appengine-pstore.rb - A PStore compatible interface using the Datastore.

appengine-pstore.rb

# = AppEngine::PStore -- 
#
# A PStore compatible interface using the Datastore.
# This module is based on pstore.rb written by matz.
#
require 'pstore'
require 'digest/md5'
require 'rubygems'
require 'appengine-apis/datastore'

module AppEngine; end

# AppEngine::PStore is compatible with PStore interface.
# It uses the Datastore to privide a persistance mechanism.
class AppEngine::PStore < PStore

  # call-seq:
  #   db = AppEngine::PStore.new('foo/bar.pstore')
  #
  # Creates a new database identified by filename
  #
  # Args:
  #  - filename: String. A key to identify database.
  #
  def initialize(filename)
    @kind = self.class.name
    @transaction = false
    @filename = filename
    @abort = false
  end

  # call-seq:
  #   db.transaction do |db|
  #     db[:somekey] = somevalue
  #     puts db[:anotherkey]
  #   end
  #
  # Opens a new transaction for the data store.
  #
  # Args:
  #   - read_only: Boolean.
  #
  def transaction(read_only = false)
    raise PStore::Error, "nested transaction" if @transaction
    begin
      @rdonly = read_only
      @abort = false
      @transaction = true
      value = nil

      key = AppEngine::Datastore::Key.from_path(@kind, @filename)
      begin
        entity = AppEngine::Datastore.get(key)
        if !read_only
          size = entity[:content].size
          md5 = Digest::MD5.digest(entity[:content])
        end
        @table = load(entity[:content])
      rescue AppEngine::Datastore::EntityNotFound
        @table = {}
      end

      begin
        catch(:pstore_abort_transaction) do
          value = yield(self)
        end
      rescue Exception
        @abort = true
        raise
      ensure
        if !read_only and !@abort
          content = dump(@table)
          if !md5 || size != content.size || md5 != Digest::MD5.digest(content)
            entity = AppEngine::Datastore::Entity.new(key)
            entity[:content] = content
            AppEngine::Datastore.put(entity)
          end
        end
      end
    ensure
      @table = nil
      @transaction = false
    end
    value
  end

end