Ted Dunning commented on ZOOKEEPER-759:

This is a unix specific bean so don't forget to defang the test if the bean 
isn't available.

> Stop accepting connections when close to file descriptor limit
> --------------------------------------------------------------
>                 Key: ZOOKEEPER-759
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-759
>             Project: Zookeeper
>          Issue Type: Improvement
>          Components: server
>            Reporter: Travis Crawford
> Zookeeper always tries to accept new connections, throwing an exception if 
> out of file descriptors. An improvement would be denying new client 
> connections when close to the limit.
> Additionally, file-descriptor limits+usage should be exported to the 
> monitoring four-letter word, should that get implemented (see ZOOKEEPER-744).
> A Zookeeper ensemble I administer recently suffered an outage when one node 
> was restarted with the low system-default ulimit of 1024 file descriptors and 
> later ran out. File descriptor usage+max are already being monitored by the 
> following MBeans:
> - java.lang.OperatingSystem.MaxFileDescriptorCount
> - java.lang.OperatingSystem.OpenFileDescriptorCount
> They're described (rather tersely) at:
> http://java.sun.com/javase/6/docs/jre/api/management/extension/com/sun/management/UnixOperatingSystemMXBean.html
> This feature request is for the following:
> (a) Stop accepting new connections when OpenFileDescriptorCount is close to 
> MaxFileDescriptorCount, defaulting to 95% FD usage. New connections should be 
> denied, logged to disk at debug level, and increment a 
> ``ConnectionDeniedCount`` MBean counter.
> (b) Begin accepting new connections when usage drops below some configurable 
> threshold, defaulting to 90% of FD usage, basically the high/low watermark 
> model.
> (c) Update the administrators guide with a comment about using an appropriate 
> FD limit.
> (d) Extra credit: if ZOOKEEPER-744 is implemented export statistics for:
> zookeeper_open_file_descriptor_count
> zookeeper_max_file_descriptor_count
> zookeeper_max_file_descriptor_mismatch - boolean, exported by leader, if not 
> all zk's have the same max FD value

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