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From Michael Han <h...@cloudera.com>
Subject Re: are ephemeral nodes removed when client receives session expiration
Date Tue, 07 Feb 2017 23:21:29 GMT
Hi Ryan,

Thanks for the elaborations. I see why you are confused. Your case is
valid, except in step 5:

>>  Client now knows its session expired but for some reason it tries to
connect to another server B and issues a read.

At this point of time, there are two possible cases:
* Client A's session has been cleaned up. As part of the session clean up,
the ephemeral nodes appertain with A will be gone. After A reconnect to the
quorum, it will not see the old ephemeral nodes.
* Client A's session has not finished cleaned up. In this case, if client A
is trying to issue a read, it has to first establish a new session with
quorum. Because the session clean up transaction of A is already in flight,
and because of the sequential consistency guarantee provided by ZooKeeper,
client A will not be able to reconnect to the quorum / establish the
session, until the previous session up transaction has finished. So we will
be back to first case here, which also guarantee the old ephemeral nodes
are gone.

On Tue, Feb 7, 2017 at 2:02 PM, Ryan Zhang <yangzhangrice@hotmail.com>

> Hi, Michael,
> Thank you for your clarification. What you said seems to be consistent
> with what I saw in the code. However, I am still confused by your
> conclusion. My conclusion from the code is that it is totally possible for
> a client to get session expired and reconnect to another sever to still see
> the ephemeral node. The exact reason is caused by the code I pasted
> below.  I am not sure if we are on the same page as you seem to suggest
> that it is not possible. Let me elaborate a bit on how this can happen.
> 1.  Client A connected to Leader with session S
> 2.  Session S expired on Leader, which according to the code below it will
> set the session as closing
> 3.  Leader then send out a close session request to its own first
> processor which will go through the usual pipeline as you mentioned
> 4. Client A send a request which will go through the checkSession code I
> pasted below and it should get a SessionExpiredException.
> 5. Client now knows its session expired but for some reason it tries to
> connect to another server B and issues a read.
> Now this is a race between the following two chain of operations
> 1. The closesession request needs to go through quorum and get the
> majority and server B get the commit (or inform) request and actually kill
> the session in the final request processor
> 2. The read operation that goes through learnerZK process chain in memory
> (as far as I see, there is no session check on read at all so it won’t know
> until it reads the tree)
> The first chain can get stuck on a various places like some quorum nodes
> are stuck on some other proposals so it looks to me that the second chain
> is very much likely to win as it does not need to go through any quorum
> operation. The key issue is the client needs to reconnect fast (or in
> parallel as the original post seems to indicate). I think I can definitely
> simulate this with a test but it will be tricky to make it pass/fail
> deterministically so I didn’t try.
> Am I missing something?
> -Ryan
> On Feb 7, 2017, at 1:24 PM, Michael Han <hanm@cloudera.com> wrote:
> Hi Ryan,
> I am not sure what you were confused about regarding session cleaning up
> code. Here is my understanding, hope it helps.
> * Session clean up is started from marking the state of a session as
> closed, as you noticed. This is because each session clean up will take a
> while so we need make sure that during session clean up, server will not
> continue processing the requests from the client that appertain with this
> session.
> * Once session is marked as closing, we will send a request so the closing
> of the session not only applies to the leader but also on the quorum
> servers. This is just like all other requests that will go through the
> normal requests process pipeline.
> * A valid session is a prerequisite for any of the client operations
> (including read operations), so the liveness of the session is validated
> before processing a read operation.
> On Wed, Jan 25, 2017 at 2:25 PM, Ryan Zhang <yangzhangrice@hotmail.com>
> wrote:
>> I am a bit confused by the code
>> On Jan 25, 2017, at 1:33 PM, Michael Han <hanm@cloudera.com<mailto:hanm
>> @cloudera.com>> wrote:
>> Does ZK guarantee that ephemeral nodes from a client are removed on the
>> sever by the time the client receives a session expiration event?
>> "the server" is a vague definition, as ZooKeeper ensemble is composed of
>> multiple servers :).
>> Therefore, it seems to be possible for a client to connect to another
>> server to see the node there.
>> This seems the only case I can think of that lead to the inconsistent view
>> from client side. I'll elaborate as follows, first the guarantees of
>> ZooKeeper that's relevant to this case:
>> * ZooKeeper quorum should have already committed the transaction of
>> closing
>> the session when a client receives the session expire event.
>> Here are the code that throws KeeperException.SessionExpiredException
>> public synchronized void checkSession(long sessionId, Object owner)
>>         throws KeeperException.SessionExpiredException,
>>         KeeperException.SessionMovedException,
>>         KeeperException.UnknownSessionException {
>>     ……….
>>     if (session.isClosing()) {
>>         throw new KeeperException.SessionExpiredException();
>>     }
>> }
>> Here is the code to set it to be closing directly
>> synchronized public void setSessionClosing(long sessionId) {
>>     if (LOG.isTraceEnabled()) {
>>         LOG.trace("Session closing: 0x" + Long.toHexString(sessionId));
>>     }
>>     SessionImpl s = sessionsById.get(sessionId);
>>     if (s == null) {
>>         return;
>>     }
>>     s.isClosing = true;
>> }
>> and here is the code that call the above
>> public void runImpl() throws InterruptedException {
>>     while (running) {
>>        ……...
>>         for (SessionImpl s : sessionExpiryQueue.poll()) {
>>             setSessionClosing(s.sessionId);
>>             expirer.expire(s);
>>         }
>>     }
>> }
>> the expire function looks like this
>> public void expire(Session session) {
>>     long sessionId = session.getSessionId();
>>     close(sessionId);
>> }
>> and close function here
>> private void close(long sessionId) {
>>     Request si = new Request(null, sessionId, 0, OpCode.closeSession,
>> null, null);
>>     setLocalSessionFlag(si);
>>     submitRequest(si);
>> }
>> so it looks to me that the session is marked as closing first and then
>> the closeSession is send. This will happen on
>> the lead only though as the checkSession is only called on the lead but
>> it is called even on read operation.
>> May I missing something?
>> * Clean up of ephemeral nodes associated with the session is part of the
>> closing session transaction, so for the quorum of servers who have already
>> committed the transaction, the ephemeral nodes should have gone already,
>> on
>> those servers.
>> * ZooKeeper quorum would not have processed the new session establishment
>> request for the same client, until after the closing session request has
>> been processed because transactions are ordered across quorum.
>> Given these guarantees, if a client reestablishes a new session via
>> connecting to a server which was the quorum of servers that committed the
>> closing session transaction, then the client should not see the old
>> ephemeral node upon new session established.
>> ZooKeeper does not guarantee a write transaction occur synchronously
>> across
>> all of the servers, since a write request only requires a quorum of
>> servers
>> to acknowledge. As a result, it is valid that some servers might lag
>> behind
>> the state of the quorum. I suspect this case is possible:
>> * Client receives session expire event, and client close its connection to
>> server A.
>> * Client reconnects to server B, which lags behind quorum, that does not
>> contain the changes to the data tree regarding ephemeral nodes.
>> * Client sees the ephemeral node so it does nothing. Later the node is
>> cleaned up when server B sync with quorum.
>> Client can ensure it always see the state of truth of the quorum by
>> issuing
>> a sync() request before issuing a read request. A sync request will force
>> the server it's connecting to sync with the quorum. If Kafka does this,
>> will the bug go away? Of course, retry creating ephemeral nodes can also
>> solve the problem (there are possible other solutions as well, by having
>> client to do some book keeping work to differentiate versions between
>> ephemeral nodes).
>> On Wed, Jan 25, 2017 at 11:32 AM, Ryan Zhang <yangzhangrice@hotmail.com
>> <mailto:yangzhangrice@hotmail.com>>
>> wrote:
>> Good question, AFAIK, it’s not the case.
>> The server will throw an SessionExpiredException during checkSession call
>> as soon as the session is marked as isClosing. However, session expiration
>> actually requires a transaction (of type OpCode.closeSession) which will
>> be
>> send to the leader to go through the quorum.  The session and ephemeral
>> node will only be removed after the transaction is  committed and
>> processed
>> in the final processor on other nodes. Therefore, it seems to be possible
>> for a client to connect to another server to see the node there. I am not
>> entirely sure if it can use the same session id though, it seems possible
>> as the session close is only based on the session expire time and there
>> can
>> be delays in session pings.
>> On Jan 25, 2017, at 8:53 AM, Jun Rao <junrao@gmail.com<mailto:junra
>> o@gmail.com><mailto:junra
>> o@gmail.com<mailto:o@gmail.com>>> wrote:
>> Hi,
>> Does ZK guarantee that ephemeral nodes from a client are removed on the
>> sever by the time the client receives a session expiration event? I am
>> getting conflicting info on this (
>> https://issues.apache.org/jira/browse/KAFKA-4277). Could someone clarify?
>> Thanks,
>> Jun
>> --
>> Cheers
>> Michael.
> --
> Cheers
> Michael.


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