The following references and ideas may be of use to people interested in writing an SQL back end for Subversion. * Glenn Thompson's pluggable FS document A new filesystem implementation has to plug into Subversion alongside the BDB one. Currently, we have a framework allowing complete reimplementations of the FS API to plug into Subversion alongside each other, but it may (or may not) be more prudent to abstract the BDB implementation at the table level so that the same code can use multiple database implementations. Glenn Thompson wrote a document about FS abstraction, located at <http://www.cdrguys.com/subversion/>. This document predates the FSFS implementation, so it is a bit dated, but it is still highly relevant to the hypothetical table-level abstraction described above. The document posits two abstraction layers, the FSAP (filesystem abstract provider) layer which abstracts at the API level, and the FSP (filesystem provider) layer which abstracts at the table level. The FSP layer would only apply to libsvn_fs_base, which implements the BDB back end. The document's FSAP abstraction proposal was used as a basis (though not as gospel) for the API-level abstraction we currently have. * Edmund Horner's prototype MySQL implementation http://homepages.paradise.net.nz/~ejrh/subversion/mysql/ describes a prototype implementation of a MySQL back end, also made before the FSFS implementation. There is a little more about this back end in the mail thread at http://www.contactor.se/~dast/svn/archive-2004-02/0470.shtml.