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Create a Reusable Service

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Create a Reusable Service

Create a Reusable Service

March 2, 2021
Created by
April 14, 2020
Create a service that will later on be reused in another CAP Java project.

You will learn

  • How to write an entity definition
  • How to use some use generic CAP artifacts like aspects and types
  • What associations and compositions are
  • How to deploy to a SQLite database
  • How to make the project reusable
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The previous tutorial was about quickly setting up a working CAP application and read/write some mock data. This tutorial is about learning how to extend the application to a complete products service.

You’ll take advantage of all the out-of-the-box features provided by the CAP Java SDK such as using SQLite as a database for local development. As a result, you’ll remove your custom handlers written in the first tutorial. In subsequent tutorials you will swap out SQLite with the SAP HANA service, when preparing your application for the cloud.

Step 1: Define the domain model

In the previous tutorial, you defined a service, which defined its own entity. When modeling with CDS the best practice is to separate services from the domain model.

Therefore, you will now define the complete domain model that is used by the products service. The domain model is stored in the db folder of your CAP application.

  1. Go to your db folder and create a file called schema.cds.

    schema cds file creation
  2. Add the following code to your newly created schema.cds file and make sure you Save the file:

        namespace sap.capire.products;
        using { Currency, cuid, managed, sap.common.CodeList } from '@sap/cds/common';
        entity Products : cuid, managed {
            title    : localized String(111);
            descr    : localized String(1111);
            stock    : Integer;
            price    : Decimal(9,2);
            currency : Currency;
            category : Association to Categories;
        entity Categories : CodeList {
            key ID   : Integer;
            parent   : Association to Categories;
            children : Composition of many Categories on children.parent = $self;
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Step 2: Understand keywords

As you can see, the domain model defines two entities:

  • Products
  • Categories

It also imports various common definitions from the @sap/cds/common package (a globally available reuse package):

  • Currency
  • cuid
  • managed
  • CodeList

In addition, the domain model uses the CDS keywords localized, Association , and Composition.
Let’s explain these imports and keywords in more detail:

The localized Keyword

The localized keyword can be used to mark elements, which require translation. The ability to store translations for different languages and to store a default fallback translation is automatically handled by CDS for you. You will see this in action in more detail in the next tutorial.

Associations and Compositions

Associations and compositions can be used to define relationships between entities. They often allow you to define these relationships without explicitly working with foreign keys.

While associations define a rather loose coupling between the entities, compositions define a containment relationship. Compositions can also be thought of as defining deep structures. You can perform deep inserts and upserts along these structures.

In your domain model, the Categories entities define a parent and children element. This enables a hierarchy of categories. The children of a category are modelled as a composition. A category with all of its children defines a deep nested structure. Deleting a category would automatically delete all of its children. However, the parent of a category is modelled as an association. Deleting a category obviously shouldn’t delete its parent.

The cuid and managed Aspects

Both cuid and managed are aspects. Aspects extend an entity with additional elements. The cuid aspect adds a key element ID of type UUID to the entity.

The managed aspect adds four additional elements to the entity. These capture the time of the creation and last update of the entity, and the user, which performed the creation and last update.

The CodeList Aspect and the Currency Type

CodeLists can be used to store global, translatable definitions based on codes, such as currencies, countries, or languages. Especially for UIs, a CodeList can be useful to provide a value help for certain input fields.

The Currency definition is a type. It defines an association to a Currencies entity. The Currencies entity is based on ISO 4217 and uses three-letter alpha codes as keys such as EUR or USD and provides the possibility to store the corresponding currency symbol such as or $.

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Step 3: Get more information about @sap/cds/common

Look at these explained keywords yourself and learn more about it.

  1. To jump to the imported definitions directly in your editor, press CTRL, hover over the keyword, and click. This will open the source file in a separate editor tab.

  2. Hold CTRL and hover over a keyword, then move the hand cursor over the keyword. This opens up you a tiny overlay with the definition of this particular item.

    overlay of definition
  3. A right-click (or F12) on the definition opens up the context menu. There you can find, for example, Peek definition to get a much bigger overlay. Not only the definition of this particular item is displayed, you also have the ability to navigate through the whole source file of this definition without opening the file itself.

    right-click to peek definition information
As you have now learned how to have a look at definitions coming from @sap/cds/common. How many properties does the Products entity effectively have?
Step 4: Rewrite the AdminService

In the first tutorial, you defined a simple service, called AdminService, which directly defined the entity Products. As you now have defined the Products entity in your domain model, the AdminService just needs to expose it. In addition, you defined the Categories entity, which should also be part of your service.

This can be achieved by using projections. Services expose projections of the entities defined in the domain model. These projections can be used to include (and also exclude) only certain elements of an entity or to rename the entity’s elements.

In this example you will use the most simple projection, which exposes the domain model entity without any changes.

  1. Go to your srv folder and open the admin-service.cds file.

  2. Replace the content with the following code and make sure you Save the file:

    using { sap.capire.products as db } from '../db/schema';
    service AdminService {
        entity Products   as projection on db.Products;
        entity Categories as projection on db.Categories;
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Step 5: Deploy the domain model

Let’s deploy the domain model to a database. Now, you will use SQLite, a light-weight file-based database, which fits the needs for local development perfectly.

  1. First, install SQLite to the project. Go to the terminal where your application is running and use CTRL+C to stop the application.

    Execute the following command in the terminal:

    npm install --save-dev sqlite3
  2. To initialize the database with the defined domain model, execute the following command in the terminal:

    cds deploy --to sqlite
    cds deploy

    This will create a file called sqlite.db in your project root. The name of this database is defined by an entry in your package.json.

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Step 6: Configure CAP application to use SQLite database

Right now, the Java application itself isn’t aware of the SQLite database.

Configure your Java application to use the sqlite.db database file.

  1. Go to srv/src/main/resources, locate, and open the application.yaml file. This file was created when you initialized the application.

  2. For the field url, replace the string "jdbc:sqlite::memory:?cache=shared" with a reference to your local database file "jdbc:sqlite:/home/user/projects/products-service/sqlite.db"

  3. Set the value of initialization-mode from always to never, because you’ve already initialized the database when running cds deploy --to sqlite.

Your application.yaml file should look like this:

  profiles: default
    url: "jdbc:sqlite:/home/user/projects/products-service/sqlite.db"
    driver-class-name: org.sqlite.JDBC
    initialization-mode: never
      maximum-pool-size: 1
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Step 7: Use CAP's generic persistence handling

The CAP Java SDK has a Persistence Service that provides out-of-the-box capabilities to store and retrieve entities from a database. Therefore, no custom coding is required for this. The entities defined in your AdminService will be automatically served via OData and you can just delete the file that was created earlier.

Delete the file in the handlers folder.

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Step 8: Run and test your application
  1. Start your application by running mvn spring-boot:run in the terminal and open it in a new tab.

  2. Test your application by using Curl from the terminal.

    You can open additional terminals by choosing Terminal > New Terminal from the main menu.

    This request will create multiple, nested categories, at once through a deep insert:

    curl -X POST http://localhost:8080/odata/v4/AdminService/Categories \
    -H "Content-Type: application/json" \
    -d '{"ID": 1, "name": "TechEd", "descr": "TechEd related topics", "children": [{"ID": 10, "name": "CAP Java", "descr": "Run on Java"}, {"ID": 11, "name": "CAP Node.js", "descr": "Run on Node.js"}]}'
  3. Try to query individual categories, for example by adding the following to the end of your app URL:


  4. Restart your application by switching to the terminal, using CTRL+C, and running mvn spring-boot:run again.

  5. Query the categories again using the URL from step 3. You’ll see that the categories are stored in a persistent database.

  6. You can also expand the nested structures. Add the following to the end of your app URL:

    • /odata/v4/AdminService/Categories?$expand=children
    • /odata/v4/AdminService/Categories(10)?$expand=parent
    • /odata/v4/AdminService/Categories(1)?$expand=children
  7. Make sure to stop your application after testing it by using CTRL+C.

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Step 9: Set up for reuse

In the following tutorial, the application will be reused by a bookstore application. The reuse of models can be achieved by publishing NPM modules with the models and defining dependencies to these NPM modules in other applications. There are a two steps we need to perform to prepare the products-service application for reuse.

  1. The name of the products-service reuse module, should be @sap/capire-products. Therefore, open the package.json file within the ~/projects/products-service folder and change the value of name field from products-service-cds to @sap/capire-products. If you want to you can also provide a meaningful description in the description field.

    adjust the package.json in the root folder of your project
  2. To make it easier to reuse the module, an index.cds file can be added to the products-service. This ensures a better decoupling from other applications.

    Create a new file index.cds in the ~/projects/products-service folder and place the following content inside this file, making sure you Save the file:

    using from './db/schema';
    using from './srv/admin-service';
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Congratulations! You have successfully developed the products service application, which is based on a CDS domain model and service definition.

In the next tutorial, you will build a bookstore application, reusing the products service application. You will later extend the bookstore application with custom business logic and deploy it to the cloud, using SAP HANA as the database.

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