Passenger architectural overview
================================

This document describes Passenger's architure in a global way. The purpose of
this document is to lower the barrier to entry for new contributors, as well
as to explain (some of the) design choices that we have made.

Or it can be a fun read for people who just want to know how Passenger works.


About the involved technologies
-------------------------------

[[typical_web_applications]]
=== Typical web applications ===

Before we describe Passenger, it is important to understand how typical web
applications work, from the point of view of someone who wants to connect the
application to a web server.

A typical, isolated, web application accepts an HTTP request from some I/O
channel, processes it internally, and outputs an HTTP response, which is sent
back to the client. This is done in a loop, until the application is commanded
to exit. This does not necessarily mean that the web application speaks HTTP
directly: it just means that the web application accepts some kind of
representation of an HTTP request.

image:images/typical_isolated_web_application.png[Architecture of a typical
web application in isolation]

Few web applications are accessible directly by HTTP clients. Common setups
are:

1. The web application is contained in an application server. This application
server may or may not be able to contain multiple web applications. The
application server is then connected to the web server. The web server
dispatches requests to the application server, which in turn dispatches
requests to the correct web application, in a format that the web application
understands. Conversely, HTTP responses outputted by the web application are
sent to the application server, which in turn sends them to the web server,
and eventually to the HTTP client.
+
A typical example of such a setup is a J2EE application, contained in the
Tomcat web server, behind the Apache web server.

2. The web application is contained in a web server. In this case, the web
server acts like an application server. This is the case for PHP applications,
on Apache servers with 'mod_php'. Note that this does not necessarily mean
that the web application is run inside the same process as the web server:
it just means that the web server manages applications.

3. The web application *is* a web server, and can accept HTTP requests
directly. This is the case for the Trac bug tracking system, running in its
standalone server. In many setups, such web applications sit behind a different
web server, instead of accepting HTTP requests directly. The frontend web
server acts like a reverse HTTP proxy.

4. The web application does not speak HTTP directly, but is connected directly
to the web server through some communication adapter. CGI, FastCGI and SCGI
are good examples of this.

These descriptions are true for virtually all web applications, whether they're
based on PHP, Django, J2EE, ASP.NET, Ruby on Rails, or whatever. Note that all
of these setups provide the same functionality, i.e. no setup can do something
that a different setup can't. The critical reader will notice that all of these
setups are identical to the one described in the first diagram, if the
combination of web servers, application servers, web applications etc. are
considered to be a single entity; a black box if you will.

It should also be noted that these setups do not enforce any particular
I/O processing implementation. The web servers, application servers, web
applications, etc. could process I/O serially (i.e. one request at a time),
could multiplex I/O with a single thread (e.g. by using `select(2)` or
`poll(2)`) or it could process I/O with multiple threads and/or multiple
processes.

Of course, there are many variations possible. For example, load balancers
could be used. But that is outside the scope of this document.


=== Ruby on Rails ===

Every Ruby on Rails application has a 'dispatcher'. This dispatcher is
responsible for processing HTTP requests. It does not speak HTTP directly.
Instead, it accepts data structures that contain the information of an
HTTP request. Thus, the dispatcher is particularly interesting to
developers who wish to develop software which connects Ruby on Rails to an
HTTP processing layer (e.g. a web server).

The Ruby on Rails dispatcher can only process requests serially, i.e. one
at a time. It is not possible to process two requests at the same time
with threads, because parts of Ruby on Rails are not thread-safe. (In
practice, this isn't as big of a problem as some people imagine. This will
be elaborated further in <<concurrent_requests,Handling of concurrent
requests>>.)

A particularly interesting thing to note, is that a lot of the memory
occupied by Ruby on Rails applications is spent on storing the program code
(i.e. the link:http://en.wikipedia.org/wiki/Abstract_syntax_tree[abstract
syntax tree (AST)]) in memory. This is observed through the use of the
memory statistics function in link:http://www.rubyenterpriseedition.com/[Ruby
Enterprise Edition]. Also, a lot of the startup time of a Ruby on Rails
application is spent on bootstrapping the Rails framework.


=== Apache ===

The Apache web server has a pluggable I/O multiprocessing (the ability to
handle more than 1 concurrent HTTP client at the same time) architecture. An
Apache module which implements a particular multiprocessing strategy, is called
a Multi-Processing Module (MPM). The
link:http://httpd.apache.org/docs/2.0/mod/prefork.html[prefork MPM] -- which
also happens to be the default -- appears to be the most popular one. This MPM
spawns multiple worker child processes. HTTP requests are first accepted by a
so-called control process, and then forwarded to one of the worker processes.
The next section contains a diagram which shows the prefork MPM's architecture.


Passenger architecture
----------------------

=== Overview ===

Passenger's architecture is a lot like setup #2 described in
<<typical_web_applications,Typical web applications>>. In other words,
Passenger extends Apache and allows it to act like an application server.
Passenger's architecture -- assuming Apache 2 with the prefork MPM is used --
is shown in the following diagram:

image:images/passenger_architecture.png[Passenger's architecture]

Passenger consists of an Apache module, 'mod_passenger'. This is written in
C++, and can be found in the directory 'ext/apache2'. The module is active in
the Apache control process and in all the Apache worker processes. When an
HTTP request comes in, 'mod_passenger' will check whether the request should
be handled by a Ruby on Rails application. If so, then 'mod_passenger' will
spawn the corresponding Rails application (if necessary) and forward the
request to that application.

It should be noted that the Ruby on Rails application does *not* run in the
same address space as Apache. This differentiates Passenger from other
application-server-inside-web-server software such as mod_php, mod_perl and
mod_ruby. If the Rails application crashes or leak memory, it will have no
effect on Apache. In fact, stability is one of our highest goals. Passenger
is carefully designed and implemented so that Apache shouldn't crash because
of Passenger.

=== Spawning and caching of code and applications ===

A very naive implementation of Passenger would spawn a Ruby on Rails
application every time an HTTP request is received, just like CGI would.
However, spawning Ruby on Rails applications is expensive. It can take 1 or 2
seconds on a modern PC, and possibly much longer on a heavily loaded server.
This overhead is particularily unacceptable on shared hosts. A less naive
implementation would keep spawned Ruby on Rails application instances alive,
similar to how Lighttpd's FastCGI implementation works.
However, this still has several problems:

1. The first request to a Rails website will be slow, and subsequent requests
   will be fast. But the first request to a different Rails website - on the
   same web server - will still be slow.
2. As we've explained earlier in this article, a lot of memory in a Rails
   application is spent on storing the AST of the Ruby on Rails framework and
   the application. Especially on shared hosts and on memory-constrained
   Virtual Private Servers (VPS), this can be a problem.

Both of these problems are very much solvable, and we've chosen to do just
that.

The first problem can be solved by preloading Rails applications, i.e. by
running the Rails application before a request is ever made to that website.
This is the approach taken by most Rails hosts, for example in the form of a
Mongrel cluster which is running all the time. However, this is unacceptable
for a shared host: such an application would just sit there and waste memory
even if it's not doing anything. Instead, we've chosen to take a different
approach, which solves both of the aforementioned problems.

We spawn Rails applications via a 'spawn server'. The spawn server caches Ruby
on Rails framework code and application code in memory. Spawning a Rails
application for the first time will still be slow, but subsequent spawn
attempts will be very fast. Furthermore, because the framework code is cached
independently from the application code, spawning a different Rails application
will also be very fast, as long as that application is using a Rails framework
version that has already been cached.

Another implication of the spawn server is that different Ruby on Rails will
share memory with each other, thus solving problem #2. This is described in
detail in <<spawn_server, the next section>>.

But despite the caching of framework code and application code, spawning is
still expensive compared to an HTTP request. We want to avoid spawning whenever
possible. This is why we've introduced the *application pool*. Spawned
application instances are kept alive, and their handles are stored into this
pool, allowing each application instance to be reused later. Thus, Passenger
has very good average case performance.

The application pool is shared between different worker processes. Because the
worker processes cannot share memory with each other, either shared memory must
be used to implement the application pool, or a client/server architecture must
be implemented. We've chosen the latter because it is easier
to implement. The Apache control process acts like a server for the application
pool. However, this does not mean that all HTTP request/response data go
through the control process. A worker process queries the pool for a connection
session with a Rails application. Once this session has been obtained, the
worker process will communicate directly with the Rails application.

The application pool is implemented inside 'mod_passenger'. One can find
detailed documentation about it in
link:cxxapi/index.html[+++the C++ API documentation+++],
in particular the documentation about the `ApplicationPool`,
`StandardApplicationPool` and `ApplicationPoolServer` classes.

The application pool is responsible for spawning applications, caching
spawned applications' handles, and cleaning up applications which have been
idle for an extended period of time.

[[spawn_server]]
=== The spawn server ===

The spawn server is written in Ruby, and its code can be found in the directory
'lib/passenger'. Its main executable is 'bin/passenger-spawn-server'.
link:rdoc/index.html[The spawn server's RDoc documentation] documents the
implementation in detail.

The spawn server consists of 3 logical layers:

1. *The spawn manager.* This is the topmost layer, and acts like a fascade for
   all the underlying layers. Clients who use the spawn server only communicate
   with this layer.
2. *The framework spawner server.* The spawn manager spawns a framework spawner
   server for each unique Ruby on Rails framework version. Each framework
   spawner server caches the code for exactly one Ruby on Rails framework
   version. A spawn request for an application is forwarded to the framework
   spawner server that contains the correct Ruby on Rails version for the
   application.
3. *The application spawner server.* This is to the framework spawner server
   what the framework spawner server is to the spawn manager. The framework
   spawner server spawns an application spawner server for each unique Ruby on
   Rails application (here ``application'' does not mean a running process, but
   a set of (source code) files). An application spawner server caches the
   code for exactly one application.

image:images/spawn_server_architecture.png[The spawn server's architecture]

As you can see, we have two layers of code caching: when the spawn server
receives a request to spawn a new application instance, it will forward the
request to the correct framework spawner server (and will spawn that framework
spawner server if it doesn't already exist), which -- in turn -- will forward
it to the correct application spawner server (which will, again, be created if
it doesn't already exist).

Each layer is only responsible for the layer directly below. The spawn manager
only knows about framework spawner servers, and a framework spawner server only
knows about its application spawner servers. The application spawner server is,
however, not responsible for managing spawned application instances. If an
application instance is spawned by mod_passenger, its information will be sent
back to mod_passenger, which will be fully responsible for managing the
application instance's life time (through the application pool).

Also note that each layer is a seperate process. This is required because a
single Ruby process can only load a single Ruby on Rails framework and a
single application.

==== Memory sharing ====

On most modern Unix operating systems, when a child process is created, it will
share most of its memory with the parent process. Processes are not supposed to
be able to access each others' memory, so the operating system makes a copy of
a piece of memory when it is written to by the parent process or the child
process. This is called copy-on-write (COW). Detailed background information
can be found on link:http://www.rubyenterpriseedition.com/[Ruby Enterprise
Edition's website].

The spawn server makes use of this useful fact. Each layer shares its Ruby AST
memory with all of its lower layers, as long as the AST nodes in question
haven't been written to. This means that all spawned Rails applications will
-- if possible -- share the Ruby on Rails framework's code, as well as its own
application code, with each other. This results in a dramatic reduction in
memory usage.

[NOTE]
==================================================================
Sharing memory only works if link:http://www.rubyenterpriseedition.com/[Ruby
Enterprise Edition] is used. This is because the standard Ruby interpreter's
garbage collector isn't copy-on-write friendly. Please visit the Ruby
Enterprise Edition website for technical details.

Passenger works fine with standard Ruby. You still get to enjoy reduced Rails
startup times. You just won't be able to benefit from memory sharing.
==================================================================

Note that link:http://rubini.us/[Rubinius]'s garbage collector is already
copy-on-write friendly.


[[concurrent_requests]]
=== Handling of concurrent requests ===

As explained earlier, a single Rails application instance can only handle a
single request at the same time. This is obviously undesirable. But before we
dive into the solution, let us take a look how the ``competition'' solves this
problem. PHP has similar problems: a single PHP script can also process only
one HTTP request at a time.

- mod_php ``solves'' this problem by using Apache's MPM. In other words,
  mod_php doesn't do anything by itself at all. A single Apache worker
  process/thread can only handle 1 PHP request at a time, but Apache spawns
  multiple worker processes/threads.
- PHP-FastCGI solves the problem by spawning multiple persistent PHP servers.
  The number of PHP servers is independent from the number of Apache worker
  processes/threads. This approach is a lot like existing Rails setups, in
  which a frontend web server proxies requests to a persistent Mongrel cluster.

Passenger cannot use the mod_php way because it would force us to spawn a new
Rails application for each request, which is -- as explained earlier --
unacceptably slow. Instead, Passenger uses the PHP-FastCGI approach. We
maintain a pool of application instances, and whenever a request is received,
we forward the request to one of the application instances in the pool. The
size of the pool is configurable, which is useful for administrators of servers
that are either heavily loaded or have little memory.

The reader might also be interested in studying the application pool's
algorithm, which is non-trivial. The algorithm is documented in detail in
link:ApplicationPool%20algorithm.txt[ApplicationPool algorithm.txt].


== Appendix A: About this document ==

The text of this document is licensed under the
link:http://creativecommons.org/licenses/by-sa/3.0/[Creative Commons
Attribution-Share Alike 3.0 Unported License].

image:images/by_sa.png[link="link:http://creativecommons.org/licenses/by-sa/3.0/"]

