3 Preferences

The ADS user interface is customized through the use of so-called HTTP persistent cookies (see [Kristol & Montulli 1997]). These "cookies'' are a means of identifying individual users. They are strings that are created by the server software. Web browsers that accept cookies store these identification strings locally on the user's computer. Anytime a user makes a request, the ADS software checks whether the requesting browser is capable of accepting cookies. If so, it sends a unique string to the browser and asks it to store this string as an identifier for that user. From then on, every time the same user accesses the ADS from that account, the browser sends this cookie back to the ADS server. The ADS software contains a database with a data structure for each cookie that the ADS has issued. The data structure associated with each cookie contains information such as the type of display the user prefers, whether tables should be used to format data, which mirror sites the user prefers for certain external data providers, the preferred printing format for ADS articles, and which journal volumes the user has read. It also can store the email address of the user and a fax number, in case the user wants to retrieve articles via email or fax.

The preference settings form includes a field for the user name as well as the email address. However, neither is necessary for the functioning of any of the features of the ADS. The system is completely anonymous. None of the information stored through this cookie system is made available to anybody outside the ADS. There is no open interface to this database and the database files are not accessible through the WWW. Any particular user can only access their own preferences, not the preferences set by any other user.

Most of these preferences can be set by the user through a WWW forms interface. Some fields in a user's preference record are for ADS internal use only. For instance the system is being used to display announcements to users in a pop-up window. The cookie database remembers when the message was last displayed, so that each message is displayed only once to each user.

This preference saving system also allows each user to store a query form with filled-in fields. This enables users to quickly query the ADS for a frequently used set of criteria.

The cookie identification system is implemented as a Berkeley DBM (DataBase Manager) database with the cookie as the record key. The data block that is stored in the database is a C structure. The binary settings (e.g. "Use Tables'', or "Use graphical ToC Page'') are stored as bits in several preference bytes. Other preferences are stored as bytes, short integer, or long integer, depending on their dynamic range.