- CasaOS specific metadata, also called *store info*, are stored under [extension](https://docs.docker.com/compose/compose-file/#extension) property `x-casaos` at two positions.
1. Service level
A `docker-compose.yml` file can contain one or more `services`. Each [service](https://docs.docker.com/compose/compose-file/#services-top-level-element) can have its own store info.
For the same example, at the buttom of the `syncthing` service in the [`docker-compose.yml` of Syncthing](Apps/Syncthing/docker-compose.yml)
en_us: Syncthing is a continuous file synchronization program. It synchronizes files between two or more computers in real time, safely protected from prying eyes. Your data is your data alone and you deserve to choose where it is stored, whether it is shared with some third party, and how it's transmitted over the internet.
**Note**: The features is only working in casaos 0.4.4 and newer version.
For resolves some cases. Casaos provide some magic value to power your application:
- Any environment variable that user to set
- $WEBUI_PORT
##### Environment variable
> for developer
your application can read environment variable that user set, such as `OPENAI_API_KEY` from environment variable. It is store in `/etc/casaos/env`. User can set only once and using anywhere. It can be change by api, after change, all application will re up to inject new env var.
**Note**: change the config didn't change the env var of current container. To set env var, you should use cli to set it.
#### WEBUI_PORT
> for application maintainer
your `docker-compose.yml` can use `WEBUI_PORT` to set webui port. Casaos will assign a available port for your application. You can use it like this:
```yaml
...
ports:
- target: 5230
published: ${WEBUI_PORT}
protocol: tcp
...
x-casaos:
architectures:
- amd64
- arm64
- arm
...
port_map: ${WEBUI_PORT}
```
or
```yaml
...
ports:
- target: 5230
published: ${WEBUI_PORT:-5230}
protocol: tcp
...
x-casaos:
architectures:
- amd64
- arm64
- arm
...
port_map: ${WEBUI_PORT:-5230}
```
**Note**: the WEBUI_PORT only allocated once. It promise the port is available when allocated. If the port be used by other application. It didn't reallocate a new port.
Once in a while, we pick certain apps as featured apps and display them at the AppStore front. The standard for apps to be featured is a bit higher than rest of the apps:
