Resource Timing API
Resource Timing interfaces enable retrieving and analyzing detailed network timing data regarding the loading of an application's resource(s). An application can use the timing metrics to determine, for example, the length of time it takes to load a specific resource, such as an
<SVG>, image, or script.
The interface's properties create a resource loading timeline with high-resolution timestamps for network events such as redirect start and end times, DNS lookup start and end times, request start, response start and end times, etc. The interface also includes other properties that provide data about the size of the fetched resource as well as the type of resource that initiated the fetch.
This document provides an overview of the
Resource Timing interfaces. For more details about the interfaces including examples see each interface's reference page, Using the Resource Timing API, and the references in the See also section. For a graphical representation of the resource timing processing model see the resource timing phases figure.
Note: This feature is available in Web Workers
Several of the
Resource Timing properties return high-resolution timestamps. These timestamps have a
type and as its name implies, they represent a high-resolution point in time. This type is a
double and its value is a discrete point in time or the difference in time between two discrete points in time.
The unit of
DOMHighResTimeStamp is milliseconds and should be accurate to 5 µs (microseconds). However, If the browser is unable to provide a time value accurate to 5 µs (because, for example, due to hardware or software constraints), the browser can represent a the value as a time in milliseconds accurate to a millisecond.
An application can get timestamps for the various stages used to load a resource. The first property in the processing model is
startTime which returns the timestamp immediately before the resource loading process begins. The
fetchStart timestamps follows and redirect processing (if applicable) and precedes DNS lookup. The next stages are
connectEnd which are the timestamps immediately before and after connecting to the server, respectively. The last three timestamps are, in order:
requestStart - the timestamp before the browser starts requesting the resource from the server;
responseStart - the timestamp after the browser receives the first byte of the response from the server; and
responseEnd - the timestamp after the browser receives the last byte of the resource. If the resource is loaded via a secure connection a
secureConnectionStart timestamp will be available between the connection start and end events.
When CORS is in effect, many of these values are returned as zero unless the server's access policy permits these values to be shared. This requires the server providing the resource to send the
Timing-Allow-Origin HTTP response header with a value specifying the origin or origins which are allowed to get the restricted timestamp values.
The properties which are returned as 0 by default when loading a resource from a domain other than the one of the web page itself:
interface also includes several network timing properties. The
redirectEnd properties return
timestamps for redirect start and end times, respectively. Likewise, the
domainLookupEnd properties return
timestamps for DNS lookup start and end times, respectively.
This would be a nice place to have a diagram showing the relationships between these segments of the resource loading time.
PerformanceResourceTiming interface has three properties that can be used to obtain size data about a resource. The
transferSize property returns the size (in octets) of the fetched resource including the response header fields plus the response payload body.
encodedBodySize property returns the size (in octets) received from the fetch (HTTP or cache), of the payload body, before removing any applied content-codings.
decodedBodySize returns the size (in octets) received from the fetch (HTTP or cache) of the message body, after removing any applied content-codings.
nextHopProtocol property returns the network protocol used to fetch the resource.
initiatorType property returns the type of resource that initiated the performance entry such as "
css" for a CSS resource, "
xmlhttprequest" for an XMLHttpRequest and "
img" for an image (such as a JPEG).
The Resource Timing API includes two methods that extend the
Performance interface. The
clearResourceTimings() method removes all "
resource" type performance entries from the browser's resource performance entry buffer. The
setResourceTimingBufferSize() method sets the resource performance entry buffer size to the specified number of resource
As shown in the
PerformanceResourceTiming interface's Browser Compatibility table, most of these interfaces are broadly implemented by desktop browsers. However, note that some properties have little to no implementation so see each property's "Browser compatibility" section for more specific interoperability data.
To test your browser's support for these interfaces, run the
- Resource Timing Standard; W3C Editor's Draft
- CanIUse data
- Resource Timing practical tips; Steve Souders; 2014 August 21
- Measuring network performance with Resource Timing API; Ilya Grigorik; 2013 December 11
- A Primer for Web Performance Timing APIs; Xiaoqian Wu; W3C Editor's Draft