Class: CRDT::PNCounter

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

Overview

A positive negative counter

This counter can be incremented up or down. Each node should only adjust it’s up and down counters. The current value of the counter is calculated by taking the sum of all the positive counters and subtracting the sum of all the negative counters

# Efficiency: value in counter: n, number of nodes: m, number of changes: k Local changes (+/-) are O(1) Merging changes are O(m) The space cost is O(m) The space cost of synchronization is O(m)

# Implementation notes: This implementation is a CvRDT. That means it sends a full copy of the entire structure, rather than messages

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(node_identity = Thread.current.object_id, base_value = 0) ⇒ PNCounter

Create a new counter

Parameters:

  • node_identity (defaults to: Thread.current.object_id) —

    Identifier for this node, used for tracking changes to the counter. Defaults to the current Thread’s object ID



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# File 'lib/crdt/pn_counter.rb', line 55

def initialize(node_identity = Thread.current.object_id, base_value = 0)
  @base_value = base_value
  @cached_value = base_value
  @positive_counters = {}
  @negative_counters = {}
  @node_identity = node_identity
end

Instance Attribute Details

#negative_counters ⇒ Object

Returns the value of attribute negative_counters.



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# File 'lib/crdt/pn_counter.rb', line 63

def negative_counters
  @negative_counters
end

#positive_counters ⇒ Object

Returns the value of attribute positive_counters.



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# File 'lib/crdt/pn_counter.rb', line 63

def positive_counters
  @positive_counters
end

Class Method Details

.from_h(hash) ⇒ Object

Expects a Hash in the following format: {

"positive" => {
  "1" => 15,
  "3" => 4
},
"negative" => {
}

}

Parameters:

  • hash (Hash) —

    a serialized PNCounter, conforming to the format here



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# File 'lib/crdt/pn_counter.rb', line 28

def self.from_h(hash)
  counter = PNCounter.new(hash["node_identity"], hash["base_value"])

  hash["positive"].each do |source, amount|
    counter.increase(amount, source)
  end
  hash["negative"].each do |source, amount|
    counter.decrease(amount, source)
  end

  return counter
end

Instance Method Details

#+(other) ⇒ Object

Add something to this counter

Parameters:

  • other (Number) —

    the amount to add to this counter



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# File 'lib/crdt/pn_counter.rb', line 92

def +(other)
  if other > 0
    increase(other)
  else
    decrease(- other)
  end
  self
end

#-(other) ⇒ Object

Subtract something from this counter

Parameters:

  • other (Number) —

    the amount to subtract from this counter



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# File 'lib/crdt/pn_counter.rb', line 104

def -(other)
  if other > 0
    decrease(other)
  else
    increase(- other)
  end
  self
end

#decrease(amount, source = nil) ⇒ Object

Decrease this counter by the given amount

Parameters:

  • amount (Number) —

    a non-negative amount to decrease this counter by



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# File 'lib/crdt/pn_counter.rb', line 80

def decrease(amount, source = nil)
  source ||= @node_identity
  negative_counters[source] ||= 0
  negative_counters[source] += amount
  @cached_value -= amount

  return self
end

#gc(node) ⇒ Object

Garbage collect a node, removing its counters and folding them into the new base value.

This should only be called if your cluster management has indicated that a node has left the cluster permanently.



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# File 'lib/crdt/pn_counter.rb', line 150

def gc(node)
  @base_value += @positive_counters[node]
  @base_value -= @negative_counters[node]
  @positive_counters.delete(node)
  @negative_counters.delete(node)
end

#increase(amount, source = nil) ⇒ Object

Increase this counter by the given amount

Parameters:

  • amount (Number) —

    a non-negative amount to decrease this counter by



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# File 'lib/crdt/pn_counter.rb', line 68

def increase(amount, source = nil)
  source ||= @node_identity
  positive_counters[source] ||= 0
  positive_counters[source] += amount
  @cached_value += amount

  return self
end

#merge(other) ⇒ Object

Merge the counters from the other PNCounter into this one



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# File 'lib/crdt/pn_counter.rb', line 122

def merge(other)
  other.positive_counters.each do |source, amount|
    current_amount = @positive_counters[source]
    if current_amount
      if current_amount < amount
        @positive_counters[source] = amount
      end
    else
      @positive_counters[source] = amount
    end
  end
  other.negative_counters.each do |source, amount|
    current_amount = @negative_counters[source]
    if current_amount
      if current_amount < amount
        @negative_counters[source] = amount
      end
    else
      @negative_counters[source] = amount
    end
  end

  return self
end

#to_h ⇒ Object

Get a hash representation of this object, which is suitable for serialization to JSON



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# File 'lib/crdt/pn_counter.rb', line 42

def to_h
  return {
    node_identity: @node_identity,
    base_value: @base_value,
    cached_value: @cached_value,
    positive: @positive_counters,
    negative: @negative_counters,
  }
end

#to_i ⇒ Object 



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# File 'lib/crdt/pn_counter.rb', line 117

def to_i
  @cached_value.to_i
end

#value ⇒ Object 



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# File 'lib/crdt/pn_counter.rb', line 113

def value
  @cached_value
end