ROW is absolutely a valid configuration.
The thing you've probably overlooked is that Server B needs
log_slave_updates enabled, or it won't replicate anything from A to C.
But to address the question at hand, consider the following table, which describes the comparison between configured
@@binlog_format and what is actually written to the server's binlog. Assume A is master, B is slave of A, no changes are made directly to B, and of course
log_slave_updates is enabled on B:
A binlog_format | A actually writes | B binlog_format | B actually writes
STATEMENT STATEMENT STATEMENT STATEMENT
STATEMENT STATEMENT MIXED MIXED
STATEMENT STATEMENT ROW ROW
MIXED MIXED STATEMENT TOTALLY FAILS EVENTUALLY
MIXED MIXED MIXED MIXED
MIXED MIXED ROW ROW
ROW ROW STATEMENT TOTALLY FAILS VERY QUICKLY
ROW ROW MIXED ROW
ROW ROW ROW ROW
What's going on here?
When B writes to its binary log, it writes an entry to replicate the change it made. It writes these entries in the format its local optimizer determines should be used. When
ROW mode is in effect, that's the only choice, so incoming
STATEMENT becomes outgoing
ROW, in your case. Server B easily translates because it executes the incoming event and then simply logs which rows it inserted/updated/deleted in its local data, and the values of the columns in those rows.
Each MySQL Server can set its own and only its own binary logging format (true whether
binlog_format is set with global or session scope). This means that changing the logging format on a replication master does not cause a slave to change its logging format to match. [...]
Changing the binary logging format on the master while replication is ongoing, or without also changing it on the slave can cause replication to fail with errors such as Error executing row event:
'Cannot execute statement: impossible to write to binary log since statement is in row format and BINLOG_FORMAT = STATEMENT.'
That's what I was referring to, above, with
TOTALLY FAILS. The only invalid configuration is for a server to be in
STATEMENT mode while at least one upstream master is in
Why? Every possible query that can be represented in
STATEMENT format can always be subsequently represented in
ROW format, because
ROW format is absolutely deterministic. Conversely, no query replicated in
ROW format can ever be subsequently replicated in
MIXED format simply gives the local optimizer the option on a query by query basis of how to replicate --
ROW -- but if the incoming replication event is
ROW the only outgoing choice is also
STATEMENT mode is seriously terrible. Get away from it as soon as you can. Before then, it's vitally critical to verify that the data on B is identical to the data on A in every way, because the data on B can slip out of sync much easier and go undetected when A is in