2012_09_14.txt

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Ex. 1

We are speaking of passive replication, what happens is that when a client performs an operation on the primary server then the state
of the system will be updated on all the backup servers. After all backups have been updated, the primary will return the asnwer to 
the client. Crash of a primary is managed specifically for each situation that may happen:
- (to be continued)
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Ex. 2

Regular register (1,N) specifications
	- Termination: if a correct process invokes an operation, then the operation eventually receives the corresponding confirmation.
	- Validity: a read operation returns the value stored in the registry or the value cuncurrently written on it.

Idea behind regular registers (1,N) is simple, each process stores a local copy of the register. Each read operation returns
the value stored in its local copy of the register. Each write operation updates the value locally stored at each process the
writer consider to have not crashed. A write completes when the writer receives an ACK from each process that has not crashed.

Atomic register (1,N) specifications
	- Termination: as RR
	- Validity: as RR
	- Ordering: if a process reads a message v2 after it a read that precedes it has returned v1 then v2 could not have been written 
				before v1.
The atomic register works differently: a (1,1) atomic register is built upon a (1,N) regular register. Then we use a set of (1,1)
atomic register to build a (1,N) atomic register.
Say p1 is the writer and p2 is the reader. Each write operation on the atomic register writes the pair (value, timestamp) into the
underlying regular register.
The reader tracks the timestamp of previously read values to avoid read something old.
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Ex. 3

1 --> 2  FIFO Order
3 --> 4  FIFO Order
3 --> 5  happen before

1) Satisify Non-Uniform Reliable Broadcast:

	- m1, m2, m3. m4

2) Satisify Uniform Reliable Broadcast:

	- m1, m2, m3, m4, m5 iff m5 is delivered by p2.

3)	1234 satisfies Causal Order and TO(NUA,WUTO) (if the faulty process has spurious message m5 and misses the delivery of m4)
	12345 TO(UA,SUTO).

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Ex. 4

upon event <Init> do
	for all x in II do
		VCi[x] = 0
	toDeliver = 0
	delivered = 0

upon event <deltaCBBroadcast | m> do
	VCi[i] = CKi
	for all Px in II
		trigger <fpl, send | Px, [m, VCi]

upon event <fplDeliver | Pj, [m, VCj]> do
	if m !in delivered && CKi <== VCj[j] + delta
		toDeliver := toDeliver U {m, VCj, Pj}
		VCi[j] := VCj[j]
	else if m !in delivered && CKi >== VCj[j] + delta
		VCi[j] := VCj[j]	

upon exists {s, Pj, VCj} s.t. VCj <= VCi
	delivered := delivered U {m}
	toDeliver := toDeliver \ {s, Pj, VCj}
	trigger <deltaCBDeliver | m>
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