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  • pstore.rb

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PStore

PStore implements a file based persistence 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 updated 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

Transaction modes

By default, file integrity is only ensured as long as the operating system (and the underlying hardware) doesn’t raise any unexpected I/O errors. If an I/O error occurs while PStore is writing to its file, then the file will become corrupted.

You can prevent this by setting pstore.ultra_safe = true. However, this results in a minor performance loss, and only works on platforms that support atomic file renames. Please consult the documentation for ultra_safe for details.

Needless to say, if you’re storing valuable data with PStore, then you should backup the PStore files from time to time.

Constants

EMPTY_MARSHAL_CHECKSUM
EMPTY_MARSHAL_DATA
EMPTY_STRING

Constant for relieving Ruby’s garbage collector.

RDWR_ACCESS
RD_ACCESS
WR_ACCESS

Attributes

ultra_safe[RW]

Whether PStore should do its best to prevent file corruptions, even when under unlikely-to-occur error conditions such as out-of-space conditions and other unusual OS filesystem errors. Setting this flag comes at the price in the form of a performance loss.

This flag only has effect on platforms on which file renames are atomic (e.g. all POSIX platforms: Linux, MacOS X, FreeBSD, etc). The default value is false.

Public Class Methods

new(file, thread_safe = false) click to toggle source

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

PStore objects are always reentrant. But if thread_safe is set to true, then it will become thread-safe at the cost of a minor performance hit.

 
               # File pstore.rb, line 122
def initialize(file, thread_safe = false)
  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
  @filename = file
  @abort = false
  @ultra_safe = false
  @thread_safe = thread_safe
  @lock = Mutex.new
end
            

Public Instance Methods

[](name) click to toggle source

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 #transaction. It will raise PStore::Error if called at any other time.

 
               # File pstore.rb, line 158
def [](name)
  in_transaction
  @table[name]
end
            
[]=(name, value) click to toggle source

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 #transaction and it cannot be read-only. It will raise PStore::Error if called at any other time.

 
               # File pstore.rb, line 203
def []=(name, value)
  in_transaction_wr()
  @table[name] = value
end
            
abort() click to toggle source

Ends the current #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 #transaction. It will raise PStore::Error if called at any other time.

 
               # File pstore.rb, line 291
def abort
  in_transaction
  @abort = true
  throw :pstore_abort_transaction
end
            
commit() click to toggle source

Ends the current #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 #transaction. It will raise PStore::Error if called at any other time.

 
               # File pstore.rb, line 265
def commit
  in_transaction
  @abort = false
  throw :pstore_abort_transaction
end
            
delete(name) click to toggle source

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

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

 
               # File pstore.rb, line 213
def delete(name)
  in_transaction_wr()
  @table.delete name
end
            
fetch(name, default=PStore::Error) click to toggle source

This method is just like #[], 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 #transaction. It will raise PStore::Error if called at any other time.

 
               # File pstore.rb, line 172
def fetch(name, default=PStore::Error)
  in_transaction
  unless @table.key? name
    if default == PStore::Error
      raise PStore::Error, format("undefined root name `%s'", name)
    else
      return default
    end
  end
  @table[name]
end
            
path() click to toggle source

Returns the path to the data store file.

 
               # File pstore.rb, line 239
def path
  @filename
end
            
root?(name) click to toggle source

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

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

 
               # File pstore.rb, line 234
def root?(name)
  in_transaction
  @table.key? name
end
            
roots() click to toggle source

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

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

 
               # File pstore.rb, line 224
def roots
  in_transaction
  @table.keys
end
            
transaction(read_only = false) click to toggle source

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 #commit or #abort. See those methods for details about how changes are handled. Raising an uncaught Exception in the block is equivalent to calling #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.

 
               # File pstore.rb, line 314
def transaction(read_only = false, &block)  # :yields:  pstore
  value = nil
  raise PStore::Error, "nested transaction" if !@thread_safe && @lock.locked?
  @lock.synchronize do
    @rdonly = read_only
    @abort = false
    file = open_and_lock_file(@filename, read_only)
    if file
      begin
        @table, checksum, original_data_size = load_data(file, read_only)

        catch(:pstore_abort_transaction) do
          value = yield(self)
        end

        if !@abort && !read_only
          save_data(checksum, original_data_size, file)
        end
      ensure
        file.close if !file.closed?
      end
    else
      # This can only occur if read_only == true.
      @table = {}
      catch(:pstore_abort_transaction) do
        value = yield(self)
      end
    end
  end
  value
rescue ThreadError
  raise PStore::Error, "nested transaction"
end
            

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