> For the complete documentation index, see [llms.txt](https://docs.heisenware.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.heisenware.com/~/changes/eYCCpn9cCBKOYnZA6a8f/building-apps/data-integration/calling-custom-code.md).

# Calling custom code

By calling custom code, you can accomplish various use cases, including:

* Integration of legacy systems lacking other interfaces
* IoT integration for embedded devices/microcontrollers
* Integration of software libraries to introduce new functionality

{% hint style="info" %}
This advanced feature requires programming knowledge. If uncertain about its applicability or how to use it, please [talk to us](mailto:support@heisenware.com), and we'll be happy to assist you.
{% endhint %}

## VRPC

VRPC (Variadic Remote Procedure Calls) is the open-source technology that enables calling custom code in Heisenware. Developed and maintained by the Heisenware team, VRPC seamlessly integrates into the Heisenware platform and apps developed with it. It allows direct calls between functions written in any programming language. Functions integrated with VRPC appear as custom functions in the App Builder's functions panel.

To make custom functions available in the App Builder, you need to implement an adapter, sometimes referred to as code adapter, within the code to be integrated. The adapter wraps around existing functions non-intrusively and standardizes their interface. Refer to the [VRPC developer documentation](https://vrpc.io/docs) for details on implementing an adapter and calling custom code. 

Adapters are available for: 

* C++
* Node.js
* R
* React
* Arduino

Adapters for Python and other programming languages can be provided upon request.


---

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