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    Feature Toggles

    Toggled features are pre-built extensions built by the provider of a SaaS application, which can be switched on selectively per subscriber.

    Introduction and Overview

    CAP feature-toggled aspects allow SaaS providers to create pre-built features as CDS models, extending the base models with new fields, entities, as well as annotations for SAP Fiori UIs. These features can be assigned to individual SaaS customers (tenants), users, and requests and are then activated dynamically at runtime, as illustrated in the following figure.

    feature-toggles.drawio

    Get cloud-cap-samples-java for step-by-step Exercises

    The following steps will extend the CAP samples for Java app to demonstrate how features can extend data models, services, as well as SAP Fiori UIs. If you want to exercise these steps, get cloud-cap-samples-java before, and prepare to extend the Fiori app:

    git clone https://github.com/SAP-samples/cloud-cap-samples-java
    cd cloud-cap-samples-java
    mvn clean install
    

    Now, open the app in your editor, for example, for VS Code type:

    code .
    

    Get cap/samples for Step-By-Step Exercises

    The following steps will extend the cap/samples/fiori app to demonstrate how features can extend data models, services, as well as SAP Fiori UIs. If you want to exercise these steps, get cap/samples before, and prepare to extend the fiori app:

    git clone https://github.com/sap-samples/cloud-cap-samples samples
    cd samples
    npm install
    

    Now, open the fiori app in your editor, for example, by this if you’re using VS Code on MacOS:

    code fiori
    

    Enable Feature Toggles

    Add @sap/cds-mtxs Package Dependency

    For example, like this:

    npm add @sap/cds-mtxs
    

    Switch on cds.requires.toggles

    Switch on feature toggle support by adding this to your package.json, in the fiori app in cap/samples:

    "cds":{
      "requires": {
        "toggles": true
      }
    }
    

    Adding Features in CDS

    Add a subfolder per feature to folder fts and put .cds files into it. The name of the folder is the name you later on use in feature toggles to switch the feature on/off. In our samples app, we add two features isbn and reviews as depicted in the following screenshot:

    image-20220628101642511

    The name of the .cds files within the fts/ subfolders can be freely chosen. All .cds files found in there will be served, with special handling for index.cds files, as usual.

    Feature fts/isbn

    Create a file fiori/fts/isbn/schema.cds with this content:

    using { CatalogService, sap.capire.bookshop.Books }
    from '../../app/browse/fiori-service';
    
    // Add new field `isbn` to Books
    extend Books with {
      isbn : String @title:'ISBN';
    }
    
    // Display that new field in list on Fiori UI
    annotate CatalogService.Books with @(
      UI.LineItem: [... up to {Value:author}, {Value:isbn}, ...]
    );
    

    This feature adds a new field isbn to entity Books and extends corresponding SAP Fiori annotations to display this field in the Browse Books list view.

    Feature fts/reviews

    Create a file fiori/fts/reviews/schema.cds with this content:

    using { CatalogService } from '../../app/browse/fiori-service';
    
    // Display existing field `rating` in list on Fiori UI
    annotate CatalogService.Books with @(
      UI.LineItem: [... up to {Value:author}, {Value:rating}, ...]
    );
    

    This feature extends corresponding SAP Fiori annotations to display already existing field rating in the Browse Books list view.

    Limitations

    Please note the following limitations for .cds files in features:

    • no .cds files in subfolders, e.g. fts/isbn/sub/file.cds
    • no using dependencies between features
    • further limitations re extend aspect → to be documented

    Toggling Features

    In principal features can be toggled per request, per user, or per tenant; most commonly they’ll be toggled per tenant, as demonstrated in the following.

    In Development

    CAP Node.js’ mocked-auth strategy has built-in support for toggling features per tenant, per user, or per request. To demonstrate toggling features per tenant, or user, you can add these lines of configuration to our package.json of the SAP Fiori app:

    {"cds":{
      "requires": {
        "auth": {
          "users": {
            "carol": { "tenant": "t1" },
            "erin":  { "tenant": "t2" },
            "fred":  { "tenant": "t2", "features":[] }
          },
          "tenants": {
            "t1": { "features": ["isbn"] },
            "t2": { "features": "*" }
          }
        }
      }
    }}
    

    CAP Java’s Mock User Authentication with Spring Boot allows to assign feature toggles to users based on the mock user configuration. To demonstrate toggling features per user, you can add these lines to the mock user configuration in the srv/src/main/resources/application.yaml file:

    cds:
      security.mock.users:
        - name: carol
          features:
          - isbn
        - name: erin
          features:
          - isbn
          - reviews
        - name: fred
          features:
    

    In effect of this, for the user carol the feature isbn is enabled, for erin, the features isbn and reviews are enabled, and for the user fred all features are disabled.

    In Production

    No features toggling for production yet — Note that the previous sample is only for demonstration purposes. As user and tenant management is outside of CAP’s scope, there’s no out-of-the-box feature toggles provider for production yet. → Learn more about that in the following section Feature Vector Providers.

    For productive use, the mock user configuration must not be used. The set of active features is determined per request by the Feature Toggles Info Provider.

    You can register a Custom Implementation as a Spring bean that computes the active feature set based on the request’s UserInfo and ParameterInfo.

    Test-Drive Locally

    To test feature toggles, just run your CAP server as usual, then log on with different users, assigned to different tenants, to see the effects.

    Run cds watch

    Start the CAP server with cds watch as usual:

    cds watch
    

    → in the log output, note the line reporting:

    [cds] - serving cds.xt.ModelProviderService {
      path: '/-/cds/model-provider',
      impl: '@sap/cds/srv/model-provider.js'
    }
    

    The ModelProviderService is used by the runtime to get feature-enhanced models.

    See Effects in SAP Fiori UIs

    To see the effects in the UIs open three anonymous browser windows, one for each user to log in, and:

    1. Open SAP Fiori app in browser and go to Browse Books.
    2. Log in as carol and see ISBN column in list.
    3. Log in as erin and see Ratings and ISBN columns in list.
    4. Log in as fred and no features for Fred, even though same tenant as Erin.

    For example the displayed UI should look like that for erin:

    image-20220630132726831

    Model Provider in Sidecar

    The ModelProviderService, which is used for toggling features, is implemented in Node.js only. To use it with CAP Java apps, you run it in a so-called MTX sidecar. For a CAP Node.js project, this service is always run embedded with the main application.

    Create Sidecar as Node.js Project

    An MTX sidecar is a standard, yet minimalistic Node.js CAP project. By default it’s added to a subfolder mtx/sidecar within your main project, containing just a package.json file, as shown in the following screenshot.

    image-20220630134350664

    1. Create a folder named mtx/sidecar.
    2. Add a package.json in there with the following content:
    {
      "name": "mtx-sidecar", "version": "0.0.0",
      "dependencies": {
        "@sap/cds": "^6",
        "@sap/cds-mtxs": "^1",
        "express": "^4"
      },
      "cds": {
        "requires": {
          "cds.xt.ModelProviderService": "in-sidecar"
        },
        "[development]": {
          "requires": { "auth": "dummy" },
          "server": { "port": 4005 }
        }
      }
    }
    

    Learn more about setting up MTX sidecars.

    In Node.js apps you usually don’t consume services from the sidecar. The ModelProviderService is served both, embedded in the main app as well as in the sidecar. The following is documented for the sake of completeness only…

    You can use the from-sidecar preset to tell the CAP runtime to use the remote model provider from the sidecar:

     "cds":{
      "requires": {
        "toggles": true,
        "cds.xt.ModelProviderService": "from-sidecar"
      }
    

    Learn more about configuring ModelProviderService.

    You need to configure the CAP Java application to request the CDS model from the Model Provider Service. This is done in the application.yaml file of your application. To enable the Model Provider Service for local development, add the following configuration to the default profile:

    cds:
      model:
        provider:
          url: http://localhost:4005
    

    Test-Drive Sidecar Locally

    With the setup as described in place, you can run the main app locally with the Model Provider as sidecar. Simply start the main app and the sidecar in two separate shells:

    First, start the sidecar as the main app now depends on the sidecar:

    cds watch mtx/sidecar
    

    Then, start the main app in the second shell:

    cds watch
    
    mvn spring-boot:run
    

    Remote getCsn() Calls to Sidecar at Runtime

    When you now run and use our application again as described in the previous section See Effects in SAP Fiori UIs, you can see in the trace logs that the main app sends getCsn requests to the sidecar, which in response to that reads and returns the main app’s models. That means, the models from two levels up the folder hierarchy as configured by root: ../.. for development.

    See Effects in SAP Fiori UIs

    To see the effects in the UIs open three anonymous browser windows, one for each user to log in, and:

    1. Open SAP Fiori app in browser and go to Browse Books.
    2. Log in as carol and see ISBN column in list.
    3. Log in as erin and see Ratings and ISBN columns in list.
    4. Log in as fred and no features for Fred, even though same tenant as Erin.

    For example the displayed UI should look like that for erin:

    image-20220630132726831

    Feature Vector Providers

    In principle, features can be toggled per request using the req.features property (req being the standard http req object here, not the CAP runtimes req object). This property is expected to contain one of the following:

    • An array with feature names, for example, ['isbn','reviews'].
    • A string with comma-separated feature names, for example, 'isbn,reviews'.
    • An object with keys being feature names, for example, {isbn:true,reviews:true}.

    So, to add support for a specific feature toggles management you can add a simple Express.js middleware as follows, for example, in your server.js:

    const cds = require ('@sap/cds')
    cds.on('bootstrap', app => app.use ((req,res,next) => {
      req.features = req.headers.features || 'isbn'
      next()
    }))
    

    Feature-Toggled Custom Logic

    Evaluate the FeatureTogglesInfo in custom code to check if a feature is enabled:

    @Autowired FeatureTogglesInfo features;
    
    ...
    
    if (features.isEnabled("discount")) {
      // specific coding when feature 'discount' is enabled...
    }
    

    Within your service implementations, you can react on feature toggles by inspecting cds.context.features like so:

    const { features } = cds.context
    if ('isbn' in features) {
      // specific coding when feature 'isbn' is enabled...
    }
    if ('reviews' in features) {
      // specific coding when feature 'reviews' is enabled...
    }
    // common coding...
    

    Or alternatively:

    const { isbn, reviews } = cds.context.features
    if (isbn) {
      // specific coding when feature 'isbn' is enabled...
    }
    if (reviews) {
      // specific coding when feature 'reviews' is enabled...
    }
    // common coding...