Developer.mozilla.org

Functional Requirements

• FR1: Platform can ingest metrics (CPU, memory, latency, custom counters) from services.
• FR2: Users can query and visualize metrics on dashboards with filters, aggregations, and time ranges.
• FR3: Users can define alert rules with thresholds over time windows, e.g., “alert if P99 latency > 500ms for 5min”.
• FR4: Users can receive notifications when alerts fire, e.g., email, Slack.

Scale: 500K servers, 5M metrics per second, ~1GB/s raw ingestion.

Query Languages for Data

If you have a list of animal species and you want to return only the sharks in the list, a relational algebra expression would be \( \text{sharks} = \sigma_{\text{family = “Sharks”}}(\text{animals}) \). SQL queries follow the structure of relational algebra closely:

SELECT * FROM animals WHERE family = 'Sharks';

… while an imperative query would be:

function getSharks() {
  let sharks = [];
  for (let i = 0; i < animals.length; i++) {
    if (animals[i].family === "Sharks") {
      sharks.push(animals[i]);
    }
  }
  return sharks;
}

As the web evolves, where do practitioners congregate and exchange ideas?

Working Groups

develops standards and guidelines to help everyone build a web based on the principles of accessibility, internationalization, privacy and security.

publishes standards and drafts , which are then implemented by browser vendors, e.g., Chromium, Firefox, and Safari. Topics of interest: accessibility, browser, CSS, data, DOM, digital publishing, graphics, HTML, HTTP, internationalization, media, performance, privacy, protocol, security, web API, web fonts, Web of Things (WoT), and XML.

Mixins

A mix-in is an abstract subclass. This technique is especially useful in languages where a class can only have a single superclass.

const calculatorMixin = (Base) =>
  class extends Base {
    calc() {}
  };

const randomizerMixin = (Base) =>
  class extends Base {
    randomize() {}
  };

class Foo {}
class Bar extends calculatorMixin(randomizerMixin(Foo)) {}

UI Design

This time we’ll use the more semantic and a11y-friendly <dialog> element . Centering it in the page is done by the browser, and that saves us a bit of hassle – thought it would have been feasible with how <search-bar>’s <ul> floats above the page.

CardsManager Interface

The previous/next buttons make use of the CardsManager object that has the API:

How to handle redirects without setting window.location.href?

Right now, there’s a pattern of doing:

sendHTTPRequest("POST", "/login/", {})
  .then((_) => {
      window.location.href = "/";
  })
  .catch((err) => { console.error(err); });

Isn’t this something that the server can do? In response, why not issue a redirect?

Back when I wrote this, the motivation for using localStorage was to reduce the trips to the server so that the app is usable offline. However, with two data stores (localStorage and the server), the former has a possibility of going stale. What usage is correct and how can we avoid stale data?

localStorage['session_info']

getAccountInfo: () => AuthenticateUser | null fetches the session_info entry and JSON.parses it into an AuthenticateUser. This is a possible failure point because the parsed JSON cannot be trusted to be a valid AuthenticateUser instance. This bit should be removed.