From: matz@... (Yukihiro Matsumoto)
Date: 2001-04-11T11:56:03+09:00
Subject: [ruby-talk:13602] Re: persistent ruby?

Hi,

In message "[ruby-talk:13595] Re: persistent ruby?"
    on 01/04/10, Anders Bengtsson <ndrsbngtssn@yahoo.se> writes:

|Does anyone know how ObjectDBM compares to PStore?

It uses DBM, read/write cache, plus it allows nested transactions.
So theoretically, it must be far better than PStore, but I'm not sure
because I haven't used it.  PStore has been enough for my tasks.

|When I looked for a persistence mechanism I was a bit
|dissapointed by PStore, since it works by writing the
|entire object hierarchy to disk for each transaction.

PStore is just a proof-of-concept chunk of code.  Remember it's
implemented in just 143 lines of code (remarkable, isn't it?).
Thus avoiding unnecessary file update and saving each root data entry
into dbm or any other database would improve its performance
dramatically.

Here's the version which avoids unnecessary update using MD5.
This will be standard PStore in 1.7.

#
# How to use:
#
# db = PStore.new("/tmp/foo")
# db.transaction do
#   p db.roots
#   ary = db["root"] = [1,2,3,4]
#   ary[0] = [1,1.5]
# end

# db.transaction do
#   p db["root"]
# end

require "ftools"
require "md5"

class PStore
  class Error < StandardError
  end

  def initialize(file)
    dir = File::dirname(file)
    unless File::directory? dir
      raise PStore::Error, format("directory %s does not exist", dir)
    end
    unless File::writable? dir
      raise PStore::Error, format("directory %s not writable", dir)
    end
    if File::exist? file and not File::readable? file
      raise PStore::Error, format("file %s not readable", file)
    end
    @transaction = false
    @filename = file
    @abort = false
  end

  def in_transaction
    raise PStore::Error, "not in transaction" unless @transaction
  end
  private :in_transaction

  def [](name)
    in_transaction
    value = @table[name]
    if value == nil
      raise PStore::Error, format("undefined root name `%s'", name)
    end
    value
  end
  def []=(name, value)
    in_transaction
    @table[name] = value
  end
  def delete(name)
    in_transaction
    @table.delete name
  end

  def roots
    in_transaction
    @table.keys
  end
  def root?(name)
    in_transaction
    @table.key? name
  end
  def path
    @filename
  end

  def commit
    @abort = false
    throw :pstore_abort_transaction
  end
  def abort
    @abort = true
    throw :pstore_abort_transaction
  end

  def transaction
    raise PStore::Error, "nested transaction" if @transaction
    begin
      @transaction = true
      value = nil
      backup = @filename+"~"
      begin
	file = File::open(@filename, "r+")
	orig = true
      rescue Errno::ENOENT
	file = File::open(@filename, "w+")
      end
      file.flock(File::LOCK_EX)
      if orig
	content = file.read
	@table = Marshal::load(content)
	size = content.size
	md5 = MD5.new(content).digest
	content = nil		# unreference huge data
      else
	@table = {}
      end
      begin
	catch(:pstore_abort_transaction) do
	  value = yield(self)
	end
      rescue Exception
	@abort = true
	raise
      ensure
	unless @abort
	  file.rewind
	  content = Marshal::dump(@table)
	  if !md5 || size != content.size || md5 != MD5.new(content).digest
	    File::copy @filename, backup
	    begin
	      file.write(content)
	      file.truncate(file.pos)
	      content = nil		# unreference huge data
	    rescue
	      File::rename backup, @filename if File::exist?(backup)
	      raise
	    end
	  end
	end
	@abort = false
      end
    ensure
      @table = nil
      @transaction = false
      file.close
    end
    value
  end
end

if __FILE__ == $0
  db = PStore.new("/tmp/foo")
  db.transaction do
    p db.roots
    ary = db["root"] = [1,2,3,4]
    ary[1] = [1,1.5]
  end

  1000.times do
    db.transaction do
      db["root"][0] += 1
      p db["root"][0]
    end
  end

  db.transaction do
    p db["root"]
  end
end
---
							matz.