mysql半主从复制

mysql半主从复制

在聊主从半同步前我们先聊一下mysql主从同步的原理

一、mysql的复制原理至少需要两个节点，一主（master）一从（slave）那么从节点如何从主节点获得数据呢？1、主节点开启二进制日志，记录了修改数据的所有语句2、从节点扮演mysql客户端请求主节点的二进制日志，默认是第一个文件的最开始位置，通常要指定二进制文件的指定位置。从节点收到二进制日志后保存在本地的中继日志中，并且会记录已经读取到哪个二进制日志的哪个位置，用作再次请求二进制日志的起始位置。从节点会读取中继日志中的事件完成重放后从而形成一模一样的数据保存在数据库中。3、slave节点上负责从master节点上请求二进制日志的线程叫做IO_THREAD，从节点收到二进制日志后，保存在本地中继日志中并由SQL_THREAD做重放后将数据保存在数据库中。4、主节点会为每个请求二进制日志的从服务器线程IO_THREAD启动一个响应线程dump_thread，该线程负责读取主服务器上的二进制日志并发送给从服务器。
在传统的mysql主从同步中，当mysql主服务器写入数据、记入二进制日志后就直接回复客户端写入完成了，是不会等待从服务器写入后再回复确认给客户端。换而言之，如果从服务器没有收到二进制日志而主服务器宕机，那么新写入的数据会来不及同步给从服务器而造成丢失的。因此，比较稳妥的做法是，当主服务器写入数据后还应该等待至少一台从服务器也写入完成后才回复客户端写入完成，也就是说，主、从服务器写入是同步进行而非异步。多台从服务器时，通常仅指定一台从服务器为同步模式，其余从服务器为异步模式——这种mysql主从同步模式成为mysql主从半同步复制。
实现mysql主从半同步复制模式是由google开发的插件完成的，它们是semisync_master.so和semisync_slave.so，前者工作在mysql主服务器，后者工作在mysql从服务器。这对插件在安装mariadb-server的rpm包时默认提供。在配置mysql主从时，安装启用此插件再启动slave复制线程即可完成主从半同步复制。

二、mysql半主从复制配置演示

node3: 172.16.92.3/16 mariadb主服务器
node4: 172.16.92.4/16 mariadb从服务器
以上节点均为CentOS 7.1

配置环境
1. 配置好光盘yum源
2. 关闭selinux和iptables

node3: mariadb主服务器

[root@node3 ~]# yum -y install mariadb-server
[root@node3 ~]# vim /etc/my.cnf
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0
# Settings user and group are ignored when systemd is used.
# If you need to run mysqld under a different user or group,
# customize your systemd unit file for mariadb according to the
# instructions in http://fedoraproject.org/wiki/Systemd 
#######以下的内容为添加########
#二进制变更日志
log-bin=mysql-bin
#二进制日志格式为混合模式
binlog_format=mixed
#为主服务器node3的ID值
server-id = 1
port = 3306
skip-external-locking
key_buffer_size = 256M
max_allowed_packet = 1M
table_open_cache = 256
sort_buffer_size = 1M
read_buffer_size = 1M
read_rnd_buffer_size = 4M
myisam_sort_buffer_size = 64M
thread_cache_size = 8
query_cache_size= 16M
thread_concurrency = 4
innodb_file_per_table = on
skip_name_resolve = on
###############################

###### 以下的内容可选 ########
[mysqldump]
quick
max_allowed_packet = 16M

[mysql]
no-auto-rehash

[myisamchk]
key_buffer_size = 128M
sort_buffer_size = 128M
read_buffer = 2M
write_buffer = 2M

[mysqlhotcopy]
interactive-timeout
#############################

[mysqld_safe]
log-error=/var/log/mariadb/mariadb.log
pid-file=/var/run/mariadb/mariadb.pid

#
# include all files from the config directory
#
!includedir /etc/my.cnf.d
############### End for my.cnf #################

[root@node3 ~]# rpm -ql mariadb-server | grep 'semisync'
/usr/lib64/mysql/plugin/semisync_master.so    #用于Master服务器安装的半同步插件
/usr/lib64/mysql/plugin/semisync_slave.so        #用于Slave服务器安装的半同步插件

[root@node3 ~]# systemctl start mariadb
[root@node3 ~]# mysql
MariaDB [(none)]> install plugin rpl_semi_sync_master soname 'semisync_master.so';     #安装Master半同步插件
MariaDB [(none)]> set global rpl_semi_sync_master_enabled = 1;    #开启Master半同步功能
MariaDB [(none)]> set global rpl_semi_sync_master_timeout = 2000;    #超时时间毫秒级
MariaDB [(none)]> grant replication client,replication slave on *.* to 'repluser'@'172.16.92.4' identified by 'replpass';
MariaDB [(none)]> flush privileges;
MariaDB [(none)]> show master status\G
*************************** 1. row ***************************
File: mysql-bin.000003
Position: 497
Binlog_Do_DB:
Binlog_Ignore_DB:

##### 记下mysql-bin.000003 和 497 , 设置从服务器中继日志时有用 ####

node4: mariadb从服务器

[root@node4 ~]# yum -y install mariadb-server
[root@node4 ~]# vim /etc/my.cnf
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0
# Settings user and group are ignored when systemd is used.
# If you need to run mysqld under a different user or group,
# customize your systemd unit file for mariadb according to the
# instructions in http://fedoraproject.org/wiki/Systemd 
########## 添加以下内容 ##########
log-bin=mysql-bin
binlog_format=mixed
server-id = 2
relay-log = relay-bin
log_slave_updates = 1
read_only = on

port = 3306
skip-external-locking
key_buffer_size = 256M
max_allowed_packet = 1M
table_open_cache = 256
sort_buffer_size = 1M
read_buffer_size = 1M
read_rnd_buffer_size = 4M
myisam_sort_buffer_size = 64M
thread_cache_size = 8
query_cache_size= 16M
thread_concurrency = 4
innodb_file_per_table = on
skip_name_resolve = on
###################################

######### 以下内容可选 ############
[mysqldump]
quick
max_allowed_packet = 16M

[mysql]
no-auto-rehash

[myisamchk]
key_buffer_size = 128M
sort_buffer_size = 128M
read_buffer = 2M
write_buffer = 2M

[mysqlhotcopy]
interactive-timeout
####################################

[mysqld_safe]
log-error=/var/log/mariadb/mariadb.log
pid-file=/var/run/mariadb/mariadb.pid

#
# include all files from the config directory
#
!includedir /etc/my.cnf.d

############# End of my.cnf ###############

[root@node4 ~]# systemctl start mariadb
[root@node4 ~]# mysql
MariaDB [(none)]> show global variables like '%read_only%'\G
*************************** 1. row ***************************
Variable_name: read_only
Value: ON

MariaDB [(none)]> change master to master_host='172.16.92.3',master_user='repluser',master_password='replpass',master_log_file='mysql-bin.000003',master_log_pos=497,master_connect_retry=5,master_heartbeat_period=2;
MariaDB [(none)]> install plugin rpl_semi_sync_slave soname 'semisync_slave.so';    #安装Slave半同步插件
MariaDB [(none)]> set global rpl_semi_sync_slave_enabled = 1;        #开启Slave半同步功能
MariaDB [(none)]> start slave;        #重启IO线程生效

MariaDB [(none)]> show slave status\G
*************************** 1. row ***************************
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
........ 其余信息略 ........

MariaDB [(none)]> show processlist\G
*************************** 3. row ***************************
State: Slave has read all relay log; waiting for the slave I/O thread to update it
........ 其余信息略 ........说明: 从节点已经接收到所有的中继日志

MariaDB [(none)]> show global status like '%semi%';
| Rpl_semi_sync_slave_status | ON |#半同步状态已经开启了；

MariaDB [(none)]> show global variables like '%rpl%';
| rpl_semi_sync_slave_enabled | ON |#从服务器节点的半同步已经开启；

node3 主节点上可查看到此进程
MariaDB [(none)]> show processlist\G
*************************** 2. row ***************************
State: Master has sent all binlog to slave; waiting for binlog to be updated
........ 其余信息略 ........

MariaDB [(none)]> show global status like 'rpl_semi%';
| Rpl_semi_sync_master_clients | 1 |    #可以看出有一个半同步客户端了；
........ 其余信息略 ........

在主节点上创建数据库测试是否能主从半同步
MariaDB [(none)]> create database testdb;

在从节点上可看到testdb数据库, 说明主从同步成功!
MariaDB [(none)]> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| test               |
| testdb             |
+--------------------+

再来看一下 从节点 的状态
MariaDB [(none)]> show slave status\G
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 172.16.92.3
Master_User: repluser
Master_Port: 3306
Connect_Retry: 5
Master_Log_File: mysql-bin.000003
Read_Master_Log_Pos: 584        #497->584
Relay_Log_File: relay-bin.000002
Relay_Log_Pos: 616            #529->616
Relay_Master_Log_File: mysql-bin.000003
Slave_IO_Running: Yes
Slave_SQL_Running: Yes

############# mysql半主从复制结束 ##############