Hello,

I am trying to find a solution for the following scenario. The topic is
replication/high-availability/redundancy and sync.

I have three computers, A, B and C. "A" and "B" are operating important
hardware (astronomical telescopes TEL_A and TEL_B), and the operation
parameters as well as logs of operations are stored in a MySQL database. I
prefer having a central database (DB_C) for this purpose, in fact located
on "C", from which I can overview parameters and logs of "A" and "B"
simultaneously. During normal operation, "A" and "B" reads/writes only
DB_C:

        C -> DB_C
             | |
     ---><---  ----><-----
     |                   |
     A -> TEL_A          B -> TEL_B
    (|)                  (|)
    (|)                  (|)
    DB_A                 DB_B


However, if for some reason the central "C" host goes down, or internet
connection is broken, I would like to achieve the following: "A" and "B"
to continues standalone operation (read-write) with the freshest possible
local DB:

        C -> DB_C
             | |
     X X X X X X X X X X X
     |                   |
     A -> TELESCOPE_A    B -> TEL_B
     |                   |
    DB_A                 DB_B

I guess this has to be done with continuous slave-master
replication between DB_A -><- DB_C and DB_B -><- DB_C while the
internet connection is alive.

When the internet connection is resumed, or the central DB is available
again, I would like [to realize that DB_C is available again, and] DB_C
to be syncronized with the changes made in DB_A and DB_B (the changes,
of course, are different in the two replicas).

In other words, we have a synced system of A, B and C, and we have the
possibility of A and B slowly diverging from C temporarily. Then we'd
like to sync up the system again.

What would you recommend? Maybe there is a problem with my concept.

All the best,
Gaspar

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