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 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