class 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 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.

There are three important concepts here (details at the links):

  • Store: a store is an instance of PStore.

  • Entries: the store is hash-like; each entry is the key for a stored object.

  • Transactions: each transaction is a collection of prospective changes to the store; a transaction is defined in the block given with a call to PStore#transaction.

About the Examples

Examples on this page need a store that has known properties. They can get a new (and populated) store by calling thus:

example_store do |store|
  # Example code using store goes here.
end

All we really need to know about example_store is that it yields a fresh store with a known population of entries; its implementation:

require 'pstore'
require 'tempfile'
# Yield a pristine store for use in examples.
def example_store
  # Create the store in a temporary file.
  Tempfile.create do |file|
    store = PStore.new(file)
    # Populate the store.
    store.transaction do
      store[:foo] = 0
      store[:bar] = 1
      store[:baz] = 2
    end
    yield store
  end
end

The Store

The contents of the store are maintained in a file whose path is specified when the store is created (see PStore.new). The objects are stored and retrieved using module Marshal, which means that certain objects cannot be added to the store; see Marshal::dump.

Entries

A store may have any number of entries. Each entry has a key and a value, just as in a hash:

  • Key: as in a hash, the key can be (almost) any object; see Hash. You may find it convenient to keep it simple by using only symbols or strings as keys.

  • Value: the value may be any object that can be marshalled by Marshal (see Marshal::dump) and in fact may be a collection (e.g., an array, a hash, a set, a range, etc). That collection may in turn contain nested objects, including collections, to any depth; those objects must also be Marshal-able. See Hierarchical Values.

Transactions

The Transaction Block

The block given with a call to method transaction# contains a transaction, which consists of calls to PStore methods that read from or write to the store (that is, all PStore methods except transaction itself, path, and Pstore.new):

example_store do |store|
  store.transaction do
    store.keys # => [:foo, :bar, :baz]
    store[:bat] = 3
    store.keys # => [:foo, :bar, :baz, :bat]
  end
end

Execution of the transaction is deferred until the block exits, and is executed atomically (all-or-nothing): either all transaction calls are executed, or none are. This maintains the integrity of the store.

Other code in the block (including even calls to path and PStore.new) is executed immediately, not deferred.

The transaction block:

  • May not contain a nested call to transaction.

  • Is the only context where methods that read from or write to the store are allowed.

As seen above, changes in a transaction are made automatically when the block exits. The block may be exited early by calling method commit or abort.

  • Method commit triggers the update to the store and exits the block:

    example_store do |store|
      store.transaction do
        store.keys # => [:foo, :bar, :baz]
        store[:bat] = 3
        store.commit
        fail 'Cannot get here'
      end
      store.transaction do
        # Update was completed.
        store.keys # => [:foo, :bar, :baz, :bat]
      end
    end
    
  • Method abort discards the update to the store and exits the block:

    example_store do |store|
      store.transaction do
        store.keys # => [:foo, :bar, :baz]
        store[:bat] = 3
        store.abort
        fail 'Cannot get here'
      end
      store.transaction do
        # Update was not completed.
        store.keys # => [:foo, :bar, :baz]
      end
    end
    

Read-Only Transactions

By default, a transaction allows both reading from and writing to the store:

store.transaction do
  # Read-write transaction.
  # Any code except a call to #transaction is allowed here.
end

If argument read_only is passed as true, only reading is allowed:

store.transaction(true) do
  # Read-only transaction:
  # Calls to #transaction, #[]=, and #delete are not allowed here.
end

Hierarchical Values

The value for an entry may be a simple object (as seen above). It may also be a hierarchy of objects nested to any depth:

deep_store = PStore.new('deep.store')
deep_store.transaction do
  array_of_hashes = [{}, {}, {}]
  deep_store[:array_of_hashes] = array_of_hashes
  deep_store[:array_of_hashes] # => [{}, {}, {}]
  hash_of_arrays = {foo: [], bar: [], baz: []}
  deep_store[:hash_of_arrays] = hash_of_arrays
  deep_store[:hash_of_arrays]  # => {:foo=>[], :bar=>[], :baz=>[]}
  deep_store[:hash_of_arrays][:foo].push(:bat)
  deep_store[:hash_of_arrays]  # => {:foo=>[:bat], :bar=>[], :baz=>[]}
end

And recall that you can use dig methods in a returned hierarchy of objects.

Working with the Store

Creating a Store

Use method PStore.new to create a store. The new store creates or opens its containing file:

store = PStore.new('t.store')

Modifying the Store

Use method []= to update or create an entry:

example_store do |store|
  store.transaction do
    store[:foo] = 1 # Update.
    store[:bam] = 1 # Create.
  end
end

Use method delete to remove an entry:

example_store do |store|
  store.transaction do
    store.delete(:foo)
    store[:foo] # => nil
  end
end

Retrieving Values

Use method fetch (allows default) or [] (defaults to nil) to retrieve an entry:

example_store do |store|
  store.transaction do
    store[:foo]             # => 0
    store[:nope]            # => nil
    store.fetch(:baz)       # => 2
    store.fetch(:nope, nil) # => nil
    store.fetch(:nope)      # Raises exception.
  end
end

Querying the Store

Use method key? to determine whether a given key exists:

example_store do |store|
  store.transaction do
    store.key?(:foo) # => true
  end
end

Use method keys to retrieve keys:

example_store do |store|
  store.transaction do
    store.keys # => [:foo, :bar, :baz]
  end
end

Use method path to retrieve the path to the store’s underlying file; this method may be called from outside a transaction block:

store = PStore.new('t.store')
store.path # => "t.store"

Transaction Safety

For transaction safety, see:

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

An Example Store

require "pstore"

# A mock wiki object.
class WikiPage

  attr_reader :page_name

  def initialize(page_name, author, contents)
    @page_name = page_name
    @revisions = Array.new
    add_revision(author, contents)
  end

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

wiki = PStore.new("wiki_pages.pstore")
# Update page data and the index together, or not at all.
wiki.transaction do
  # Store page.
  wiki[home_page.page_name] = home_page
  # Create page index.
  wiki[:wiki_index] ||= Array.new
  # Update wiki index.
  wiki[:wiki_index].push(*home_page.wiki_page_references)
end

# Read wiki data, setting argument read_only to true.
wiki.transaction(true) do
  wiki.keys.each do |key|
    puts key
    puts wiki[key]
  end
end

Constants

CHECKSUM_ALGO

Constant for relieving Ruby’s garbage collector.

EMPTY_MARSHAL_CHECKSUM
EMPTY_MARSHAL_DATA
EMPTY_STRING
RDWR_ACCESS
RD_ACCESS
VERSION
WR_ACCESS

Attributes

ultra_safe[RW]

Whether PStore should do its best to prevent file corruptions, even when an unlikely error (such as memory-error or filesystem error) occurs:

  • true: changes are posted by creating a temporary file, writing the updated data to it, then renaming the file to the given path. File integrity is maintained. Note: has effect only if the filesystem has atomic file rename (as do POSIX platforms Linux, MacOS, FreeBSD and others).

  • false (the default): changes are posted by rewinding the open file and writing the updated data. File integrity is maintained if the filesystem raises no unexpected I/O error; if such an error occurs during a write to the store, the file may become corrupted.

Public Class Methods

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

Returns a new PStore object.

Argument file is the path to the file in which objects are to be stored; if the file exists, it should be one that was written by PStore.

path = 't.store'
store = PStore.new(path)

A PStore object is reentrant. If argument thread_safe is given as true, the object is also thread-safe (at the cost of a small performance penalty):

store = PStore.new(path, true)
# File pstore.rb, line 372
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 = Thread::Mutex.new
end

Public Instance Methods

[](key) click to toggle source

Returns the value for the given key if the key exists. nil otherwise; if not nil, the returned value is an object or a hierarchy of objects:

example_store do |store|
  store.transaction do
    store[:foo]  # => 0
    store[:nope] # => nil
  end
end

Returns nil if there is no such key.

See also Hierarchical Values.

Raises an exception if called outside a transaction block.

# File pstore.rb, line 417
def [](key)
  in_transaction
  @table[key]
end
[]=(key, value) click to toggle source

Creates or replaces the value for the given key:

example_store do |store|
  temp.transaction do
    temp[:bat] = 3
  end
end

See also Hierarchical Values.

Raises an exception if called outside a transaction block.

# File pstore.rb, line 459
def []=(key, value)
  in_transaction_wr
  @table[key] = value
end
abort() click to toggle source

Exits the current transaction block, discarding any changes specified in the transaction block. See Committing or Aborting.

Raises an exception if called outside a transaction block.

# File pstore.rb, line 539
def abort
  in_transaction
  @abort = true
  throw :pstore_abort_transaction
end
commit() click to toggle source

Exits the current transaction block, committing any changes specified in the transaction block. See Committing or Aborting.

Raises an exception if called outside a transaction block.

# File pstore.rb, line 528
def commit
  in_transaction
  @abort = false
  throw :pstore_abort_transaction
end
delete(key) click to toggle source

Removes and returns the value at key if it exists:

example_store do |store|
  store.transaction do
    store[:bat] = 3
    store.delete(:bat)
  end
end

Returns nil if there is no such key.

Raises an exception if called outside a transaction block.

# File pstore.rb, line 476
def delete(key)
  in_transaction_wr
  @table.delete key
end
fetch(key, default=PStore::Error) click to toggle source

Like [], except that it accepts a default value for the store. If the key does not exist:

  • Raises an exception if default is PStore::Error.

  • Returns the value of default otherwise:

    example_store do |store|
      store.transaction do
        store.fetch(:nope, nil) # => nil
        store.fetch(:nope)      # Raises an exception.
      end
    end
    

Raises an exception if called outside a transaction block.

# File pstore.rb, line 436
def fetch(key, default=PStore::Error)
  in_transaction
  unless @table.key? key
    if default == PStore::Error
      raise PStore::Error, format("undefined key `%s'", key)
    else
      return default
    end
  end
  @table[key]
end
key?(key) click to toggle source

Returns true if key exists, false otherwise:

example_store do |store|
  store.transaction do
    store.key?(:foo) # => true
  end
end

Raises an exception if called outside a transaction block.

PStore#root? is an alias for PStore#key?.

# File pstore.rb, line 509
def key?(key)
  in_transaction
  @table.key? key
end
Also aliased as: root?
keys() click to toggle source

Returns an array of the existing keys:

example_store do |store|
  store.transaction do
    store.keys # => [:foo, :bar, :baz]
  end
end

Raises an exception if called outside a transaction block.

PStore#roots is an alias for PStore#keys.

# File pstore.rb, line 492
def keys
  in_transaction
  @table.keys
end
Also aliased as: roots
path() click to toggle source

Returns the string file path used to create the store:

store.path # => "flat.store"
# File pstore.rb, line 519
def path
  @filename
end
root?(key)
Alias for: key?
roots()
Alias for: keys
transaction(read_only = false) { |pstore| ... } click to toggle source

Opens a transaction block for the store. See Transactions.

With argument read_only as false, the block may both read from and write to the store.

With argument read_only as true, the block may not include calls to transaction, []=, or delete.

Raises an exception if called within a transaction block.

# File pstore.rb, line 555
def transaction(read_only = false)  # :yields:  pstore
  value = nil
  if !@thread_safe
    raise PStore::Error, "nested transaction" unless @lock.try_lock
  else
    begin
      @lock.lock
    rescue ThreadError
      raise PStore::Error, "nested transaction"
    end
  end
  begin
    @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
      end
    else
      # This can only occur if read_only == true.
      @table = {}
      catch(:pstore_abort_transaction) do
        value = yield(self)
      end
    end
  ensure
    @lock.unlock
  end
  value
end

Private Instance Methods

empty_marshal_checksum() click to toggle source
# File pstore.rb, line 732
def empty_marshal_checksum
  EMPTY_MARSHAL_CHECKSUM
end
empty_marshal_data() click to toggle source
# File pstore.rb, line 729
def empty_marshal_data
  EMPTY_MARSHAL_DATA
end
in_transaction() click to toggle source

Raises PStore::Error if the calling code is not in a PStore#transaction.

# File pstore.rb, line 388
def in_transaction
  raise PStore::Error, "not in transaction" unless @lock.locked?
end
in_transaction_wr() click to toggle source

Raises PStore::Error if the calling code is not in a PStore#transaction or if the code is in a read-only PStore#transaction.

# File pstore.rb, line 395
def in_transaction_wr
  in_transaction
  raise PStore::Error, "in read-only transaction" if @rdonly
end
load_data(file, read_only) click to toggle source

Load the given PStore file. If read_only is true, the unmarshalled Hash will be returned. If read_only is false, a 3-tuple will be returned: the unmarshalled Hash, a checksum of the data, and the size of the data.

# File pstore.rb, line 643
def load_data(file, read_only)
  if read_only
    begin
      table = load(file)
      raise Error, "PStore file seems to be corrupted." unless table.is_a?(Hash)
    rescue EOFError
      # This seems to be a newly-created file.
      table = {}
    end
    table
  else
    data = file.read
    if data.empty?
      # This seems to be a newly-created file.
      table = {}
      checksum = empty_marshal_checksum
      size = empty_marshal_data.bytesize
    else
      table = load(data)
      checksum = CHECKSUM_ALGO.digest(data)
      size = data.bytesize
      raise Error, "PStore file seems to be corrupted." unless table.is_a?(Hash)
    end
    data.replace(EMPTY_STRING)
    [table, checksum, size]
  end
end
on_windows?() click to toggle source
# File pstore.rb, line 671
def on_windows?
  is_windows = RUBY_PLATFORM =~ /mswin|mingw|bccwin|wince/
  self.class.__send__(:define_method, :on_windows?) do
    is_windows
  end
  is_windows
end
open_and_lock_file(filename, read_only) click to toggle source

Open the specified filename (either in read-only mode or in read-write mode) and lock it for reading or writing.

The opened File object will be returned. If read_only is true, and the file does not exist, then nil will be returned.

All exceptions are propagated.

# File pstore.rb, line 618
def open_and_lock_file(filename, read_only)
  if read_only
    begin
      file = File.new(filename, **RD_ACCESS)
      begin
        file.flock(File::LOCK_SH)
        return file
      rescue
        file.close
        raise
      end
    rescue Errno::ENOENT
      return nil
    end
  else
    file = File.new(filename, **RDWR_ACCESS)
    file.flock(File::LOCK_EX)
    return file
  end
end
save_data(original_checksum, original_file_size, file) click to toggle source
# File pstore.rb, line 679
def save_data(original_checksum, original_file_size, file)
  new_data = dump(@table)

  if new_data.bytesize != original_file_size || CHECKSUM_ALGO.digest(new_data) != original_checksum
    if @ultra_safe && !on_windows?
      # Windows doesn't support atomic file renames.
      save_data_with_atomic_file_rename_strategy(new_data, file)
    else
      save_data_with_fast_strategy(new_data, file)
    end
  end

  new_data.replace(EMPTY_STRING)
end
save_data_with_atomic_file_rename_strategy(data, file) click to toggle source
# File pstore.rb, line 694
def save_data_with_atomic_file_rename_strategy(data, file)
  temp_filename = "#{@filename}.tmp.#{Process.pid}.#{rand 1000000}"
  temp_file = File.new(temp_filename, **WR_ACCESS)
  begin
    temp_file.flock(File::LOCK_EX)
    temp_file.write(data)
    temp_file.flush
    File.rename(temp_filename, @filename)
  rescue
    File.unlink(temp_file) rescue nil
    raise
  ensure
    temp_file.close
  end
end
save_data_with_fast_strategy(data, file) click to toggle source
# File pstore.rb, line 710
def save_data_with_fast_strategy(data, file)
  file.rewind
  file.write(data)
  file.truncate(data.bytesize)
end