3 Direct and indirect links to libraries

From the CASL summary [LD98]:

  LIB-ID           ::= DIRECT-LINK | INDIRECT-LINK
  DIRECT-LINK      ::= direct-link URL
  INDIRECT-LINK    ::= indirect-link SIMPLE-ID+

A direct link to another library is simply written as its URL. The location of a library is always a directory, giving access not only to the the individual specifications defined by the current version of the library but also to archived versions, various indexes, and other documentation. An indirect link is written:

SI_1/.../SI_n

where each simple identifier SI_i is a valid file name as if for use in a URL.

An indirect link is interpreted as a URL with reference to the current global library directory.

Typically, libraries are referenced by indirect links. The global registration procedure includes the assignment of a unique indirect link to each library. Indirect links are structured like paths in file systems, allowing a hierarchical structuring of all the registered libraries. There is a global library directory, mapping a path to the URL pointing to the place where the library can actually be obtained.

Typically, the local file system of a site contains both registered libraries (resp. versions of libraries) which are publicly available, and local libraries (resp. versions of libraries) that are not publicly available, for example because they are still under construction. Still, the local libraries (or versions) need to be accessed from other sites when different sites develop a joint project. Therefore, it is possible to use direct links to other URLs.

The directory of the global library repository can be mirrored at different sites to prevent it from becoming a bottleneck due to server overcharge or failure. Immediate registration is not so crucial since it is always possible to refer to a local version.

Acknowledgements This study note is the outcome of a lunch discussion during ETAPS 98 in Lisbon with Peter D. Mosses, Grigore Rosu, Stefan Westmeier and the author. It benefitted from further discussions with Peter D. Mosses and Stefan Westmeier.

Disclaimer I am not at all an expert for version management, I just happen to know one (namely Stefan Westmeier). If some member of CoFI has better experiences with version management, please tell me.