The sql queries are given in two text files, defined by options promotion_sql_file
and demotion_sql_file. The files must exist when monitor starts. The files are read
line by line, ignoring empty lines and lines starting with '#'. All other lines
are sent to the server being promoted, demoted or rejoined. Any error in opening
a file, reading it or executing the contents will cause the entire operation to
fail.
The filed defined in demotion_sql_file is also ran when rejoining a server. This
is to ensure a previously failed master is "demoted" properly when it joins the
cluster.
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