How to declare a global variable in mendix?

I want to declare a variable whose value can be displayed anywhere in app (on any page) and can be modified from any micro flow. how can we do that??

As all mutable values in mendix are represented by attributes in an entity, you need to create an entity in order to be able to modify a value. The closest thing to a global variable in Mendix is an attribute on a singleton entity.

Let's suppose we want to be able to change the some settings of your app through its UI or within a microflow. To do this we can create an 'AppSettings' entity with attributes for all the different "global variables" that need to be set.

To make it a singleton entity we need to make sure that there is only one object of its kind in the database. To do this it's a common practice to implement a 'GetOrCreate' microflow that retrieves the 'AppConfiguration' object from the database and creates one if there is none yet.

We can now use 'GetOrCreateAppConfiguration' anywhere, where we need to read or modify our app settings, such as a microflow.

Using'GetOrCreateAppConfiguration' we could also create and settings page, where admins can modify the AppConfiguration attributes using a DataView with a Microflow retrieve.

We can also use a dataview to display the AppName "global variable" to users and use conditional visibility based on feature flag "global variables" to show or hide UI elements. Note that this means that we should probably not to give regular users write access to 'AppConfiguration' attributes.