<?xml version="1.0"?>

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

<module name="HTTP MP4 Module" id="http_mp4_module">

<section name="Summary">

<para>
The module <code>ngx_http_mp4_module</code> provides pseudo-streaming
server-side support for H.264/AAC files typically having filename extensions
<pathname>.mp4</pathname>, <pathname>.m4v</pathname>,
and <pathname>.m4a</pathname>.
</para>

<para>
Pseudo-streaming works in alliance with conformant Flash players.
A player sends an HTTP request to the server with a start time
argument in the request URI’s query string (named simply
<parameter>start</parameter>
and specified in seconds), and the server responds with a stream
so that its start position corresponds to the requested time,
for example:
<example>
http://example.com/elephants_dream.mp4?start=238.88
</example>
This allows for a random seeking at any time, or starting playback
in the middle of a 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
<parameter>start=0</parameter>
argument.  Many encoding software will insert the metadata at
the end of the file.  This is bad for pseudo-streaming:
the metadata needs to be located at the beginning of the file,
or else the entire file will have to be downloaded before it
starts playing.  If a file is well-formed (with metadata at the
beginning of a file), nginx just sends back the contents of a file.
Otherwise, it has to read the file and prepare a new stream so that
metadata comes before 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
<parameter>start</parameter>
argument, nginx will read 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
<parameter>start</parameter>
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
<command>--with-http_mp4_module</command>
configuration parameter.
<note>
If you were using the third-party mp4 module, be sure to disable it.
</note>
</para>

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

</section>


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

<para>
<example>
location /video/ {
    mp4;
    mp4_buffer_size     1m;
    mp4_max_buffer_size 5m;
}
</example>
</para>

</section>


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

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

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

</directive>


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

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

</directive>


<directive name="mp4_max_buffer_size">
<syntax>mp4_max_buffer_size <argument>size</argument></syntax>
<default>mp4_max_buffer_size 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 <argument>size</argument>,
or else nginx will return the server error
<http-status code="500" text="Internal Server Error"/>,
and log the following:
<example>
"/some/movie/file.mp4" mp4 moov atom is too large:
12583268, you may want to increase mp4_max_buffer_size
</example>
</para>

</directive>

</section>

</module>