doc: add Application Timeouts document

Hopefully this leads to fewer worker processes being killed.

Author: Eric Wong

.document

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 ISSUES
 Sandbox
 Links
+Application_Timeouts

Application_Timeouts

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+= Application Timeouts
+
+This article focuses on _application_ setup for Rack applications, but
+can be expanded to all applications that connect to external resources
+and expect short response times.
+
+This article is not specific to \Unicorn, but exists to discourage
+the overuse of the built-in
+{timeout}[link:Unicorn/Configurator.html#method-i-timeout] directive
+in \Unicorn.
+
+== ALL External Resources Are Considered Unreliable
+
+Network reliability can _never_ be guaranteed.  Network failures cannot
+be detected reliably by the client (Rack application) in a reasonable
+timeframe, not even on a LAN.
+
+Thus, application authors must configure timeouts when interacting with
+external resources.
+
+Most database adapters allow configurable timeouts.
+
+Net::HTTP and Net::SMTP in the Ruby standard library allow
+configurable timeouts.
+
+Even for things as fast as {memcached}[http://memcached.org/],
+{dalli}[http://rubygems.org/gems/dalli],
+{memcached}[http://rubygems.org/gems/memcached] and
+{memcache-client}[http://rubygems.org/gems/memcache-client] RubyGems all
+offer configurable timeouts.
+
+Consult the relevant documentation for the libraries you use on
+how to configure these timeouts.
+
+== Rolling Your Own Socket Code
+
+Use non-blocking I/O and IO.select with a timeout to wait on sockets.
+
+== Timeout module in the Ruby standard library
+
+Ruby offers a Timeout module in its standard library.  It has several
+caveats and is not always reliable:
+
+* /Some/ Ruby C extensions are not interrupted/timed-out gracefully by
+  this module (report these bugs to extension authors, please) but
+  pure-Ruby components should be.
+
+* Long-running tasks may run inside `ensure' clauses after timeout
+  fires, causing the timeout to be ineffective.
+
+The Timeout module is a second-to-last-resort solution, timeouts using
+IO.select (or similar) are more reliable.  If you depend on libraries
+that do not offer timeouts when connecting to external resources, kindly
+ask those library authors to provide configurable timeouts.
+
+=== A Note About Filesystems
+
+Most operations to regular files on POSIX filesystems are NOT
+interruptable.  Thus, the "timeout" module in the Ruby standard library
+can not reliably timeout systems with massive amounts of iowait.
+
+If your app relies on the filesystem, ensure all the data your
+application works with is small enough to fit in the kernel page cache.
+Otherwise increase the amount of physical memory you have to match, or
+employ a fast, low-latency storage system (solid state).
+
+Volumes mounted over NFS (and thus a potentially unreliable network)
+must be mounted with timeouts and applications must be prepared to
+handle network/server failures.
+
+== The Last Line Of Defense
+
+The {timeout}[link:Unicorn/Configurator.html#method-i-timeout] mechanism
+in \Unicorn is an extreme solution that should be avoided whenever
+possible.  It will help catch bugs in your application where and when
+your application forgets to use timeouts, but it is expensive as it
+kills and respawns a worker process.