Thin Air

Everything about versioning

Questions on the versioning model

Bruce Badger posted a comment in response to my post on the versioning model used in Monticello 2. He has some questions about methods:

  • What is the identity (or primary key) of a method?
  • Within what scope is the identity unique?
  • If I wanted to use a particular version of a particular method in two classes, could I (setting asside the question of whether this is a good idea or not)?

The short answer is that Monticello two uses the same semantics that the Smallltalk runtime uses. The identity of a MethodElement is class name and selector; it's only guaranteed to be unique within a given image. You couldn't put the same method in two classes, it would have to be copied.

Now, Avi and I have kicked around ideas for a deeper model of Smalltalk code. Rather than identifying elements by name, they'd each have UUIDs. Method sources would be versioned as an AST. The nodes for variable references would have the UUIDs of the elements the variables are bound to in the compiled method.

This would have two advantages:

First, it would help with platform independence. Rather than depending on names to bind variables during compilation, we'd be relying on UUIDs. This would make it easier to transform the names when moving code back and forth between dialects. This would make it easier to handle Namespaces in VW, for example, or differences in platform libraries.

Second, it would allow us to provide a more accurate reproduction of code between images. We'd be restoring methods to their compiled states rather than just their source code. This is one of the things that's so compelling about Spoon, and it would allow Bruce's scenario of the same method version being used in two different classes.

On the other hand, it's that much more code and complexity. It would require a custom parser, an AST able to handle all the syntactic quirks of the various dialects of Smalltalk where Monticello will run, and a compiler back end for each platform. Monticello 2 is already an ambitious project, and a significant improvement over Monticello 1. Our goal for now is to get the current version up to production quality so we can start using it. Maybe some of these ideas will be part of Monticello 3.

Posted in monticello versioning smalltalk