Options not specified retain their value, except as indicated in the following discussion. Thus, in most
cases, there is no need to specify options that do not change. For example, if the password to connect
to your MySQL master has changed, you just need to issue these statements to tell the slave about the
new password:
STOP SLAVE; -- if replication was running
CHANGE MASTER TO MASTER_PASSWORD='new3cret';
START SLAVE; -- if you want to restart replication
MASTER_HOST, MASTER_USER, MASTER_PASSWORD, and MASTER_PORT provide information to the
slave about how to connect to its master:
Note: Replication cannot use Unix socket files. You must be able to connect to the
master MySQL server using TCP/IP.
If you specify the MASTER_HOST or MASTER_PORT option, the slave assumes that the master
server is different from before (even if the option value is the same as its current value.) In this
case, the old values for the master binary log file name and position are considered no longer
applicable, so if you do not specify MASTER_LOG_FILE and MASTER_LOG_POS in the statement,
MASTER_LOG_FILE='' and MASTER_LOG_POS=4 are silently appended to it.
Setting MASTER_HOST='' (that is, setting its value explicitly to an empty string) is not the same as
not setting MASTER_HOST at all. Beginning with MySQL 5.5, trying to set MASTER_HOST to an empty
string fails with an error. Previously, setting MASTER_HOST to an empty string caused START SLAVE
subsequently to fail. (Bug #28796)