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http_mp4 module

http_mp4 module#

The module provides pseudo-streaming server-side support for MP4 files. Such files typically have the .mp4, .m4v, or .m4a filename extensions.

Pseudo-streaming works in alliance with a compatible media player. The player sends an HTTP request to the server with the start time specified in the query string argument (named simply start and specified in seconds), and the server responds with the stream such that its start position corresponds to the requested time, for example:

http://example.com/elephants_dream.mp4?start=238.88

This allows performing a random seeking at any time, or starting playback in the middle of the timeline.

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

To start playback, the player first needs to read metadata. This is done by sending a special request with the start=0 argument. A lot of encoding software insert the metadata at the end of the file. This is suboptimal for pseudo-streaming, because the player has to download the entire file before starting playback. If the metadata are located at the beginning of the file, it is enough for Angie to simply start sending back the file contents. If the metadata are located at the end of the file, Angie must read the entire file and prepare a new stream so that the metadata come before the media data. This involves some CPU, memory, and disk I/O overhead, so it is a good idea to prepare an original file for pseudo-streaming in advance, rather than having Angie do this on every such request.

The module also supports the end argument of an HTTP request which sets the end point of playback. The end argument can be specified with the start argument or separately:

http://example.com/elephants_dream.mp4?start=238.88&end=555.55

For a matching request with a non-zero start or end argument, Angie will read the metadata from the file, prepare the stream with the requested time range, and send it to the client. This has the same overhead as described above.

If the start argument points to a non-key video frame, the beginning of such video will be broken. To fix this issue, the video can be prepended with the key frame before start point and with all intermediate frames between them. These frames will be hidden from playback using an edit list.

If a matching request does not include the start and end arguments, there is no overhead, and the file is sent simply as a static resource. Some players also support byte-range requests, and thus do not require this module.

This module isn’t built by default; it should be enabled with the ‑‑with‑http_mp4_module configuration parameter.

Packages in our repositories have this module built.

Caution

If a third-party mp4 module was previously used, it should be disabled.

A similar pseudo-streaming support for FLV files is provided by the http_flv module.

Example Configuration#

set_real_ip_from  192.168.1.0/24;
set_real_ip_from  192.168.2.1;
set_real_ip_from  2001:0db8::/32;
real_ip_header    X-Forwarded-For;
real_ip_recursive on;

Directives#

mp4#

Syntax:

mp4;

Default:

—

Context:

location

Turns on module processing in a surrounding location.

mp4_buffer_size#

Syntax:

mp4_buffer_size size;

Default:

mp4_buffer_size 512K;

Context:

http, server, location

Sets the initial size of the buffer used for processing MP4 files.

mp4_max_buffer_size#

Syntax:

mp4_max_buffer_size size;

Default:

mp4_max_buffer_size 10M;

Context:

http, server, location

During metadata processing, a larger buffer may become necessary. Its size cannot exceed the specified size, or else Angie will return the 500 (Internal Server Error) server error, and log the following message:

“/some/movie/file.mp4” mp4 moov atom is too large: 12583268, you may want to increase mp4_max_buffer_size

mp4_limit_rate#

Syntax:

mp4_limit_rate on | off | factor;

Default:

mp4_limit_rate off;

Context:

http, server, location

Rate-limits the transfer of the requested MP4 file to the client. To calculate the limit, the factor is multiplied by the average bitrate of the file.

  • The off value disables rate limiting.

  • The on value sets a factor of 1.1.

  • The limit is applied after reaching the value set by mp4_limit_rate_after.

The requests are rate limited individually: if the client opens two connections, the resulting rate doubles. In this regard, consider using limit_conn and accompanying directives.

mp4_limit_rate_after#

Syntax:

mp4_limit_rate_after time;

Default:

mp4_limit_rate_after 60s;

Context:

http, server, location

Sets (in terms of playback time) the amount of mediadata transferred that triggers the rate limit set by mp4_limit_rate.

mp4_start_key_frame#

Syntax:

mp4_start_key_frame on | off;

Default:

mp4_start_key_frame off;

Context:

http, server, location

Forces output video to always start with a key video frame. If the start argument does not point to a key frame, initial frames are hidden using an mp4 edit list. Edit lists are supported by major players and browsers such as Chrome, Safari, QuickTime and ffmpeg, partially supported by Firefox.

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