CREATE CONSTANT

Synopsis

CREATE [ OR MODIFY ] CONSTANT module.name TYPE data_type [ DEFAULT value ]

CREATE CONSTANT defines a named constant in a module. Constants hold configuration values (API URLs, feature flags, limits) that can differ between environments. In Mendix, constant values can be overridden per runtime configuration without changing the model.

The TYPE clause specifies the constant’s data type. Supported types include String, Integer, Long, Decimal, Boolean, and DateTime.

The optional DEFAULT clause sets the constant’s default value. This is the value used at runtime unless overridden by environment configuration.

If OR MODIFY is specified, the statement is idempotent. If the constant already exists, its type and default value are updated.

Constant values can also be overridden per deployment configuration using ALTER SETTINGS CONSTANT.

Parameters

OR MODIFY: Makes the statement idempotent. If the constant already exists, its definition is updated. Without this clause, creating a duplicate constant is an error.
module.name: The qualified name of the constant in the form Module.ConstantName. The module must already exist.
data_type: The constant’s data type. One of: String, Integer, Long, Decimal, Boolean, DateTime.
DEFAULT value: The default value for the constant. String values are single-quoted. Numeric values are bare. Boolean values are true or false.

Examples

String constant for API URL

CREATE CONSTANT MyModule.ApiBaseUrl TYPE String DEFAULT 'https://api.example.com';

Integer constant for configuration

CREATE CONSTANT MyModule.MaxRetries TYPE Integer DEFAULT 3;

Boolean feature flag

CREATE CONSTANT MyModule.EnableLogging TYPE Boolean DEFAULT true;

Idempotent with OR MODIFY

CREATE OR MODIFY CONSTANT MyModule.ApiBaseUrl TYPE String DEFAULT 'https://api.example.com/v2';

Constant with an empty default

A DEFAULT is always required. For secrets, use an empty default and set the real value per configuration (see below):

CREATE CONSTANT MyModule.DatabasePassword TYPE String DEFAULT '';

Override constant per configuration

-- Create the constant
CREATE CONSTANT MyModule.ApiBaseUrl TYPE String DEFAULT 'https://api.example.com';

-- Override in a specific runtime configuration
ALTER SETTINGS CONSTANT 'MyModule.ApiBaseUrl' VALUE 'https://staging.example.com' IN CONFIGURATION 'Staging';

Shared and private values

An override holds its value one of two ways:

Shared — stored in the model, so it travels with the project in version control and every developer gets it.
Private — stored on the developer’s own workstation and deliberately kept out of the repository. This is the answer for a development secret: the constant and the override are shared, the value is not.

MDL preserves that choice but never changes it — the shared/private decision belongs to the constant, and configurations just respect it:

Statement	On a private override
`ALTER SETTINGS CONSTANT … VALUE …`	refused — setting a value would convert it to shared and publish a deliberately-local value into version control
`ALTER SETTINGS DROP CONSTANT …`	allowed — removes the whole override, which is what was asked for
`SHOW CONSTANT VALUES`	reports `(private)` rather than a blank cell
`DESCRIBE SETTINGS`	emits a comment, not a re-executable statement

To make a private value shared (or the reverse), change it in Studio Pro.

mxcli Documentation