messenger support in CDP described

Author: julitafalcondusza

docs/cdp/cdp_data_export_schedule.md

@@ -65,4 +65,47 @@ php bin/console ibexa:cdp:stream-product-data --help
 
 ```bash
 php bin/console ibexa:cdp:stream-content-data --help
-```
+```
+
+## [[= product_name_base =]] Messenger support for large batches of data
+
+CDP uses [[= product_name_base =]] Messenger to process data.
+This approach improves performance and reliability when processing large amounts of CDP user records.
+For more information, see [Background tasks: How it works](background_tasks.md#how-it-works).
+
+By using Messenger while working with large batches of data, requests are queued instead of being processed synchronously:
+
+- queuing items starts automatically once a certain number of actions is reached (below this number, items are processed in a single request, using the standard synchronous behavior)
+- every single data is recorded in the database
+- a background worker retrieves records from the queue, processing them one by one or in batches, depending on the [Messenger](https://symfony.com/doc/current/messenger.html) configuration
+- processing happens at set intervals to avoid timeouts and keep the system stable
+
+Messenger requires a dedicated database table.
+This table is not created automatically and must be added manually using a database update script.
+
+Follow the [database update procedure](https://doc.ibexa.co/en/latest/update_and_migration/from_5.0/update_from_5.0/#database-update) to proceed.
+
+1\. Make sure that the transport layer is [defined properly](background_tasks.md#configure-package) in [[= product_name_base =]] Messenger configuration.
+
+2\. Add `bulk_async_threshold` setting in the `config/packages/ibexa_cdp.yaml` configuration:
+
+``` bash
+ibexa_cdp:
+      bulk_async_threshold: 100  # Default: 100 items
+```
+
+Available options:
+
+- `bulk_async_threshold` (integer, default: 100) - minimum number of items required to trigger asynchronous processing
+    - below threshold - items are processed immediately in a single request, using the standard synchronous behavior
+    - at/above threshold - items are automatically dispatched to the asynchronous queue for background processing
+
+3\. Make sure that the [worker starts](background_tasks.md#start-worker) together with the application to watch the transport bus:
+
+``` bash
+php bin/console messenger:consume ibexa.messenger.transport --bus=ibexa.messenger.bus
+```
+
+!!! note "Deploying Symfony Messenger"
+
+    For more information about deploying the Messenger to production, see [Symfony documentation]([[= symfony_doc =]]/messenger.html#deploying-to-production).