In this blog I will talk about the different scenarios within 'In-process' AIF Message and how to approach & evaluate them to find the root cause of errors.
Introduction
SAP Application Interface Framework (AIF) enables to monitor and manage all the replication related messages at one place. These message can have status - Successful, Error (Technical/ Application) or In-Process. Documents with Successful message does not need attention, while failed messages are presented with the failure reasons to act upon.
Records that has “in process” status is difficult to comprehend as you do not find the direct reason associated to it. . Further , it stays in same status until the process is successfully completed or ended in error through manual intervention
Scenarios
There can be 4 probable Scenarios related to In-Process messages:
1.GUIDs which turned into Success message after you attempt restart. You can continue processing such message directly into success

GUID Processed successfully
2.GUIDs which have the waiting for predecessor Status. Example: ‘
A predecessor exists (with MSGGUID XXXXXX); message cannot be processed’. These depends on the processing of the other GUIDs. You can go to ‘Root Predecessor’ in AIF and process Predecessor first followed by actual message

GUID - waiting for Predecessor
3.GUIDs which have AIF Log as empty. These are those which were not executed before. In this case it is more likely to be connected to the scenario 2 as it would be necessary to review why these GUIDs weren't processed at the time they arrived at AIF.

GUID without Log Message
4.GUIDs which are with status "In Process" and have AIF Logs, without any message. These are although have AIF Log, there is no message in it. Usually, when the log is empty in AIF, it would be related to a GUID which faced a dump during its processing. This need further examination of ST22.

GUID with log but without Message
Background
Messages are transferred in batches from Source system via SLT to Central Finance target system. These batches are put into a background job for processing. Sometimes, a background job can get interrupted / dumps. Since there are multiple messages in the background job, it might happen, that some messages in that job has not been processed yet (status is stuck
"In process - I") and hence they will not get processed further because the job they were assigned to has dumped.

Status
In such cases a manual restart is required from the end users to process. There can be unfortunately no log available because the message actually had no processing, hence no log writing.

Sub Status

Table: CFIN_AIF_IDX_TBL
Solution
Above can be checked in interface single index table: CFIN_AIF_IDX_TBL. Check the Processing status as well as sub-status field. The Sub-status will indicate there was an error in the job processing : Type E - Error in job processing
Pay attention to sub-status "P" – processing, in the batch (may be date), because most probably this was the actual message which dumped.
Restart this message alone in /AIF/ERR, and check ST22 and SM37 for your user to see if there is a dump happening.
From release SAP S4HANA 1909, SAP has enhance AIF/ERR report to include sub status within in-process document status. However there is additional config in AIF to bring these sub status which is not the scope of this blog.

Sub Process Filter option in AIF
Conclusion
It is important to have the holistic approach of analysing the entire batch of In-process errors to find the root cause - often dump and restart the messages individually case by case basis.
For details on how to activate AIF sub-status for Inprocess message , Pls refer to blog from Anirban Roy-
https://blogs.sap.com/2020/09/16/aif-sub-status-for-inprocess-messages-in-1909-s4-hana/
Keep Exploring!!