I have pasted pseudocode for a paxos algorithm below and was wondering if someone could point me in the right direction. I am trying to implement the algorithm below, but I’m a confused what exactly “views” represents below. I know the comment says it is a “map of past view numbers to values”, but if someone could explain to me what exactly these “values” are and what “view numbers” are.
state:
num_h: highest proposal # seen in a prepare
num_a, val_a: highest value and proposal # which node has accepted
my_num: the last proposal # the node has used in this round of Paxos
inst_h: highest view number we have accepted (round number)
views: map of past view numbers to values
done: leader says agreement was reached, we can start new view
on each view change, initialize state:
num_a = 0
num_h = 0
my_num = 0
val_a = () // empty list
Paxos Phase 1
a node (maybe more than one...) decides to be leader (need not be in current view):
my_num = max(num_h, my_num)+1, append node ID // unique proposal number
done = false
sends prepare(inst_h+1, my_num) to all nodes in {views[inst_h], initial contact node, itself}
if node receives prepare(vid, n):
if vid <= inst_h:
return OLD_VIEW(vid, views[vid]) // views[vid] is the winner for vid
else if n > num_h:
num_h = n
done = false
return PROMISE(num_a, val_a)
else:
return REJECT()
Paxos Phase 2
if leader gets OLD_VIEW(vid, v):
views[vid] = v
inst_h = vid
view change
restart paxos
else if leader gets REJECT():
delay and restart paxos
else if leader gets PROMISE from majority of nodes in views[inst_h]:
if any PROMISE(n_i, v_i) exists such that v_i is not empty:
v = non-empty value v_i corresponding to highest n_i received
else leader gets to choose a value:
v = set of pingable nodes (including self)
send ACCEPT(inst_h+1, my_num, v) to all responders
else:
delay and restart paxos
if node gets ACCEPT(vid, n, v):
if vid <= inst_h:
return OLD_VIEW(vid, views[vid])
else if n >= num_h:
num_a = n
val_a = v
return ACCEPTED()
else
return REJECT()
Paxos Phase 3
if leader gets OLD_VIEW(vid, v):
views[vid] = v
inst_h = vid
view change
restart paxos
else if leader gets ACCEPTED from a majority of nodes in views[inst_h]:
send DECIDE(inst_h+1, val_a) to all (including self)
else:
delay and restart paxos
if node gets decide(vid, v):
if vid <= inst_h:
return OLD_VIEW(vid, views[vid])
else:
done = true
primary is lowest-numbered node in v
views[vid] = v
inst_h = vid
view change
The views correspond to rounds of the algorithm. They are called views because every node in the algorithm is asynchronous and thus every node’s understanding of the global system state can be outdated. The nodes use views to synchronize the execution of the algorithm. Look at the comment that says views = rounds.