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 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
- CHECKSUM_ALGO
Constant for relieving Ruby's garbage collector.
- EMPTY_MARSHAL_CHECKSUM
- EMPTY_MARSHAL_DATA
- EMPTY_STRING
- RDWR_ACCESS
- RD_ACCESS
- WR_ACCESS
Attributes
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
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 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 = Thread::Mutex.new 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.
# File lib/pstore.rb, line 155 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_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 lib/pstore.rb, line 200 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.
# File lib/pstore.rb, line 288 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.
# File lib/pstore.rb, line 262 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.
# File lib/pstore.rb, line 210 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.
# File 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
Returns the path to the data store file.
# File lib/pstore.rb, line 236 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.
# File lib/pstore.rb, line 231 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.
# File lib/pstore.rb, line 221 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.
# File 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 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
# File lib/pstore.rb, line 488 def empty_marshal_checksum EMPTY_MARSHAL_CHECKSUM end
# File lib/pstore.rb, line 485 def empty_marshal_data EMPTY_MARSHAL_DATA end
Raises PStore::Error
if the calling code is not in a PStore#transaction
.
# File 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
or if the code is in a read-only PStore#transaction
.
# File lib/pstore.rb, line 142 def in_transaction_wr in_transaction raise PStore::Error, "in read-only transaction" if @rdonly end
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 lib/pstore.rb, line 399 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
# File lib/pstore.rb, line 427 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 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 lib/pstore.rb, line 374 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
# File lib/pstore.rb, line 435 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
# File lib/pstore.rb, line 450 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 lib/pstore.rb, line 466 def save_data_with_fast_strategy(data, file) file.rewind file.write(data) file.truncate(data.bytesize) end