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From "Jaakko Laine (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-562) Handle pending range clash gracefully
Date Thu, 19 Nov 2009 14:39:39 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780004#action_12780004
] 

Jaakko Laine commented on CASSANDRA-562:
----------------------------------------

> operator has to let gossip from previous bootstrap finish before starting another one
(this is only a few seconds -- much less stringent than "only one node at a time")

I'm not exactly sure what you mean by operator. Do you mean bootstrap source? Bootstrap source
might delay the bootstrap until previous one has completed and then continue to give another
token of its new range to the following bootstrapper. There are some problems in this, I think:
(1) Boostrap source needs a mechanism to abort the first bootstrap in case the node fails
to complete bootstrap. (2) This is very likely to end up in a suboptimal load balance as more
than one node move to same range almost simultaneously. It would be better to reject the second
bootstrap right away (it will be tried again with new load information soon).

I was probably not very clear with my wording. With "one node at a time" I meant that only
one node is bootstrapping to one range at one time. Once the bootstrap is done, another node
is free to bootstrap there immediately.

> if you are adding M nodes to a ring of N where M > N, you can either add N and wait
for those to complete before adding the rest, or manually assign tokens to M - N since the
autoguess won't work after the first N.

Yeah, but if we have a mechanism to tell the bootstrapping node that the operation cannot
be done now, it can wait for random delay and start over again. Before long all nodes will
be able to enter the ring.


> Handle pending range clash gracefully
> -------------------------------------
>
>                 Key: CASSANDRA-562
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-562
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 0.5
>            Reporter: Jaakko Laine
>             Fix For: 0.5
>
>
> I think there are currently some problems with bootstrapping & leaving. The inherent
problem is that a node one-sidedly announces that it is going to leave/take a token without
making sure it will not cause conflict, and we do not have proper mechanism to clean up after
a conflict. There are currently ways a simultaneous bootstrap or leaving can leave the cluster
(tokenmetadata) in an inconsistent state and we'll need either a mechanism to resolve which
operation wins or make sure only one operation is in process for affected ranges at one time.
> 1st option (resolve & clean conflicts as they happen):
> We could add local timestamp to bootstrap/leave gossip and resolve conflicts based on
that. This would allow us to choose one of the operations unambiguously and reject all but
the one that was first. Theoretically, if different data centers are not in perfect clock
sync, this might always favor one DC over the other in race situations, but this would hardly
create any noticeable bias. Problem is, this approach would probably end up being horrendously
complex (if not impossible) to do properly.
> 2nd option (make sure only one operation is in process at one time for affected ranges):
> Add new messaging "channel" to be used to agree beforehand who is going to move. (1)
Before a node can start bootstrapping, it must ask permission from the node whose range it
is going to bootstrap to. The request will be accepted if no other node is currently bootstrapping
there. Nodes of course check their own token metadata before bootstrapping, but this does
not guard against two nodes bootstrapping simultaneously (that is, before they see each others'
gossip). The only node able to answer this reliably is bootstrap source. (2) For leaving,
the node should first check with all nodes that are going to have pending ranges if it is
OK to leave. If it receives "OK" from all of them, then it can leave. If bootstrapping or
leaving operation is rejected, the node will wait for random time and start over. Eventually
all nodes will be able to bootstrap.
> Good in this approach would be that with a relatively small change (adding one messaging
exchange before actual move) we could make sure that all parties involved agree that the operation
is OK. This is IMHO also the "cleanest" way. Downside is that we will need token lease times
(how long the node owns "rights" to the token) and timers to make sure that we do not end
in a deadlock (or lock ranges) in case the node will not complete the operation. Network partitions,
delays and clock skews might create very difficult border cases to handle.
> 3rd option (another approach to preventing conflicts from happening):
> (3) We might take a simplistic approach and add two new states: ABOUT_TO_BOOTSTRAP and
ABOUT_TO_LEAVE. Whenever a node wants to bootstrap or leave, it will first gossip this state
with token info and wait for some time. After the wait, it will check if it is OK to carry
on with the operation (that is, it has not received any bootstrap/leaving gossip that would
contradict with its own plans). Also in this case, if the operation cannot be done due to
conflict, the node waits for random period and start over.
> This would be easiest to implement, but it would not completely remove the problem as
network partitions and other delays might still cause two nodes to clash without knowledge
of each others' intentions. Also, adding two more gossip states will expand the state machine
and make it more fragile.
> Don't know if I'm thinking about this in a wrong way, but to me it seems resolving conflicts
is very difficult, so the only option is to avoid them by some mechinism that makes sure only
one node is moving within affected ranges.

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