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eradio

An Erlang HTTP server which streams data to multiple clients, lossily time-synchronized with low latency.

HTTP/1 and HTTP/2 use TCP as their transport protocol, which is problematic for "live streaming" (i.e. low-latency time-synchronized streaming). Since TCP is a lossless protocol usually optimized for high throughput by default, the operating system usually maintains a large buffer of unacknowledged outgoing data. This buffering may induce latency on unreliable network connections. Furthermore, some popular browser HTTP media streaming implementations don't allow efficiently seeking past bursts of buffered data to keep playback as close to the most recent data as possible.

eradio uses a couple techniques to avoid this TCP-induced latency. The first is adjusting the send buffer size. When data is sent by an application using an ordinary Socket API, it must be buffered by the OS until the receiver acknowledges receipt of the data. This is called the "send buffer", and eradio tries to minimize its capacity. Second, since some operating systems such as Linux also maintain separate buffers preceding the send buffer which are not application-controllable, eradio tries to query the current amount of data in these buffers before sending and stops sending until the buffers have emptied. This technique has been analyzed in "Low-Latency Adaptive Streaming over TCP" by Goel et. al.

Development

Internal API Documentation
Integration Test Reports (common_test)

Building with docker

Building with the container-build script requires python3 and docker, and builds the project within the erlang:latest Docker image.

$ scripts/container-build make

Building without docker

Prerequisites:

  • GNU Make
  • Erlang >= 23
  • rebar3
  • tsc (Typescript compiler)
$ make all

The compiled Typescript is checked into source control at apps/eradio/priv/htdocs/js/, so it only needs to be compiled if it has changed. To compile only the Typescript code:

$ make ts

To compile only the Erlang and C code:

$ rebar3 compile

Running with docker

Firstly, ensure that the Erlang OTP version used to build the project closely matches that of the erlang:latest image, which is the image the included Dockerfile uses.

To run the server in the background:

$ container=$(docker run -p 8080:8080 --detach $(docker build -q .))

To start a remote Erlang shell on the node:

$ docker exec -it $container erl -remsh eradio@localhost -hidden

To re-compile and hot-load code changes to modules on the running node (the eradio_api_handler module in this example):

$ docker cp apps $container:/home/erlang/
$ docker exec -u root:root $container chown -R erlang:erlang /home/erlang/apps
$ docker exec -it $container erl -remsh eradio@localhost -hidden
(eradio@localhost)1> l(eradio_api_handler).

Only hot-load code when you know the new code will be compatible with any running state in the system.

Running without docker

Prerequisites:

  • Erlang (same version as built with)
$ ERL_LIBS=_build/default/lib erl -config config/sys.config -s eradio_app

The _build/default/lib directory contains the compiled .beam and files necessary to run eradio. However, rebar3 symlinks _build/default/lib/eradio/priv to the apps/eradio/priv/ directory, which contains the compiled eradio.so, along with the default index.html and associated Javascript. The eradio.so library contains the Erlang NIFs necessary for maintaining lower listener stream latency, but it is not required.

eradio may be configured using config/sys.config, which is read at startup. The configuration keys are:

  • listen_ip (optional): tuple containing the IP to listen on, as an IPv4 address in octets, or an IPv6 address in 16-bit integers (inet:ip_address()). Listens on all interfaces if omitted.
  • listen_port (default 8080): integer port to listen on for HTTP connections.
  • sndbuf (default 8000): positive integer number of bytes to set as the send buffer size on listener streams. This affects their maximum latency.
  • webroot (optional): string path to a directory containing files to be served as requested over HTTP. Defaults to $ERL_LIBS/eradio/priv/htdocs.

By default, $ERL_LIBS/eradio/priv/source is executed to provide source data on its standard output to broadcast to all listeners. The source data must be sent in frames, each preceded by a 32-bit big-endian frame length. Note that since frames of source data can be dropped depending on network conditions, the source data itself should be framed such that listeners can be recover after missing frames. The first byte of each frame's data specifies the frame's type:

  • Frame type 1: log message. The first byte specifies the log level, and the rest is the log message.
    • Log Level 1: error
    • Log Level 2: warning
    • Log Level 3: info
    • Log Level 4: debug
  • Frame type 2: one frame of stream data for broadcast.
  • Frame type 3: stream started notification. Additional metadata about the stream may optionally be present:
    • URI prefixed by its 16-bit big-endian length.
    • Name prefixed by its 16-bit big-endian length.
  • Frame type 4: stream stopped notification.

Using the Erlang remote shell

eradio must be started with the -name/-sname flag to enable Erlang dist. Note that connecting to the dist port allows running arbitrary Erlang code. To listen only on the loopback interface, with the node name 'eradio@localhost':

$ ERL_LIBS=_build/default/lib erl -config config/sys.config -sname eradio@localhost -kernel inet_dist_use_interface '{127,0,0,1}' -s eradio_app

Then, to connect to the 'eradio@localhost' node with an Erlang remote shell:

$ erl -remsh eradio@localhost -hidden

Note that when starting the Erlang remote shell, $HOME/.erlang.cookie must match that which eradio was started with.

Reloading code

When connected to the Erlang remote shell, the gen_cluster_code module may be used as a convenience to hot-reload modified code in .beam files:

(eradio@localhost)1> gen_cluster_code:load_all().
{ok,[eradio_app, eradio_server]}

Note that existing internal state is currently not guaranteed to be compatible with the newly loaded code, which may cause crashes or strange behaviour.

API

The HTTP server exposes an endpoint to listen for stream data, a REST API to fetch metadata about the stream, and a websocket to listen for changes to the stream metadata. Currently, the best way to use this API is by using or referencing api.ts.