[UserTiming] Feedback on latest draft

User Timing

The IE-9 Beta has a running implementation of the first draft [1] available here [2].

Requirements discussed

*         R1: High precision marks accessible by mark name, within 1ms accuracy

*         R2: Ease of store and access of marks

*         R3: Does not burden the browser, efficient use of CPU and Memory

Use cases discussed

*         UC1: A web developer makes unique marks in their web-page / web-app.

*         UC2: A web developer uses marks in repeated or iterative patterns. For example, a web developer may want to measure successive clicks denoted by a single name called "my_action".

*         UC3: Analytics companies report on standardized markers

*         UC4: Developer tools consume user marks interface


Nice to have, as discussed

*         NH1: Measures - achievable through JavaScript, we can create a best practice library that leverages the implementation of mark to create measures, which is a delta between marks and/or a date time.

With the latest User Timing spec [3] we have the following concerns.

Issue 1 - Does not meet UC2.
The '.user' attribute currently does not support the ability to retrieve a list of marks to meet Use Case 2 (UC2).  The getMarks() API solves this by returning an array of all marks that match that name.  The web-developer can get the latest single mark using getMark().

Issue 2 - Ease of use, possible confusion
*Assuming we support the ability to retrieve multiple marks from the same name.

If we change from the getMark()/getMarks() API to a '.user' attribute, we're overloading that attribute getter for multiple use cases: filtering and indexing.  This might lead to confusion.

For example:
Use Cases

getMark()/getMarks()

.userTiming attribute

usage

returns

usage

returns

Get all marks

performance.getMarks()

array

performance.user

array

Get the first mark

performance.getMarks()[0]

mark

performance.user[0]

mark

Get all marks named "foo"

performance.getMarks("foo")

array

performance.user["foo"]

array

Get the first "foo" mark

performance.getMarks("foo")[0]

mark

performance.user["foo"][0]

mark

Get all marks named "0"

performance.getMarks("0")

array

performance.user["0"]

array

Get the latest "foo" mark

performance.getMark("foo")

mark

performance.user["foo"][ performance.user["foo"].length-1]

mark


There can be confusion may come from the fact that if .user is indexed by a number, it is a single mark (the nth mark).  But if it's indexed by a string, it's an array of marks that match that name.

The getMark()/getMarks() API [1], on the other hand, separates the filtering, based on the API parameters, from indexing which web developers can access in a standard JavaScript array returned from the API.  getMarks() always returns an array, which can be iterated over the standard way.  The getMark() interface returns the latest mark for that name and is slightly easier than the .user way (performance.userTiming["foo"][ performance.userTiming["foo"].length-1]);

Issue 3 - Efficiency of data structures, Requirement 3 (R3)
In terms of design efficiency data structure of having a "PerformanceMark" does not necessarily require that it can also be a measure.  If it's just a mark, it carries additional 'type' and 'measure' attributes that are not appropriate.  If we do add measures, we should consider splitting them into another object type.

Issue 4 - Naming
The attribute name 'user' is ambiguous.

[1] http://dvcs.w3.org/hg/webperf/raw-file/d1498d8e0e5e/specs/UserTiming/Overview.html<https://dvcs.w3.org/hg/webperf/raw-file/d1498d8e0e5e/specs/UserTiming/Overview.html>
[2] http://ie.microsoft.com/testdrive/info/downloads/Default.html
[3] http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/UserTiming/Overview.html

Best Regards,
Anderson, Nic and Karen
Internet Explorer