docs: [JS] add IAS App-to-App

docs-js/features/connectivity/destination-cache-isolation.mdx

@@ -73,8 +73,8 @@ The default cache isolation strategy depends on the provided JWT:
   The value for the tenant is the provider account tenant.
 - JWT without `user_id`: Isolation is set to `tenant`.
   The value for the tenant is the `zid` property of the JWT.
-- JWT with `user_id`: Isolation is set to `tenant-user`.
-  The and values are taken from `zid` and `user_id`.
+- JWT with `user_uuid` or `user_id`: Isolation is set to `tenant-user`.
+  The tenant and user values are taken from `zid` and `user_uuid` (preferred) or `user_id`.
 
 The first two cases (no JWT and JWT wihout `user_id`) are typical for user-independent authentication types, like `BasicAuthentication`, `ClientCertificateAuthentication` or `OAuth2ClientCredentials`.
 This is a safe choice which prevents privilege escalation due to caching.

docs-js/features/connectivity/destination.mdx

@@ -191,6 +191,7 @@ The SAP Cloud SDK provides a default implementation for the transformation of se
 - `service-manager`
 - `xsuaa`
 - `aicore`
+- `identity`
 
 The default implementation also retrieves a service token, if needed.
 

docs-js/features/connectivity/ias.mdx

@@ -0,0 +1,248 @@
+---
+id: identity-authentication-service
+title: Identity Authentication Service
+hide_title: false
+hide_table_of_contents: false
+sidebar_label: Identity Authentication Service
+description: This article describes how to use the Identity Authentication Service (IAS) with the SAP Cloud SDK.
+keywords:
+  - sap
+  - cloud
+  - sdk
+  - ias
+  - identity authentication service
+  - authentication
+  - app2app
+  - app-to-app
+  - oauth2
+  - btp
+  - JavaScript
+  - TypeScript
+---
+
+:::warning
+
+Only IAS App2App authentication is supported by the SAP Cloud SDK.
+Other scenarios such as App2Service are not fully supported yet.
+
+:::
+
+The SAP Cloud SDK supports the Identity Authentication Service (IAS) for App2App authentication scenarios.
+In this scenario, a consumer application requests tokens scoped to specific provider applications through pre-configured dependencies in IAS.
+
+## App2App Authentication
+
+App2App authentication allows secure service-to-service communication where tokens are scoped to specific provider applications.
+The consumer and provider applications must have a pre-configured dependency relationship in IAS.
+
+At runtime, the consumer requests a token using the [`resource` parameter](#app2app-resources), which references the provider dependency.
+The IAS broker validates the relationship and issues a scoped token that only works for the specified provider.
+
+### Configuration
+
+:::note
+
+The destination includes `mtlsKeyPair` with x509 credentials from the IAS service binding, if present.
+The SAP Cloud SDK uses these credentials for mTLS communication with the provider system.
+
+The SAP Cloud SDK supports IAS service bindings with the following credential types:
+
+- **`X509_GENERATED`**: automatically generated X.509 certificates.
+- **`SECRET`**: client secret credentials.
+
+:::
+
+Provider applications register a "provides" configuration on the IAS broker service, defining which APIs are exposed.
+Consumer applications create a service binding to IAS with dependencies on the required provider resources.
+
+The dependency name configured in IAS is used as `resource.name` in the SAP Cloud SDK.
+
+```mermaid
+sequenceDiagram
+    participant Consumer as Consumer App
+    participant SDK as SAP Cloud SDK
+    participant IAS as IAS Broker
+    participant Provider as Provider App
+
+    Note over Consumer,Provider: Configuration Phase (Setup)
+    Provider->>IAS: Register "provides" configuration
+    Consumer->>IAS: Configure dependency on provider
+
+    Note over Consumer,Provider: Runtime Phase (Token Exchange)
+    Consumer->>SDK: Request token with resource parameter
+    SDK->>IAS: Token request (client credentials + resource)
+    IAS->>IAS: Validate dependency relationship
+    IAS->>SDK: Issue scoped token
+    SDK->>Consumer: Return token
+    Consumer->>Provider: API call with scoped token
+    Provider->>Provider: Validate token
+    Provider->>Consumer: API response
+```
+
+### Creating Destinations
+
+Use [`getDestinationFromServiceBinding()`](pathname:///api/v4/functions/sap-cloud-sdk_connectivity.getDestinationFromServiceBinding.html) to connect to a system that is registered as an application within IAS.
+The parameter `iasOptions` contains:
+
+- `targetUrl`: The URL of the system where the target application resides.
+- `resource`: The dependency identified by its name or identifier configured in IAS (see [App2App Resources](#app2app-resources)) section.
+
+#### Technical User Authentication
+
+For service-to-service communication with client credentials:
+
+```typescript
+import { getDestinationFromServiceBinding } from '@sap-cloud-sdk/connectivity';
+
+const destination = await getDestinationFromServiceBinding({
+  destinationName: 'my-identity-service',
+  iasOptions: {
+    targetUrl: 'https://backend-provider.example.com',
+    resource: { name: 'backend-api' }
+  }
+});
+```
+
+For multi-tenant scenarios, see the [Multi-Tenant Support](#multi-tenant-support) section.
+
+Technical user token requests will be cached by default.
+To disable caching, set the `useCache` option to `false` in the destination request:
+
+```typescript
+const destination = await getDestinationFromServiceBinding({
+  destinationName: 'my-identity-service',
+  useCache: false,
+  iasOptions: {
+    targetUrl: 'https://backend-provider.example.com',
+    resource: { name: 'backend-api' }
+  }
+});
+```
+
+#### Business User Authentication
+
+:::warning
+
+When using business user authentication, token requests are not cached.
+
+:::
+:::info
+
+Setting `authenticationType` to `OAuth2JWTBearer` is required to trigger Business User authentication.
+
+:::
+
+For user context propagation, provide the JWT and set the authentication type.
+When you provide a JWT to the function, it automatically uses it as the assertion for token exchange.
+This will happen when no explicit `assertion` is provided in `iasOptions`:
+
+```typescript
+const destination = await getDestinationFromServiceBinding({
+  destinationName: 'my-identity-service',
+  jwt: userToken,
+  iasOptions: {
+    authenticationType: 'OAuth2JWTBearer',
+    targetUrl: 'https://backend-provider.example.com',
+    resource: { name: 'backend-api' }
+    // assertion is automatically set to userToken
+  }
+});
+```
+
+Multi-tenant scenarios are supported as well, refer to the [JWT-Based Tenant Extraction](#jwt-based-tenant-extraction) section for more details.
+
+### App2App Resources
+
+The [`IasResource`](pathname:///api/v4/types/sap-cloud-sdk_connectivity.IasResource.html) type identifies provider dependencies configured in IAS.
+It can be specified by dependency name or provider client identifier:
+
+```typescript
+type IasResource =
+  | {
+      name: string;
+    }
+  | {
+      providerClientId: string;
+      providerTenantId?: string;
+    };
+```
+
+The `name` property refers to the dependency name configured in IAS and is the recommended way to identify resources.
+Alternatively, the `providerClientId` property can be used to specify the provider application's client identifier.
+Providing `providerClientId` grants access to all dependencies associated with that provider.
+
+### Multi-Tenant Support
+
+In multi-tenant scenarios, you can control the tenant context using the `requestAs` parameter or explicitly provide the tenant identifier.
+
+#### Current Tenant and Provider Tenant options
+
+:::warning
+
+The `requestAs` parameter only affects technical user authentication (client credentials flow).
+
+:::
+
+The `requestAs` parameter determines which tenant (`app_tid`) context is used for the token request:
+
+- **`'current-tenant'`** (default): Uses the tenant from the provided JWT (if any), otherwise falls back to the service binding credentials' `app_tid`
+- **`'provider-tenant'`**: Always uses the tenant from the service binding credentials
+
+```typescript
+// Request as current tenant (uses JWT's app_tid)
+const jwt = '<user-jwt>'; // Placeholder for user JWT
+const destination = await getDestinationFromServiceBinding({
+  destinationName: 'my-identity-service',
+  jwt, // JWT's app_tid will be used
+  iasOptions: {
+    targetUrl: 'https://backend-provider.example.com',
+    resource: { name: 'backend-api' },
+    requestAs: 'current-tenant' // default
+  }
+});
+```
+
+```typescript
+// Request as provider tenant (uses service binding's app_tid)
+const destination = await getDestinationFromServiceBinding({
+  destinationName: 'my-identity-service',
+  iasOptions: {
+    targetUrl: 'https://backend-provider.example.com',
+    resource: { name: 'backend-api' },
+    requestAs: 'provider-tenant'
+  }
+});
+```
+
+#### Explicit Tenant Identifier
+
+For client credentials flows in multi-tenant scenarios, you can explicitly specify the consumer tenant identifier using `appTid`:
+
+```typescript
+const destination = await getDestinationFromServiceBinding({
+  destinationName: 'my-identity-service',
+  iasOptions: {
+    targetUrl: 'https://backend-provider.example.com',
+    resource: { name: 'backend-api' },
+    appTid: 'subscriber-tenant-id'
+  }
+});
+```
+
+#### JWT-Based Tenant Extraction
+
+When a JWT is supplied in the `options`, the SAP Cloud SDK automatically extracts the tenant information from the JWT assertion and routes token requests to the correct IAS tenant.
+JWT-Based tenant extraction is enabled for technical user authentication (`OAuth2ClientCredentials`) as the current tenant (default) and business user authentication (`OAuth2JWTBearer`):
+
+```typescript
+const destination = await getDestinationFromServiceBinding({
+  destinationName: 'my-identity-service',
+  jwt: subscriberUserJwt,
+  iasOptions: {
+    authenticationType: 'OAuth2JWTBearer',
+    targetUrl: 'https://backend-provider.example.com',
+    resource: { name: 'backend-api' }
+  }
+});
+// Token request is automatically routed to the subscriber's IAS tenant
+```

docs-js/guides/how-to-retrieve-jwt.mdx

@@ -28,17 +28,17 @@ Therefore, JWTs are used to exchange authorization and authentication informatio
 
 ## JWT on SAP BTP
 
-The retrieval of a JWT is done by the approuter together with the XSUAA.
+The retrieval of a JWT is done by the approuter together with XSUAA or IAS.
 [This guide](./how-to-use-the-approuter.mdx) explains these concepts.
 Find a complete setup in the sample applications for [Cloud Foundry](https://github.com/SAP-samples/cloud-sdk-js/tree/main/samples/cf-sample-application) and [k8s](https://github.com/SAP-samples/cloud-sdk-js/tree/main/samples/k8s-sample-application).
 The flow is as follows:
 
 - user requests a resource and is not yet authenticated
 - approuter redirects the request to the identity provider (IdP)
-- XSUAA issues a JWT - see [here](#obtain-jwt-programmatically) for details
+- XSUAA or IAS issues a JWT - see [here](#obtain-jwt-programmatically) for details
 - approuter adds the JWT to the request headers and redirects the request to the initially requested resource
 
-A JWT issued this way contains a `JKU` header property.
+A JWT issued by XSUAA contains a `JKU` header property.
 This property points to the URL where you can obtain the certificate to verify the JWT.
 The URL must be from the XSUAA domain so that it is trusted and the JWT validation does not fail.
 
@@ -48,6 +48,10 @@ The URL must be from the XSUAA domain so that it is trusted and the JWT validati
 Since `v4.1.0` SAP Cloud SDK no longer verifies the JWT issued by XSUAA.
 :::
 
+:::info IAS App-to-App Authentication
+For app-to-app authentication scenarios using the Identity Authentication Service (IAS), see the [Identity Authentication Service documentation](../features/connectivity/ias.mdx).
+:::
+
 Sometimes you want to use a JWT which is not issued by the XSUAA and approuter.
 Such tokens do not contain a `JKU` at all or a value with a different domain.
 The destination service validates the token in this case.

docusaurus.config.js

@@ -26,6 +26,10 @@ module.exports = {
   organizationName: 'SAP',
   projectName: 'cloud-sdk',
   trailingSlash: false,
+  markdown: {
+    mermaid: true
+  },
+  themes: ['@docusaurus/theme-mermaid'],
   themeConfig: {
     colorMode: {
       respectPrefersColorScheme: true,