> 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/app-builder/build-backend/functions/extensions/custom-extensions.md).
# Custom Extensions
Custom Extensions let you extend the Heisenware platform to your specific needs. We provide a project setup into which you add your custom functionality in a completely non-intrusive fashion. Built on our [VRPC](https://github.com/heisenware/heisenware-docs/tree/master/advanced/vrpc.md) library, you write plain Node.js code (no APIs to learn) and make it ready for visual programming in minutes.
{% hint style="info" %}
We are actively working on the same idea for C++ and Python.
{% endhint %}
The best starting point is our [docker-extension-starter-js](https://github.com/heisenware/heisenware-docker-extension-starter-js). We recommend [downloading this project](https://github.com/heisenware/heisenware-docker-extension-starter-js/archive/refs/heads/master.zip) as a scaffold, changing it to your needs, and placing it under your version control.
You end up creating a Docker image whose containers integrate into the platform in one of two ways.
## Running inside the platform
Once your Docker image is built, pushed, and publicly accessible ([contact us](mailto:support@heisenware.com) for private registry support), load it as a Custom Extension.
Once installed, and given your code is syntactically correct, it immediately appears in the [Function Explorer](/app-builder/build-backend/functions/function-explorer.md). To apply a new version, install it again (works even with the same label).
{% hint style="info" %}
Any instances you create are automatically persisted and restarted. You find them in the [File Explorer](/app-builder/build-backend/file-explorer.md) under `extensions/my-extension/...`
{% endhint %}
## Running outside the platform
This lets you run your custom code on-premises while we bridge it automatically, seamlessly, and securely into the cloud. Start a container of your image locally and configure it with the correct credentials using environment variables:
```bash
docker run -it \
-e HW_DOMAIN=. \
-e HW_BROKER=mqtts://.heisenware.cloud \
-e HW_USERNAME= \
-e HW_PASSWORD= \
myusername/myimage:1.0.0
```
To retrieve a valid username and password, add a VRPC integration under [Integrations (inbound)](/app-manager/inbound-integrations.md) in the App Manager.
Example: For an account named `my-company`, an integration with username `agentRunner`, and a password `secret`, the call would be:
```bash
docker run -it \
-e HW_DOMAIN=my-company.default \
-e HW_BROKER=mqtts://my-company.heisenware.cloud \
-e HW_USERNAME=agentRunner \
-e HW_PASSWORD=secret \
myusername/myimage:1.0.0
```
When everything is set up correctly, you should see something like this on your console:
---
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