From ff8493ef9d33e7c348293587637c70f82bb90aba Mon Sep 17 00:00:00 2001 From: Laurent Bercot Date: Fri, 19 Dec 2014 00:25:16 +0000 Subject: s6-notifywhen up now sends U and exits on the first newline. Doc updated to reflect it. --- doc/notifywhenup.html | 18 +++++++++++++----- 1 file changed, 13 insertions(+), 5 deletions(-) (limited to 'doc/notifywhenup.html') 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.
  • Daemons can use the ftrigw_notify() function, provided in the ftrigw library. This is extremely simple and efficient, but requires specific s6 support in the daemon.
    • -
  • Daemons can write something to a file descriptor of their choice, +
  • 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 s6-notifywhenup, 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.
    • +with a 'U' event, meaning that the service is now up.

    + Note that there is still a small race condition remaining: +if the daemon writes a line then instantly dies, and the supervisor +picks up the death before the s6-notifywhenup +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. +

    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.

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