ngx_http_mp4

Module ngx_http_mp4_module

The ngx_http_mp4_module 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 Flash 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 nginx to simply start sending back the file contents. If the metadata are located at the end of the file, nginx 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 nginx do this on every such request.

The module also supports the end argument of an HTTP request (1.5.13) 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, nginx 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 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 is not built by default, it should be enabled with the --with-http_mp4_module configuration parameter.