- #PXC-666: DDL operations protected via RSU can change the database during SST #90
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…during SST Currently DDL operations that are protected via the RSU can be performed during SST. Thus, the database can be changed during execution of the SST process. Therefore, the SST process may transfer to another node the incorrect snapshot. To avoid this problem, we should pause new desynchronization requests (wsrep->desync() API calls), which are intiated by the RSU, until completion of the SST. Unfortunately there are many options for implicit completion of the SST, and Replicator layer does not always receive clear notification about it. But it always gets some kind of message, which changes its state. Therefore, to track any exit from the desynchronization mode, which was established for the SST, and to continue execution of the thread that wants to make a new desynchronization on behalf of the RSU, I used the mechanism of action linked to the finite state machine transitions, which is already implemented in the Galera (but currently not used in practice for other purposes).
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Currently DDL operations that are protected via the RSU can
be performed during SST. Thus, the database can be changed
during execution of the SST process.
Therefore, the SST process may transfer to another node
the incorrect snapshot. To avoid this problem, we should
pause new desynchronization requests (wsrep->desync() API
calls), which are intiated by the RSU, until completion
of the SST.
Unfortunately there are many options for implicit
completion of the SST, and Replicator layer does not
always receive clear notification about it. But it
always gets some kind of message, which changes
its state. Therefore, to track any exit from the
desynchronization mode, which was established for
the SST, and to continue execution of the thread
that wants to make a new desynchronization on behalf
of the RSU, I used the mechanism of action linked
to the finite state machine transitions, which is
already implemented in the Galera (but currently
not used in practice for other purposes).