From: Erik Veenstra <erikveen@...>
Date: 2007-06-27T22:46:02+09:00
Subject: Re: Fun about inspect

> What do you think?

In one specific situation, I use a journal to resemble a small
DB. The reason to use a journal is the requirement to be able
to go back to _any_ moment in time.

When adding, changing or deleting an object to the hash, the
corresponding line is written to the journal. For speed, the
hash and the length and MD5 checksum of the journal are dumped
(using Marshal) to a file at the end of the program. On
startup, this file is loaded and the length and MD5 checksum of
the journal are checked. If the length and MD5 checksum don't
match or Marshal.load fails for whatever reason, the database
is rebuild using the journal (which might take a while... :}).
This gives us the possibility to go back in time. And it's a
nice way to ensure that all data gets effectively written when
the application ends.

I even added commits to the journal. When loading the journal,
everything after the last commit is disregarded.

Loading the journal is done with the code below (simplified).

As long as the objects inspect to Ruby syntax, this approach
works fine.

gegroet,
Erik V. - http://www.erikveen.dds.nl/

----------------------------------------------------------------

 # journal

 [{:action=>:added, :timestamp=>"20070627140031"},
{:id=>"aaa", :something=>17}]
 [{:action=>:added, :timestamp=>"20070627140031"},
{:id=>"bbb", :something=>18}]
 [{:action=>:changed, :timestamp=>"20070627140031"},
{:id=>"bbb", :something=>19}]
 [{:action=>:deleted, :timestamp=>"20070627140033"}, {:id=>"bbb"}]

----------------------------------------------------------------

 # read_journal.rb

 items   = {}

 File.open("journal") do |f|
   f.readlines.each do |line|
     raise "line doesn't end with \\n, probably caused by a previous
IO error [#{line}]" unless line[-1..-1] == "\n"

     action, item        = Thread.new{$SAFE=4; eval(line,
Module.new.module_eval{binding})}.value

     case action[:action]
     when :added         then items[item[:id]] = item
     when :changed       then items[item[:id]] = item
     when :deleted       then items.delete(item[:id])
     else
       raise "unknown action [#{action.inspect}]"
     end
   end
 end

 p items

----------------------------------------------------------------