Metakit is a C++ database library, and is used as the persistence engine underneath WiKit. If it works as intended, you'll never have to deal with any of it, because one of its goals is to help you stay out of trouble.
It was developed, and is actively supported, by jcw.
As of 2012-11-30, the most recent release, 2.4.9.7, does not build against Tcl-8.6. Use the latest repository code instead.
DGP:
For what it's worth, here are the points I found most useful in the Roseman intro:
CMcC locally reformatted Metakit Documentation
MetaKit: Quick and Easy Storage for your Tcl Application1 ,a tutorial ,Mark Roseman ,2002-04-26
Relational Algebra & Metakit ,Brian Kelly ,Dr. Dobb's ,2004-12-01
Metakit C++ Tutorial , Riccardo Cohen ,2003-09-19
http://www.equi4.com/metakit/wiki.cgi/ was an official wiki for Metakit
"... many of the integrity checks required in traditional database use are not required in Metakit, because Metakit offers nested views -- a cleaner way to represent the relationships often enforced by integrity constraints."
"cleaner" is a matter of opinion, ask a relational database theorist. I don't want to start those arguments here, just point out that if you use Metakit for a complex system, you should design first, and be very careful. - EMJ
DKF: "You should design first"? That's actually true of any non-trivial program, you know... ;^) An ounce of thought saves a ton of effort.
alexshpilkin Metakit now has a extension for the Jim interpreter, too (not yet released as of Sep 3 2011, but available in Git) -- couldn't get in touch with jcw, so no search support for now. The documentation is at http://jim.berlios.de/documentation/metakit
jcw 2005-11-09: There were two pages called "Metakit" in this wiki, due to some old renaming glitches. I've manually adjusted things to resolve this (the "lost" copy was on page 478). Below is the original text.''
Metakit was previously the persistence engine for Wikit.
Roy Terry 2003-05-09: It was claimed here that Mk4tcl is part of the [ActiveTcl Batteries included release. This does not appear to be the case as of 8.4.2.
Hof: Metakit is included in lib/mk4tcl as of tcl 8.4.4, but without documentation, it seems.