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---
layout: page
title: "Counters"
by: "Aviral Goel"
---
Counters are replicated integers. They are the most basic distributed object. This chapter describes the different variations of counter CRDT in both state based and operation based form.
## G-counter - Increment only counter
As the name suggests, these counters only support increment operation. They can be used to implement the `like` button functionality of social media websites.
### CmRDT: Operation based G-counter
In the operation based implementation, the increment operation is transmitted to all other replicas.
This is straightforward to implement as there is only one update operation.
#### Specification
```python
class CmRDT:
pass
class Counter(CmRDT):
def __init__(self): # constructor function
self._count = 0
def value(self): # query function
return self._count
def increment(self): # update function
self._count += 1
for replica in self.replicas():
self.transmit("increment", replica)
```
#### Figure
![Operation based increment only counter][operation-based-increment-only-counter]
### CvRDT: State based specification
In the state based implementation, the counter state is transmitted to all other replicas.
But how do we model the state? Of course, the counter's count is its state.
Since the count always increases, modeling the state as count automatically makes it a monotonic semilattice.
#### Specification
```python
class CvRDT:
pass
class Counter(CvRDT):
def __init__(self, count = 0): # constructor function
self._count = count
def value(self): # query function
return self._count
def increment(self): # update function
self._count += 1
def compare(self, other): # comparison function
return self.value() <= other.value()
def merge(self, other): # merge function
return Counter(max(self.value(), other.value()))
```
#### Figure
![State based increment only counter (incorrect)][state-based-increment-only-counter-incorrect]
As it can be seen from the figure, the clients issue a total of 4 increment requests. However, the replicas finally converge to 2.
Something is clearly amiss! When two replicas are incremented, they should together converge to 2. But the `merge` function computes the maximum of two states.
#### Specification
```python
class CvRDT:
pass
class Counter(CvRDT):
def __init__(self, counts = None): # constructor function
if counts is None:
self._counts = [0] * length(self.replicas())
else:
self._counts = counts
def value(self): # query function
return sum(self._counts)
def counts(self): # query function
return list(self._counts) # return a clone
def increment(self): # update function
self._counts[self.replicaId()] += 1
def compare(self, other): # comparison function
return all(v1 <= v2 for (v1, v2) in
zip(self.counts(),
other.counts()))
def merge(self, other): # merge function
return Counter(map(max, zip(self.counts(),
other.counts())))
```
#### Figure
![State based increment only counter (correct)][state-based-increment-only-counter-correct]
As it can be seen from the figure, the clients issue a total of 4 increment requests. However, the replicas finally converge to 2.
Something is clearly amiss! When two replicas are incremented, they should together converge to 2. But the `merge` function computes the maximum of two states.
## PN-counter - Increment and Decrement counter
### CmRDT: Operation based specification
#### Specification
```python
class CmRDT:
pass
class Counter(CmRDT):
def __init__(self): # constructor function
self._count = 0
def value(self): # query function
return self._count
def increment(self): # update function
self._count += 1
for replica in self.replicas():
self.transmit("increment", replica)
def decrement(self): # update function
self._count -= 1
for replica in self.replicas():
self.transmit("decrement", replica)
```
#### Figure
![Operation based increment and decrement counter][operation-based-increment-and-decrement-counter]
### CvRDT: State based specification
#### Specification
```python
class CvRDT:
pass
class Counter(CvRDT):
def __init__(self, counts = None): # constructor function
if counts is None:
self._counts = [0] * length(self.replicas())
else:
self._counts = counts
def value(self): # query function
return sum(self._counts)
def counts(self): # query function
return list(self._counts) # return a clone
def increment(self): # update function
self._counts[self.replicaId()] += 1
def decrement(self): # update function
self._counts[self.replicaId()] -= 1
def compare(self, other): # comparison function
return all(v1 <= v2 for (v1, v2) in
zip(self.counts(),
other.counts()))
def merge(self, other): # merge function
return Counter(map(max, zip(self.counts(),
other.counts())))
```
#### Figure
![State based increment and decrement counter (incorrect)][state-based-increment-and-decrement-counter-incorrect]
#### Specification
```python
class CvRDT:
pass
class Counter(CvRDT):
def __init__(self,
increments = None,
decrements = None): # constructor function
if increments is None:
self._increments = [0] * length(replicas())
else:
self._increments = increments
if decrements is None:
self._decrements = [0] * length(replicas())
else:
self._decrements = decrements
def increments(self): # query function
return list(self._increments) # return a clone
def decrements(self): # query function
return list(self._decrements) # return a clone
def value(self): # query function
return (sum(self.increments()) -
sum(self.decrements()))
def increment(self): # update function
self._increments[self.replicaId()] += 1
def decrement(self): # update function
self._decrements[self.replicaId()] += 1
def compare(self, other): # comparison function
return (all(v1 <= v2 for (v1, v2) in
zip(self.increments(),
other.increments()))
and
all(v1 <= v2 for (v1, v2) in
zip(self.decrements(),
other.decrements())))
def merge(self, other): # merge function
return Counter(increments = map(max, zip(self.increments(),
other.increments())),
decrements = map(max, zip(self.decrements(),
other.decrements())))
```
#### Figure
![State based increment and decrement counter (correct)][state-based-increment-and-decrement-counter-correct]
[operation-based-increment-only-counter]: resources/images/counters/operation-based-increment-only-counter.png
[state-based-increment-only-counter-incorrect]: resources/images/counters/state-based-increment-only-counter-incorrect.png
[state-based-increment-only-counter-correct]: resources/images/counters/state-based-increment-only-counter-correct.png
[operation-based-increment-and-decrement-counter]: resources/images/counters/operation-based-increment-and-decrement-counter.png
[state-based-increment-and-decrement-counter-incorrect]: resources/images/counters/state-based-increment-and-decrement-counter-incorrect.png
[state-based-increment-and-decrement-counter-correct]: resources/images/counters/state-based-increment-and-decrement-counter-correct.png
|
