<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> <HTML> <HEAD> <META NAME="Generator" CONTENT="MS Exchange Server version 6.0.6249.1"> <TITLE>[plt-scheme] Re: Shared resources for servlets</TITLE> </HEAD> <BODY dir=ltr> <DIV>I'm not sure that would work.</DIV> <DIV>Consider this:</DIV> <DIV>3 readers are reading. A writer comes and blocks. Then a reader comes and joins in, blocking the writer even longer. This could block indefinitely, which won't be very good, would it?</DIV> <BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px"> <DIV>-----Original Message----- From: Keith Frost [mailto:keithf@amnis.com] Sent: ג 09/09/2003 18:31 To: plt-scheme@list.cs.brown.edu Cc: Subject: [plt-scheme] Re: Shared resources for servlets </DIV>   For list-related administrative tasks:   <A href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</A> If I'm not mistaken, you've answered your own question:  just use a module. When subsequent servlets 'require' the same module, it is not reloaded, and any local state in the module is preserved.  If you need to serialize access to resources controlled by the module, you can create a semaphore to do so (see 'MzScheme Language Manual: Section 7.4'), so that servlets may block while waiting for the resource to become available. Which brings to mind a question I've had in the back of my mind, and I expect someone on this list could help me with.  Consider the common resource model "One writer -- many readers".  In other words, if a reader goes to access the resource, it needs only check whether there is a writer accessing it.  If so, it blocks until the writer releases it.  If not, it may go ahead, so that any number of readers can be simultaneously accessing the resource (a database, or even a file, say).  If a writer goes to access the resource, it has to check if there is another writer, or any readers, currently accessing it.  If so, it blocks until all readers and any writer have released it. What is the simplest, or a recommended, way to achieve this resource pattern using MzScheme's threads and semaphores? / / </BLOCKQUOTE> </BODY> </HTML>