What is MDL?

MDL (Mendix Definition Language) is a text-based, SQL-like language for describing and manipulating Mendix application models. It provides a human-readable representation of Mendix project elements – entities, microflows, pages, security rules, and more – at the same abstraction level as the visual models in Studio Pro.

Why a Text-Based Language?

Mendix projects are stored in a binary format (.mpr files containing BSON documents in SQLite). While Studio Pro provides a visual interface, there is no standard text format for Mendix models. This creates problems for:

• AI assistants that need to read and generate model elements
• Code review of changes to Mendix projects
• Scripting and automation of repetitive modeling tasks
• Documentation of project structure and design patterns

MDL addresses these problems by providing a syntax that is both human-readable and machine-parseable.

Characteristics of MDL

• SQL-like syntax – uses familiar keywords like CREATE, ALTER, DROP, SHOW, DESCRIBE, GRANT.
• Declarative – you describe what the model should look like, not how to construct it step by step.
• Complete – covers domain models, microflows, pages, security, navigation, workflows, and business events.
• Validated – mxcli can check MDL scripts for syntax errors and reference validity before applying them.

Token Efficiency

When working with AI coding assistants, context window size is a critical constraint. MDL’s compact syntax uses significantly fewer tokens than equivalent JSON model representations:

Fewer tokens means lower API costs, more application context fits in a single prompt, and AI assistants produce more accurate output because there is less noise in their input.

Example: JSON vs MDL

A simple entity definition illustrates the difference.

JSON representation (~180 tokens):

{
  "entity": {
    "name": "Customer",
    "documentation": "Customer master data",
    "attributes": [
      {"name": "Name", "type": {"type": "String", "length": 200}},
      {"name": "Email", "type": {"type": "String", "length": 200}}
    ]
  }
}

MDL representation (~35 tokens):

/** Customer master data */
CREATE PERSISTENT ENTITY Sales.Customer (
    Name: String(200),
    Email: String(200)
);

The MDL version conveys the same information in roughly one-fifth the space, and its structure is immediately recognizable to anyone familiar with SQL DDL.

What MDL Covers

MDL provides statements for the following areas of a Mendix project:

See Part II (The MDL Language) for a complete guide, or Part VI (MDL Statement Reference) for detailed syntax of each statement.