CREATE CONSTANT

Synopsis

[ /** documentation */ ]
CREATE [ OR MODIFY ] CONSTANT module.name [ FOLDER 'path' ] (
    Type: data_type,
    DefaultValue: value
    [ , ExposedToClient: true ]
)

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 properties are a ( Key: value ) list, with the names Studio Pro gives them. Type is the constant’s data type. Supported types include String, Integer, Long, Decimal, Boolean, and DateTime.

DefaultValue is the value used at runtime unless overridden by environment configuration. Type and DefaultValue are required. ExposedToClient: true makes the value available to nanoflows and client-side code; it defaults to false.

The clause form TYPE data_type DEFAULT value [ EXPOSED TO CLIENT ] is the deprecated alias of the property list: it still parses and warns MDL-DEPR136, and mxcli fmt --upgrade rewrites it.

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.

FOLDER 'path' goes right after the name, as on every document. A FOLDER after the properties is the old position: it still parses and warns MDL-DEPR134, and mxcli fmt --upgrade moves it.

There is no PRIVATE modifier. Older scripts wrote one after the value; it was never stored and protected nothing. Without the mdl 1; header it still parses, does nothing and warns MDL-DEPR138, and mxcli fmt --upgrade deletes it; under mdl 1; it is an error. To keep a secret out of the model, leave DefaultValue empty and set the value per machine with mxcli constant set Module.Name <value>.

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.
Type: data_type: The constant’s data type. One of: String, Integer, Long, Decimal, Boolean, DateTime.
DefaultValue: value: The default value for the constant. String values are single-quoted. Numeric values are bare. Boolean values are true or false.
ExposedToClient: true | false: Whether client-side code can read the value. Optional; false when left out.

Examples

String constant for API URL

CREATE CONSTANT MyModule.ApiBaseUrl ( Type: String, DefaultValue: 'https://api.example.com' );

Integer constant for configuration

CREATE CONSTANT MyModule.MaxRetries ( Type: Integer, DefaultValue: 3 );

Boolean feature flag

CREATE CONSTANT MyModule.EnableLogging ( Type: Boolean, DefaultValue: true );

Idempotent with OR MODIFY

CREATE OR MODIFY CONSTANT MyModule.ApiBaseUrl ( Type: String, DefaultValue: '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, DefaultValue: '' );

Override constant per configuration

mdl 1;
-- Create the constant
CREATE CONSTANT MyModule.ApiBaseUrl ( Type: String, DefaultValue: '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
`LIST 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