MTA deployment steps for Postgres (#394)

* Addition of detailed MTA deployment actions

* Update guides/databases-postgres.md

Co-authored-by: Daniel Hutzel <daniel.hutzel@sap.com>

* editing

---------

Co-authored-by: René Jeglinsky <rene.jeglinsky@sap.com>
Co-authored-by: Daniel Hutzel <daniel.hutzel@sap.com>

guides/databases-postgres.md

@@ -311,6 +311,104 @@ When deploying to Cloud Foundry, this can be accomplished by providing a simple
 
 5. Finally, package and deploy that, for example using [MTA-based deployment](deployment/to-cf#build-mta).
 
+## Setup with Existing Projects
+
+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:
+    ::: code-group
+    ```yaml [mta.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
+    ::: code-group
+    ```yaml [mta.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:
+    ::: code-group
+    ```yaml [mta.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](deployment/to-cf#build-mta).
+
+
 ## 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: