HTML: The Living Standard

Edition for Web Developers — Last Updated 16 January 2024

1. 1. 9.2 Server-sent events
      1. 9.2.1 Introduction
      2. 9.2.2 The EventSource interface
      3. 9.2.3 The `Last-Event-ID` header
      4. 9.2.4 The event stream format
      5. 9.2.5 Authoring notes

9.2 Server-sent events

9.2.1 Introduction

To enable servers to push data to web pages over HTTP or using dedicated server-push protocols, this specification introduces the EventSource interface.

Using this API consists of creating an EventSource object and registering an event listener.

var source = new EventSource('updates.cgi');
source.onmessage = function (event) {
  alert(event.data);
};

On the server-side, the script ("updates.cgi" in this case) sends messages in the following form, with the text/event-stream MIME type:

data: This is the first message.

data: This is the second message, it
data: has two lines.

data: This is the third message.

Authors can separate events by using different event types. Here is a stream that has two event types, "add" and "remove":

event: add
data: 73857293

event: remove
data: 2153

event: add
data: 113411

The script to handle such a stream would look like this (where addHandler and removeHandler are functions that take one argument, the event):

var source = new EventSource('updates.cgi');
source.addEventListener('add', addHandler, false);
source.addEventListener('remove', removeHandler, false);

The default event type is "message".

Event streams are always decoded as UTF-8. There is no way to specify another character encoding.

Event stream requests can be redirected using HTTP 301 and 307 redirects as with normal HTTP requests. Clients will reconnect if the connection is closed; a client can be told to stop reconnecting using the HTTP 204 No Content response code.

Using this API rather than emulating it using XMLHttpRequest or an iframe allows the user agent to make better use of network resources in cases where the user agent implementer and the network operator are able to coordinate in advance. Amongst other benefits, this can result in significant savings in battery life on portable devices. This is discussed further in the section below on connectionless push.

9.2.2 The EventSource interface

source = new EventSource( url [, { withCredentials: true } ])

Creates a new EventSource object.

url is a string giving the URL that will provide the event stream.

Setting withCredentials to true will set the credentials mode for connection requests to url to "include".

source.close()

Aborts any instances of the fetch algorithm started for this EventSource object, and sets the readyState attribute to CLOSED.

source.url

Returns the URL providing the event stream.

source.withCredentials

Returns true if the credentials mode for connection requests to the URL providing the event stream is set to "include", and false otherwise.

source.readyState

Returns the state of this EventSource object's connection. It can have the values described below.

CONNECTING (numeric value 0)

The connection has not yet been established, or it was closed and the user agent is reconnecting.

OPEN (numeric value 1)

The user agent has an open connection and is dispatching events as it receives them.

CLOSED (numeric value 2)

The connection is not open, and the user agent is not trying to reconnect. Either there was a fatal error or the close() method was invoked.

The following are the event handlers (and their corresponding event handler event types) supported, as event handler IDL attributes, by all objects implementing the EventSource interface:

9.2.3 The `Last-Event-ID` header

The Last-Event-ID` HTTP request header reports an EventSource object's last event ID string to the server when the user agent is to reestablish the connection.

See whatwg/html issue #7363 to define the value space better. It is essentially any UTF-8 encoded string, that does not contain U+0000 NULL, U+000A LF, or U+000D CR.

9.2.4 The event stream format

This event stream format's MIME type is text/event-stream.

The event stream format is as described by the stream production of the following ABNF, the character set for which is Unicode. [ABNF]

stream        = [ bom ] *event
event         = *( comment / field ) end-of-line
comment       = colon *any-char end-of-line
field         = 1*name-char [ colon [ space ] *any-char ] end-of-line
end-of-line   = ( cr lf / cr / lf )

; characters
lf            = %x000A ; U+000A LINE FEED (LF)
cr            = %x000D ; U+000D CARRIAGE RETURN (CR)
space         = %x0020 ; U+0020 SPACE
colon         = %x003A ; U+003A COLON (:)
bom           = %xFEFF ; U+FEFF BYTE ORDER MARK
name-char     = %x0000-0009 / %x000B-000C / %x000E-0039 / %x003B-10FFFF
                ; a scalar value other than U+000A LINE FEED (LF), U+000D CARRIAGE RETURN (CR), or U+003A COLON (:)
any-char      = %x0000-0009 / %x000B-000C / %x000E-10FFFF
                ; a scalar value other than U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR)

Event streams in this format must always be encoded as UTF-8. [ENCODING]

Lines must be separated by either a U+000D CARRIAGE RETURN U+000A LINE FEED (CRLF) character pair, a single U+000A LINE FEED (LF) character, or a single U+000D CARRIAGE RETURN (CR) character.

data: YHOO
data: +2
data: 10

var stocks = new EventSource("https://stocks.example.com/ticker.php");
stocks.onmessage = function (event) {
  var data = event.data.split('\n');
  updateStocks(data[0], data[1], data[2]);
};

function updateStocks(symbol, delta, value) { ... }

: test stream

data: first event
id: 1

data:second event
id

data:  third event

data

data
data

data:

data:test

data: test

9.2.5 Authoring notes

Legacy proxy servers are known to, in certain cases, drop HTTP connections after a short timeout. To protect against such proxy servers, authors can include a comment line (one starting with a ':' character) every 15 seconds or so.

Authors wishing to relate event source connections to each other or to specific documents previously served might find that relying on IP addresses doesn't work, as individual clients can have multiple IP addresses (due to having multiple proxy servers) and individual IP addresses can have multiple clients (due to sharing a proxy server). It is better to include a unique identifier in the document when it is served and then pass that identifier as part of the URL when the connection is established.

Authors are also cautioned that HTTP chunking can have unexpected negative effects on the reliability of this protocol, in particular if the chunking is done by a different layer unaware of the timing requirements. If this is a problem, chunking can be disabled for serving event streams.

Clients that support HTTP's per-server connection limitation might run into trouble when opening multiple pages from a site if each page has an EventSource to the same domain. Authors can avoid this using the relatively complex mechanism of using unique domain names per connection, or by allowing the user to enable or disable the EventSource functionality on a per-page basis, or by sharing a single EventSource object using a shared worker.