Extension:NavigationTiming

The NavigationTiming extension allows logging perceived latency measurements, exposed by browsers as part of the W3C Navigation Timing spec, which "defines an interface for web applications to access timing information related to navigation and elements".

Scope
The extension collects anonimised information from a sample of page views. The collection uses JavaScript and thus is limited to pageviews that:


 * 1) are from a modern browser, as defined by the Grade A capability check (90%+ of traffic).
 * 2) have the Navigation Timing API, (98% of browser traffic in 2022, 95% in Nov 2017 )
 * 3) reach the "page loaded" event (99%+ of page loads).
 * 4) are in the random sample (configurable).

How is the information used?
Engineers at Wikimedia Foundation intend to use this data to assess the impact (positive and negative) of changes to application code and server configuration. Thus guiding on-going work on site performance.

Dependencies

 * EventLogging

Oversampling
Oversampling allows us to selectively sample a larger portion of the traffic coming from a particular geography, or with a specific User Agent string in the browser. Use cases for this include: getting detailed observations of the change in performance based on factors like data center location; or evaluating the change in performance due to things like browser upgrades (eg, the release of Firefox 57, which had significant rewrites that were intended to improve performance).

Oversampling is configured using the wgNavigationTimingOversampleFactor variable. This variable is an associative array, and the keys that are checked are "geo" and "userAgent".

If the user's country code is included in the 'geo' array, then a sample is taken with frequency 1/samplerate. If the user's browser has a User Agent string that matches a key in the userAgent array, then a sample is taken with frequency 1/samplerate. No more than 1 oversample is taken for a given request.

If the request is oversampled, the emitted event will include these properties:
 * isOversample: true
 * oversampleReason: string, whose contents are a JSON array of reasons that the oversample happened. These will be of the form "geo:XX" (where XX is the country code that matched), or "ua:ABCDEF" (where ABCDEF is the string that was found in the browser's User Agent string)