[improve] PIP-411: Expose Peek Messages API to Non-Admin Clients

[sandeep-mst]

Fixes apache#24051

Main Issue: apache#24051

PIP: apache#411

Motivation

Apache Pulsar currently exposes the Peek Messages API only under the Admin API, which prevents non-admin applications from using this functionality. Some non-admin applications—such as monitoring tools or custom client applications—need to peek at messages without consuming them. Exposing this functionality via a new endpoint will:

• Enhance usability for a broader range of applications.
• Simplify the development of non-admin clients by providing direct access to message peeking.
• Maintain consistency by leveraging the existing internal Peek Messages logic.

Modifications

• Endpoint Creation:
  A new REST endpoint (e.g., /lookup/peek) will be added within the Lookup API, enabling non-admin clients to access the Peek Messages functionality.

• Authorization Checks:
  The new endpoint will include proper authorization checks to ensure that only permitted non-admin clients can use it.

• Internal Delegation:
  Requests received on the new endpoint will be internally delegated to the existing Peek Messages logic, avoiding code duplication while preserving consistent behavior.

• Documentation Update:
  The API documentation and reference materials will be updated to include details about the new endpoint, request parameters, response formats, and error conditions.

Verifying this change

• Added unit and integration tests to validate:

  • The new endpoint correctly delegates to the internal Peek Messages logic.
  • Authorization checks are enforced as expected.
  • Backward compatibility is maintained (existing Admin API remains unchanged).

• Manual testing will be conducted to verify non-admin clients can access the new endpoint while admin clients continue to use the current API.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

This PR affects the REST endpoints (by adding a new endpoint) and updates the API documentation accordingly.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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