Class PStore
In: pstore.rb
Parent: Object

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

Methods

[]   []=   abort   commit   delete   fetch   new   path   root?   roots   transaction  

Classes and Modules

Class PStore::Error

Public Class methods

To construct a PStore object, pass in the file path where you would like the data to be stored.

[Source]

# File pstore.rb, line 89
  def initialize(file)
    dir = File::dirname(file)
    unless File::directory? dir
      raise PStore::Error, format("directory %s does not exist", 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

Public Instance methods

Retrieves a value from the PStore file data, by name. The hierarchy of Ruby objects stored under that root name will be returned.

WARNING: This method is only valid in a PStore#transaction. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 123
  def [](name)
    in_transaction
    @table[name]
  end

Stores an individual Ruby object or a hierarchy of Ruby objects in the data store file under the root name. Assigning to a name already in the data store clobbers the old data.

Example:

 require "pstore"

 store = PStore.new("data_file.pstore")
 store.transaction do  # begin transaction
   # load some data into the store...
   store[:single_object] = "My data..."
   store[:obj_heirarchy] = { "Kev Jackson" => ["rational.rb", "pstore.rb"],
                             "James Gray"  => ["erb.rb", "pstore.rb"] }
 end                   # commit changes to data store file

WARNING: This method is only valid in a PStore#transaction and it cannot be read-only. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 167
  def []=(name, value)
    in_transaction_wr()
    @table[name] = value
  end

Ends the current PStore#transaction, discarding any changes to the data store.

Example:

 require "pstore"

 store = PStore.new("data_file.pstore")
 store.transaction do  # begin transaction
   store[:one] = 1     # this change is not applied, see below...
   store[:two] = 2     # this change is not applied, see below...

   store.abort         # end transaction here, discard all changes

   store[:three] = 3   # this change is never reached
 end

WARNING: This method is only valid in a PStore#transaction. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 255
  def abort
    in_transaction
    @abort = true
    throw :pstore_abort_transaction
  end

Ends the current PStore#transaction, committing any changes to the data store immediately.

Example:

 require "pstore"

 store = PStore.new("data_file.pstore")
 store.transaction do  # begin transaction
   # load some data into the store...
   store[:one] = 1
   store[:two] = 2

   store.commit        # end transaction here, committing changes

   store[:three] = 3   # this change is never reached
 end

WARNING: This method is only valid in a PStore#transaction. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 229
  def commit
    in_transaction
    @abort = false
    throw :pstore_abort_transaction
  end

Removes an object hierarchy from the data store, by name.

WARNING: This method is only valid in a PStore#transaction and it cannot be read-only. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 177
  def delete(name)
    in_transaction_wr()
    @table.delete name
  end

This method is just like PStore#[], save that you may also provide a default value for the object. In the event the specified name is not found in the data store, your default will be returned instead. If you do not specify a default, PStore::Error will be raised if the object is not found.

WARNING: This method is only valid in a PStore#transaction. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 137
  def fetch(name, default=PStore::Error)
    unless @table.key? name
      if default==PStore::Error
        raise PStore::Error, format("undefined root name `%s'", name)
      else
        default
      end
    end
    self[name]
  end

Returns the path to the data store file.

[Source]

# File pstore.rb, line 203
  def path
    @filename
  end

Returns true if the supplied name is currently in the data store.

WARNING: This method is only valid in a PStore#transaction. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 198
  def root?(name)
    in_transaction
    @table.key? name
  end

Returns the names of all object hierarchies currently in the store.

WARNING: This method is only valid in a PStore#transaction. It will raise PStore::Error if called at any other time.

[Source]

# File pstore.rb, line 188
  def roots
    in_transaction
    @table.keys
  end

Opens a new transaction for the data store. Code executed inside a block passed to this method may read and write data to and from the data store file.

At the end of the block, changes are committed to the data store automatically. You may exit the transaction early with a call to either PStore#commit or PStore#abort. See those methods for details about how changes are handled. Raising an uncaught Exception in the block is equivalent to calling PStore#abort.

If read_only is set to true, you will only be allowed to read from the data store during the transaction and any attempts to change the data will raise a PStore::Error.

Note that PStore does not support nested transactions.

[Source]

# File pstore.rb, line 278
  def transaction(read_only=false)  # :yields:  pstore
    raise PStore::Error, "nested transaction" if @transaction
    begin
      @rdonly = read_only
      @abort = false
      @transaction = true
      value = nil
      new_file = @filename + ".new"

      content = nil
      unless read_only
        file = File.open(@filename, File::RDWR | File::CREAT)
        file.binmode
        file.flock(File::LOCK_EX)
        commit_new(file) if FileTest.exist?(new_file)
        content = file.read()
      else
        begin
          file = File.open(@filename, File::RDONLY)
          file.binmode
          file.flock(File::LOCK_SH)
          content = (File.read(new_file) rescue file.read())
        rescue Errno::ENOENT
          content = ""
        end
      end

      if content != ""
        @table = load(content)
        if !read_only
          size = content.size
          md5 = Digest::MD5.digest(content)
        end
      else
        @table = {}
      end
      content = nil             # unreference huge data

      begin
        catch(:pstore_abort_transaction) do
          value = yield(self)
        end
      rescue Exception
        @abort = true
        raise
      ensure
        if !read_only and !@abort
          tmp_file = @filename + ".tmp"
          content = dump(@table)
          if !md5 || size != content.size || md5 != Digest::MD5.digest(content)
            File.open(tmp_file, "w") {|t|
              t.binmode
              t.write(content)
            }
            File.rename(tmp_file, new_file)
            commit_new(file)
          end
          content = nil         # unreference huge data
        end
      end
    ensure
      @table = nil
      @transaction = false
      file.close if file
    end
    value
  end

[Validate]