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https://issues.apache.org/jira/browse/NET-258?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12993958#comment-12993958
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Sebb commented on NET-258:
--------------------------
If any of the FTP classes have any mutable instance data (which I'm sure they
do) then if one thread updates the data, another thread may not see the updated
data - or it could even be partially updated. This is the same problem as with
the boolean stopKeepAlive flag.
In most cases the data will be updated properly, but it's not guaranteed by the
Java Memory Model, and with faster multi-cpu systems and more agressive JVM
optimisation the problem will surface at some point - and will probably be
extremely difficult to debug.
There are probably other solutions which can be implemented within the control
thread, e.g. loop with timeout whilst waiting for completion.
If timeout occurs, send NOOP, and wait again.
> Implement A Keepalive Mechanism
> -------------------------------
>
> Key: NET-258
> URL: https://issues.apache.org/jira/browse/NET-258
> Project: Commons Net
> Issue Type: Improvement
> Affects Versions: 2.0
> Reporter: Rory Winston
> Assignee: Rory Winston
> Attachments: ftp-keepalive.diff
>
>
> For routers/firewalls that terminate idle connections, a separate heartbeat
> mechanism may need to be implemented to keep the control connection active.
> Some potential issues:
> * Synchronization between a heartbeat write and a __getReply() on an active
> control connection command;
> * Should use the NOOP command as a heartbeat signal;
> * Make the timeout configurable;
> * Default SocketImpl::setKeepAlive() wont do here.
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