Integrate Sentry's Web Vitals into Slack.

Introduction

Web Vitals is an initiative proposed by Google that aims to provide various metrics to quantify the performance of a website. Previously, there was a video on the Google Chrome Developer channel introducing Core Web Vitals in Taiwanese, which includes some very interesting examples that I recommend checking out ↓

Nowadays, many websites are implementing processes to measure Web Vitals, identifying areas for improvement through quantifiable indicators. There are quite a few tools available to help developers measure Web Vitals, such as:

• Lighthouse
• PageSpeed Insights
• CrUX
• Search Console

Sentry’s Web Vitals Statistics Feature

However, what fewer people know is that Sentry also launched a Web Vitals statistics feature earlier¹, which can track various metrics like LCP, FP, and CLS. This allows developers to view related charts in the backend, even labeling the averages, which is quite thoughtful.

Nonetheless, it seems that Sentry only retains data for 30 days, and it does not support comparison features—for instance, comparing last week’s metrics with this week’s metrics. The charts can only be viewed on Sentry, creating a switching cost for other teams. As developers, it’s natural to want to automate the entire process.

Upon further investigation, I found that although Sentry offers API integration (if you have a token), it seems there is no specific API documentation for Web Vitals. However, out of curiosity, I opened the console and noticed that the charts are generated using an API, and after copying the API endpoint and parameters, I successfully called it using CURL! It appears that it just wasn’t documented.

There are two main APIs: * (Note that since there is no documentation, parameters or paths may change) *

• /events-measurements-histogram/?...
• /eventsv2

Automation Process Design

Now that we have the data, we can integrate it into Slack for regular tracking, making it easy to share with others in different teams. For more detailed information, one can refer back to the Sentry dashboard. The entire process is as follows:

By using a cron job to call the API (or writing it as serverless), we can obtain Sentry’s data, upload it, and generate reports.

One crucial point to note is the presence of + in the query parameters; directly sending it to encodeURIComponent or using URLSearchParams will convert it to %2B, resulting in an error from Sentry. Therefore, it’s essential to either extract the + or find a way to prevent it from being encoded.

For example, the API’s query parameters might look like this: field=percentile(measurements.fp%2C+0.75)&field=percentile(measurements.fcp%2C+0.75)&.... You can define the data you want to return in a function-like manner, such as percentile(measurements.fp,+0.75), which returns the 75th percentile of FP data (measured in milliseconds). Notice that while , is encoded as %2C, the + remains unencoded. If there are errors when creating the query parameters, this is a good area to investigate.

The returned data structure will look like this:

{
  "path": "/my-page",
  "data": [
    {
      "count": 10,
      "bin": 2000
    },
    ...
  ]
}

Next, we generate a .json file and upload it to S3, or any other storage solution. This way, we can compare it with yesterday’s data (or earlier data) and generate a report sent to Slack:

Additionally, if you want to generate charts, you can use chart.js and chartjs-node-canvas in Node.js to implement this. This allows you to create charts without needing a browser (canvas). Just be aware that if the corresponding fonts are not installed on the server, it may lead to formatting issues or font mismatches.

Regarding cron job selection, there are many mature solutions available now, and even hardcoding a server into crontab is an option. However, here we utilized the cron job feature from Drone CI to implement this. The main reason is that the configuration file for Drone is very convenient to write, and our company has a dedicated Drone CI server, plus we cannot use external services casually. Therefore, integrating with Drone CI was the simplest way to make everything easy to set up.

However, for some reason, every time the cron event is triggered, other pipelines also get triggered. So later, we modified the .yaml to call the API by opening an additional branch:

steps:
  - name: upload and report
    image: byrnedo/alpine-curl
    commands:
      - curl -X POST YOUR_SERVER_ENDPOINT

With that, the entire automation implementation is complete. Now we can see reports sent to Slack daily, making it easier for developers to observe changes and receive feedback.