This specification defines an interface that web developers can use to schedule asynchronous and non-blocking delivery of data that minimizes resource contention with other time-critical operations, while ensuring that such requests are still processed and delivered to destination.
Status of This Document
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W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community. This Candidate Recommendation is expected to advance to Proposed Recommendation no earlier than 11 May 2017. All comments are welcome.
The Working Group expects to demonstrate 2 implementations of the features listed in this specification by the end of the Candidate Recommendation phase. Please see the Working Group's implementation report.
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Web applications often need to issue requests that report events, state updates, and analytics to one or more servers. Such requests typically do not require response processing on the client (e.g. result in 204, or 200 HTTP response codes with an empty response body), and should not compete for network and compute resources with other high priority operations such as fetching critical resources, reacting to input, running animations, and so on. However, such one-way requests (beacons), are also responsible for delivering critical application and measurement data, forcing developers to use costly methods to ensure their delivery:
Developers opt for immediate delivery of each beacon, instead of coalescing and deferring their delivery because this provides improved delivery rates. Deferring delivery may mean that the beacon request may not have sufficient time to complete successfully, which leads to loss of important application data.
Developers opt for issuing blocking requests via synchronous XMLHttpRequest's, inserting no-op busy loops, or using other techniques that block the user agent from executing time-critical operations (e.g. click, unload, and other handlers) and hurt the user experience. The blocking behavior is used to provide improved delivery rate, as it prevents the user agent and the operating system from cancelling the request if the page is unloaded, suspended, or killed by the system.
The mismatch between above delivery and processing requirements leaves most developers with a tough choice and widespread adoption of blocking techniques that hurt the user experience. This specification defines an interface that web developers can use to schedule asynchronous and non-blocking delivery of data that minimizes resource contention with other time-critical operations, while ensuring that such requests are still processed and delivered to destination:
Beacon requests are prioritized to avoid competing with time-critical operations and higher priority network requests.
Beacon requests may be efficiently coalesced by the user agent to optimize energy use on mobile devices.
Beacon requests are guaranteed to be initiated before page is unloaded and are allowed to run to completion without requiring blocking requests or other techniques that block processing of user interactive events.
The following example shows use of the sendBeacon method to deliver event, click, and analytics data:
Example 1
<html><script>// emit non-blocking beacon to record client-side eventfunctionreportEvent(event) {
var data = JSON.stringify({
event: event,
time: performance.now()
});
navigator.sendBeacon('/collector', data);
}
// emit non-blocking beacon with session analytics as the page// transitions to background state (Page Visibility API)document.addEventListener('visibilitychange', function() {
if (document.visiblityState === 'hidden') {
var sessionData = buildSessionReport();
navigator.sendBeacon('/collector', sessionData);
}
});
</script><body><ahref='http://www.w3.org/'onclick='reportEvent(this)'><buttononclick="reportEvent('some event')">Click me</button></body></html>
Note
Above example uses visibilitychange event defined in [PAGE-VISIBILITY] to trigger delivery of session data. This event is the only event that is guaranteed to fire on mobile devices when the page transitions to background state (e.g. when user switches to a different application, goes to homescreen, etc), or is being unloaded. Developers should avoid relying on unload event because it will not fire whenever a page in background state (i.e.
visiblityState equal to hidden) and the process is terminated by the mobile OS.
The requests initiated via the sendBeacon method do not block or compete with time-critical work, may be prioritized by the user agent to improve network efficiency, and eliminate the need to use blocking operations to ensure delivery of beacon data.
What sendBeacon does not do and is not intended to solve:
The sendBeacon method does not provide special handling for offline storage or delivery. A beacon request is like any other request and may be combined with [SERVICE-WORKERS] to provide offline functionality where necessary.
The sendBeacon method is not intended to provide background synchronization or transfer capabilities. The user agent restricts the maximum accepted payload size to ensure that beacon requests are able to complete quickly and in a timely manner.
The sendBeacon method does not provide ability to customize the request method, provide custom request headers, or change other processing properties of the request and response. Applications that require non-default settings for such requests should use the [FETCH] API with
keepalive flag set to
true.
2. Conformance requirements
All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "
RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in [RFC2119]. For readability, these words do not appear in all uppercase letters in this specification.
Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.
Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on the user agent.
Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)
2.1 Dependencies
DOM
The following terms are defined in the DOM specification: [
DOM]
The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification [WebIDL].
The following terms are defined in the Web IDL specification:
The sendBeacon method transmits data provided by the data parameter to the URL provided by the url parameter:
The user agent MUST restrict the maximum data size to ensure that beacon requests are able to complete quickly and in a timely manner.
The user agent MUST schedule immediate transmission of all beacon requests when the document visiblityState ([
PAGE-VISIBILITY]) transitions to hidden, and must allow all such requests to run to completion without blocking other time-critical and high-priority work.
The user agent SHOULD schedule transmission of provided data to minimize resource (CPU and network) contention with other time-critical and high priority work.
The user agent MAY delay transmission of provided data to optimize network and energy efficiency - e.g. deliver immediately if the network is active, or wait until network interface is active. However, the user agent SHOULD NOT delay transmission indefinitely and ensure that pending transmissions are periodically flushed even if there is no other network activity.
Note
Beacon API does not provide a response callback. The server is encouraged to omit returning a response body for such requests (e.g. respond with 204 No Content).
Parameters
url
The url parameter indicates the URL where the data is to be transmitted.
data
The data parameter is the BodyInit data that is to be transmitted.
Return Value
The sendBeacon method returns true if the user agent is able to successfully queue the data for transfer. Otherwise it returns false.
Note
The user agent imposes limits on the amount of data that can be sent via this API: this helps ensure that such requests are delivered successfully and with minimal impact on other user and browser activity. If the amount of data to be queued exceeds the user agent limit, this method returns false; a return value of true implies the browser has queued the data for transfer. However, since the actual data transfer happens asynchronously, this method does not provide any information whether the data transfer has succeeded or not.
3.2 Processing Model
On calling the sendBeacon method with
url and optional data, the following steps must be run:
Set parsedUrl to the result of the URL parser steps with url and base. If the algorithm returns an error, or if parsedUrl's scheme is not "http" or "https", throw a "
TypeError" exception and terminate these steps.
If data is not null:
Extract object's byte stream (transmittedData) and MIME type (mimeType).
If the amount of data that can be queued to be sent by
keepalive enabled requests is exceeded by the size of transmittedData, set the return value to false and terminate these steps.
Requests initiated via the Beacon API automatically set the keepalive flag, and developers can similarly set the same flag manually when using the Fetch API. All requests with this flag set share the same in-flight quota restrictions that is enforced within the Fetch API.
The sendBeacon interface provides an asynchronous and non-blocking mechanism for delivery of data. This API can be used to:
Report client-side events to the server. The delivery is prioritized and scheduled by the user agent such that it does not block other interactive work and makes efficient use of system resources.
Report session data when the page transitions to background state or is being unloaded, without blocking the user agent.
Other use cases that require delivery of small payloads and do not expect a response callback.
The delivered data might contain potentially sensitive information, for example, data about a user's activity on a web page, to a server. While this can have privacy implications for the user, existing methods, such as scripted form-submit, image beacons, and XHR/fetch requests provide similar capabilities, but come with various and costly performance tradeoffs: the requests can be aborted by the user agent unless the developer blocks the user agent from processing other events (e.g. by invoking a synchronous request, or spinning in an empty loop), and the user agent is unable to prioritize and coalesce such requests to optimize use of system resources.
A request initiated by sendBeacon is subject to following properties:
If the payload is one of application/x-www-form-urlencoded, multipart/form-data, or text/plain, then the request is a simple request that does not require an additional CORS-preflight; same as scripted form-post or XHR/fetch.
Otherwise, if the payload is a Blob and the resulting Content-Type is not a simple header, then a CORS preflight is made and the server needs to first allow such requests by returning the appropriate set of CORS headers (Access-Control-Allow-Credentials, Access-Control-Allow-Origin, Access-Control-Allow-Headers); same as XHR/fetch.
As such, from the security perspective, the Beacon API is subject to all the same security policies as the current methods in use by developers. Similarly, from the privacy perspective, the resulting requests are initiated immediately when the API is called, or upon a page visibility change, which restricts the exposed information (e.g. user's IP address) to existing lifecycle events accessible to the developers. However, user agents might consider alternative methods to surface such requests to provide transparency to users.
Compared to the alternatives, the sendBeacon does apply two restrictions: there is no callback method, and the payload size can be restricted by the user agent. Otherwise, the sendBeacon API is not subject to any additional restrictions. The user agent ought not skip or throttle processing of sendBeacon calls, as they can contain critical application state, events, and analytics data. Similarly, the user agent ought not disable sendBeacon when in "private browsing" or equivalent mode, both to avoid breaking the application and to avoid leaking that the user is in such mode.
3.4 Acknowledgments
Thanks to Alois Reitbauer, Arvind Jain, Anne van Kesteren, Boris Zbarsky, Chase Douglas, Daniel Austin, Jatinder Mann, James Simonsen, Jason Weber, Jonas Sicking, Nick Doty, Philippe Le Hegaret, Todd Reifsteck, Tony Gentilcore, William Chan, and Yoav Weiss for their contributions to this work.
