Bug #84008 GR+MTS: ER1755 can be improved for MTS LC -> MTS DATABASE
Submitted: 29 Nov 2016 18:24 Modified: 31 Mar 2017 23:58
Reporter: Narendra Chauhan Email Updates:
Status: Closed Impact on me:
None 
Category:MySQL Server: Group Replication Severity:S3 (Non-critical)
Version:8.0.1 OS:Any
Assigned to: CPU Architecture:Any

[29 Nov 2016 18:24] Narendra Chauhan
Description:
Scenario:- Test replaying GR member to normal server with default parallel_type ('database') using ASYNC setup.
M1 <-> M2 -> S1
Actual Output:- Error message of ER1755 (upon seeing View_change) is not fully correct.

"2016-11-29T18:01:57.136895Z 43 [Warning] Slave: Cannot execute the current event group in the parallel mode. Encountered event View_change, relay-log name ./server-relay-log-ch1_2.000003, position 601 which prevents execution of this event group in parallel mode. Reason: the event is a part of a group that is unsupported in the parallel execution mode. Error_code: 1755"

This message part doesn't look good i.e. "which prevents execution of this event group in parallel mode. Reason: the event is a part of a group that is unsupported in the parallel execution mode."

Concerns/questions:-
1. In GR MTS type "DATABASE" is not supported. Thus, what should happen in topology GR (M1) -ASYN RPL-> S1 (MTS:database)? i.e. Is ER1755 OK? or Will it be good if this event gets logged as empty transaction?
2. Should we improve ER1755 in this case?

How to repeat:
Details:-
==========
MySQL Version: 8.0.1

Steps to repro:-
=================
Setup M1(GR) -ASYNC CHANNEL-> S1 (MTS type: database)
M1> START GROUP_REPLICATION;
S1> START SLAVE;
S1> SHOW SLAVE STATUS;
....
               Last_SQL_Errno: 1755
               Last_SQL_Error: Cannot execute the current event group in the parallel mode. Encountered event View_change, relay-log name ./server-relay-log-ch1_2.000003, position 601 which prevents execution of this event group in parallel mode. Reason: the event is a part of a group that is unsupported in the parallel execution mode.
....
[31 Mar 2017 23:58] Daniel So
Posted by developer:
 
Added the following entry to the MySQL server 5.7.18 and 8.0.1 changelogs:

"Replicating from a Group Replication group to an external multithreaded slave (MTS) is now supported."