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.
Constant for relieving Ruby’s garbage collector.
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 119
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
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 155
def [](name)
in_transaction
@table[name]
end
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 200
def []=(name, value)
in_transaction_wr
@table[name] = value
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_hierarchy] = { "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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 288
def abort
in_transaction
@abort = true
throw :pstore_abort_transaction
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 262
def commit
in_transaction
@abort = false
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 210
def delete(name)
in_transaction_wr
@table.delete name
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 482
def empty_marshal_checksum
EMPTY_MARSHAL_CHECKSUM
end
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 479
def empty_marshal_data
EMPTY_MARSHAL_DATA
end
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 169
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
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 135
def in_transaction
raise PStore::Error, "not in transaction" unless @lock.locked?
end
Raises PStore::Error if the calling code is not in a PStore#transaction.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 142
def in_transaction_wr
in_transaction
raise PStore::Error, "in read-only transaction" if @rdonly
end
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 tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 393
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 = Digest::MD5.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
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 421
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
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 368
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
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 tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 236
def path
@filename
end
Returns the path to the data store file.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 231
def root?(name)
in_transaction
@table.key? name
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 221
def roots
in_transaction
@table.keys
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.
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 429
def save_data(original_checksum, original_file_size, file)
new_data = dump(@table)
if new_data.bytesize != original_file_size || Digest::MD5.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
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 444
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
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 460
def save_data_with_fast_strategy(data, file)
file.rewind
file.write(data)
file.truncate(data.bytesize)
end
# File tmp/rubies/ruby-2.3.8/lib/pstore.rb, line 311
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 if !file.closed?
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
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.