Metamusing on object persistence

The need to "open up languages" has led to object-oriented programming languages with object-oriented implementations. By encapsulating the fundamental aspects of a language semantics within a set of default classes and giving t h e programmer t h e flexibility of deriving new versions of these base classes, a language whose semantics can be tailored to the needs of individual programmers can be provided. The degree of success in designing a language in this way to achieve flexibility and efficiency simultaneously is an open question. The Common Lisp Object System is designed with these techniques and we address this question by reporting our experience with the CLOS metaobject protocol in incorporating support for persistence into CLOS. For many aspects of our implementation we found that the metaobject protocol was perfectly suitable. In other cases we had to variously extend the protocol, pay an unacceptable performance penalty, or modify the language implementation directly. Based on our experience we propose some improvements to the protocol. We also present some performance measurements that reveal the need for improved language implementation techniques.