-browser though (not a concern for my company). It should work fine to
> add this to all your JSP's, assuming your app isn't frame-based, I think
> it'll work just the same. But, maybe you can live with the session
> lingering if the window is closed anyway. It's probably not a big concern
> if the timeout period is short enough, but you need to recognize that you
> may see more than one session per user for a few minutes if they log on
> again.
>
> Hope that helps!
>
> Frank
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