ScopedPerformance

Safely use the Performance API User timing features without fear of having name collisions with another concurrently running code.

Problem

When working with the User Timing Performance API on a Deno server with async handlers. You may encounter interferences, as the performance instance (and entries) are shared for all the concurrently executing handlers on the same isolate.

The following code poses a problem because the same names might be used for different requests if they overlap in time:

Deno.serve(async (_req: Request => {
  performance.mark('start');

  // some async work here ...

  const measureEntry = performance.measure('total', 'start');

  return new Response(...);
});

Using clearMarks()/clearMeasures() does not help. It might be even worse as you may remove the entries created by another request!

Solution

We can avoid the problem by making a new ScopedPerformance instance for each request, which will automatically take care of prefixing the mark/measure names with a unique prefix:

import { ScopedPerformance } from 'https://deno.land/x/scoped_performance/mod.ts';

Deno.serve(async (_req: Request => {
  const scoped = new ScopedPerformance();
  scoped.mark('start');

  // some async work here ...

  const measureEntry = scoped.measure('total', 'start');

  return new Response(...);
});

This effectively avoid name collisions among concurrent requests.

Executing getEntries() in the ScopedPerformance instance will only return the entries created with that instance:

import { ScopedPerformance } from 'https://deno.land/x/scoped_performance/mod.ts';

// Let's make a mark in the global original `performance` instance
performance.mark('global-start');

// Then proceed to make an scoped instance, and some scoped marks
const scoped = new ScopedPerformance();
scoped.mark('start');
scoped.measure('total', 'start');

// When retrieving the entries of the scoped instance, we just get those.
// The "name" of the entry will be unscoped/unprefixed back to the original name.
console.log(scoped.getEntries());

/*
[
  PerformanceMark {
  name: "start",
  entryType: "mark",
  startTime: 24,
  duration: 0,
  detail: null
},
  PerformanceMeasure {
  name: "total",
  entryType: "measure",
  startTime: 24,
  duration: 0,
  detail: null
}
]
*/

// The global `performance` instance of course contains all the marks.
// Here we can see that the "name" of the entries created with the scoped instance
// are internally scoped/prefixed with an unique id to avoid collisions.
console.log(performance.getEntries());

/*
[
  PerformanceMark {
  name: "global-start",
  entryType: "mark",
  startTime: 24,
  duration: 0,
  detail: null
},
  PerformanceMark {
  name: "f5a8be11-610b-4745-afdd-98fa8ae0e6d0::start",
  entryType: "mark",
  startTime: 24,
  duration: 0,
  detail: null
},
  PerformanceMeasure {
  name: "f5a8be11-610b-4745-afdd-98fa8ae0e6d0::total",
  entryType: "measure",
  startTime: 24,
  duration: 0,
  detail: null
}
]
*/