[ZOOKEEPER-1676] C client zookeeper_interest returning ZOK on Connection Loss

• Type: Bug
• Status: Closed
• Priority: Blocker
• Resolution: Not A Problem
• Affects Version/s: 3.4.3
• Component/s: c client

Environment

All

• Release Note: Ok, let's resolve this one and document it.
• Assignee: Yunong Xiao
• Reporter: Yunong Xiao
• Created: 28/Mar/13 17:09
• Updated: 04/Sep/16 04:57
• Resolved: 22/May/16 22:18

Description

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.

I have included a patch for the client which fixes this for release 3.4.3 6b35e96 in this branch: https://github.com/yunong/zookeeper/tree/release-3.4.3-patched Here's the patch https://gist.github.com/yunong/efe869a0345867d54adf

For more information, please see this email thread. http://mail-archives.apache.org/mod_mbox/zookeeper-dev/201211.mbox/[e-mail]%3E

Issue Links

• is related to: ZOOKEEPER-2432 Document how to use single thread C client API to monitor connection status.
• relates to: ZOOKEEPER-2432 Document how to use single thread C client API to monitor connection status.