Re: Notification for new navigation timing entries

I'm working on trying to define the proposed interface [1] within
Performance Timeline, some thoughts as I'm trying to spec it...

>From a developers perspective, the proposed API is as follows:
var observer = new PerformanceObserver(function(events) {
  // events is LazyPerformanceEntryList with getEntries, getEntriesByType,
etc, methods.

observer.observe({eventTypes: ['render', 'composite', 'resource']})

The events (LazyPerformanceEntryList) object might contain one or more
PerformanceEntry objects, and the application can choose to process the
list immediately, or defer processing until the "time is right" -- e.g. you
probably want to wait until you have some idle time to avoid competing with
app-critical processing. In fact, especially with Frame Timing, the latter
(deferred) use case is the recommended route.. but the API ergonomics for
this are not great:

(a) As a naive developer I'm likely to just create an array and start
pushing the LazyPerformanceEntryList's onto it, such that I can process
them later. However, now I have an array of "lazy lists", each of which
supports the getEntries{ByName, ByType, ..}() methods, but I can't query
the array itself with the same methods. Now I have a nested foreach and
this feels awkward...

(b) Perhaps we could extend the LazyPerformanceEntryList to be
"appendable"? Now, as a developer I get the LazyPerformanceEntryList on
first callback, to which I can retain the reference and push other lists
onto it? This allows me to construct a single "lazy list" which I can query
with getEntries* methods. This feels a bit better...?

(c) What if the UA automates step (b)? One way to approach it: once the
PerformanceObserver is registered it starts appending observed entries into
a single LazyPerformanceEntryList; the callback is fired as it would
previously, but instead of a new list within each callback we simply return
a reference to the same list owned by that PerformanceObserver.. In effect,
the PerformanceObserver has a "local lazy timeline" which automatically
buffers the events and provides same access methods (getEntries*) as the
global Performance Timeline. This makes it very simple to work with for a

+ It removes the burden of efficient implementation from the developers;
they can't get it wrong.
+ It is simple to explain and work with: each observer maintains a local
timeline that's active while the observer is registered.
+ It allows efficient buffering and minimizes number of created objects on
both ends.
+ It works with buffered + immediate delta processing approaches:
  * Buffer until you're ready to process, then apply your logic and call
clear() to reset.. repeat.
  * You can also process in each callback and immediately call clear() to
reset each time.

Thoughts, comments? </vigorous handwaving>


P.S. I have the beginnings of a very rough attempt at (c) here:


On Fri, Feb 13, 2015 at 10:49 AM, Ilya Grigorik <>

> On Fri, Feb 13, 2015 at 10:45 AM, Nat Duca <> wrote:
>> interface LazyPerformanceEntryList {
>>>   bool HasEntryType(string);
>>>   PerformanceEntryList getEntries();
>>>   PerformanceEntryList getEntriesByType(DOMString entryType);
>>> }
>> This sounds awesome. I updated the doc with this text.
> Would it also make sense to expose getEntriesByName(DOMString entryName)?
> For example, I have an observer for "subresource" entryType, but I only
> care about "" and it'd be nice to be able to skip
> iterating over all the ResourceTiming objects each time to check for it?

Received on Friday, 20 March 2015 19:46:23 UTC