Multiple Trace Composition and Its Uses

Traces are code attachments to variables that cause designated blocks of code to be executed on reads or writes to the given variable. Traces have numerous uses, including rules (\on do" statements), autoloading and initialization based on data access, trans- parent remote data access, paging and swapping, and persistence. Traces are usually limited to ad-hoc, hard-coded composition, making it difficult to place multiple traces on the same variable. Multiple traces are useful for putting a rule on a persistent variable, or several rules on a variable. This paper presents a design for a low-level mechanism for reasoning about and configuring multiple traces ona single variable. Although the work is based on a prototype using a modified version of Tcl's trace command, this mechanism easily applies to any language or library capable of implementing traps. This includes systems such as data breakpoints[WLG93] in C and overloadable get() and set() methods in prototype-based object-oriented systems. The specifics of the prototype are presented, including a facility for writing persistent Tcl scripts.