I failed to understand the above description, hopefully knielsen will make sense out of it.

If I understand correctly, the problem here is that a transaction originating on a mariadb 5.5 master and replicated to a mariadb 10.1 slave, does not cause the gtid_slave_pos on the slave to be updated to the GTID of the transaction from the master?

The problem is that MariaDB 5.5 is too old to support GTID. So the transaction does not get a GTID assigned when it originates on a 5.5 master. Basically, to use GTID, all involved servers need to be of version 10.0 or greater...

Hm, but maybe the idea is to use GTID to connect server 2a as a slave to server 2b as a master, using GTID with the GTIDs assigned by server 2a to transactions originating from the 5.5 server 1. The reason those GTIDs do not get into gtid_current_pos is that they were neither replicated (since they had no GTID when they arrived at the slave), nor do they have the server_id of a2 (since they originated on server 1).

I can try to think if there is a simple way around this - I guess to assign the server_id of 2a to the GTIDs replicated from server 1? But generally, GTID switchover was not designed to work with servers that do not support GTID...

How come in some cases gtid_current_pos is not updated by local writes?

For example I am here on server_id 2:

MariaDB [(none)]> show variables like 'gtid%';
-------------------------------+

-------------------------------+

-------------------------------+

I do a local write:

MariaDB [(none)]> show variables like 'gtid%';
-------------------------------+

-------------------------------+

-------------------------------+

Doc says: Such changes can either be master events (ie. local changes made by user or application)
In this case local change did not update current_pos. Is it normal?

As discussed on IRC, this issue is not caused by an old master. It is the result of RESET MASTER, which deletes the gtid history and causes GTID-0-1-616 to be considered as not having occured in the past (since RESET MASTER effectively deletes the past).

knielsen, tanj, I'm pretty sure I experienced a different issue. My gtid_current_pos did not get updated for multiple hundreds of transactions coming in from an old master.

Currently, the transaction on node 2b does get a GTID somewhere on the way. The gtid_slave_pos in domain 0 on that node increases with the transactions coming in (indirectly) from server 1. The issue I am reporting is that the same transaction on server 2a does not have a GTID (domain 0 in gtid_current_pos does not increase).
I don't need the transaction to get a new server_id or gtid_domain_id, I am fine with domain 0 and the server_id of server 1 (although back there it really wasn't a gtid transaction). If you think that's better or needed I have no problemw ith it though.

By the way, I am replicating from an ancient Percona server 5.6 (node 1) without GTID mode enabled. Sorry I did not mention that before, I don't think it makes a difference though.