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== Design
* Simplicity: Unicorn is a traditional UNIX prefork web server.
No threads are used at all, this makes applications easier to debug
and fix. When your application goes awry, a BOFH can just
"kill -9" the runaway worker process without worrying about tearing
all clients down, just one. Only UNIX-like systems supporting
fork() and file descriptor inheritance are supported.
* The Ragel+C HTTP parser is taken from Mongrel. This is the
only non-Ruby part and there are no plans to add any more
non-Ruby components.
* All HTTP protocol parsing and I/O is done much like Mongrel:
1. read/parse HTTP request headers in full
2. call Rack application
3. write HTTP response back to the client
* Like Mongrel, neither keepalive nor pipelining are supported.
These aren't needed since Unicorn is only designed to serve
fast, low-latency clients directly. Do one thing, do it well;
let nginx handle slow clients.
* Configuration is purely in Ruby and eval(). Ruby is less
ambiguous than YAML and lets lambdas for
before_fork/after_fork/before_exec hooks be defined inline. An
optional, separate config_file may be used to modify supported
configuration changes (and also gives you plenty of rope if you RTFS
:>)
* One master process spawns and reaps worker processes. The
Rack application itself is called only within the worker process (but
can be loaded within the master). A copy-on-write friendly garbage
collector like Ruby Enterprise Edition can be used to minimize memory
usage along with the "preload_app true" directive (see
Unicorn::Configurator).
* The number of worker processes should be scaled to the number of
CPUs, memory or even spindles you have. If you have an existing
Mongrel cluster on a single-threaded app, using the same amount of
processes should work. Let a full-HTTP-request-buffering reverse
proxy like nginx manage concurrency to thousands of slow clients for
you. Unicorn scaling should only be concerned about limits of your
backend system(s).
* Load balancing between worker processes is done by the OS kernel.
All workers share a common set of listener sockets and does
non-blocking accept() on them. The kernel will decide which worker
process to give a socket to and workers will sleep if there is
nothing to accept().
* Since non-blocking accept() is used, there can be a thundering
herd when an occasional client connects when application
*is not busy*. The thundering herd problem should not affect
applications that are running all the time since worker processes
will only select()/accept() outside of the application dispatch.
* Additionally, thundering herds are much smaller than with
configurations using existing prefork servers. Process counts should
only be scaled to backend resources, _never_ to the number of expected
clients like is typical with blocking prefork servers. So while we've
seen instances of popular prefork servers configured to run many
hundreds of worker processes, Unicorn deployments are typically only
2-4 processes per-core.
* On-demand scaling of worker processes never happens automatically.
Again, Unicorn is concerned about scaling to backend limits and should
never configured in a fashion where it could be waiting on slow
clients. For extremely rare circumstances, we provide TTIN and TTOU
signal handlers to increment/decrement your process counts without
reloading. Think of it as driving a car with manual transmission:
you have a lot more control if you know what you're doing.
* Blocking I/O is used for clients. This allows a simpler code path
to be followed within the Ruby interpreter and fewer syscalls.
Applications that use threads continue to work if Unicorn
is only serving LAN or localhost clients.
* Timeout implementation is done via fchmod(2) in each worker
on a shared file descriptor to update st_ctime on the inode.
Master process wakeups for checking on timeouts is throttled
one a second to minimize the performance impact and simplify
the code path within the worker. Neither futimes(2) nor
pwrite(2)/pread(2) are supported by base MRI, nor are they as
portable on UNIX systems as fchmod(2).
* SIGKILL is used to terminate the timed-out workers from misbehaving apps
as reliably as possible on a UNIX system. The default timeout is a
generous 60 seconds (same default as in Mongrel).
* The poor performance of select() on large FD sets is avoided
as few file descriptors are used in each worker.
There should be no gain from moving to highly scalable but
unportable event notification solutions for watching few
file descriptors.
* If the master process dies unexpectedly for any reason,
workers will notice within :timeout/2 seconds and follow
the master to its death.
* There is never any explicit real-time dependency or communication
between the worker processes nor to the master process.
Synchronization is handled entirely by the OS kernel and shared
resources are never accessed by the worker when it is servicing
a client.
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