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Using PostgreSQL

This guide focuses on the new PostgreSQL Service provided through @cap-js/postgres, which is based on the same new database services architecture as the new SQLite Service. This architecture brings significantly enhanced feature sets and feature parity, as documented in the Features section of the SQLite guide.

Learn about migrating from the former cds-pg in the Migration chapter.{.learn-more}

CAP Java SDK is tested on PostgreSQL 15. Most CAP features are supported on PostgreSQL.

Learn more about features and limitations of using CAP with PostgreSQL{.learn-more}

::: info This guide is available for Node.js and Java. Press v to switch, or use the toggle. :::

[[toc]]

Setup & Configuration

Run this to use PostgreSQL for production:

To run CAP Java on PostgreSQL, add a Maven dependency to the PostgreSQL feature in srv/pom.xml:

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

In order to use the CDS tooling with PostgreSQL, you also need to install the module @cap-js/postgres:

npm add @cap-js/postgres

After that, you can use the cds deploy command to deploy to a PostgreSQL database or to create a DDL script for PostgreSQL.

Auto-Wired Configuration {.impl .node}

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{.learn-more}

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: ::: code-group

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

    :::

  3. Create and run the docker container:

    docker-compose -f pg.yml up -d

::: tip With the introduction of Testcontainers support in Spring Boot 3.1, you can create PostgreSQL containers on the fly for local development or testing purposes. :::

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.

For local development provide the credentials using a suitable cds env technique, like one of the following.

Configure Connection Data {.impl .java}

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{ .learn-more}

Service Bindings for CDS Tooling {.impl .java}

Using Defaults with [pg] Profile {.impl .java}

@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 {.impl .java}

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 {.impl .node}

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.{.learn-more}

In Your private ~/.cdsrc.json

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

::: code-group

{
  "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:

::: code-group

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

:::

::: tip Using Profiles

The previous configuration examples use the cds.env profile [pg] to allow selectively testing with PostgreSQL databases from the command line as follows:

cds watch --profile pg

The profile name can be freely chosen, of course.

:::

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. Similar to SAP HANA deployer apps, it is auto-generated for PostgreSQL-enabled projects by running

cds build

::: details What cds build does…

  1. Compiles the model into gen/pg/db/csn.json.
  2. Copies required .csv files into gen/pg/db/data.
  3. Adds a gen/pg/package.json with this content: 
    {
  "dependencies": {
    "@sap/cds": "^7",
    "@cap-js/postgres": "^1"
  },
  "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. :::

Add Postgres Deployment Configuration

cds add postgres

::: details See what this does…

  1. Adds @cap-js/postgres dependency to your package.json dependencies.
  2. Sets up deployment descriptors such as mta.yaml to use a PostgreSQL instance deployer application.
  3. Wires up the PostgreSQL service to your deployer app and CAP backend. :::

Deploy

You can package and deploy that application, for example using MTA-based deployment.

Automatic Schema Evolution { #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{.good}

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

Disallowed{.bad}

  • 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)
}

::: tip

If you need to apply such disallowed changes during development, just drop and re-create your database, for example by killing it in docker and re-create it using the docker-compose command, see Using Docker.

:::

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:

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

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

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

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

    -- 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 { .impl .java }

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.

::: warning Don't change the model.sql after it has been deployed by Liquibase as the checksum of the file is validated. These files should be checked into your version control system. Follow step ② to make changes. :::

② Schema Evolution { #schema-evolution-with-liquibase }

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 { .impl .node }

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 '*'.

::: warning IMPORTANT:

Your .cds models are expected to reflect the deployed state of your database.

:::

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

::: warning

Multitenancy and extensibility aren't yet supported on PostgreSQL.

:::