Hi Michi,
> Otherwise the client > might think the session is expired when it isn't. In case of the > single-threaded client, it looks like you need to keep calling > zookeeper_interest() until select() succeeds. What happens if the server is offline? In the scenario you described, select() will never succeed, and zookeeper_interest() will never inform the upstream consumer that the server is down. That's the crux of the problem here, when the server is unavailable, the client needs to know after some amount of time that the connection has failed. Otherwise the upstream consumer has no idea the connection to zookeeper has been severed and will hang indefinitely. > Zookeeper client shouldn't return ZOO_EXPIRED_SESSION_STATE unless the > server tells the client the session is expired. That's fair, I could return ZOO_CONNECTION_LOSS here instead. On Nov 5, 2012, at 1:59 PM, Michi Mutsuzaki <mi...@cs.stanford.edu> wrote: > Hi Yunong, > > Zookeeper client shouldn't return ZOO_EXPIRED_SESSION_STATE unless the > server tells the client the session is expired. Otherwise the client > might think the session is expired when it isn't. In case of the > single-threaded client, it looks like you need to keep calling > zookeeper_interest() until select() succeeds. It would be great to > have sample code / documentation. > > --Michi > > On Mon, Nov 5, 2012 at 1:19 PM, Yunong Xiao <yjx...@gmail.com> wrote: >> Michi, >> >> I don't think just returning error after checking SO_ERROR is sufficient >> here, >> since this indicates connection loss but not session expiration. The client >> could still connect to a different zk server on future iterations of >> zookeeper_interest(), which will not occur if we return error. I think what >> is >> needed here is to keep track of the amount of time that has elapsed since the >> last time a connection was established to the server, while trying to connect >> to servers in the list. If this time exceeds session_timeout, then return an >> error such as ZOO_EXPIRED_SESSION_STATE. I've patched my local zookeeper >> client >> to keep track of this state. >> >> In addition, It's not clear to me how you would go about checking for >> SO_ERROR >> within zookeeper.c, since you'll need to check SO_ERROR after calling >> select(), >> which -- at least in my case, since I'm using the single threaded client, >> and I >> am embedding in the node.js/libuv runtime -- needs to be outside of the C >> client. I'd like to hear your thoughts on how this could be better achieved. >> I >> would propose the following patch: >> >> 1) zookeeper_interest needs to return state about it's current connection >> status, since the zhandle is opaque. This will allow consuming clients to >> check >> for the status of the non-blocking connect. >> >> 2) zookeeper_interest has to keep track of the last time it was connected to >> a >> server, and return ZOO_EXPIRED_SESSION_STATE if it's still unable to connect >> after the timeout exceeds the session timeout. >> >> 3) some sample code/documentation around checking for the status of >> non-blocking connects in user code for consumers of zookeeper.h. >> >> Does this seem reasonable? Would you guys be open to taking my patch? >> >> -Yunong >> >> On Nov 5, 2012, at 10:33 AM, Michi Mutsuzaki <mi...@cs.stanford.edu> wrote: >> >>> Hi Yunong, >>> >>> Yes, this looks like a bug. The problem is that the C client is not >>> handling the case when connect() returns EINPROGRESS or EWOULDBLOCK >>> and eventually fails. I think the right fix is to check SO_ERROR after >>> the socket becomes writable. Please go ahead and open a jira. >>> >>> Thanks! >>> --Michi >>> >>> On Sun, Nov 4, 2012 at 2:11 PM, Yunong Xiao <yjx...@gmail.com> wrote: >>>> I have a fairly simple single-threaded C client set up -- single-threaded >>>> because we are embedding zk in the node.js/libuv runtime -- which consists >>>> of >>>> the following algorithm: >>>> >>>> zookeeper_interest(); select(); >>>> // perform zookeeper api calls >>>> zookeeper_process(); >>>> >>>> I've noticed that zookeeper_interest in the C client never returns error >>>> if it >>>> is unable to connect to the zk server. >>>> >>>> From the spec of the zookeeper_interest API, I see that zookeeper_interest >>>> is >>>> supposed to return ZCONNECTIONLOSS when disconnected from the client. >>>> However, >>>> digging into the code, I see that the client is making a non-blocking >>>> connect >>>> call >>>> https://github.com/apache/zookeeper/blob/trunk/src/c/src/zookeeper.c#L1596-1613 >>>> , and returning ZOK >>>> https://github.com/apache/zookeeper/blob/trunk/src/c/src/zookeeper.c#L1684 >>>> >>>> If we assume that the server is not up, this will mean that the subsequent >>>> select() call would return 0, since the fd is not ready, and future calls >>>> to >>>> zookeeper_interest will always return 0 and not the expected >>>> ZCONNECTIONLOSS. >>>> Thus an upstream client will never be aware that the connection is lost. >>>> >>>> I don't think this is the expected behavior. I have temporarily patched >>>> the zk >>>> C client such that zookeeper_interest will return ZCONNECTIONLOSS if it's >>>> still >>>> unable to connect after session_timeout has been exceeded. >>>> >>>> Is this the right interpretation of the API? Are you guys open to taking >>>> the >>>> patch I described? >>>> >>>> -Yunong >>
