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Diffstat (limited to 'doc/notifywhenup.html')
| -rw-r--r-- | doc/notifywhenup.html | 18 |
1 files changed, 13 insertions, 5 deletions
diff --git a/doc/notifywhenup.html b/doc/notifywhenup.html index 40b0593..f73e2aa 100644 --- a/doc/notifywhenup.html +++ b/doc/notifywhenup.html @@ -54,21 +54,29 @@ is reliably up - because only they know when it is the case. <li> Daemons can use the <tt>ftrigw_notify()</tt> function, provided in <a href="libftrigw.html">the ftrigw library</a>. This is extremely simple and efficient, but requires specific s6 support in the daemon. </li> - <li> Daemons can write something to a file descriptor of their choice, + <li> Daemons can write a line to a file descriptor of their choice, then close that file descriptor, when they're ready to serve. This is a generic mechanism that some daemons already implement, and does not require anything specific in the daemon's code. The administrator can then run the daemon under <a href="s6-notifywhenup.html">s6-notifywhenup</a>, which will properly catch the daemon's message and notify all the subscribers -with a 'U' event, meaning that the service is now up with no possible race -condition. </li> +with a 'U' event, meaning that the service is now up. <br /> <br /> + Note that there is <em>still</em> a small race condition remaining: +if the daemon writes a line then instantly dies, and the supervisor +picks up the death before the <a href="s6-notifywhenup.html">s6-notifywhenup</a> +program picks up the line, it is possible for the event sequence written +to the fifodir to be wrong - 'd' before 'U'. This should be extremely +rare, but unfortunately the race condition is unavoidable. The only +way to be absolutely race-free is to have the daemon perform its +readiness notification itself, which requires specific support. + </li> </ol> <p> The second method should really be implemented in every long-running program providing a service. When it is not the case, it's impossible -to provide race-free startup notifications, and subscribers should be -content with the unreliable 'u' events provided by s6-supervise. +to provide reliable startup notifications, and subscribers should then +be content with the unreliable 'u' events provided by s6-supervise. </p> </body> |