XCom backend confusion for Operator Extra Links

[hanxdatadog]

Hi Folks. In our Airflow project we are utilizing the operator extra links feature to provide additional information following the instructions here. We dynamically set the link values via xcom as the example and we are using a custom xcom backend (XComObjectStorageBackend).
The issue we are seeing is, when the tasks are running, the links are resolved to the object storage key, but in the end they will be resolved correctly to the value. After digging into the source code, I realized:

1. when using XComObjectStorageBackend, the xcom db table, and the UI by default are only storing the object s3 path. But the xcom value in db for operator link is eventually resolved into the the value. This behavior is actually "specific" to the operator.
2. We are purposefully only using DB XCom backend for extra links. Push operator extra link xcoms to XComModel directly #48227 and the value is resolved when a task is finished.
3. Although it is only meant to use DB backend, TaskInstance doesn't expose a public method to set xcom to db directly. https://github.com/apache/airflow/blob/main/task-sdk/src/airflow/sdk/execution_time/task_runner.py#L658. This forces us to use direct call to BaseXCom which is inconvenient.

My question is, is there any specific reasons why we only use DB xcom backend for operator links? Any objections if I try to enable it for the configured custom backend?

[hanxdatadog]

As I was trying out some implementation locally to use custom xcom backend, it doesn't look like it is meant to be used in core API layer. That might be the reason why it was limited to XComModel originally.
Similar limitation happens for the UI support xcom tab. It shows the raw xcom value in DB. If so, I wonder it makes sense to have a general support on some front end purpose API endpoints using custom xcom backend for these 2 areas. Trying to understand the architecture more.

[potiuk]

Although it is only meant to use DB backend, TaskInstance doesn't expose a public method to set xcom to db directly. https://github.com/apache/airflow/blob/main/task-sdk/src/airflow/sdk/execution_time/task_runner.py#L658. This forces us to use direct call to BaseXCom which is inconvenient.

cc: @kaxil @amoghrajesh -> I think that is an important point - was it a deliberate choice and is there a way to save to DB directly from task instance ?