worker start needs to be added to diagram

[pauljroy]

Hello, I'm on the Bing performance team. Just was talking to Todd Reifsteck and we notice that Worker Start is not in the Navigation Timing diagram at the start of the processing model. Would be great if it could be added. Thanks!

[npm1]

@yoavweiss are you a good person to add this? I believe it goes after unloadEventEnd and before redirectStart.

[yoavweiss]

Sure

[nicjansma]

Is the value of workerStart lost or wrong in some redirect scenarios?

e.g.

Same Origin Redirects

When navigating to a.com which redirects to another page on a.com (all covered by a Service Worker):

1. Step (8) records workerStart
2. Step (9) starts the fetch for a.com/, which redirects to a.com/redir
3. Step (20e) sets all attributes to 0, except a list which doesn't have workerStart
4. Step (20f) goes back to Step (9)

In this case, a.com's workerStart time is cleared at step (20e), and not re-set at any point. Even though it's the same origin for the SW the whole time. I think I would expect it to be saved to its original value (from the first fetch of a.com/redir, assuming the SW doesn't "restart" during the redirection).

Cross-Origin Redirects

When navigating to a.com which redirects to b.com (both are covered by a Service Worker):

1. Step (8) records workerStart for a.com
2. Step (9) starts the fetch for a.com/, which redirects to b.com
3. Step (20a) sets a few attributes to 0 (not including workerStart), then goes to Step (9)

In this case, a.com's workerStart time is saved though the rest of the DNS->Response attributes are for b.com. I think I would expect the SW time to be for b.com instead of a.com here (unless there are privacy/security implications?).

I may be missing some security implications -- if the current steps are correct, we may want to note in the workerStart definition what it represents in redirect scenarios.

[nicjansma]

And some of this may be related to #100

[nicjansma]

Addressed this (added to the diagram) and the comments above about the processing model in #131

[dashed]

I'm looking to calculate Service Worker initial boot times.

Is the following no longer reliable if redirectStart > 0?

// measuring service worker startup time
let entry = performance.getEntriesByType('navigation')[0];
let swStartupTime = entry.fetchStart - entry.workerStart;

I've seen requestStart - workerStart referenced here https://webplatform.news/issues/2018-11-19 (and https://youtu.be/25aCD5XL1Jk?t=274 ). Unsure if this is better.

[nicjansma]

Correct, today if there are redirects, Service Worker Startup time (as fetchStart-workerStart) will include those redirect durations.

requestStart-workerStart would have the same issue.

[noamr]

It's now in the diagram.