Class: PStore

Inherits:
Object
  • Object
show all
Defined in:
lib/pstore.rb

Overview

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 Keys. 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:

  • Optional argument thread_safe at method PStore.new.

  • Attribute #ultra_safe.

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

Defined Under Namespace

Classes: Error

Constant Summary collapse

VERSION = 
"0.1.3"
RDWR_ACCESS = 
{mode: IO::RDWR | IO::CREAT | IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze
RD_ACCESS = 
{mode: IO::RDONLY | IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze
WR_ACCESS = 
{mode: IO::WRONLY | IO::CREAT | IO::TRUNC | IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(file, thread_safe = false) ⇒ PStore

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)



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# File 'lib/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

Instance Attribute Details

#ultra_safe ⇒ Object

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.



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# File 'lib/pstore.rb', line 355

def ultra_safe
  @ultra_safe
end

Instance Method Details

#[](key) ⇒ Object

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.



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# File 'lib/pstore.rb', line 417

def [](key)
  in_transaction
  @table[key]
end

#[]=(key, value) ⇒ Object

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.



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# File 'lib/pstore.rb', line 459

def []=(key, value)
  in_transaction_wr
  @table[key] = value
end

#abort ⇒ Object

Exits the current transaction block, discarding any changes specified in the transaction block.

Raises an exception if called outside a transaction block.



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# File 'lib/pstore.rb', line 535

def abort
  in_transaction
  @abort = true
  throw :pstore_abort_transaction
end

#commit ⇒ Object

Exits the current transaction block, committing any changes specified in the transaction block.

Raises an exception if called outside a transaction block.



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# File 'lib/pstore.rb', line 524

def commit
  in_transaction
  @abort = false
  throw :pstore_abort_transaction
end

#delete(key) ⇒ Object

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.



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# File 'lib/pstore.rb', line 476

def delete(key)
  in_transaction_wr
  @table.delete key
end

#fetch(key, default = PStore::Error) ⇒ Object

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.



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# File 'lib/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) ⇒ Boolean Also known as: root?

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.

Returns:

  • (Boolean) 


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# File 'lib/pstore.rb', line 505

def key?(key)
  in_transaction
  @table.key? key
end

#keys ⇒ Object Also known as: roots

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.



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# File 'lib/pstore.rb', line 490

def keys
  in_transaction
  @table.keys
end

#path ⇒ Object

Returns the string file path used to create the store:

store.path # => "flat.store"



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# File 'lib/pstore.rb', line 515

def path
  @filename
end

#transaction(read_only = false) ⇒ Object

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.



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# File 'lib/pstore.rb', line 551

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