Forum OpenACS Q&A: Re: History of OpenACS
OpenACS started as a PostgreSQL-only port of the Oracle-only ACS. I'm not sure what version of ACS OpenACS started with, but both ACS and OpenACS existed in parallel for some time, through ACS 3.2. ArsDigita continued the ACS line of development through one or two more 3.x versions, then 4.0 and 4.2, and then abandoned the codebase.
ACS 4.0 and 4.2 included a bunch of fairly radical changes from ACS 3.x, and unfortunatley as-released were never feature-complete relative to ACS 3.x. But, the OpenACS folks liked it enough to adopt it. So OpenACS picked up ACS 4.2, enhanced it to support both Oracle and PostgreSQL, made lots of other fixes and improvements, etc., and released the result as OpenACS 4.5. Then the OpenACS toolkit continued to evolve from there, to what you have today.
When ArsDigita stopped all further development of ACS 4.x, they shifted the "core team" to starting work on ACS Java. I don't remember when that was exactly, either late 2000 (Nov. or Dec.) or sometime in the first half of 2001. Also sometime in there, ArsDigita officially end-of-lifed "ACS Tcl" and pointed existing users to OpenACS.
I believe early versions of ACS Java would have had the data model in common with ACS 4.2, but probably nothing else, and then started diverging further from there. I don't really know though. I think Jun Yamog is familiar with it, ask either him, or the actual ACS Java / Byline developers.
So, the OpenACS codebase of today is the lineal descendent of ACS all the way back to ACS version 1.0 and earlier, and ACS Java was essentially a radical fork of, which the OpenACS maintainers knew about but had no interest in following.
As for why, back at the tail end of the dot-com boom, ArsDigita twice deprecated their old version of ACS before releasing a feature-complete new version, and tried to jump onto the Java bandwagon... Well, different people tell different stories. There's never been - and probably never will be - a truly definitive account, but plenty has been written on that, search around if you're curious. But again, if what you're interested in is the history of OpenACS, that's pretty peripheral.