CAS stands for Check-And-Set or Compare-And-Swap. Memcached CAS command 'checks' and 'set' data item if and only if, no other client process has updated it since last read by this client.

当我们想要在key = a， val = 2的时候将val 设为4，我们首先要利用get来获取2的hash结果，然后再使用这个2的hash结果来改变val = 4
举例，

key = a, val = 3
# server1: want change a -> 4
3, Hash3 = gets(1)
# server2: want change a -> 5
3, Hash3 = gets(1)

# server1:
cas(a, 4,Hash3) # True, succeed

# server2:
cas(a, 5,Hash3) # False,Faile, because the hash of key a is now Hash(4) not Hash(3)

Implement CAS

CACHE = {}

#return True after setting the data
def set(key, value):
    CACHE[key] = value
    return True

#return the value for key
def get(key):
    return CACHE.get(key)

#delete key from the cache
def delete(key):
    if key in CACHE:
        del CACHE[key]

#clear the entire cache
def flush():
    CACHE.clear()

# QUIZ - implement gets() and cas() below
#return a tuple of (value, h), where h is hash of the value. a simple hash
#we can use here is hash(repr(val))
def gets(key):
    ###Your gets code here.
    v = get(key)
    if v:
        return v, hash(repr(v))
        
# set key = value and return True if cas_unique matches the hash of the value
# already in the cache. if cas_unique does not match the hash of the value in
# the cache, don't set anything and return False.
def cas(key, value, cas_unique):
    ###Your cas code here.
    r = gets(key)
    if r:
        val, unique = r
        if unique == cas_unique:
            return set(key, value)
        else:
            return False
    
#print set('x', 1)
#>>> True
#
#print get('x')
#>>> 1
#
#print get('y')
#>>> None
#
#delete('x')
#print get('x')
#>>> None
#
#set('x', 2)
#print gets('x')
#>>> 2, HASH
#
#print cas('x', 3, 0)
#>>> False
#
#print cas('x', 4, HASH)
#>>> True
#
#print get('x')
#>>> 4

References:
https://www.tutorialspoint.com/memcached/memcached_cas.htm