Observability for Frontend Developers

This is a small proof of concept for how we can instrument a clientside JavaScript app for observability. I'd be happy to take PRs and discuss in issues about applying these ideas to real world frontend apps.

A lot of the inspiration for this comes from a conversation I had with Charity Majors (CTO at Honeycomb), as well as a talk at o11ycon 2018 by Emily Nakashima (Frontend engineer, now VP Eng at Honeycomb).

Steps:

We fork Excalidraw, a popular open source sketch diagramming tool built with React, and deploy it to Netlify. This gives us a working site to play around with - for example https://frontend-observability.netlify.com/.
We then create a dataset with the Honeycomb free tier. This gets us a HONEYCOMB_API_KEY and a HONEYCOMB_DATASET_NAME

We then clone our fork locally and add a serverless function (in this case using Netlify Functions) to send that data to Honeycomb:

// simplified example function without some nice error handling and netlify niceties
// see full file for details

// inject env vars if needed
if (process.env.NODE_ENV !== "production") {
  require("dotenv-safe").config();
}
const axios = require("axios");
const honeycombWriteKey = process.env.HONEYCOMB_API_KEY;
const honeycombDatasetName = process.env.HONEYCOMB_DATASET_NAME;
const honeycombEndpoint = `https://api.honeycomb.io/1/events/${encodeURIComponent(
  honeycombDatasetName,
)}`;

exports.handler = async (event, context) => {
  /* parse the string body into a useable JS object */
  const receivedData = JSON.parse(event.body);
  const options = {
    method: "POST",
    headers: { "X-Honeycomb-Team": honeycombWriteKey },
    data: receivedData,
    url: honeycombEndpoint,
  };
  await axios(options);
  return {
    statusCode: 200,
    body: "POST OK",
  };
};

This process (running a serverless function alongside a React app in local development) can be made easy with no config using Netlify Dev - note your project must first be linked to the site instance with netlify link.
We then go through the app code and call the function whenever a significant user event has occurred. Anything in a state management sore (e.g. Redux) is a good candidate.

// simplified function call from frontend to serverless fn
// chose to send as an unchained promise call so as not to block execution
fetch("/.netlify/functions/honeycomb", {
  method: "POST",
  body: JSON.stringify(infoToSend),
}).catch(console.error);

You can see the results of this action in a single commit here and this is how it looks when it shows up on the Honeycomb dashboard:

Instrumenting Load/Unload Events

The other big actionable part of Emily's o11ycon talk I found applicable to a proof of concept is tracking load/unload events (there is also an accompanying blogpost on instrumenting browser page loads) - a lot of the below code is adapted from that blogpost, but updated for current browser APIs and the build setup I have here.

In particular, pay attention to the resourceName section where I opted to track main.WEBPACKHASH.chunk.js and main.WEBPACKHASH.chunk.css - tweak that as you need be. The primary idea I was going for was to only track the primary bundle but you may have other priorities.

Anyway, it's two steps:

make a standalone js module (or insert this inline in your html)

// trackLoadUnload.ts

const perf = window.performance as Performance & {
  // nonstandard api so have to augment type
  memory: {
    usedJSHeapSize: number;
    totalJSHeapSize: number;
  };
};
const nav = navigator as Navigator & {
  connection?: {
    type: string;
    effectiveType: string;
    rtt: number;
  };
};

const honeycomb = {
  sendEvent(event: object, actionName: string) {
    return (
      // same fetch code as in the rest of the React app! hit the serverless fn
      fetch("/.netlify/functions/honeycomb", {
        method: "POST",
        body: JSON.stringify({
          actionName,
          ...event,
        }),
      }).catch(console.error)
    );
  },
};

// Randomly generate a page load ID so we can correlate load/unload events
export const pageLoadId = Math.floor(Math.random() * 100000000);
// Memory usage stats collected as soon as JS executes, so we can compare the
// delta later on page unload

export const jsHeapUsed = perf.memory && perf.memory.usedJSHeapSize;
const jsHeapTotal = perf.memory && perf.memory.totalJSHeapSize;
// // Names of static asset files we care to collect metrics about
// const trackedAssets = ["/main.css", "/main.js"];
// Returns a very wide event of perf/client stats to send to Honeycomb
const pageLoadEvent = function() {
  const nt = window.performance.timing;
  const event: Record<string, string | number | undefined> = {
    type: "page-load",
    page_load_id: pageLoadId,
    // User agent. We can parse the user agent into device, os name, os version,
    // browser name, and browser version fields server-side if we want to later.
    user_agent: window.navigator.userAgent,
    // Current window size & screen size stats
    // We use a derived column in Honeycomb to also be able to query window
    // total pixels and the ratio of window size to screen size. That way we
    // can understand whether users are making their window as large as they can
    // to try to fit Honeycomb content on screen, or whether they find a smaller
    // window size more comfortable.
    //
    // Capture how large the user has made their current window
    window_height: window.innerHeight,
    window_width: window.innerWidth,
    // Capture how large the user's entire screen is
    screen_height: window.screen && window.screen.height,
    screen_width: window.screen && window.screen.width,
    // Chrome-only (for now) information on internet connection type (4g, wifi, etc.)
    // https://developers.google.com/web/updates/2017/10/nic62
    connection_type: nav.connection && nav.connection.type,
    connection_type_effective: nav.connection && nav.connection.effectiveType,
    connection_rtt: nav.connection && nav.connection.rtt,
    // Navigation (page load) timings, transformed from timestamps into deltas
    timing_unload_ms: nt.unloadEventEnd - nt.navigationStart,
    timing_dns_end_ms: nt.domainLookupEnd - nt.navigationStart,
    timing_ssl_end_ms: nt.connectEnd - nt.navigationStart,
    timing_response_end_ms: nt.responseEnd - nt.navigationStart,
    timing_dom_interactive_ms: nt.domInteractive - nt.navigationStart,
    timing_dom_complete_ms: nt.domComplete - nt.navigationStart,
    timing_dom_loaded_ms: nt.loadEventEnd - nt.navigationStart,
    timing_ms_first_paint: nt.domComplete - nt.navigationStart, // Calculate page render time
    // Some calculated navigation timing durations, for easier graphing in Honeycomb
    // We could also use a derived column to do these calculations in the UI
    // from the above fields if we wanted to keep our event payload smaller.
    timing_dns_duration_ms: nt.domainLookupEnd - nt.domainLookupStart,
    timing_ssl_duration_ms: nt.connectEnd - nt.connectStart,
    timing_server_duration_ms: nt.responseEnd - nt.requestStart,
    timing_dom_loaded_duration_ms: nt.loadEventEnd - nt.domComplete,
    // Entire page load duration
    timing_total_duration_ms: nt.loadEventEnd - nt.connectStart,
  };
  // First paint data via PerformancePaintTiming (Chrome only for now)
  const hasPerfTimeline = !!window.performance.getEntriesByType;
  if (hasPerfTimeline) {
    const paints = window.performance.getEntriesByType("paint");
    // Loop through array of two PerformancePaintTimings and send both
    paints.forEach(paint => {
      if (paint.name === "first-paint") {
        event.timing_ms_first_paint = paint.startTime;
      } else if (paint.name === "first-contentful-paint") {
        event.timing_first_contentful_paint_ms = paint.startTime;
      }
    });
  }
  // Redirect count (inconsistent browser support)
  // Find out if the user was redirected on their way to landing on this page,
  // so we can have visibility into whether redirects are slowing down the experience
  event.redirect_count =
    window.performance.navigation &&
    window.performance.navigation.redirectCount;
  // Memory info (Chrome) — also send this on unload so we can compare heap size
  // and understand how much memory we're using as the user interacts with the page
  if (perf.memory) {
    event.js_heap_size_total_b = jsHeapTotal;
    event.js_heap_size_used_b = jsHeapUsed;
  }
  // ResourceTiming stats
  // We don't care about getting stats for every single static asset, but we do
  // care about the overall count (e.g. which pages could be slow because they
  // make a million asset requests?) and the sizes of key files (are we sending
  // our users massive js files that could slow down their experience? should we
  // be code-splitting for more manageable file sizes?).
  if (hasPerfTimeline) {
    type ExtendedPerformanceEntry = PerformanceEntry & {
      encodedBodySize: number;
      decodedBodySize: number;
      responseEnd: number;
    };
    const resources = window.performance.getEntriesByType(
      "resource",
    ) as ExtendedPerformanceEntry[];
    event.resource_count = resources.length;
    // Loop through resources looking for ones that match tracked asset names
    resources.forEach(resource => {
      const resourceNames = resource.name.split("/");
      const resourceName = resourceNames[resourceNames.length - 1];
      if (
        resourceName.startsWith("main.") &&
        resourceName.endsWith(".chunk.js")
      ) {
        // // Don't put chars like . and / in the key name
        const name = "main_chunk_js";
        event[`resource_${name}_encoded_size_kb`] = resource.encodedBodySize;
        event[`resource_${name}_decoded_size_kb`] = resource.decodedBodySize;
        event[`resource_${name}_timing_duration_ms`] =
          resource.responseEnd - resource.startTime;
      } else if (
        resourceName.startsWith("main.") &&
        resourceName.endsWith(".chunk.css")
      ) {
        // // Don't put chars like . and / in the key name
        const name = "main_chunk_css";
        event[`resource_${name}_encoded_size_kb`] = resource.encodedBodySize;
        event[`resource_${name}_decoded_size_kb`] = resource.decodedBodySize;
        event[`resource_${name}_timing_duration_ms`] =
          resource.responseEnd - resource.startTime;
      }
    });
  }
  return event;
};
// Send this wide event we've constructed after the page has fully loaded
window.addEventListener("load", function() {
  // Wait a tick so this all runs after any onload handlers
  setTimeout(function() {
    // Sends the event to our servers for forwarding on to api.honeycomb.io
    honeycomb.sendEvent(pageLoadEvent(), "onPageLoad");
  }, 0);
});

add it somewhere in your bundler asset graph (or, again, insert inline in your html)

// index.tsx
import React from "react";
import ReactDOM from "react-dom";
import "./trackLoadUnload"; // new

Here's an example of a raw event sent to Honeycomb:

I had a lot more trouble with unload events - I tried all variations of window.addEventListener("beforeunload") and window.onbeforeunload and window.onunload and so on but couldnt reliably get them to fire when I closed the tab (rather than refreshed). This one is something I hope to revisit in future.

Querying

When instrumenting is done and shipped to production, we'll start to receive a bunch of data. Of course, we don't really have production traffic on this Proof of Concept, so I can't show as much. But you can start to do queries like correlating screen height and width (and you can derive aspect ratio later):

A lot of these fields (a mature setup will have 300-500 fields tracked) won't make sense at first, so some experience will be needed to figure out how to massage data to answer questions you need (and to instrument more things you didn't think of the first time around!)

A killer feature of Honeycomb is the concept of Heatmaps and BubbleUp charts. The idea is that you often have a group of interesting datapoints and you want to find out what sets them apart from "normal" datapoints. Honeycomb lets you click and highlight a box over those points of interests, and then displays the quick diff for you to figure out what is unique among the metrics you track. This is helpful potentially for support and developer-adjacent usecases as well.

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Observability for Frontend Developers

Instrumenting Load/Unload Events

Querying

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Observability for Frontend Developers

Instrumenting Load/Unload Events

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