主从复制架构下，默认Slave是只读的，如果写入则会报错：
127.0.0.1:6379> set foo bar(error) READONLY You can't write against a read only slave.
127.0.0.1:6379> set foo bar
(error) READONLY You can't write against a read only slave.
注意这个行为是可以修改的，虽然这样的修改没有意义：
127.0.0.1:6379> CONFIG SET slave-read-only noOK127.0.0.1:6379> set foo barOK
127.0.0.1:6379> CONFIG SET slave-read-only no
OK
