Comments - HandlerSocket Installation
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In stock Ubuntu 14.04LTS, installing the plugin alone doesn't enable the listener as the defaults for handlersocket_address, handlersocket_port and handlersocket_port_wr are set to empty strings, rather than the expected values.
You need to edit the /etc/mysql/my.cnf file and add the appropriate values, then restart the server to get the handlersocket listeners started.
Thanks for the feedback on the outdated docs, they've been updated.