Daidong, There are several key differences between distributed transactions and the replication problem we solve in ZooKeeper, and if you are interested in understanding them, you might start by having a look at the Paxos Commit work of Gray and Lamport. They have a TR available online, just use your favorite search engine.
-Flavio On Apr 23, 2011, at 6:55 AM, daidong wrote: Hi, Alex Thanks for your reply and Flavio's I think i finally get the idea. :) Would it be appropriate to see the ZAB as a 3PC without the READY/WAIT status? As all the participators will reply VOTE_COMMIT (they do not abort...). I will read the source code and hope can do some stuff with ZAB. Thanks a lot for all the replies. -- daidong On 2011年4月22日星期五 at 上午3:54, Alexander Shraer [via zookeeper-user] wrote: Hi Daidong,
In addition to Flavio's response, I'll try to address some of your specific questions.
In my opinion, an atomic broadcast protocol must guarantee all the non-
faulty servers have the same status eventually. So in the 2PC protocol,
the coordinator must block until "all" the servers reply "ok".
Designed this way, the protocol wouldn't be able to tolerate any failures - the leader could block
waiting for a response from a server that had crashed. The idea is to receive enough "ok" messages
to guarantee that even if a minority of servers crash, the information is still not lost. That's why
the leader waits for a majority of acks. Messages are still sent to all followers, so they will eventually
get them (or if they disconnect they will later reconnect and synch with the leader automatically).
Regarding your second question - formally, sequential consistency guarantees that operations of each client take effect in the order
they were submitted by the client - so a client's read is guaranteed to see its own last complete write.
In the example you mention, the client first executes a create() and then getChildren(). If clients C1 and C2 both submit a create()
concurrently, one of these requests will reach the leader and will be scheduled by the leader before the other one, suppose the create() request of C1.
Then, when C2 is notified about the completion of its own create, FIFO ensures that it also finds out about any operation that completed before that create()
(these messages were sent by the leader earlier). So when C2 finally runs getChildren(), its local state will already have every operation that was scheduled
by the leader before its own create() completed.
In general, ZAB implements state-machine replication by executing consensus on each operation. To understand the general idea,
I recommend reading Lamport's "Paxos made simple" paper I sent earlier - it has a constructive explanation of this
(although the algorithm is somewhat different from ZAB).
Alex
-----Original Message-----
From: daidong [mailto:]
Sent: Wednesday, April 20, 2011 11:31 PM
To: [hidden email]
Subject: Re: RE: Problems about Zab protocol
Hi, Alex
Thanks for your reply. :)
I knew ZAB has two modes, but things i do not quit understand focus on
the broadcast mode. In the ZAB paper, authors said ZAB is a simple
version of two phases commit protocol because we don't have abort
actions in followers. I do not quit understand this.
In my opinion, an atomic broadcast protocol must guarantee all the non-
faulty servers have the same status eventually. So in the 2PC protocol,
the coordinator must block until "all" the servers reply "ok". If there
is not any abort too, consider the situation that we have a very slow
follower F who processes messages slower than other followers.
According TCP and FIFO channel, We can say all the messages will be
processed orderly in F, however, the messages will assemble if
coordinator continues to broadcasting. What happens if the receive
buffer in F is overflow?
Is there any mechanism i have not noticed to avoid this situation in
ZAB?
About my second questions, I read the consistency guarantees section,
thanks for your tips. I still have a question, if zookeeper do not make
sure that all the clients will see the latest value, how the lock
mechanism works? i checked the recipe example code in Zookeeper 3.3.3,
when a client try to get the write lock, it does not sync() before call
getChildren(). If other client has created a ephemeral node with the
lowest number suffix, this client does not get this information as
getChildren() do not sync with leader. Is there any possibility that
two clients will think they both got the lock?
Thanks for any words. :)
--
daidong
Sent with Sparrow
On 2011年4月21日星期四 at 上午2:30, Alexander Shraer [via zookeeper-
user] wrote:
Hi,
Regarding your first question - ZAB has two parts - the broadcast
protocol you mention,
which is executed by a leader, and the leader election protocol,
which recovers from a leader failure.
This is similar to the way other state-machine replication algorithms
work, where you have
a fast normal mode and a slower recovery mode (you don't need to
execute both all the time - only when the leader fails).
See Paxos state-machine replication for example (section 3):
http://research.microsoft.com/en-
us/um/people/lamport/pubs/pubs.html#paxos-simple
Regarding your second question - Zookeeper basically guarantees so
called "sequential consistency" semantics.
This guarantees that the real execution looks to clients like some
sequential execution in which
the operations of every client appear in the order they were
submitted. It does not guarantee that a read of one client
returns the latest value written by another client. This allows reads
to be executed locally. If you need to return the latest
state, you can use the sync() call which flushes the pending updates
between the leader and a follower.
See also the "consistency guarantees" section here:
http://hadoop.apache.org/zookeeper/docs/r3.3.1/zookeeperProgrammers.htm
l
Alex
-----Original Message-----
From: daidong [mailto:[hidden email]]
Sent: Wednesday, April 20, 2011 2:38 AM
To: [hidden email]
Subject: Problems about Zab protocol
Hi, everyone.
Recently, i read the paper "a simple total ordered broadcast
protocol"
and
there are some problems i can not figure out. Hope anyone can help
me... :P
The paper describes the Zab protocol as a 2 phase commit protocol
when
system is under broadcast mode. However some paper(Skeen 82, "A
Quorum
Based
Commit Protocol") has mentioned if we want to extend an 2PC to
adapt a
quorum based commit protocol we must introduce a three phase commit
protocol(In fact, i haven't quit understood this, :( ). However
according
Zab paper, this still can be done. Why and how to do this?
Secondly, even Zookeeper can guarantee that status in different
followers
are consistent. However, this consistency only works among a quorum
of
followers that has acked the COMMIT. As the client can connect to
any
followers when perform reading action, so what happens if the
client
happens
to connect with the follower that has not acked the COMMIT? I can
not
find
the information in this paper...
If i ask some naive question, Hope anybody can tell me where i can
find
the
answer or some suggestions, thanks :)
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