CAP Inbox not getting enabled for receiving events from SAP Cloud Application Event Hub

[Chaitanyaputhran]

What happened?

Hi Team,

We are currently integrating S/4 HANA events with the Resource Management BTP CAP Java application through SAP Cloud Application Event Hub (Event Broker).

The webhook URL is successfully triggered, and CAP event handlers are invoked as expected. However, the CAP Inbox mechanism does not appear to be enabled, even though we have explicitly configured it in our application.yaml as specified in the CAP documentation.

Our application.yaml configuration for enabling the inbox can be found here:
https://github.tools.sap/Cloud4RM/Resource-Request/blob/dev/events/project-integration-adapter/src/main/resources/application.yml#L41

We have followed the official CAP Inbox configuration guidelines as described in the documentation:
https://cap.cloud.sap/docs/guides/events/event-queues#inbox

Despite this setup, Inbox persistence does not seem to be active.

Issue Summary

• Webhook URL is successfully called (visible in cf logs)
• CAP event handlers are triggered
• When an exception is deliberately thrown from the handler, the event is retried by Event Hub itself
• No indication in logs that Inbox persistence is active

Additionally:

• When processing fails inside the handler, the HTTP response status in CF logs reflects an error (e.g., 5xx), which triggers Event Hub retry.
• If Inbox were enabled, our understanding is that the application should respond with HTTP 202 Accepted, even if processing fails internally, since the event should already be persisted in the Inbox table and handled transactionally by CAP.

Kindly look into this issue and guide us on how to proceed.

Steps to reproduce

Our application.yaml configuration for enabling the inbox can be found here:
https://github.tools.sap/Cloud4RM/Resource-Request/blob/dev/events/project-integration-adapter/src/main/resources/application.yml#L41

Versions

Your Project Name	https://github.tools.sap/Cloud4RM/Resource-Management
CAP Java SDK	5.20.0
OData version	v4
Node.js version	v20.11.0
@sap/cds	9.1.0
@sap/cds-dk	9.1.0
@sap/cds-mtxs	3.1.0
CAP version	4.1.0

OS / Environment

Cloud Foundry

Service Inventory

n/a — Test/demo app

Relevant log output

No response

[beckermarc]

Note the following information that I also shared in an internal CAP issue about this:

The event hub plugin unfortunately is still using a deprecated constructor method to instantiate the MessagingBrokerQueueListener, which implements the inbox feature. Due to the usage of this deprecated constructor the inbox feature is not available.

See here for the outdated code in the event hub plugin:

cds-feature-event-hub/cds-feature-event-hub/src/main/java/com/sap/cds/feature/messaging/eventhub/service/EventHubMessagingService.java

Line 54 in f856100

this.queueListener = new MessagingBrokerQueueListener(this, toFullyQualifiedQueueName(queue), queue, runtime, true);

[Chaitanyaputhran]

Hi @beckermarc,
Thanks for the clarification regarding the deprecated constructor and the current limitation around Inbox support in the Event Hub plugin.

Is enabling Inbox support for the Event Hub plugin considered a requirement or planned enhancement? If yes, when can we expect this feature to be available?

Understanding the timeline would help us decide our implementation approach.

In case the Inbox feature is not usable in this setup, could you please advise if there is any alternative recommended approach to achieve reliable and decoupled event processing?

Our main objective is to:
Avoid relying solely on Event Hub retry logic
Ensure reliable, idempotent event processing
Could you please guide us on the best-practice alternative in the current state of the Event Hub plugin?

[kevin-b-sap]

I added Inbox support to this open PR: #106.
We intend to include it in the upcoming release.