This is different from using an `APIRouter` as a mounted application is completely independent. The OpenAPI and docs from your main application won't include anything from the mounted application, etc.

You can read more about this in the [Advanced User Guide](../advanced/index.md){.internal-link target=_blank}.

## Details

This sub-application is just another standard FastAPI application, but this is the one that will be "mounted":

```Python hl_lines="11  14-16"
{!../../docs_src/sub_applications/tutorial001.py!}
```

### Mount the sub-application

In your top-level application, `app`, mount the sub-application, `subapi`.

In this case, it will be mounted at the path `/subapi`:

```Python hl_lines="11  19"

// under the procfs, /proc/<pid>/ns/net. In majority of cases, this is not used directly, but is rather bind mounted to
// /var/run/netns/<name>. However, this pattern is not ubiquitous. Some platforms bind mount to other places. As we run
// in a pod, we cannot just access any arbitrary file they happen to bind mount in, as we don't know ahead of time where
// it might be.
//
// Instead, we rely directly on the procfs.

# This bazelrc can build a CPU-supporting TF package.

# Convenient cache configurations
# Use a cache directory mounted to /tf/cache. Very useful!
build:sigbuild_local_cache --disk_cache=/tf/cache
# Use the public-access TF DevInfra cache (read only)
build:sigbuild_remote_cache --remote_cache="https://storage.googleapis.com/tensorflow-devinfra-bazel-cache/manylinux2014" --remote_upload_local_results=false

# This bazelrc can build a CPU-supporting TF package.

# Convenient cache configurations
# Use a cache directory mounted to /tf/cache. Very useful!
build:sigbuild_local_cache --disk_cache=/tf/cache
# Use the public-access TF DevInfra cache (read only)
build:sigbuild_remote_cache --remote_cache="https://storage.googleapis.com/tensorflow-devinfra-bazel-cache/manylinux2014" --remote_upload_local_results=false

)

// Exposed for testing "constants"
var (
	CNIBinDir     = "/opt/cni/bin"
	HostCNIBinDir = "/host/opt/cni/bin"
	// Well-known subpath we will mount any needed host-mounts under,
	// to preclude shadowing or breaking any pod-internal mounts
	HostMountsPath = "/host"

```html
<a href="/items/42">
```

## Templates and static files

You can also use `url_for()` inside of the template, and use it, for example, with the `StaticFiles` you mounted with the `name="static"`.

```jinja hl_lines="4"
{!../../docs_src/templates/templates/item.html!}
```

In this example, it would link to a CSS file at `static/styles.css` with:

```CSS hl_lines="4"

		nspathInContainer := filepath.Join(hostMountsPath, nspath)
		ns, err := OpenNetns(nspathInContainer)
		if err != nil {
			err = fmt.Errorf("failed to open netns: %w. Make sure that the netns host path %s is mounted in under %s in the container", err, nspath, hostMountsPath)
			log.Error(err.Error())
		}
		return ns, err
	}

|OUTPUT_CERT|write all fetched certificates to some directory. Used to support applications that need certificates (Prometheus) as well as rotating mTLS control plane authentication.|
|FILE_MOUNTED_CERTS|completely disable CA path, exclusively use certs mounted into the pod with set certificate file locations|
|CREDENTIAL_FETCHER_TYPE|allows using custom credential fetcher, for VMs with existing identity|

    - periodically copy the K8S JWT token for istio-cni on the host to connect to K8S.
    - injects the CNI plugin config to the CNI config file
        - CNI installer will try to look for the config file under the mounted CNI net dir based on file name extensions (`.conf`, `.conflist`)
        - the file name can be explicitly set by `CNI_CONF_NAME` env var