<?xml version="1.0"?>

<!--
  Copyright (C) Nginx, Inc.
  -->

<!DOCTYPE module SYSTEM "../../../../dtd/module.dtd">

<module name="Module ngx_http_mp4_module"
        link="/en/docs/http/ngx_http_mp4_module.html"
        lang="en"
        rev="2">

<section id="summary">

<para>
The <literal>ngx_http_mp4_module</literal> module provides pseudo-streaming
server-side support for H.264/AAC files.
Such files typically have the <path>.mp4</path>, <path>.m4v</path>,
or <path>.m4a</path> filename extensions.
</para>

<para>
The pseudo-streaming works in alliance with a compatible Flash players.
A player sends an HTTP request to the server with a start time
specified in the query string argument (named simply
<literal>start</literal>
and specified in seconds), and the server responds with the stream
such that its start position corresponds to the requested time,
for example:
<example>
http://example.com/elephants_dream.mp4?start=238.88
</example>
This allows performing a random seeking at any time, or starting playback
in the middle of the timeline.
</para>

<para>
To support seeking, H.264-based formats store the metadata
in the so-called “moov atom.”
It is a part of the file that holds the index information for the
whole file.
</para>

<para>
To start playback, a player first needs to read metadata.
This is done by sending a special request with the
<literal>start=0</literal> argument.
Much of encoding software will insert the metadata at
the end of the file.
This is bad for pseudo-streaming:
the metadata should be located at the beginning of the file,
or else the entire file will have to be downloaded to
start playing.
If a file is well-formed (with metadata at the
beginning of a file), nginx just sends back the file contents.
Otherwise, it has to read the file and prepare a new stream so that
the metadata comes before the media data.
This involves some CPU, memory, and disk I/O overhead,
so it is a good idea to
<link
url="http://flowplayer.org/plugins/streaming/pseudostreaming.html#prepare">
prepare an original file for pseudo-streaming</link>,
rather than having nginx do this on every such request.
</para>

<para>
For a matching request with a non-zero
<literal>start</literal>
argument, nginx will read the metadata from the file, prepare the
stream starting from the requested offset, and send it to a client.
This has the same overhead as described above.
</para>

<para>
If a matching request does not include the
<literal>start</literal>
argument, there is no overhead, and the file is just sent as a static resource.
Some players also support byte-range requests, and thus do not require
this module at all.
</para>

<para>
This module is not built by default, it should be enabled with the
<literal>--with-http_mp4_module</literal>
configuration parameter.
<note>
If a third-party mp4 module was previously used, it should be disabled.
</note>
</para>

<para>
A similar pseudo-streaming support for FLV files is provided by the
<link doc="ngx_http_flv_module.xml">ngx_http_flv_module</link> module.
</para>

</section>


<section id="example" name="Example Configuration">

<para>
<example>
location /video/ {
    mp4;
    mp4_buffer_size       1m;
    mp4_max_buffer_size   5m;
    mp4_limit_rate        on;
    mp4_limit_rate_after  30s;
}
</example>
</para>

</section>


<section id="directives" name="Directives">

<directive name="mp4">
<syntax/>
<default/>
<context>location</context>

<para>
Turns on module processing in a surrounding location.
</para>

</directive>


<directive name="mp4_buffer_size">
<syntax><value>size</value></syntax>
<default>512K</default>
<context>http</context>
<context>server</context>
<context>location</context>

<para>
Sets the initial size of a memory buffer used for processing MP4 files.
</para>

</directive>


<directive name="mp4_max_buffer_size">
<syntax><value>size</value></syntax>
<default>10M</default>
<context>http</context>
<context>server</context>
<context>location</context>

<para>
During metadata processing, a larger buffer may become necessary.
Its size cannot exceed the specified <value>size</value>,
or else nginx will return the
<http-status code="500" text="Internal Server Error"/> server error,
and log the following message:
<example>
"/some/movie/file.mp4" mp4 moov atom is too large:
12583268, you may want to increase mp4_max_buffer_size
</example>
</para>

</directive>


<directive name="mp4_limit_rate">
<syntax>
    <literal>on</literal> |
    <literal>off</literal> |
    <value>factor</value></syntax>
<default>off</default>
<context>http</context>
<context>server</context>
<context>location</context>

<para>
Enables or disables rate limiting based on the average bitrate of the
MP4 file served.
To calculate the rate, the bitrate is multiplied by the specified
<value>factor</value>.
The special value “<literal>on</literal>” corresponds to the factor of 1.1.
</para>

<para>
<note>
This directive is available as part of our <commercial_version/>.
</note>
</para>

</directive>


<directive name="mp4_limit_rate_after">
<syntax><value>time</value></syntax>
<default>1m</default>
<context>http</context>
<context>server</context>
<context>location</context>

<para>
Limits the rate after sending the specified amount of media data.
</para>

<para>
<note>
This directive is available as part of our <commercial_version/>.
</note>
</para>

</directive>

</section>

</module>