Q. I expect to have tens of thousands of users, probably many more than that. Is the internal database the way to go, or how do you recommend I set up the site?
A. There is no hard-coded limit for the internal database, however if you have or are planning to have more than about 10,000 users it is advisable to use an ODBC database instead.
The internal database is designed to help get administrators up and running quickly For large numbers of users a commercial grade ODBC database such as Oracle or SQL Server is more appropriate.
Note that you can check both the internal database and an external ODBC database on a single AuthentiX protected directory. If the internal database doesn't find the user, AuthentiX will do a lookup in the ODBC database.
Should this ODBC server still prove to be a bottleneck, consider moving the database to its own dedicated machine. You should be able to scale the dedicated database machine up as large as you wish according to the recommendations of the database manufacturer.
If this is still not enough, consider an IP address round-robin system such as the one Microsoft uses. Then have multiple copies of your website on several different machines ("web-heads"). Install AuthentiX on each of them and connect the AuthentiX protected directories to the appropriate DSN.