HTTP is a client-server protocol — the client makes a request and the server replies with a response. For some applications, though, the request-reply model is limiting or unsuitable. These applications tend to want server-to-client notification capabilities. While such notifications can be simulated using polling, and web-based polling can be much more efficient than one would think due to intermediary caching, it's still less efficient and less timely than a notification model.
Yaws users have a few options for notification-oriented applications:
Yaws supports an older technique called "long polling" or "Comet" where the client sends a request that the server sits on and doesn't answer until it actually has an event for the client. The problem with long polling is that it requires the client and server applications to be bound tightly to each other via the specialized ad hoc long-polling protocol they share.
The WebSockets protocol (RFC 6455) allows web client and server to upgrade their TCP connection from using HTTP to using some other protocol they agree on. The protocol they choose can be bidirectional and can transmit whatever data transfer formats they wish to use. WebSockets afford applications a great deal of freedom and flexibility, but they also require client and server to agree on specialized protocols, framing, and data formats to be able to communicate successfully.
Server-Sent Events (SSE) is a W3C working draft that unlike long polling is on a path to standardization and unlike WebSockets is pretty simple. Despite being a working draft, it's already fairly widely used. With SSE, a client sends a standard HTTP request asking for an event stream, and the server responds initially with a standard HTTP response and holds the connection open. When appropriate, the server sends standard text-based event data back to the client as part of the initial response, and continues to do so until either end closes the connection. Clients can disconnect and later reconnect, indicate the last event they received, and pick up new events from that point.
Currently, Chrome, Firefox, Opera, and Safari support SSE. Older browsers do not support SSE directly, but they can be made to do so with suitable JavaScript packages.
Yaws supports SSE through its streaming
capabilities. SSE applications typically consist of an entry point
page and an appmod. The entry page returns HTML and JavaScript that acts
as the SSE client, with the JavaScript invoking the appmod's
out/1 function that creates a streaming process responsible
for sending events back to the client. The appmod uses the
yaws_sse module to properly format and send its event
data.
Yaws supplies an example that uses SSE to return the server's time of day clock to the client. Each second, the server sends a new event to the client updating its time of day, which the client dynamically displays in a web page.
Note: if you're running this on your own Yaws installation, make sure your server configuration includes the following appmod entry:
appmods = <"/sse", server_sent_events>
and also make sure server_sent_events.beam is on the
Yaws load path, which it should be if you followed regular installation
procedures.
First, the entry HTML page is here: server_sent_events.html. It presents
a page title and a placeholder for the server date string. It also
supplies a bit of JavaScript that receives events from Yaws, using the
browser's EventSource JavaScript class to receive them. It
then pulls the data out of the event and displays it dynamically in the
HTML.
Next, the server appmod code is here:
server_sent_events.erl. Its out/1 function create a
gen_server event generation process, returning the pid in a
streamcontent_from_pid directive to Yaws along with suitable
headers. Note that it obtains the desired out/1 return value
via the yaws_sse:headers/1 function. Its gen_server is
fairly simple in that it creates a timer that, once per second, generates
a time of day string and sends it as an event to the client formatted via
the yaws_sse:data/1 function.
The yaws_sse module supplies all the SSE primitives
required for formatting event data, event identifiers, and event retry
settings. See the Server-Sent
Events working draft for more details on using these features.
The yaws_sse module also supplies functions for
formatting and sending event data on a socket. If you're using the
yaws_sse module outside of a Yaws streaming application, you
should use the arity 3 version of yaws_sse:send_events and
pass fun yaws:gen_tcp_send/2 as the third argument.
Note: be aware that because the W3C Server-Sent Events spec is still a working draft, any future changes in it might cause API-incompatible changes in how Yaws supports it.
