Malloy HTML Data Apps

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In-Package HTML Data Apps

A package becomes a web app by adding a public/ directory. Publisher serves those files and gives the page Publisher.query(...) to run Malloy against the package's models. No build step, no npm, no framework.

When this is the right tool

Pick an HTML data app when the user wants full control of the markup and only plain web files.

Package anatomy

my-package/
  publisher.json        # name, version, description
  subscriptions.malloy  # the model(s), stays private
  subscriptions.parquet # data, stays private
  public/               # ONLY this directory is web-served
    index.html
    app.js
    vendor/             # chart library, vendored rather than loaded from a CDN
      chart.umd.js

Only public/ is reachable over the web, at /environments/<env>/packages/<pkg>/<file>. Models, data, and publisher.json are private and reached only through the query API, which still applies the model's filters, access modifiers, and authorize rules. There is no flag to set: a public/ directory is what makes a package an app.

Build sequence

The agent orchestrates these. Each query and chart step hands off to a focused skill.

1. DESIGN IT FIRST. Read reference/design.md: who opens it, the decision it serves, the archetype, the form each tile takes, and the depth plan. Skipping this is what produces an app that enumerates the model - a KPI row and a chart grid, whatever the domain - instead of one that answers a question. Hand the brief to the user before scaffolding.
2. READ THE MODEL. Get the model's real source and view names, through your environment's context tool if it has one, or by opening the .malloy file directly. Never guess field or view names.
3. SCAFFOLD the package (template below).
4. WRITE THE QUERIES. Read reference/runtime.md. Validate each before pasting it into the page, using whatever query tool your environment provides or a running Publisher (see reference/runtime.md). Malloy syntax questions go to skill:malloy-queries.
5. CHOOSE CHARTS with skill:malloy-charts when rendering through <malloy-render>; otherwise it is your own chart library drawing the returned rows. Vendor any chart library into public/ and load it locally, not from a CDN. Two reasons: embedded author JavaScript runs with the viewing user's data authority, and a blocked CDN (agent sandboxes and many corporate networks block them) is easy to miss, because the script never runs and the charts come up empty. The storefront example ships its chart library in public/vendor/ and loads it from public/index.html as ./vendor/chart.umd.js. Copy that, but resolve the path against the page's own directory: a page in a subdirectory (public/reports/index.html) needs ../vendor/chart.umd.js. A wrong relative path 404s and leaves the charts blank, which is the failure you are trying to avoid.
6. EMBED (optional). Read reference/embedding.md.
7. PREVIEW with the local authoring loop (below).
8. VERIFY before you call it done (see "What 'done' means" below). This step is not optional.

The scaffold in step 3 only proves the wiring. It is the start, not the deliverable. What you ship is a production app that meets the recipe below.

What "done" means (production recipe)

A data app you can defend has all of these. Build to this list, not to the scaffold.

• Real names, never guessed. Every source, view, and field name comes from the model you read in step 2. A name you derived or assumed is a bug waiting to surface as an empty tile.
• A real name is not the same as a correct measure. Using the model's declared measures is right by default, but a wrong one fails far more quietly than a wrong name: an empty tile is obvious, a plausible wrong number is not. Before a measure reaches a headline, confirm it against the raw column values (reference/design.md step 2 of the inventory). The case that bites is a predicate like != null over a column whose "absent" value is a sentinel string - measured on a real build, that reported 95.6% of police cases cleared where the true figure was 16.6%, and every check in this skill still passed.
• DOM-only - never innerHTML with interpolated values. Build every element with createElement + textContent; do not assign innerHTML (or insertAdjacentHTML, document.write) with any string that contains a model value. Query results render any markup they contain - an XSS vector, and blocked outright under a Trusted-Types CSP. This is a hard build rule, not a lint suggestion: an app that interpolates a model value into innerHTML is not done.
• Modular, not one inline blob. Split the page into modules per reference/runtime.md (pure formatting helpers, a chart layer, your tile/query definitions as data, a thin entry point). One source of truth for each tile's model/source/view, no parallel maps that drift.
• Every tile handles loading, empty, and error on its own. One failing query must not blank the page. (reference/runtime.md.)
• Defensible numbers. Missing ≠ zero (omit the point, don't plot a fake 0); show the latest non-null value for "current"; guard division with nullif; convert units explicitly. (reference/runtime.md.)
• Visible assumptions. When you assume something (two sources joined by month, an in-month proxy that differs from a certified definition) or a metric is incomplete, say so in the app: a caption, a footnote, a placeholder card with the reason. The non-technical user cannot see your reasoning; bury a caveat and you have misled them. Don't silently drop a metric you couldn't model. Show a placeholder that names what's missing and why.
• Built to the design brief, not to the model's shape. The app matches the archetype, forms, and depth plan from step 1 (reference/design.md), carries a real token contract (surfaces, text ramp, semantic colors, a categorical chart palette) that the chart code reads at runtime rather than hardcoding, and puts visible weight on the two or three numbers that lead. A uniform grid of same-size cards, or a tile per view, means the brief was skipped.
• Vendored libraries. Chart and helper libraries live in public/, loaded locally (step 5).
• Lazy-load below the fold, once the page has more than about eight query-backed tiles. ("Many" is not decidable; that is the threshold.) Tiles sitting below the fold is why the technique works, not a second trigger for it - almost every real data app scrolls, so reading it as one makes the rule "always" and contradicts the count. Under that threshold, firing every tile at once is cheaper than the machinery. Don't fire every tile's query on load. reference/lazy-load.md is the recipe: IntersectionObserver (rootMargin ~240px) + a small concurrency cap + reserve each tile's height so lazy tiles don't reflow. Includes the verification trap - on a short/tall-default viewport all tiles intersect at once and you get a false "everything deferred" pass, so test on a deliberately small viewport.