From 90b12bd71bb9fc79a4640b9112c13ef529d0196a Mon Sep 17 00:00:00 2001 From: Laurent Bercot Date: Fri, 5 Dec 2014 22:26:11 +0000 Subject: Initial commit --- doc/notifywhenup.html | 75 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) create mode 100644 doc/notifywhenup.html (limited to 'doc/notifywhenup.html') diff --git a/doc/notifywhenup.html b/doc/notifywhenup.html new file mode 100644 index 0000000..40b0593 --- /dev/null +++ b/doc/notifywhenup.html @@ -0,0 +1,75 @@ + + + + + s6: service startup notifications + + + + + + +

+s6
+Software
+skarnet.org +

+ +

Service startup notifications

+ +

+ It is easy for a process supervision suite to know when a service that was up +is now down: the long-lived process implementing the service is dead. The +supervisor, running as the daemon's parent, is instantly notified via a SIGCHLD. +When it happens, s6-supervise sends a 'd' event +to its ./event fifodir, so every subscriber +knows that the service is down. All is well. +

+ +

+ It is much trickier for a process supervision suite to know when a service +that was down is now up. The supervisor forks and execs the +daemon, and knows when the exec has succeeded; but after that point, it's all +up to the daemon itself. Some daemons do a lot of initialization work before +they're actually ready to serve, and it is impossible for the supervisor to +know exactly when the service is really ready. +s6-supervise sends a 'u' event to its +./event fifodir when it successfully +spawns the daemon, but any subscriber +reacting to 'u' is subject to a race condition - the service provided by the +daemon may not be ready yet. +

+ +

+ Reliable startup notifications need support from the daemons themselves. +Daemons should notify the outside world when the service they are providing +is reliably up - because only they know when it is the case. +

+ +

+ s6 provides two ways for daemons to perform startup notification. +

+ +
    +
  1. 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.
    2. +
  2. Daemons can write something 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.
    3. +
+ +

+ 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. +

+ + + -- cgit v1.2.3