Getting Started on APPUiO Cloud with the Terminal using C

This tutorial explains how to run applications written in the C programming language on APPUiO Cloud using the command line.

If you aren’t familiar with issuing commands on a terminal session, you might want to try the Getting Started with C on the OpenShift Web Console guide instead.

Requirements

To follow this guide, please make sure that you have the following tools installed:

oc: You can download the OpenShift command directly from APPUiO Cloud, selecting the help menu (marked as a question mark) and selecting the "Command line tools" entry.
docker or podman: You can download them from www.docker.com and podman.io.
curl: Available from curl.se.

About the Application

To demo how to run applications written in the C programming language on APPUiO Cloud, we will use a small demo application using the GNU libmicrohttpd library, bundled as a container thanks to its corresponding Dockerfile, and ready to be used on APPUiO Cloud.

You can browse the full source code of this application on GitLab.

The "Fortune in C" application does a few simple things:

When invoked on the browser, it returns a random number and a funny quote.
When invoked with an HTTP request including the Accept: application/json header, it returns the same information in JSON format.
Finally, when invoked with an HTTP request including the Accept: text/plain header, it returns the same information in plain text format.

Learn more about the fortune-c application, including how to edit and build it, on the project README.

The Application Router

The code below shows the main router of the application in the C programming language, located at src/main.c, courtesy of the GNU libmicrohttpd library.

int handler(void *cls, struct MHD_Connection *connection,
                         const char *url,
                         const char *method, const char *version,
                         const char *upload_data,
                         size_t *upload_data_size, void **con_cls)
{
    char fortune[100000] = { "" };
    int result = execute_command(fortune, "fortune");
    char hostname[100] = { "" };
    result = execute_command(hostname, "hostname");
    int random = rand() % 1000;

    char page[200000] = { "" };
    const char *accept_header = MHD_lookup_connection_value (connection, MHD_HEADER_KIND, "Accept");
    if (strcmp(accept_header, "application/json") == 0) {
        result = respond_json(page, fortune, hostname, random);
    }
    else if (strcmp(accept_header, "text/plain") == 0) {
        result = respond_text(page, fortune, random);
    }
    else {
        result = respond_html(page, fortune, hostname, random);
    }
    struct MHD_Response *response;
    int ret;

    response = MHD_create_response_from_buffer(strlen(page),
                                               (void *)page, MHD_RESPMEM_PERSISTENT);
    ret = MHD_queue_response(connection, MHD_HTTP_OK, response);
    MHD_destroy_response(response);

    return ret;
}

Run the Container

This step is optional; you can easily test the application locally with Docker:

docker run --rm --publish 8080:8080 registry.gitlab.com/vshn/applications/fortune-c:latest

Or with Podman:

podman run --rm --publish 8080:8080 registry.gitlab.com/vshn/applications/fortune-c:latest

Now you can test the application with curl, for example to get some JSON:

curl http://localhost:8080 --header "Accept: application/json"

Or to see a plain text version:

curl http://localhost:8080 --header "Accept: text/plain"

You can also open a browser and navigate to localhost:8080 to get your fortune cookie of the day.

Dockerfile

The application includes a Dockerfile ready to build a container fully compatible with OpenShift. The snippet below shows the instructions used to build the actual running image:

# Step 2: Production image
FROM busybox:glibc
COPY --from=builder /builder/build/c-fortune /usr/local/bin/c-fortune
COPY --from=builder /usr/local/lib/libjson-c.so.5 /usr/local/lib/libjson-c.so.5
COPY --from=builder /usr/games/fortune /usr/local/bin/fortune
COPY --from=builder /usr/share/games/fortunes /usr/share/games/fortunes
COPY --from=builder /usr/lib/x86_64-linux-gnu/libmicrohttpd.so.12 /usr/lib/x86_64-linux-gnu/libmicrohttpd.so.12
COPY --from=builder /usr/lib/x86_64-linux-gnu/libgnutls.so.30 /usr/lib/x86_64-linux-gnu/libgnutls.so.30
COPY --from=builder /usr/lib/x86_64-linux-gnu/libp11-kit.so.0 /usr/lib/x86_64-linux-gnu/libp11-kit.so.0
COPY --from=builder /usr/lib/x86_64-linux-gnu/libidn2.so.0 /usr/lib/x86_64-linux-gnu/libidn2.so.0
COPY --from=builder /usr/lib/x86_64-linux-gnu/libunistring.so.2 /usr/lib/x86_64-linux-gnu/libunistring.so.2
COPY --from=builder /usr/lib/x86_64-linux-gnu/libtasn1.so.6 /usr/lib/x86_64-linux-gnu/libtasn1.so.6
COPY --from=builder /usr/lib/x86_64-linux-gnu/libnettle.so.8 /usr/lib/x86_64-linux-gnu/libnettle.so.8
COPY --from=builder /usr/lib/x86_64-linux-gnu/libhogweed.so.6 /usr/lib/x86_64-linux-gnu/libhogweed.so.6
COPY --from=builder /usr/lib/x86_64-linux-gnu/libgmp.so.10 /usr/lib/x86_64-linux-gnu/libgmp.so.10
COPY --from=builder /usr/lib/x86_64-linux-gnu/libffi.so.7 /usr/lib/x86_64-linux-gnu/libffi.so.7
COPY --from=builder /usr/lib/x86_64-linux-gnu/librecode.so.0 /usr/lib/x86_64-linux-gnu/librecode.so.0
COPY --from=builder /lib/x86_64-linux-gnu/libdl.so.2 /lib/x86_64-linux-gnu/libdl.so.2

EXPOSE 8080

(1)
USER 1001:0

CMD ["/usr/local/bin/c-fortune"]

1	This explicitly prevents the container from running as root; this is a requirement of OpenShift, and a good practice for images in general.

You can use the Dockerfile above to build your own copy of the container, which you can then push to the registry of your choice. Clone the repo:

git clone https://gitlab.com/vshn/applications/fortune-c.git

cd into it:

cd fortune-c

And build your image with Docker:

docker build -t fortune-c .

Or with Podman instead:

podman build -t fortune-c .

Step 1: Create a Project

Follow these steps to login to APPUiO Cloud on your terminal:

oc login --server=https://api.${zone}.appuio.cloud:6443

You can find the exact URL of your chosen zone in the APPUiO Cloud Portal.

This command displays a URL on your terminal:

You must obtain an API token by visiting
https://oauth-openshift.apps.${zone}.appuio.cloud/oauth/token/request

Click on the link above and open it in your browser.
Click "Display token" and copy the login command shown as "Log in with this token"

Paste the oc login command on the terminal:

oc login --token=sha256~_xxxxxx_xxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxx-X \
    --server=https://api.${zone}.appuio.cloud:6443

Create a new project called "[YOUR_USERNAME]-fortune-c"
```
oc new-project [YOUR_USERNAME]-fortune-c
```

To deploy the application we will use a standard Kubernetes Deployment object. Save the following YAML in a file called deployment.yaml:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: fortune-c
spec:
  template:
    spec:
      containers:
      - image: registry.gitlab.com/vshn/applications/fortune-c:latest
        imagePullPolicy: Always
        name: fortune-container
        ports:
        - containerPort: 8080
    metadata:
      labels:
        app: fortune-c
  selector:
    matchLabels:
      app: fortune-c
  strategy:
    type: Recreate
---
apiVersion: v1
kind: Service
metadata:
  name: fortune-c
spec:
  ports:
    - port: 8080
      targetPort: 8080
  selector:
    app: fortune-c
  type: ClusterIP

Then apply the deployment to your APPUiO Cloud project:
```
oc apply -f deployment.yaml
```
And wait until your pod appears with the status "Running":
```
oc get pods --watch
```

Step 2: Publish your Application

At the moment your container is running but it’s not available from the Internet. To be able to access our application, we must create an Ingress object.

Create another file called ingress.yaml with the following contents, customizing the parts marked as [YOUR_USERNAME] and [YOUR_CHOSEN_ZONE] to your liking (and according to the Zones documentation page):

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    cert-manager.io/cluster-issuer: letsencrypt-production
  name: fortune-c-ingress
spec:
  rules:
  - host: [YOUR_USERNAME]-fortune-c.apps.[YOUR_CHOSEN_ZONE].appuio.cloud (1)
    http:
      paths:
      - pathType: Prefix
        path: /
        backend:
          service:
            name: fortune-c
            port:
              number: 8080
  tls:
  - hosts:
    - [YOUR_USERNAME]-fortune-c.apps.[YOUR_CHOSEN_ZONE].appuio.cloud
    secretName: fortune-c-cert

1	Replace the placeholders `YOUR_USERNAME` and `YOUR_CHOSEN_ZONE` with valid values.

About URL lengths

Make sure that the total length of the prefix string [YOUR_USERNAME]-fortune-[LANGUAGE] used in the Ingress YAML above isn’t longer than 63 characters. This is due for two reasons:

As per the DNS RFC, each label (for example the prefix in this case) must be equal or less than 63 characters long (octets to be exact), and the whole domain name must be equal or shorter than 255 octets.
These limits also apply to most Kubernetes resource names.

If your Ingress doesn’t generate a route after deploying your application, shorten the URL in your YAML and redeploy.

Apply the ingress object to your APPUiO Cloud project and wait until you route shows as available.
```
oc apply -f ingress.yaml
```
And wait for your route to be ready:
```
oc get routes --watch
```

After a few seconds, you should be able to get your daily fortune message using curl in plain text!

curl https://[YOUR_USERNAME]-fortune-c.apps.[YOUR_CHOSEN_ZONE].appuio.cloud --header "Accept: text/plain"

Or in JSON instead:

curl https://[YOUR_USERNAME]-fortune-c.apps.[YOUR_CHOSEN_ZONE].appuio.cloud --header "Accept: application/json"

Step 3: There’s no Step 3!

The "Fortune in C" application is now running on APPUiO Cloud. Congratulations! Hit the R key in your keyboard to see a new fortune message, or just wait 10 seconds to get a new one automatically.

What’s next? To run your own application written in C or using the GNU libmicrohttpd library on APPUiO Cloud, follow these steps:

Containerize the application making sure it’s compatible with APPUiO Cloud. The Dockerfile above can serve as a starting point.
Enhance the deployment for your application with liveness and health probes, or better yet, create a Helm chart.
Configure your CI/CD system to automatically deploy your application to your preferred APPUiO Cloud zone.

Finally, when you’re done testing the fortune application, delete the fortune-c project with the following command:

oc delete project [YOUR_USERNAME]-fortune-c