>From a WWC project, I have a global variable which indicates if all my files are opened or not. On the first transaction, it'll open the required files and the flag will be set.
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>However, from a Web Service, should we use the same approach? Is the same technique applicable?

No. WWWC sessions are able to hold state because a process keeps running between hits (with file based), but you cannot assume the same thing with a Web Service. With both ASP and ISAPI based web services, plan on your object being killed after every hit, closing all tables and losing all state.