2014_07_22.txt

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Ex.1

Statement is false, logical scalar clock can't say that an event e1 "happened-before" another event e2
because the definition of a logical says that
if e1 --> e2 then L(e1) < L(e2)

Statement is true if we are talking about vector clocks.
e1 --> e2 <=> L(e1) < L(e2)

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Ex.2

          UA, SUTO 
	NUA, SUTO	UA WUTO
		|	  /		|
	NUA WUTO		|
		|		   /
	  NUA,WNUTO/SNUTO

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Ex.3

1 --> 2 FIFO 
1 --> 2 happened-before
2 --> 4 happenede-before

Causal:
1,2,3,4
3,1,2,4
1,3,2,4
1,2,4,3

FIFO but not causal:
1,3,4,2
1,4,3,2
4,1,2,3
3,1,4,2
3,4,1,2
4,3,1,2
4,1,3,2

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Ex.4

upon event <Init> do
	LEFT := p_x
	RIGHT := p_j
	pending := empty
	memstatus := ok
	state := nop
	grant := 0

upon event <accessMem | R> do
	state := REQ_CS
	trigger <FIFOSend | RIGHT, p_i, state>
	trigger <FIFOSend | LEFT, P_i, state>

upon event <FIFODeliver | LEFT/RIGHT, p_i, s>
	if s == REQ_CS
		pending.addLast(p_i) <--- ordered list
		trigger <FIFOSend | RIGHT/LEFT, p_i, s>

upon exist pending != empty && memstatus != busy do
	p_j := pending.pop()
	trigger <pp2pSend | RIGHT, [GRANT, p_i]>
	trigger <pp2pSend | LEFT, [GRANT, p_i]>
	memstatus := busy

upon event <pp2pDeliver | RIGHT/LEFT, [GRANT], p_j> do
	if self == p_j then
		grant := grant U {p_j}
	else
		trigger <pp2pSend | LEFT/RIGHT, [GRANT, p_j]

upon event <pp2pDeliver | RIGHT/LEFT, s, p_j> do
	if (s == DONE) then
		memstatus := ok
		trigger <pp2pSend | LEFT/RIGHT, s, p_j]>

upon correct <= grant && state := REQ_CS do
	executeOP(R)
	state := DONE
	trigger <pp2pSend | RIGHT, state, p_i>
	trigger <pp2pSend | LEFT, state, p_i>
	grant := 0

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Ex.5

CAP theorem states that in a distributed system scenario, if a network fails occurs then it is not
possible to achieve Consistency, Availability and Partition tolerance thus one of the 3 must be 
dropped. Since we are in a distributed system scenario, we cannot drop Partition Tolerance, thus
the choose must be among the two.
AP == Amazon Dynamo