Implements a request filter that automatically collects performance notifications during a request and adds them to the Server-Timing heading in the response, this enables the web development tools to show server timing information to better analyze the timing of a request/response cycle.

Add a WAPerformanceTimingFilter to your request handler as follows:

myApp := (WADispatcher default handlerAt: 'myApp')
            addFilter: WAPerformanceTimingFilter new

Then in your code you signal performance notifications as follows:

(WAPerformanceTiming name: 'callback' description: 'Callback procesing' duration: 80 milliseconds) signal

You can also do:

WAPerformanceTiming measure: 'metricName' during: aBlock

Metacello new
	baseline: 'SeasidePerformanceTiming';
	repository: 'github://eMaringolo/seaside-server-timing/src';
	onConflictUseLoaded; "Just in case you have an older version of Seaside Installed"
	load.

Seaside's WAHeaderFields object corectly supports adding multiple values for the same header name via aWAHeaderFields at: key add: value or via aWAResponse headerAt: key append: value.

But the ZnZincServerAdaptor>>responseFrom: aRequestContext iteration over such headers only adds the last one. For this reason the filter manually updates the header value each time, breaking somehow the the encapsulation of WAHeaderFields. Once this is solved at the ZnZincServerAdaptor side, this behavior should be removed from the WAPerformanceTimingFilter.

Each timing notification gets immediately added to the request header, so there is no "counter" functionality of each indicator. It is, if you signal a WAPerformanceTiming with the same name twice, this will be added twice as well to the headers. So it doesn't work as a performance counter of any sort.

• Server-Timing Working Draft
• Server-Timing MDN