Controlled access on an open project
Problem
You want to constrain respondents to a single questionnaire but also want to allow unknown respondents in.
Solution
You could use # Control by cookie but another, maybe more robust, solution is to add a font-end project to control access.
Discussion
The # Control by cookie activates a cookie-based control system which prevents the creation of a second questionnaire by a given computer. However, it is limited to fully open projects, works in callweb.cgi only, not in cwx.cgi, and is not immuned to users deleting the cookie or using a second browser to access more questionnaires.
The general solution proposed here is to create a small project (hereby called the gatekeeper) that is used to control access to the second, real questionnaire. The gatekeeper project requests the respondent's email address and emails a one-time use code to authenticate the user at that address. Once the code is correctly entered, the gatekeeper project looks up the _telkey corresponding to the email address in the second project or creates a record in the second project, and flows the respondent to that record in the main project. This can work even if the second project is closed. It can also allow or disallow access by email addresses that are not already prepopped into the second project.
Gatekeeper project
Here are example code snipplets.
Main project
The requirements of the main project are minimal.