Welcome to your new project. It contains a few files and folders following our recommended project layout:

The following sections are a quick walkthrough of essential tasks:

• Define your first Service and Run it
• Add Data Models
• Add Databases
• Add Initial Data
• Add Custom Logic
• More...

Create a file srv/cat-service.cds and define a service in there as follows:

using { Country, managed } from '@sap/cds/common';

service CatalogService {

  entity Books {
    key ID : Integer;
    title  : localized String;
    author : Association to Authors;
    stock  : Integer;
  }

  entity Authors {
    key ID : Integer;
    name   : String;
    books  : Association to many Books on books.author = $self;
  }

  entity Orders : managed {
    key ID  : UUID;
    book    : Association to Books;
    country : Country;
    amount  : Integer;
  }
}

Run that in a terminal to start a generic server:

> cds run

Cmd/Ctrl-click on the link http://localhost:4004 in the output to open a browser and send requests to your service using the provided links.

Above we used a simplistic 'all-in-one' service definition for a quick start. Usually, you would put your entity definitions into a separate data model and have your services expose views on that entities.

Create a file named db/data-model.cds and fill it with that:

namespace my.bookshop;
using { Country, managed } from '@sap/cds/common';

entity Books {
  key ID : Integer;
  title  : localized String;
  author : Association to Authors;
  stock  : Integer;
}

entity Authors {
  key ID : Integer;
  name   : String;
  books  : Association to many Books on books.author = $self;
}

entity Orders : managed {
  key ID  : UUID;
  book    : Association to Books;
  country : Country;
  amount  : Integer;
}

With that we can adapt our srv/cat-service.cds to exposing views as follows:

using my.bookshop as my from '../db/data-model';

service CatalogService {
  entity Books    @readonly as projection on my.Books;
  entity Authors  @readonly as projection on my.Authors;
  entity Orders @insertonly as projection on my.Orders;
}

The cds runtime ships with built-in generic handlers which automatically serve all CRUD requests SQL databases. We choose sqlite for dev usage as that is already available on Macs (→ install it on Windows):

npm i sqlite3 -D

Now we can deploy our data model to a sqlite database:

cds deploy --to sqlite:my.db

This creates a sqlite database file at my.db. In addition, the given configuration is stored to your package.json as your default data source. With that, you can run cds deploy subsquently without any arguments.

Add plain CSV files under db/csv to fill your database tables with initial data.

1. Add a filed called db/csv/my.bookshop-Authors.csv and add the following data:

ID;name
101;Emily Brontë
107;Charlote Brontë
150;Edgar Allen Poe
170;Richard Carpenter

2. Add a file called db/csv/my.bookshop-Books.csv and add the following data

ID;title;author_ID;stock
201;Wuthering Heights;101;12
207;Jane Eyre;107;11
251;The Raven;150;333
252;Eleonora;150;555
271;Catweazle;170;22

Run cds deploy again to have the data filled in.

Then cds run the server again and see the data returned.

As we have now a fully cabable SQL database connected, we can leverage the querying capabilities of the generic handlers with requests like that: .../Authors?$expand=books($select=ID,title)

( Example for Node.js, skip for Java )

So far, all requests were served automatically by built-in generic service providers. You can hook in to these providers to add your domain-specific logic. Just add an equally named .js file next to your service definition, i.e. srv/cat-service.js, and fill this in:

module.exports = (srv) => {

  const {Books} = cds.entities ('my.bookshop')

  // Reduce stock of ordered books
  srv.before ('CREATE', 'Orders', async (req) => {
    const order = req.data
    if (!order.amount || order.amount <= 0)  return req.error (400, 'Order at least 1 book')
    const tx = cds.transaction(req)
    const affectedRows = await tx.run (
      UPDATE (Books)
        .set   ({ stock: {'-=': order.amount}})
        .where ({ stock: {'>=': order.amount},/*and*/ ID: order.book_ID})
    )
    if (affectedRows === 0)  req.error (409, "Sold out, sorry")
  })

  // Add some discount for overstocked books
  srv.after ('READ', 'Books', each => {
    if (each.stock > 111)  each.title += ' -- 11% discount!'
  })

}

• Just open .../Books in the browser to test the handler for Books.

• For the Orders handler, you can use this curl command (or use Postman with the data from below):

  curl http://localhost:4004/catalog/Orders -X POST -H "Content-Type: application/json" -d '{"book_ID":201, "amount":6}'

  After a few more requests the Sold out response is returned since the stock would drop below 0.

Find more in the help.sap.com