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From: noren@dinl.uucp (Charles Noren)
Newsgroups: comp.object,comp.databases
Subject: Re: Object-Oriented Databases and Memory Mapped Files
Message-ID: <1693@dinl.mmc.UUCP>
Date: 13 Aug 90 21:38:10 GMT
References: <1990Aug13.095316@shambhala.Berkeley.EDU>
Reply-To: noren@dinl.UUCP (Charles Noren)
Organization: Martin Marietta I&CS, Denver CO.
Lines: 40

In article <1990Aug13.095316@shambhala.Berkeley.EDU> ricks@berkeley.edu writes:
>One quick method of implementing a persistent store for a language 
>is to layout the persistent store just like main memory (including
>pointers) and use memory mapped files or shared memory to
>transparently map the data from disk to the appropriate location in
>memory...

This will provide a persistent store for a single process.  Other things
are needed to make it a database, notably concurrancy protection (read
and write locks) to prevent bad things from happening when more than
one process access the same object in memory.

At least one Object-Oriented Database (OODB) company does this,
Object Design Inc. (Eastern US phone: (617) 270-9797, Western U.S.
phone: (213) 412-8411).  Their product, ObjectStore (which is in
Beta right now, currently works only on Sun 3's and 4's with more to
come soon) puts database fetches into physical memory and maps the
process address space into it.  The C++ process referencing the data
uses C++ dereferencing mechanisms to access the data making for very
fast access for data in cache memory.  For data referenced outside of
cache, the ObjectStore runtime environment catches the segmentation
violation signal and determines what to bring in an remaps the
process address space into it.

We are in the process of purchasing ObjectStore from Object Design
because our evaluation of it (and particularly, the evaluation done
by another project in our company) has found this database to be very
fast.  The database is also extremely easy to use, basically you're
just writing C++ code to use the database.

...now if I can just coax Objective-C to use ObjectStore in the same way,
what a combination that would make!


-- 
Chuck Noren
NET:     dinl!noren@ncar.ucar.edu
US-MAIL: Martin Marietta I&CS, MS XL8058, P.O. Box 1260,
         Denver, CO 80201-1260
Phone:   (303) 971-7930