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Re: handling multi-request fetches in Nav & Resource Timing

From: Nottingham, Mark <mnotting@akamai.com>
Date: Thu, 30 Apr 2015 02:54:28 +0000
To: Ilya Grigorik <igrigorik@google.com>
CC: public-web-perf <public-web-perf@w3.org>
Message-ID: <D3017B04-BEA5-47F3-8481-72489EB8003B@akamai.com>
Generally +1. I do wonder if a more radical change would be worth it, e.g., changing the model to be effectively a queue of logged events, without restrictions on cardinality, so that you can have multiple requests and responses (along with other things like SW) associated with one fetch event.

Also, consider that some requests will have two or more responses (when 1xx status codes are used).

Cheers,


> On 19 Feb 2015, at 6:45 am, Ilya Grigorik <igrigorik@google.com> wrote:
> 
> Problem: some fetches incur multiple requests before they are fulfilled - e.g. HTTP redirects, permission preflights, resumed upload/downloads, etc. Today, both NT and RT only show the last request in the sequence and often hide a lot of critical timing data even for that last request - see [1], [2]. This is a big spec gap and one that we need to address: having access to accurate request timing data for *all requests in the sequence* is critical to optimizing performance.
> 
> There are many different ways we can go about addressing this, and I'd like to propose some overarching principles to help narrow down the space:
> 
> 1) Performance Timeline should record *every* request as a separate entity, regardless of whether it was initiated by the application, or done automatically by the browser (i.e. redirects, preflights, etc). 
> 2) Timing data for each request entity should be subject to regular TAO restrictions - i.e. you can always observe "duration", and access to more detailed timing data is subject to that origin emitting the appropriate TAO opt-in policy.
> 3) In the case where multiple requests were incurred to satisfy the fetch, the application should have a way to "detect and follow" the chain of requests.
> 
> --- Examples ---
> 
> a) A navigation to example.com triggers a redirect chain:
> 
> performance.getEntriesByType("navigation") -> [
>    PerformanceTiming{name: "http://example.com", <timing data>},
>    PerformanceTiming{name: "https://example.com", <timing data>},
>    PerformanceTiming{name: "https://example.com/en-us", <timing data>}
> ]
> 
> Each PerformanceEntry object contains full timing data, but access to detailed timing data is subject to TAO. For example, in above chain the final destination (https://example.com/en-us) can observe that there have been three redirects (which are recorded in the sequence they occured in) and it can access full timing data for the last two because they are on same origin. To get access to the detailed timing data (aka, beyond duration) for the first one, the {http://example.com} origin would need to emit a TAO opt-in header allowing {https://example.com} to gather it.
> 
> b) A subresource fetch triggers a redirect:
> 
> performance.getEntriesByType("resource") -> [
>    ... <other timing entries > ...,
>    PerformanceResourceTiming{name: "http://example.com/thing.js", <timing data>},
>    PerformanceResourceTiming{name: "https://example.com/thing.js", <timing data>},
>    ... <other timing entries > ...
> ]
> 
> Similar to above, each request is recorded separately and is subject to the same TAO restrictions. However, unlike a navigation request, the getEntriesByType("resource") returns an array of *all* subresource requests... Hence, we need a way to tell the application that a request for "http://example.com/thing.js" triggered another request due to a redirect. A simple way to do this would be to add a pointer to a subsequent request - e.g. "nextName" attribute, or some such. To follow the chain after making the request the application would: 
> 
> reqs = []
> reqs.push(performance.getEntriesByName("http://example.com/thing.js"))
> while(r.nextName != '') { 
> 	reqs.push(performance.getEntriesByName(r.nextName))	
> }
> processTiming(reqs)
> 
> c) A preflight request is triggered for a cross-origin request:
> 
> performance.getEntriesByType("resource") -> [
>    PerformanceResourceTiming{name: "https://other.com/thing.js", <timing attrs>},
>    PerformanceResourceTiming{name: "https://other.com/thing.js", <timing attrs>},
>    ... <other entries > ...
> ]
> 
> Similar to above subrequest case, but the request URL remains the same across multiple requests required to fulfill this fetch. The application would: 
> 
> reqs = performance.getEntriesByName("https://other.com/thing.js") // returns an array with two PerformanceResourceTiming entries
> processTiming(reqs)
> 
> (note: there is an implicit assumption here that requests are recorded in the timeline in the same order as they occur)
> 
> d) Auto-resumed download/upload: effectively the same as cross-origin case... If the browser triggers multiple requests to transfer the data, it would simply record the timing data for each request as a standalone entry. This allows the application to identify how many requests were made and how long each took.
> 
> --- ~Delta from current specs ---
> 
> * Record every request into timeline
> * Add "nextName" attribute (or some such) to follow cases where URL changes between requests
> * Drop redirect{Start,End} from NavTiming and ResourceTiming?
> * Drop current cross-origin "hide data" if redirect logic in NT/RT
> 
> There are other nitpicks we'd have to address, but I think the above covers the main changes we'd need to make. Removing redirect{Start,End} is not ideal from compatibility perspective, but they would be unnecessary under this new model. Alternatively, we can keep them around and make then return 0's.
> 
> Thoughts, other ideas? Anything I'm not accounting for in terms of use cases or API surface?
> 
> [1] https://lists.w3.org/Archives/Public/public-web-perf/2015Jan/0006.html
> [2] https://lists.w3.org/Archives/Public/public-web-perf/2015Feb/0054.html

--
Mark Nottingham    mnot@akamai.com    https://www.mnot.net/
Received on Thursday, 30 April 2015 02:55:04 UTC

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