
=encoding utf8

=head1 NAME

Mojolicious::Guides::FAQ - Frequently Asked Questions

=head1 OVERVIEW

This document contains answers for the most frequently asked questions about
L<Mojolicious>.

=head1 QUESTIONS

We hope these answers are to your satisfaction.

=head2 How does Mojolicious compare to other Perl web frameworks?

The short answer is "it doesn't", because we interpret the words
"web framework" much more literally than others. With the emergence of the
real-time web and new technologies such as WebSockets, we are facing new
challenges that go way beyond what commonly used modules like L<LWP> were
designed for. Because of this, L<Mojolicious> contains a whole new HTTP
client/server stack called L<Mojo>, which was heavily inspired by the original
LWPng effort and carefully designed with these new requirements in mind. So
while some of the higher abstraction layers might look similar to other web
frameworks, it actually defines a whole new category and could even be the
foundation for more advanced ones in the future.

=head2 Why doesn't Mojolicious have any dependencies?

We are optimizing L<Mojolicious> for user-friendliness and development speed,
without compromises. While there are no rules in
L<Mojolicious::Guides::Contributing> that forbid dependencies, we do currently
discourage adding non-optional ones in favor of a faster and more painless
installation process. And we do in fact already use several optional CPAN
modules such as L<EV>, L<IO::Socket::IP>, L<IO::Socket::Socks>,
L<IO::Socket::SSL> and L<Plack> to provide advanced functionality if they are
installed.

=head2 Why reinvent wheels?

Because we can make them rounder. Components specifically designed for
user-friendliness and development speed are not easy to come by. We are strong
believers of the Perl mantra "There is more than one way to do it", and our
quest is to develop the best possible solutions for these two criteria.

=head2 What about backwards compatibility?

In conformance with L<Mojolicious::Guides::Contributing>, we will always
deprecate a feature before removing or changing it in incompatible ways
between major releases. New features can however be marked as experimental to
explicitly exclude them from these rules. This gives us the necessary freedom
to ensure a healthy future for L<Mojolicious>. So, as long as you are not
using anything marked experimental, untested or undocumented, you can always
count on backwards compatibility, everything else would be considered a bug.

=head2 Why not split up Mojolicious into many smaller distributions?

Because there are no advantages, it drastically increases maintenance costs
and installation times without giving us anything in return. It would only
make sense if we wanted to pass ownership of a module to a new maintainer,
which we already have done in the past.

=head2 Which versions of Perl are supported by Mojolicious?

First of all, you need to be aware that according to the L<perlpolicy>, only
the two most recent stable release series of Perl are supported by the
community and receive bug fixes, which are currently 5.20.x and 5.18.x.
L<Mojolicious> follows this model and fully supports these two release series.
In addition we will also keep the distribution installable up to a certain
legacy version that we deem worthy of supporting, but not specifically
optimize for it, this is currently 5.10.1.

=head2 Do I need to clean my environment before testing Mojolicious?

Mojolicious uses many environment variables both internally and externally,
notably (but not exclusively) those starting with the prefix C<MOJO_*>. The
test suite expects a clean environment; testing with a non-standard
environment is unsupported and is unlikely to succeed. Therefore when
installing or upgrading Mojolicious and when running its tests, we highly
recommend using an environment which does not set these variables.

=head2 What is the difference between blocking and non-blocking operations?

A I<blocking> operation is a subroutine that blocks the execution of the
calling subroutine until the subroutine is finished.

  my $result = blocking_subroutine();
  ...

A I<non-blocking> operation on the other hand lets the calling subroutine
continue execution even though the subroutine is not yet finished. Instead of
waiting, the calling subroutine passes along a callback to be executed once
the subroutine is finished, this is called continuation-passing style.

  non_blocking_subroutine(sub {
    my $result = shift;
    ...
  });
  ...

=head2 Will my code magically become non-blocking with Mojolicious?

No, it is not possible to magically make Perl code non-blocking. While
L<Mojolicious> has been designed from the ground up for non-blocking I/O and
event loops, taking advantage of this requires specialized code available
through modules like L<Mojo::IOLoop> and L<Mojo::UserAgent>, or third-party
event loops. In the documentation we often refer to this as real-time web, for
more information see also L<Mojolicious::Guides::Cookbook/"REAL-TIME WEB">.

=head2 What is an event loop?

An event loop is basically a loop that continually tests for external events
and executes the appropriate callbacks to handle them, it is often the main
loop in a program. Non-blocking tests for readability/writability of file
descriptors and timers are commonly used events for highly scalable network
servers, because they allow a single process to handle thousands of client
connections concurrently.

  while (1) {
    my @readable = test_fds_for_readability();
    handle_readable_fds(@readable);

    my @writable = test_fds_for_writability();
    handle_writable_fds(@writable);

    my @expired = test_timers();
    handle_timers(@expired);
  }

In L<Mojolicious> this event loop is L<Mojo::IOLoop>.

=head2 What does the error "Maximum message size exceeded" mean?

To protect your applications from excessively large requests and responses,
our HTTP parser has a cap after which it will automatically stop accepting new
data, and in most cases force the connection to be closed. This limit is
around 10MB by default, you can use the attribute
L<Mojo::Message/"max_message_size"> or C<MOJO_MAX_MESSAGE_SIZE> environment
variable to change this value.

=head2 What does the error "Maximum line size exceeded" mean?

This is a very similar protection mechanism to the one described in the
previous answer, but a little more specific. It limits the maximum length of
any C<\x0d\x0a> terminated part of a HTTP message, such as request line,
status line and headers. This limit is around 10KB by default, you can use the
attributes L<Mojo::Message/"max_line_size"> and
L<Mojo::Headers/"max_line_size"> or C<MOJO_MAX_LINE_SIZE> environment variable
to change this value.

=head2 What does the error "Maximum buffer size exceeded" mean?

This protection mechanism is very similar to those mentioned in the two
previous answers. It limits how much content the HTTP parser is allowed to
buffer when parsing chunked, compressed and multipart messages. This limit is
around 256KB by default, you can use the attribute
L<Mojo::Content/"max_buffer_size"> or C<MOJO_MAX_BUFFER_SIZE> environment
variable to change this value.

=head2 What does the error "EV does not work with ithreads" mean?

The L<Mojolicious> user agent and web servers are based on an event loop that
supports multiple reactor backends. One of these backends is L<EV>, it is very
fast and will be automatically used if installed. On Windows however, the
C<ithreads> based C<fork()> emulation can interfere with it, and you may have
to use the C<MOJO_REACTOR> environment variable to enforce a more portable
one.

  MOJO_REACTOR=Mojo::Reactor::Poll

=head2 What does "Your secret passphrase needs to be changed" mean?

L<Mojolicious> uses secret passphrases for security features such as signed
cookies. It defaults to using the moniker of your application, which is not
very secure, so we added this log message as a reminder. You can change the
passphrase with the attribute L<Mojolicious/"secrets">.

  app->secrets(['My very secret passphrase.']);

=head2 What does "Nothing has been rendered, expecting delayed response" mean?

L<Mojolicious> has been designed from the ground up for non-blocking I/O and
event loops. So when a new request comes in and no response is generated right
away, it will assume that this was intentional and return control to the web
server, which can then handle other requests while waiting for events such as
timers to finally generate a response.

=head2 What does "Inactivity timeout" mean?

To protect your applications from denial-of-service attacks, all connections
have an inactivity timeout which limits how long a connection may be inactive
before being closed automatically. It defaults to C<20> seconds for the user
agent and C<15> seconds for all built-in web servers, and can be changed with
the attributes L<Mojo::UserAgent/"inactivity_timeout"> and
L<Mojo::Server::Daemon/"inactivity_timeout"> or the C<MOJO_INACTIVITY_TIMEOUT>
environment variable. This timeout always applies, so you might have to tweak
it for applications that take a long time to process a request.

=head2 What does "Premature connection close" mean?

This error message is often related to the one above, and means that the web
server closed the connection before the user agent could receive the whole
response or that the user agent got destroyed, which forces all connections to
be closed immediately.

=head2 What does "Worker 31842 has no heartbeat, restarting" mean?

As long as they are accepting new connections, worker processes of all
built-in preforking web servers send heartbeat messages to the manager process
at regular intervals, to signal that they are still responsive. A blocking
operation such as an infinite loop in your application can prevent this, and
will force the affected worker to be restarted after a timeout. This timeout
defaults to C<20> seconds and can be extended with the attribute
L<Mojo::Server::Prefork/"heartbeat_interval"> if your application requires it.

=head1 MORE

You can continue with L<Mojolicious::Guides> now or take a look at the
L<Mojolicious wiki|http://github.com/kraih/mojo/wiki>, which contains a lot
more documentation and examples by many different authors.

=head1 SUPPORT

If you have any questions the documentation might not yet answer, don't
hesitate to ask on the
L<mailing-list|http://groups.google.com/group/mojolicious> or the official IRC
channel C<#mojo> on C<irc.perl.org>.

=cut
