Chapter 5: App Builder & UIs

Welcome to Chapter 5: App Builder & UIs. In this part of Windmill Tutorial: Scripts to Webhooks, Workflows, and UIs, you will build internal tools and dashboards using Windmill's drag-and-drop App Builder, powered by your scripts and flows from previous chapters.

Build internal tools with drag-and-drop components backed by your Windmill scripts and flows.

Overview

The App Builder lets you create rich, interactive internal tools without writing frontend code. You drag components onto a canvas, wire them to scripts and flows as backends, and publish shareable apps. Every component can read from and write to a reactive state, making complex UIs possible without a frontend framework.

flowchart TB
    subgraph AppBuilder["App Builder"]
        direction TB
        C1[Table Component]
        C2[Form Component]
        C3[Chart Component]
        C4[Button Component]
        C5[Text / HTML Component]
    end

    subgraph Backend["Backend (Scripts & Flows)"]
        S1[Fetch Users Script]
        S2[Update User Script]
        S3[Analytics Flow]
    end

    subgraph State["Reactive State"]
        ST[Component State Store]
    end

    C1 -->|"onSelect"| ST
    C2 -->|"onSubmit → run"| S2
    C4 -->|"onClick → run"| S3
    S1 -->|"result → data"| C1
    S3 -->|"result → data"| C3
    ST -->|"selected row"| C2

    classDef component fill:#e1f5fe,stroke:#01579b
    classDef backend fill:#e8f5e8,stroke:#1b5e20
    classDef state fill:#fff3e0,stroke:#ef6c00

    class C1,C2,C3,C4,C5 component
    class S1,S2,S3 backend
    class ST state

Creating an App

1. Navigate to Home and click + App
2. You see a canvas with a grid layout
3. Drag components from the left panel
4. Configure each component's data source and behavior in the right panel

Available Components

Category	Components
Layout	Container, Tabs, Drawer, Modal, Stepper, Horizontal/Vertical Split
Display	Text, HTML, Image, Icon, Map, PDF Viewer, Log Display
Input	Text Input, Number, Select, Multi-select, Date Picker, File Upload, Toggle, Slider, Rich Text Editor
Data	Table, AgGrid Table, List, JSON Viewer
Charts	Bar, Line, Pie, Scatter, Timeseries (via Plotly / Chart.js)
Action	Button, Form, Download Button, Approve Button

Example: User Management Dashboard

Step 1: Backend Scripts

First, create the scripts that power the app (see Chapter 3):

// f/scripts/list_users
type Postgresql = {
  host: string;
  port: number;
  user: string;
  password: string;
  dbname: string;
};

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

export async function main(
  db: Postgresql,
  search: string = "",
  limit: number = 100
): Promise<object[]> {
  const client = new Client({
    hostname: db.host,
    port: db.port,
    user: db.user,
    password: db.password,
    database: db.dbname,
  });
  await client.connect();

  try {
    const result = await client.queryObject(
      `SELECT id, name, email, role, status, created_at
       FROM users
       WHERE (\$1 = '' OR name ILIKE '%' || \$1 || '%' OR email ILIKE '%' || \$1 || '%')
       ORDER BY created_at DESC
       LIMIT \$2`,
      [search, limit]
    );
    return result.rows;
  } finally {
    await client.end();
  }
}

// f/scripts/update_user_status

type Postgresql = {
  host: string;
  port: number;
  user: string;
  password: string;
  dbname: string;
};

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

export async function main(
  db: Postgresql,
  user_id: number,
  new_status: "active" | "suspended" | "archived"
): Promise<string> {
  const client = new Client({
    hostname: db.host,
    port: db.port,
    user: db.user,
    password: db.password,
    database: db.dbname,
  });
  await client.connect();

  try {
    await client.queryObject(
      `UPDATE users SET status = \$1, updated_at = NOW() WHERE id = \$2`,
      [new_status, user_id]
    );
    return `User ${user_id} status updated to ${new_status}`;
  } finally {
    await client.end();
  }
}

Step 2: App Layout

The app definition (simplified JSON):

{
  "grid": [
    {
      "id": "search_bar",
      "type": "textinputcomponent",
      "config": {
        "placeholder": "Search users by name or email...",
        "defaultValue": ""
      },
      "position": { "x": 0, "y": 0, "w": 8, "h": 1 }
    },
    {
      "id": "search_button",
      "type": "buttoncomponent",
      "config": {
        "label": "Search",
        "color": "blue",
        "onClickAction": {
          "type": "runnableByPath",
          "path": "f/scripts/list_users",
          "inputTransforms": {
            "db": { "type": "resource", "value": "f/resources/main_db" },
            "search": { "type": "eval", "expr": "search_bar.result" }
          }
        }
      },
      "position": { "x": 8, "y": 0, "w": 4, "h": 1 }
    },
    {
      "id": "users_table",
      "type": "tablecomponent",
      "config": {
        "dataSource": {
          "type": "runnableByPath",
          "path": "f/scripts/list_users",
          "runOnAppLoad": true,
          "inputTransforms": {
            "db": { "type": "resource", "value": "f/resources/main_db" },
            "search": { "type": "static", "value": "" }
          }
        },
        "columns": [
          { "key": "id", "header": "ID", "width": 60 },
          { "key": "name", "header": "Name" },
          { "key": "email", "header": "Email" },
          { "key": "role", "header": "Role", "width": 100 },
          { "key": "status", "header": "Status", "width": 100 }
        ],
        "selectableRows": true
      },
      "position": { "x": 0, "y": 1, "w": 12, "h": 6 }
    },
    {
      "id": "detail_panel",
      "type": "containercomponent",
      "config": {
        "title": "User Details"
      },
      "subgrid": [
        {
          "id": "user_name_display",
          "type": "textcomponent",
          "config": {
            "content": {
              "type": "eval",
              "expr": "'Selected: ' + (users_table.selectedRow?.name || 'None')"
            }
          }
        },
        {
          "id": "status_select",
          "type": "selectcomponent",
          "config": {
            "items": ["active", "suspended", "archived"],
            "defaultValue": {
              "type": "eval",
              "expr": "users_table.selectedRow?.status"
            }
          }
        },
        {
          "id": "update_button",
          "type": "buttoncomponent",
          "config": {
            "label": "Update Status",
            "color": "green",
            "onClickAction": {
              "type": "runnableByPath",
              "path": "f/scripts/update_user_status",
              "inputTransforms": {
                "db": { "type": "resource", "value": "f/resources/main_db" },
                "user_id": {
                  "type": "eval",
                  "expr": "users_table.selectedRow?.id"
                },
                "new_status": {
                  "type": "eval",
                  "expr": "status_select.result"
                }
              },
              "recomputeOnSuccess": ["users_table"]
            }
          }
        }
      ],
      "position": { "x": 0, "y": 7, "w": 12, "h": 4 }
    }
  ]
}

Step 3: Reactive Wiring

flowchart LR
    SI[Search Input] -->|"value"| SB[Search Button]
    SB -->|"run script"| LS[list_users Script]
    LS -->|"result"| TBL[Users Table]
    TBL -->|"selectedRow"| DP[Detail Panel]
    DP -->|"status_select + user_id"| UB[Update Button]
    UB -->|"run script"| US[update_user_status]
    US -->|"recompute"| TBL

    classDef component fill:#e1f5fe,stroke:#01579b
    classDef script fill:#e8f5e8,stroke:#1b5e20

    class SI,SB,TBL,DP,UB component
    class LS,US script

The data flow is:

1. On app load, list_users runs and populates the table
2. User types in search bar and clicks Search -- table refreshes
3. User clicks a table row -- detail panel updates with selected row data
4. User changes status and clicks Update -- script runs, then table recomputes

Background Runnables

Apps can have background runnables that run on load or on a timer:

{
  "backgroundRunnables": [
    {
      "id": "bg_stats",
      "type": "runnableByPath",
      "path": "f/scripts/get_dashboard_stats",
      "runOnAppLoad": true,
      "autoRefreshSeconds": 30,
      "inputTransforms": {
        "db": { "type": "resource", "value": "f/resources/main_db" }
      }
    }
  ]
}

Components can reference background runnable results:

// In a Text component's content expression:
`Active Users: ${bg_stats.result.active_count} | Total: ${bg_stats.result.total_count}`

Charts and Visualizations

Bar Chart from Script Data

# f/scripts/get_monthly_signups

def main(db: dict, months: int = 12) -> list:
    """Get monthly signup counts for charting."""
    import psycopg2

    conn = psycopg2.connect(
        host=db["host"], port=db["port"],
        user=db["user"], password=db["password"],
        dbname=db["dbname"]
    )
    cur = conn.cursor()
    cur.execute("""
        SELECT
            TO_CHAR(created_at, 'YYYY-MM') as month,
            COUNT(*) as signups
        FROM users
        WHERE created_at >= NOW() - INTERVAL '%s months'
        GROUP BY 1
        ORDER BY 1
    """, (months,))

    results = [{"month": r[0], "signups": r[1]} for r in cur.fetchall()]
    conn.close()
    return results

In the App Builder, add a Bar Chart component and set:

• Data source: f/scripts/get_monthly_signups
• X-axis: month
• Y-axis: signups

Styling and Theming

Apps support CSS customization per component:

{
  "id": "header_text",
  "type": "textcomponent",
  "config": {
    "content": "User Management Dashboard",
    "style": {
      "fontSize": "24px",
      "fontWeight": "bold",
      "color": "#1a1a2e",
      "padding": "16px 0"
    }
  }
}

Global CSS can be added in the app settings for consistent styling across all components.

Publishing and Permissions

Action	Description
Preview	Test the app in the editor
Publish	Make the app available at its URL
Share	Set permissions per user/group/folder
Public	Make accessible without authentication

Published apps are accessible at: http://localhost:8000/apps/get/f/apps/user_dashboard

Source Code Walkthrough

App frontend — frontend/src/lib/components/apps/

The App Builder UI components live in frontend/src/lib/components/apps/. This directory contains the drag-and-drop canvas, individual component renderers (Button, Table, Chart, etc.), and the component configuration panels. The Svelte components show exactly how apps are serialized and rendered.

App runtime — backend/windmill-api/src/apps.rs

backend/windmill-api/src/apps.rs handles app publication, versioning, and the policy system that controls what script paths an app can call. This governs the permissions model for published internal tools.

What You Learned

In this chapter you:

1. Built an interactive dashboard with table, form, and chart components
2. Wired components to backend scripts with input transforms
3. Implemented reactive data flow (select row, update, recompute)
4. Added background runnables for auto-refreshing data
5. Styled and published the app

The key insight: Windmill apps are thin reactive UIs over your scripts. The frontend is declarative configuration; the backend is the same scripts you already wrote. No React, no bundler, no deployment pipeline -- just wire components to functions.

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Next: Chapter 6: Scheduling & Triggers -- automate script and flow execution with cron schedules, webhooks, and event triggers.

Back to Tutorial Index | Previous: Chapter 4 | Next: Chapter 6

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