Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!pt.cs.cmu.edu!rochester!fulk
From: fulk@cs.rochester.edu (Mark Fulk)
Newsgroups: comp.sys.mac
Subject: Re: America Online
Message-ID: <1989Dec13.163928.23588@cs.rochester.edu>
Date: 13 Dec 89 16:39:28 GMT
References: <1989Dec9.170635.22759@cs.rochester.edu> <24891@cup.portal.com> <1989Dec11.165834.7605@cs.rochester.edu> <24941@cup.portal.com>
Reply-To: fulk@cs.rochester.edu (Mark Fulk)
Organization: University of Rochester Computer Science Department
Lines: 55


>"So what solution suggests itself?  Storing the menus on the user's Mac is
>clearly needed." Unfortunately, this would do away with one of AO's main
>features: the ability to dynamically change the way screens look. AO can
>these days, though). Enjoy, -Russ

Once again, you didn't read the whole posting.  Caching means storing
the most recent version on disk, AS RETRIEVED FROM AMERICA ONLINE.
This does not mean permanently storing menus.  The cache must be kept
up-to-date; the current buzzword for this notion is "coherent memory".
The algorithm is simplicity itself:

WHEN the user calls up item-name DO

  IF an item with name item-name is in a file THEN
    SEND <"IS" item-name "DATED" item-date "CURRENT?"> to america-online;
    GET ao-message FROM america-online;
    IF ao-message = <"UPDATE" item-name "DATE" new-date "INFO" update-info>
      THEN
      apply update info to the file of the item;
      set date of the file of the item to new-date;
    ELSE
      (*message is "ITEM" item-name "UP-TO-DATE"*)
      DO NOTHING;
    END IF
  ELSE (*no version on disk*)
    SEND <"SENDME" item-name> to america-online;
    GET item FROM america-online;
    WRITE item in a file;
  END IF
  DISPLAY item from its file;

END WHEN

Floppy-only systems only send SENDME messages, and never store items on
the disk.  The choice is a preference item.

There is a second version of the algorithm which would work better if
turnaround to the AO computers is poor:

  When the user signs on, send the creation dates of all the menus
  and most-used on-line files.

  When a menu or file gets updated, broadcast the news to the machines of all
  those people who are logged on.

  When a user requests a menu or frequently-used file, his computer would
  know whether or not it is up-to-date, and wouldn't have to have a
  conversation with AO to establish the case.  In other cases, the
  previous algorithm is used.

This would lengthen the sign-on process by several seconds, but, since
it is long anyway, that won't matter much.

Mark