Using PostgreSQL ​

Setup & Configuration ​

<dependency>
    <groupId>com.sap.cds</groupId>
    <artifactId>cds-feature-postgresql</artifactId>
    <scope>runtime</scope>
</dependency>

npm add @cap-js/postgres

Auto-Wired Configuration ​

The @cap-js/postgres package uses cds-plugin technique to auto-configure your application and use a PostgreSQL database for production.

You can inspect the effective configuration using cds env:

cds env requires.db --for production

Output:

{
  impl: '@cap-js/postgres',
  dialect: 'postgres',
  kind: 'postgres'
}

See also the general information on installing database packages

Provisioning a DB Instance ​

To connect to a PostgreSQL offering from the cloud provider in Production, leverage the PostgreSQL on SAP BTP, hyperscaler option.

For local development and testing convenience, you can run PostgreSQL in a docker container.

Using Docker ​

You can use Docker to run a PostgreSQL database locally as follows:

1. Install and run Docker Desktop

2. Create the following file in your project root directory:

   pg.yml

   yaml

   services:
     db:
       image: postgres:alpine
       environment: { POSTGRES_PASSWORD: postgres }
       ports: [ '5432:5432' ]
       restart: always

3. Create and run the docker container:

   sh

   docker-compose -f pg.yml up -d

Service Bindings ​

You need a service binding to connect to the PostgreSQL database.

In the cloud, use given techniques to bind a cloud-based instance of PostgreSQL to your application.

Configure Connection Data ​

If a PostgreSQL service binding exists, the corresponding DataSource is auto-configured.

You can also explicitly configure the connection data of your PostgreSQL database in the application.yaml file. If you run the PostgreSQL database in a docker container your connection data might look like this:

spring:
  config.activate.on-profile: postgres-docker
  datasource:
    url: jdbc:postgresql://localhost:5432/postgres
    username: postgres
    password: postgres
    driver-class-name: org.postgresql.Driver

To start the application with the new profile postgres-docker, the spring-boot-maven-plugin can be used: mvn spring-boot:run -Dspring-boot.run.profiles=postgres-docker. Learn more about the configuration of a PostgreSQL database

Service Bindings for CDS Tooling ​

Using Defaults with [pg] Profile ​

@cds-js/postgres comes with a set of default credentials under the profile [pg] that matches the defaults used in the docker setup. So, if you stick to these defaults you can skip to deploying your database with:

cds deploy --profile pg

In Your Private .cdsrc-private.json ​

If you don't use the default credentials and want to use just cds deploy, you need to configure the service bindings (connection data) for the CDS tooling. Add the connection data to your private .cdsrc-private.json:

{
  "requires": {
    "db": {
      "kind": "postgres",
      "credentials": {
        "host": "localhost", "port": 5432,
        "user": "postgres",
        "password": "postgres",
        "database": "postgres"
      }
    }
  }
}

Configure Service Bindings ​

Using Defaults with [pg] Profile ​

The @cds-js/postgres comes with default credentials under profile [pg] that match the defaults used in the docker setup. So, in case you stick to these defaults you can skip the next sections and just go ahead, deploy your database:

cds deploy --profile pg

Run your application:

cds watch --profile pg

Learn more about that in the Deployment chapter below.

In Your private ~/.cdsrc.json ​

Add it to your private ~/.cdsrc.json if you want to use these credentials on your local machine only:

{
  "requires": {
    "db": {
      "[pg]": {
        "kind": "postgres",
        "credentials": {
          "host": "localhost", "port": 5432,
          "user": "postgres",
          "password": "postgres",
          "database": "postgres"
        }
      }
    }
  }
}

In Project .env Files ​

Alternatively, use a .env file in your project's root folder if you want to share the same credentials with your team:

cds.requires.db.[pg].kind = postgres
cds.requires.db.[pg].credentials.host = localhost
cds.requires.db.[pg].credentials.port = 5432
cds.requires.db.[pg].credentials.user = postgres
cds.requires.db.[pg].credentials.password = postgres
cds.requires.db.[pg].credentials.database = postgres

cds watch --profile pg

Deployment ​

Using cds deploy ​

Deploy your database as usual with that:

cds deploy

Or with that if you used profile [pg] as introduced in the setup chapter above:

cds deploy --profile pg

With a Deployer App ​

When deploying to Cloud Foundry, this can be accomplished by providing a simple deployer app, which you can construct as follows:

1. Create a new folder named gen/pg/db:

   sh

   mkdir -p gen/pg/db

2. Generate a precompiled cds model:

   sh

   cds compile '*' > gen/pg/db/csn.json

3. Add required .csv files, for example:

   sh

   cp -r db/data gen/pg/db/data

4. Add a package.json to gen/pg with this content:

   gen/pg/package.json

   json

   {
     "dependencies": {
       "@sap/cds": "*",
       "@cap-js/postgres": "*"
     },
     "scripts": {
       "start": "cds-deploy"
     }
   }

   Note the dash in cds-deploy, which is required as we don't use @cds-dk for deployment and runtime, so the cds CLI executable isn't available.

5. Finally, package and deploy that, for example using MTA-based deployment.

Step-by-Step Instructions ​

Here's a step by step guide to add PostgreSQL to an existing project and deploy to SAP BTP. We assume that the following prerequiistes are fulfilled:

1. An existing instance of PostgreSQL running. For this example the instance name my-postgres-db is used.
2. Service definition(s) and data model are in place (content in /srv and /db folder)

Add Postgres dependencies ​

npm install @cap-js/postgres

This automatically hooks itself into the production profile of CAP. Once the CAP service is deployed in the BTP and the production profile is active, the Postgres adapter is used.

Add Standard CAP Dependencies ​

cds add xsuaa,mta --for production

Modify the mta.yaml ​

1. Add the Postgres instance as existing service to the resource section:

   mta.yaml

   yaml

   - name: my-postgres-db
       type: org.cloudfoundry.existing-service

2. Add a deployer task/module, to deploy the data model to the Postgres instance as part of the standard deployment.

   yaml

   - name: pg-db-deployer
       type: hdb
       path: gen/pg
       parameters:
           buildpack: nodejs_buildpack
       requires:
           - name: my-postgres-db

   • Make sure to use the type hdb and NOT nodejs as the nodejs type will try to restart the service over and over again.
   • The deployer path points to a gen/pg directory we need to create as part of the deployment process. See next step.
   • The deployer also defines the dependency/binding to the postgres instance to have the credentials available at deploy time.

3. Add dependencies to your CAP service module

   mta.yaml

   yaml

   requires:
           - name: my-postgres-db
           - name: pg-db-deployer

   This configuration creates a binding to the Postgres instance and waits for the deployer to finish before deploying the service.

4. To generate the content into the gen/pg folder, we reference a shell script in the custom builder section. The complete section should look like this:

   mta.yaml

   yaml

   build-parameters:
       before-all:
         - builder: custom
           commands:
             - npx cds build --production
             - ./scripts/pgbuild.sh

Create the Shell Script ​

The shell script specified in the previous step is a simple combination of all the commands outlined in the CAP documentation. It creates the necessary artifacts in the gen/pg directory. Here are the simple steps:

1. Create a directory /scripts in the root of the project
2. Create a file pgbuild.sh in the /scripts directory and change the permissions to make it executable:

   chmod +x pgbuild.sh

3. Add the following content to the pgbuild.sh file:
   bash

   #!/usr/bin/env bash
   
    echo ** Starting Postgres build **
   
    echo - creating dir gen/pg/db -
    mkdir -p gen/pg/db
   
    echo - compiling model -
    cds compile '*' > gen/pg/db/csn.json
   
    echo - copy .csv files -
    cp -r db/data gen/pg/db/data
   
    echo '{"dependencies": { "@sap/cds": "*", "@cap-js/postgres": "*"},  "scripts": {    "start": "cds-deploy",}}' > gen/pg/package.json

Deploy ​

Package and deploy your project, for example using MTA-based deployment.

Automatic Schema Evolution ​

When redeploying after you changed your CDS models, like adding fields, automatic schema evolution is applied. Whenever you run cds deploy (or cds-deploy) it executes these steps:

1. Read a CSN of a former deployment from table cds_model.
2. Calculate the delta to current model.
3. Generate and run DDL statements with:
   • CREATE TABLE statements for new entities
   • CREATE VIEW statements for new views
   • ALTER TABLE statements for entities with new or changed elements
   • DROP & CREATE VIEW statements for views affected by changed entities
4. Fill in initial data from provided .csv files using UPSERT commands.
5. Store a CSN representation of the current model in cds_model.

You can disable automatic schema evolution, if necessary, by setting cds.requires.db.schema_evolution = false.

Limitations ​

Automatic schema evolution only allows changes without potential data loss.

Allowed ​

• Adding entities and elements
• Increasing the length of Strings
• Increasing the size of Integers

Disallowed ​

• Removing entities or elements
• Changes to primary keys
• All other type changes

For example the following type changes are allowed:

entity Foo {
   anInteger : Int64;     // from former: Int32
   aString : String(22);  // from former: String(11)
}

Dry-Run Offline ​

We can use cds deploy with option --dry to simulate and inspect how things work.

1. Capture your current model in a CSN file:

   sh

   cds deploy --dry --model-only > cds-model.csn

2. Change your models, for example in cap/samples/bookshop/db/schema.cds:

   cds

   entity Books { ...
      title : localized String(222); //> increase length from 111 to 222
      foo : Association to Foo;      //> add a new relationship
      bar : String;                  //> add a new element
   }
   entity Foo { key ID: UUID }       //> add a new entity

3. Generate delta DDL script:

   sh

   cds deploy --dry --delta-from cds-model.csn > delta.sql

4. Inspect the generated SQL script, which should look like this:

   delta.sql

   sql

   -- Drop Affected Views
   DROP VIEW localized_CatalogService_ListOfBooks;
   DROP VIEW localized_CatalogService_Books;
   DROP VIEW localized_AdminService_Books;
   DROP VIEW CatalogService_ListOfBooks;
   DROP VIEW localized_sap_capire_bookshop_Books;
   DROP VIEW CatalogService_Books_texts;
   DROP VIEW AdminService_Books_texts;
   DROP VIEW CatalogService_Books;
   DROP VIEW AdminService_Books;
   
   -- Alter Tables for New or Altered Columns
   ALTER TABLE sap_capire_bookshop_Books ALTER title TYPE VARCHAR(222);
   ALTER TABLE sap_capire_bookshop_Books_texts ALTER title TYPE VARCHAR(222);
   ALTER TABLE sap_capire_bookshop_Books ADD foo_ID VARCHAR(36);
   ALTER TABLE sap_capire_bookshop_Books ADD bar VARCHAR(255);
   
   -- Create New Tables
   CREATE TABLE sap_capire_bookshop_Foo (
     ID VARCHAR(36) NOT NULL,
     PRIMARY KEY(ID)
   );
   
   -- Re-Create Affected Views
   CREATE VIEW AdminService_Books AS SELECT ... FROM sap_capire_bookshop_Books AS Books_0;
   CREATE VIEW CatalogService_Books AS SELECT ... FROM sap_capire_bookshop_Books AS Books_0 LEFT JOIN sap_capire_bookshop_Authors AS author_1 O ... ;
   CREATE VIEW AdminService_Books_texts AS SELECT ... FROM sap_capire_bookshop_Books_texts AS texts_0;
   CREATE VIEW CatalogService_Books_texts AS SELECT ... FROM sap_capire_bookshop_Books_texts AS texts_0;
   CREATE VIEW localized_sap_capire_bookshop_Books AS SELECT ... FROM sap_capire_bookshop_Books AS L_0 LEFT JOIN sap_capire_bookshop_Books_texts AS localized_1 ON localized_1.ID = L_0.ID AND localized_1.locale = session_context( '$user.locale' );
   CREATE VIEW CatalogService_ListOfBooks AS SELECT ... FROM CatalogService_Books AS Books_0;
   CREATE VIEW localized_AdminService_Books AS SELECT ... FROM localized_sap_capire_bookshop_Books AS Books_0;
   CREATE VIEW localized_CatalogService_Books AS SELECT ... FROM localized_sap_capire_bookshop_Books AS Books_0 LEFT JOIN localized_sap_capire_bookshop_Authors AS author_1 O ... ;
   CREATE VIEW localized_CatalogService_ListOfBooks AS SELECT ... FROM localized_CatalogService_Books AS Books_0;

   Note: If you use SQLite, ALTER TYPE commands are not necessary and so, are not supported, as SQLite is essentially typeless.

Deployment Using Liquibase ​

You can also use Liquibase to control when, where, and how database changes are deployed. Liquibase lets you define database changes in an SQL file, use cds deploy to quickly generate DDL scripts which can be used by Liquibase.

Add a Maven dependency to Liquibase in srv/pom.xml:

<dependency>
    <groupId>org.liquibase</groupId>
    <artifactId>liquibase-core</artifactId>
    <scope>runtime</scope>
</dependency>

Once liquibase-core is on the classpath, Spring runs database migrations automatically on application startup and before your tests run.

① Initial Schema Version ​

Once you're ready to release an initial version of your database schema, you can create a DDL file that defines the initial database schema. Create a db/changelog subfolder under srv/src/main/resources, place the Liquibase change log file as well as the DDL scripts for the schema versions here. The change log is defined by the db/changelog/db.changelog-master.yml file:

databaseChangeLog:
   - changeSet:
       id: 1
       author: me
       changes:
       - sqlFile:
           dbms: postgresql
           path: db/changelog/v1/model.sql

Use cds deploy to create the v1/model.sql file:

cds deploy --profile pg --dry > srv/src/main/resources/db/changelog/v1/model.sql

Finally, store the CSN file, which corresponds to this schema version:

cds deploy --model-only --dry > srv/src/main/resources/db/changelog/v1/model.csn

The CSN file is needed as an input to compute the delta DDL script for the next change set.

If you start your application with mvn spring-boot:run Liquibase initializes the database schema to version v1, unless it has already been initialized.

② Schema Evolution ​

If changes of the CDS model require changes on the database, you can create a new change set that captures the necessary changes.

Use cds deploy to compute the delta DDL script based on the previous model versions (v1/model.csn) and the current model. Write the diff into a v2/delta.sql file:

cds deploy --profile pg --dry --delta-from srv/src/main/resources/db/changelog/v1/model.csn > \
                                           srv/src/main/resources/db/changelog/v2/model.sql

Next, add a corresponding change set in the changelog/db.changelog-master.yml file:

databaseChangeLog:
   - changeSet:
       id: 1
       author: me
       changes:
       - sqlFile:
           dbms: postgresql
           path: db/changelog/v1/model.sql
   - changeSet:
       id: 2
       author: me
       changes:
       - sqlFile:
           dbms: postgresql
           path: db/changelog/v2/model.sql

Finally, store the CSN file, which corresponds to this schema version:

cds deploy --model-only --dry > srv/src/main/resources/db/changelog/v2/model.csn

If you now start the application, Liquibase executes all change sets, which haven't yet been deployed to the database.

For further schema versions, repeat step ②.

Migration ​

Thanks to CAP's database-agnostic cds.ql API, we're confident that the new PostgreSQL service comes without breaking changes. Nevertheless, please check the instructions in the SQLite Migration guide, with by and large applies also to the new PostgreSQL service.

cds deploy --model-only ​

Not a breaking change, but definitely required to migrate former cds-pg databases, is to prepare it for schema evolution.

To do so run cds deploy once with the --model-only flag:

cds deploy --model-only

This will...:

• Create the cds_model table in your database.
• Fill it with the current model obtained through cds compile '*'.

With Deployer App ​

When you have a SaaS application, upgrade all your tenants using the deployer app with CLI option --model-only added to the start script command of your package.json. After having done that, don't forget to remove the --model-only option from the start script, to activate actual schema evolution.

MTX Support ​