Coming from the neo environment in SAP Cloud Platform, a so-called destination enables proxying HTTP requests at runtime. This not only fullfilled CORS requirements in a browser-based scenario, but also allowed querying data in a SAPCP-application from an on-premise system.
Even though the name destination is the same in the SAP CP Cloud Foundry (cf) environment, it’s usage is completely different. Let’s elaborate.

Concept

destinations are of substantical value in hybrid application architectures, where a cloud application wants to retrieve data from an on-premise system via the Cloud Connector.

In neo, calling

https://myapp-4711.dispatcher.hana.ondemand.com/⮐
destinations/<destination-name>/path/to/resource

would proxy /path/to/resource on the server configured in <destination-name> (can be an on-premise system!) to the application running on https://myapp-4711.dispatcher.hana.ondemand.com – a really convenient way to proxy requests fullfilling CORS requirements.

Achieving this in cf, it’s necessary to

• create a destination (sub-account level)
• create a destination instance (space level)
• create a connectivity instance (space level)
• create a xsuaa instance (space level)

Then, the connectivity instance serves as a generic reverse proxy, where as the destination instance is nothing more but a catalogue organizing proxy targets (including on-premise systems).

The xsuaa instance serves as the credentials authority, issuing JWT (via `OAuth 2.0) to allow

• querying the destination instance for an entry
• using the reverse proxy (aka connectivity instance) for actually proxying requests

So in cf:

1. retrieve token from xsuaa instance for using the destination instance
2. use token for querying a specific destination from destination instance
3. retrieve token from xsuaa instance for using the reverse proxy (aka connectivity instance)
4. use token for querying some <uri> in the destination

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Programmatic usage (nodejs)

There are 2 main blogs/tutorials out there that describe how to handle using destinations/connectivity in nodejs:

• https://blogs.sap.com/2018/10/08/using-the-destination-service-in-the-cloud-foundry-environment/
  gives a good overview of the entire process, but leaves out using the actual proxy for querying resources
• https://blogs.sap.com/2018/10/16/call-sap-cloud-platform-cloud-foundry-destinations-from-your-node.js-application
  explains the concept of command line cf along with services, bindings and instances nicely, but also doesn’t mention any proxying and is difficult to re-use

Spoiler alert That’s why I wrapped the entire process in a node module, sap-cf-destination (super alpha pre-release, yada yada).

I took the liberty and read throught the reference Java implementation at https://help.sap.com/viewer/cca91383641e40ffbe03bdc78f00f681/Cloud/en-US/313b215066a8400db461b311e01bd99b.html and ported it to a reusable node module in JavaScript.

Overall usage (all following code is flavored pidgin-code):

const callDestination = require('sap-cf-destination');

callDestination({
        // url to call on proxied server
        url: '/api/json',
        // name of the CF connectivity service instance
        connectivity_instance: 'connectivity-lite',
        // name of the CF xsuaa service instance
        uaa_instance: 'uaa-lite',
        // name of the CF destination service instance
        destination_instance: 'destination-lite',
        // name of the configured destination
        destination_name: 'my_destination'
    })
        .then(response => {
            // do sth clever from the response
            // of $server_behind_destination_'my_destination'/api/json
        })
        .catch(err => {
            // oh no ?
        })

Under the hood, an OAuth 2.0 client is used for retrieving tokens from both connectivity- and destination-instance, wrapped in a Promise:

new Promise((resolve, reject) => {
    const oAuthClient = new OAuth2(
        <either destination or connectivity client id>,
        <either destination or connectivity client secret>, 
        <url of cf xsuaa instance>, 
        '/oauth/authorize', // authorization endpoint
        'oauth/token', // token issuer endpoint
        null // no custom headers
    );
    oAuthClient.getOAuthAccessToken(
        '', 
        {grant_type: 'client_credentials'},
        (err, accessToken, refreshToken, results) => {
            if (err) {
                reject(err);
            }
            resolve(accessToken);
        }
    );
});

Utilizing the cf proxy is done via the npm request-promise http client module:

const proxy = `http://${connectivityInstance.credentials.onpremise_proxy_host}:
${connectivityInstance.credentials.onpremise_proxy_port}`;

const options = {
        url: <destination api/destination>,
        method: 'GET',
        headers: <proxy access token>,
        proxy: proxy
    };

request(options);

Code is over at https://github.com/vobu/sap-cf-destination.git – collaboration welcome!

Granted, it has the “Geschmäckle” of “why doesn’t SAP provide this anyway?!?” – but hey, @SAPCP, you never know who will contribute, right. Right?!?

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