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From Tharindu Kumara <zonik.hatkum...@gmail.com>
Subject Re: Sessions Expire due to Network partitioning in Zookeeper
Date Fri, 03 Mar 2017 05:19:45 GMT
Hi Rakesh,

I have made a small mistake in the above email.
For client's session timeout I have used 20000ms not 45000ms.
Sorry for the mistake.
Basically it waits for 28 seconds to connect to Server A. (14 seconds to
figure out disconnection and another 14 seconds to connect to Server B)




Then as soon as it connected to Server A, Server A sends the session
expiration.
Then I again did the same experiment with setting client's session
expiration time to 45000ms.
Then it took 15secs to retrieve the disconnection event to client.
After from that point, client took another 30secs to trying to connect to
Server B.
And then after that, client figures out that it cannot connect to Server B
and try to initiate a connection to Server A.
Since 15 + 30 secs is 45, when as soon as client connected to Server A,
Server A sends the session expiration event.
What is the reason for that? It looks like client is trying too much time
to connect to Server B.


Here I attached the Server B's log (Leader) and client's log.
Server A = 172.25.83.201
Server B = 172.25.83.204
Server C = 172.25.83.205
And client's session id is 0x35a926acae80000

https://drive.google.com/file/d/0B9o9GJ_CoG1_NWRTQ3hmeGpZRktrc040MkZzTk5LTDdpMi1v/view?usp=sharing
https://drive.google.com/file/d/0B9o9GJ_CoG1_VHlKa092QzB6M3NPemRORk9Lc0w1a2N5ZDdz/view?usp=sharing

And also in the above post, you mentioned that, you mentioned about a
connection-timeout.
Can you please explain a bit more?

I see that when I try with both 45000ms and 20000ms, it takes around 14 -
15 seconds to figure out a disconnection from clients end.
Looks like it is a constant and client has (session timeout - connection
loss notified time) to find a new Server. Am I correct here?
You mentioned that client connection timeout is equal to
("sessiontimeout/listed servers count").
Please explain a bit about the client ping times and connection timeout.
How these to co-relate?

On Thu, Mar 2, 2017 at 8:18 PM, Rakesh Radhakrishnan <rakeshr@apache.org>
wrote:

> >>> You mentioned that a client sends a ping every 1/3 the session timeout.
> Yes, you are correct. Again to analyse your issue, we have to consider
> re-connection timeout also, which is "sessiontimeout/listed servers count"
> https://github.com/apache/zookeeper/blob/branch-3.4/src/
> java/main/org/apache/zookeeper/ClientCnxn.java#L1292
> https://github.com/apache/zookeeper/blob/branch-3.4/src/
> java/main/org/apache/zookeeper/ClientCnxn.java#L1098
>
> Coincidentally, in your example both heartbeat interval and re-connection
> interval are same as you have three servers.
>
> >>>>> It looks like C3 has taken 14 seconds to determine the disconnected
> event
> >>>>> and another 14 seconds to that it cannot connect to Server B(C3
is
> isolated
> >>>>> from B).
>
> With this info, total elapsed time is 28 secs which is less than 45 secs
> session timeout. Now, the client has 17 secs (45 secs - 28 secs) time
> period to re-establish a connection with server A, right? Could you please
> check whether the client is connecting to A during this period?
>
> Rakesh
>
>
>
> On Thu, Mar 2, 2017 at 6:58 PM, Tharindu Kumara <zonik.hatkumara@gmail.com
> >
> wrote:
>
> > ​​
> > Hi Rakesh,
> >
> > First of all thank you for the quick reply.
> >
> > >>>>> Actually, ZooKeeper client has retry mechanism.
> > >>>>> Client sends a ping every 1/3 the session timeout (here, 3
is the
> no.
> > of listed servers, A, B, C) and then looks for a response before another
> > 1/3 elapses. This allows time to reconnect to a different server (and
> still
> > maintain the session) if the connected server becomes unavailable.
> >
> > You mentioned that a client sends a ping every 1/3 the session timeout.
> And
> > 3 is the no of listed servers.
> >
> > I doubt that. Because, I am using the C Binding and after inspecting the
> > code it looks like that 3 is a hard coded value.
> > Simply no matter what the number of clients, zk client biding is always
> > sending a ping every 1/3 session timeout.
> >
> > Can please clarify that for me?
> >
> > Here I used a tick of 3000ms and session expiration timeout of 45000ms.
> >
> > And please find the screenshot of extacted client log outout.
> >
> > https://anonimag.es/image/JT9htnL
> >
> > It looks like C3 has taken 14 seconds to determine the disconnected event
> > and another 14 seconds to that it cannot connect to Server B(C3 is
> isolated
> > from B).
> >
> >
> >
> > On Thu, Mar 2, 2017 at 4:08 PM, Rakesh Radhakrishnan <rakeshr@apache.org
> >
> > wrote:
> >
> > > >>>> According to my understanding, it looks like, when a client
trying
> > to
> > > >>>> connect to a server that it cannot connect due to a network
> > > partitioning,
> > > >>>> it uses a blocking call and it waits too much time trying
to
> > > >>>> connect to a server that it cannot communicate.
> > >
> > > Actually, ZooKeeper client has retry mechanism.
> > > Client sends a ping every 1/3 the session timeout (here, 3 is the no.
> of
> > > listed servers, A, B, C)
> > > and then looks for a response before another 1/3 elapses. This allows
> > time
> > > to reconnect to a
> > > different server (and still maintain the session) if the connected
> server
> > > becomes unavailable.
> > >
> > > Could you grep the following log message in your client log and tell me
> > how
> > > much time C3 taken for the re-connection attempts.
> > > "Client session timed out, have not heard from server in "
> > >
> > > C3 might have first attempted to reconnect to B and then A. Also, need
> to
> > > check how much time C3 taken to detect connection failure from server
> C.
> > >
> > > Could you please share the zk client log to dig more.
> > >
> > > Rakesh
> > >
> > >
> > > On Thu, Mar 2, 2017 at 11:04 AM, Tharindu Kumara <
> > > zonik.hatkumara@gmail.com>
> > > wrote:
> > >
> > > >  > ​
> > > > 1) Could you tell me the status of Server C, is this lost connection
> to
> > > the
> > > >  >     quorum and fails to join quorum continuously as B is the
> Leader
> > ?
> > > >
> > > > Yes, B the leader. C Server is completely isolated from the Leader(B)
> > > > and It cannot communicate with the Leader. C cannot continuously
> > connect
> > > to
> > > > the
> > > >
> > > > Leader.
> > > >
> > > >
> > > >  > 2) C3 is connected C. Please tell me the connection host string
> > passed
> > > > to
> > > >  >     this client. Does it contains all three servers info
> > > "A:clientport,
> > > >  >    B:clientport, C:clientport" ?
> > > >
> > > > Yes, C3's connection string contains all three servers.
> ("A:clientport,
> > > > B:clientport, C:clientport")
> > > >
> > > >
> > > >  > 3) Please check all three servers and client C3 logs to see any
> > > >  >    inconsistencies or exceptions.
> > > >
> > > > After looking at logs, it seems when the server C isolated from the
> > > Leader,
> > > >
> > > > a disconnect event fires to client C3. Then it (C3) tries too much
> time
> > > to
> > > > connect to Server B(Leader) .
> > > >
> > > > But it cannot connect to server B, as we blocked the connection
> between
> > > > Server C and
> > > >
> > > > Server B. Basically, C3 tries more than half of the session timeout
> > time
> > > to
> > > > connect to Server B.
> > > >
> > > > Then after figuring out that C3 cannot to connect to Server B, it
> tries
> > > to
> > > > connect
> > > >
> > > > to Server A, and it connects to Server A successfully. But this is
> too
> > > > late, because
> > > >
> > > > session is already expired at the time C3 connected.
> > > >
> > > > And this happens sometimes only. Because when we specify all the
> > servers
> > > in
> > > > the client's
> > > >
> > > > connect string, sometimes after C3 disconnecting from Server C,
> instead
> > > of
> > > > trying to connect to
> > > >
> > > > Server B it connects to Server A as the first attempt. In this case
> the
> > > > client C3 connects to the
> > > >
> > > > quorum successfully before the session expiration.
> > > >
> > > > According to my understanding, it looks like, when a client trying to
> > > > connect to a server that it cannot
> > > >
> > > > connect due to a network partitioning, it uses a blocking call and it
> > > waits
> > > > too much time trying to
> > > >
> > > > connect to a server that it cannot communicate.
> > > >
> > > >
> > > >
> > > >  > 4) ZooKeeper version used in your testing ?
> > > >
> > > > I used zookeeper 3.4.9 (current stable release)
> > > >
> > > >
> > > >
> > > > On Thu, Mar 2, 2017 at 7:48 AM, Rakesh Radhakrishnan <
> > rakeshr@apache.org
> > > >
> > > > wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > Could you please give few more details,
> > > > >
> > > > > ​​
> > > > > 1) Could you tell me the status of Server C, is this lost
> connection
> > to
> > > > the
> > > > > quorum and fails to join quorum continuously as B is the Leader ?
> > > > >
> > > > > 2) C3 is connected C. Please tell me the connection host string
> > passed
> > > to
> > > > > this client. Does it contains all three servers info "A:clientport,
> > > > > B:clientport, C:clientport" ?
> > > > >
> > > > > 3) Please check all three servers and client C3 logs to see any
> > > > > inconsistencies or exceptions.
> > > > >
> > > > > 4) ZooKeeper version used in your testing ?
> > > > >
> > > > >
> > > > > Rakesh
> > > > >
> > > > > On Wed, Mar 1, 2017 at 4:55 PM, Tharindu Kumara <
> > > > zonik.hatkumara@gmail.com
> > > > > >
> > > > > wrote:
> > > > >
> > > > > > ​Recently, carried out a test to to find the behavior of clients
> > > when a
> > > > > > client is partitioned from the ensemble.
> > > > > >
> > > > > > Here I used a ensemble of 3 zookeeper servers called A, B and
C.
> > And
> > > > > quorum
> > > > > > was set up like below.
> > > > > >
> > > > > > A - Follower
> > > > > > B - Leader
> > > > > > C - Follower​
> > > > > >
> > > > > > A  <---> B <---> C
> > > > > >    \____________/
> > > > > >
> > > > > > And 3 clients are connected to ensemble like below.
> > > > > >
> > > > > > C1 is connected A
> > > > > > C2 is connected B
> > > > > > C3 is connected C.
> > > > > >
> > > > > > I used iptables to remove the network link between B and C.
> > > > > >
> > > > > > command used: iptables -I INPUT -s 123.123.45.123 -j DROP
> > > > > >
> > > > > > After removing the link connections looks like below.
> > > > > >
> > > > > > A  <----> B         C
> > > > > >    \____________/
> > > > > >
> > > > > > Simply there is no way to communicate from B to C and vice versa.
> > > > > >
> > > > > > Here What I noticed is that the client connected to Zookeeper
> > Server
> > > > "C",
> > > > > > could not connect to the ensemble resulting a session expiration
> > > > timeout.
> > > > > >
> > > > > > For this experiment I used tickTime of 3000ms and client session
> > > > > expiration
> > > > > > timeout of 45000ms. And tested with different combinations also.
> > > > > >
> > > > > > Can someone please explain what is the root cause for this
> > behavior?
> > > > > >
> > > > >
> > > >
> > >
> >
>

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