PStore

PStore implements a file based persistance mechanism based on a Hash. User code can store hierarchies of Ruby objects (values) into the data store file by name (keys). An object hierarchy may be just a single object. User code may later read values back from the data store or even update data, as needed.

The transactional behavior ensures that any changes succeed or fail together. This can be used to ensure that the data store is not left in a transitory state, where some values were upated but others were not.

Behind the scenes, Ruby objects are stored to the data store file with Marshal. That carries the usual limitations. Proc objects cannot be marshalled, for example.

Usage example:

 require "pstore"

 # a mock wiki object...
 class WikiPage
   def initialize( page_name, author, contents )
     @page_name = page_name
     @revisions = Array.new

     add_revision(author, contents)
   end

   attr_reader :page_name

   def add_revision( author, contents )
     @revisions << { :created  => Time.now,
                     :author   => author,
                     :contents => contents }
   end

   def wiki_page_references
     [@page_name] + @revisions.last[:contents].scan(/\b(?:[A-Z]+[a-z]+){2,}/)
   end

   # ...
 end

 # create a new page...
 home_page = WikiPage.new( "HomePage", "James Edward Gray II",
                           "A page about the JoysOfDocumentation..." )

 # then we want to update page data and the index together, or not at all...
 wiki = PStore.new("wiki_pages.pstore")
 wiki.transaction do  # begin transaction; do all of this or none of it
   # store page...
   wiki[home_page.page_name] = home_page
   # ensure that an index has been created...
   wiki[:wiki_index] ||= Array.new
   # update wiki index...
   wiki[:wiki_index].push(*home_page.wiki_page_references)
 end                   # commit changes to wiki data store file

 ### Some time later... ###

 # read wiki data...
 wiki.transaction(true) do  # begin read-only transaction, no changes allowed
   wiki.roots.each do |data_root_name|
     p data_root_name
     p wiki[data_root_name]
   end
 end