Class: PEROBS::Cache

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

Overview

The Cache provides two functions for the PEROBS Store. It keeps some amount of objects in memory to substantially reduce read access latencies. It also stores a list of objects that haven’t been synced to the permanent store yet to accelerate object writes.

Instance Method Summary collapse

Constructor Details

#initialize(bits = 16) ⇒ Cache

Create a new Cache object.

Parameters:

  • bits (Integer) (defaults to: 16) —

    Number of bits for the cache index. This parameter heavilty affects the performance and memory consumption of the cache.



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# File 'lib/perobs/Cache.rb', line 43

def initialize(bits = 16)
  @bits = bits
  # This mask is used to access the _bits_ least significant bits of the
  # object ID.
  @mask = 2 ** bits - 1
  # Initialize the read and write cache
  reset
end

Instance Method Details

#abort_transaction ⇒ Object

Tell the cache to abort the currently active transaction. All modified objects will be restored from the storage back-end to their state before the transaction started.



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# File 'lib/perobs/Cache.rb', line 196

def abort_transaction
  if @transaction_stack.empty?
    PEROBS.log.fatal 'No ongoing transaction to abort'
  end
  @transaction_stack.pop.each do |id|
    @transaction_objects[id]._restore(@transaction_stack.length)
  end
end

#begin_transaction ⇒ Object

Tell the cache to start a new transaction. If no other transaction is active, the write cache is flushed before the transaction is started.



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# File 'lib/perobs/Cache.rb', line 153

def begin_transaction
  if @transaction_stack.empty?
    # The new transaction is the top-level transaction. Flush the write
    # buffer to save the current state of all objects.
    flush
  else
    # Save a copy of all objects that were modified during the enclosing
    # transaction.
    @transaction_stack.last.each do |id|
      @transaction_objects[id]._stash(@transaction_stack.length - 1)
    end
  end
  # Push a transaction buffer onto the transaction stack. This buffer will
  # hold a reference to all objects modified during this transaction.
  @transaction_stack.push(::Array.new)
end

#cache_read(obj) ⇒ Object

Add an PEROBS::Object to the read cache.

Parameters:



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# File 'lib/perobs/Cache.rb', line 54

def cache_read(obj)
  # This is just a safety check. It can probably be disabled in the future
  # to increase performance.
  if obj.respond_to?(:is_poxreference?)
    # If this condition triggers, we have a bug in the library.
    PEROBS.log.fatal "POXReference objects should never be cached"
  end
  @reads[index(obj)] = obj
end

#cache_write(obj) ⇒ Object

Add a PEROBS::Object to the write cache.

Parameters:



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# File 'lib/perobs/Cache.rb', line 66

def cache_write(obj)
  # This is just a safety check. It can probably be disabled in the future
  # to increase performance.
  #if obj.respond_to?(:is_poxreference?)
  #  # If this condition triggers, we have a bug in the library.
  #  PEROBS.log.fatal "POXReference objects should never be cached"
  #end

  if @transaction_stack.empty?
    # We are not in transaction mode.
    idx = index(obj)
    if (old_obj = @writes[idx]) && old_obj._id != obj._id
      # There is another old object using this cache slot. Before we can
      # re-use the slot, we need to sync it to the permanent storage.
      old_obj._sync
    end
    @writes[idx] = obj
  else
    # When a transaction is active, we don't have a write cache. The read
    # cache is used to speed up access to recently used objects.
    cache_read(obj)
    # Push the reference of the modified object into the write buffer for
    # this transaction level.
    unless @transaction_stack.last.include?(obj._id)
      @transaction_stack.last << obj._id
      @transaction_objects[obj._id] = obj
    end
  end
end

#end_transaction ⇒ Object

Tell the cache to end the currently active transaction. All write operations of the current transaction will be synced to the storage back-end.



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# File 'lib/perobs/Cache.rb', line 173

def end_transaction
  case @transaction_stack.length
  when 0
    PEROBS.log.fatal 'No ongoing transaction to end'
  when 1
    # All transactions completed successfully. Write all modified objects
    # into the backend storage.
    @transaction_stack.pop.each { |id| @transaction_objects[id]._sync }
    @transaction_objects = ::Hash.new
  else
    # A nested transaction completed successfully. We add the list of
    # modified objects to the list of the enclosing transaction.
    transactions = @transaction_stack.pop
    # Merge the two lists
    @transaction_stack.push(@transaction_stack.pop + transactions)
    # Ensure that each object ID is only included once in the list.
    @transaction_stack.last.uniq!
  end
end

#evict(id) ⇒ True/False

Evict the object with the given ID from the cache.

Parameters:

  • id (Integer) —

    ID of the cached PEROBS::ObjectBase

Returns:

  • (True/False) —

    True if object was stored in the cache. False otherwise.



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# File 'lib/perobs/Cache.rb', line 100

def evict(id)
  unless @transaction_stack.empty?
    PEROBS.log.fatal "You cannot evict entries during a transaction."
  end

  idx = id & @mask
  # The index is just a hash. We still need to check if the object IDs are
  # actually the same before we can return the object.
  if (obj = @writes[idx]) && obj._id == id
    # The object is in the write cache.
    @writes[idx] = nil
    return true
  elsif (obj = @reads[idx]) && obj._id == id
    # The object is in the read cache.
    @reads[idx] = nil
    return true
  end

  false
end

#flush ⇒ Object

Flush all pending writes to the persistant storage back-end.



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# File 'lib/perobs/Cache.rb', line 139

def flush
  @writes.each { |w| w._sync if w }
  @writes = ::Array.new(2 ** @bits)
end

#in_transaction? ⇒ true/false

Returns true if the Cache is currently handling a transaction, false otherwise.

Returns:

  • (true/false) 


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# File 'lib/perobs/Cache.rb', line 147

def in_transaction?
  !@transaction_stack.empty?
end

#inspect ⇒ Object

Don’t include the cache buffers in output of other objects that reference Cache.



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# File 'lib/perobs/Cache.rb', line 219

def inspect
end

#object_by_id(id) ⇒ Object

Return the PEROBS::Object with the specified ID or nil if not found.

Parameters:

  • id (Integer) —

    ID of the cached PEROBS::ObjectBase



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# File 'lib/perobs/Cache.rb', line 123

def object_by_id(id)
  idx = id & @mask
  # The index is just a hash. We still need to check if the object IDs are
  # actually the same before we can return the object.
  if (obj = @writes[idx]) && obj._id == id
    # The object was in the write cache.
    return obj
  elsif (obj = @reads[idx]) && obj._id == id
    # The object was in the read cache.
    return obj
  end

  nil
end

#reset ⇒ Object

Clear all cached entries. You must call flush before calling this method. Otherwise unwritten objects will be lost.



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# File 'lib/perobs/Cache.rb', line 207

def reset
  # The read and write caches are Arrays. We use the _bits_ least
  # significant bits of the PEROBS::ObjectBase ID to select the index in
  # the read or write cache Arrays.
  @reads = ::Array.new(2 ** @bits)
  @writes = ::Array.new(2 ** @bits)
  @transaction_stack = ::Array.new
  @transaction_objects = ::Hash.new
end