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這個事情怎么產生的

MyCat作為經典的分庫分表中間件，在長時間內被廣泛認為是管理超大MySQL數據庫集合的有效解決方案。近來接到客戶需求，需要將MyCat集群遷移到GreatSQL中，并且在一段時間內需要實時從MyCat中同步數據到GreatSQL中，全量同步數據比較容易操作，增量同步有如下兩個棘手的問題：

多個server，不同的庫名字，都要同步到GreatSQL一個庫中，即同步關系如下

server1:db1.tab->gdb:db.tab;
server2:db2.tab->gdb:db.tab;
server3:db3.tab->gdb:db.tab;

ddl同步多次執行會沖突。當MyCat的表中添加一個索引、添加一個字段時，實際上是后端所有db都會執行這個DDL，同步到GreatSQL時，多次執行DDL，復制會異常中斷。

為了解決上面兩個問題，經過查詢資料，發現有兩個不常用，官方也不建議使用的功能，剛好能夠滿足需求

為解決庫名映射問題：需要在配置文件中添加參數

replicate_rewrite_db="channel_1:test_rep1->test_rep"
replicate_rewrite_db="channel_2:test_rep2->test_rep"
replicate_rewrite_db="channel_3:test_rep3->test_rep"

為了解決DDL同步后重復執行導致復制中斷問題，在配置文件中添加

slave-skip-errors=ddl_exist_errors

驗證一下

為了簡化問題，MyCat集群咱們就不搭建了，簡化為多源同步復制問題。

1.初始化4個實例，同步關系如下

源端口	源DB_NAME	目標端口	目標映射DB	channel_name
3306	test_rep1	3309	test_rep	channel_3306
3307	test_rep2	3309	test_rep	channel_3307
3308	test_rep3	3309	test_rep	channel_3308

2.在3309的實例配置文件中，添加庫映射關系配置和DDL沖突忽略參數

replicate_rewrite_db="channel_3306:test_rep1->test_rep"
replicate_rewrite_db="channel_3307:test_rep2->test_rep"
replicate_rewrite_db="channel_3308:test_rep3->test_rep"
slave-skip-errors=ddl_exist_errors

4.在3309實例中，配置三個channel

greatsql> change master to MASTER_HOST = '172.17.137.91',MASTER_USER = 'greatsql',MASTER_PASSword = 'greatsql',MASTER_PORT = 3306,MASTER_AUTO_POSITION = 1 FOR CHANNEL 'channel_3306';

greatsql> change master to MASTER_HOST = '172.17.137.91',MASTER_USER = 'greatsql',MASTER_PASSWORD = 'greatsql',MASTER_PORT = 3307,MASTER_AUTO_POSITION = 1 FOR CHANNEL 'channel_3307';

greatsql> change master to MASTER_HOST = '172.17.137.91',MASTER_USER = 'greatsql',MASTER_PASSWORD = 'greatsql',MASTER_PORT = 3308,MASTER_AUTO_POSITION = 1 FOR CHANNEL 'channel_3308';

greatsql> start slave;

3.檢查channel配置狀態

greatsql> show slave status G
*************************** 1. row ***************************
               Slave_IO_State: WAIting for source to send event
                  Master_Host: 172.17.137.91
                  Master_User: greatsql
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: binlog.000002
          Read_Master_Log_Pos: 1119
               Relay_Log_File: relaylog-channel_3306.000007
                Relay_Log_Pos: 397
        Relay_Master_Log_File: binlog.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 1119
              Relay_Log_Space: 606
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 3306
                  Master_UUID: 5facacd7-9ed6-11ee-b76b-00163e5af5d6
             Master_Info_File: mysql.slave_master_info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Replica has read all relay log; waiting for more updates
           Master_Retry_Count: 86400
                  Master_Bind: 
      Last_IO_Error_Timestamp: 
     Last_SQL_Error_Timestamp: 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: 
            Executed_Gtid_Set: 49b66af0-9ed6-11ee-ae4f-00163e5af5d6:3,
5a369d0b-9ed6-11ee-b3d1-00163e5af5d6:3,
5facacd7-9ed6-11ee-b76b-00163e5af5d6:1-4,
9a5f9c26-4262-11ee-85fd-00163e5af5d6:1-16571224
                Auto_Position: 1
         Replicate_Rewrite_DB: (test_rep1,test_rep)
                 Channel_Name: channel_3306
           Master_TLS_Version: 
       Master_public_key_path: 
        Get_master_public_key: 0
           .NETwork_Namespace: 
*************************** 2. row ***************************
               Slave_IO_State: Waiting for source to send event
                  Master_Host: 172.17.137.91
                  Master_User: greatsql
                  Master_Port: 3307
                Connect_Retry: 60
              Master_Log_File: binlog.000002
          Read_Master_Log_Pos: 1119
               Relay_Log_File: relaylog-channel_3307.000004
                Relay_Log_Pos: 1034
        Relay_Master_Log_File: binlog.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 1119
              Relay_Log_Space: 1243
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 3307
                  Master_UUID: 5a369d0b-9ed6-11ee-b3d1-00163e5af5d6
             Master_Info_File: mysql.slave_master_info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Replica has read all relay log; waiting for more updates
           Master_Retry_Count: 86400
                  Master_Bind: 
      Last_IO_Error_Timestamp: 
     Last_SQL_Error_Timestamp: 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: 5a369d0b-9ed6-11ee-b3d1-00163e5af5d6:1-2:4
            Executed_Gtid_Set: 49b66af0-9ed6-11ee-ae4f-00163e5af5d6:3,
5a369d0b-9ed6-11ee-b3d1-00163e5af5d6:3,
5facacd7-9ed6-11ee-b76b-00163e5af5d6:1-4,
9a5f9c26-4262-11ee-85fd-00163e5af5d6:1-16571224
                Auto_Position: 1
         Replicate_Rewrite_DB: (test_rep2,test_rep)
                 Channel_Name: channel_3307
           Master_TLS_Version: 
       Master_public_key_path: 
        Get_master_public_key: 0
            Network_Namespace: 
*************************** 3. row ***************************
               Slave_IO_State: Waiting for source to send event
                  Master_Host: 172.17.137.91
                  Master_User: greatsql
                  Master_Port: 3308
                Connect_Retry: 60
              Master_Log_File: binlog.000002
          Read_Master_Log_Pos: 1119
               Relay_Log_File: relaylog-channel_3308.000004
                Relay_Log_Pos: 1034
        Relay_Master_Log_File: binlog.000002
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 1119
              Relay_Log_Space: 1243
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 3308
                  Master_UUID: 49b66af0-9ed6-11ee-ae4f-00163e5af5d6
             Master_Info_File: mysql.slave_master_info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Replica has read all relay log; waiting for more updates
           Master_Retry_Count: 86400
                  Master_Bind: 
      Last_IO_Error_Timestamp: 
     Last_SQL_Error_Timestamp: 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: 49b66af0-9ed6-11ee-ae4f-00163e5af5d6:1-2:4
            Executed_Gtid_Set: 49b66af0-9ed6-11ee-ae4f-00163e5af5d6:3,
5a369d0b-9ed6-11ee-b3d1-00163e5af5d6:3,
5facacd7-9ed6-11ee-b76b-00163e5af5d6:1-4,
9a5f9c26-4262-11ee-85fd-00163e5af5d6:1-16571224
                Auto_Position: 1
         Replicate_Rewrite_DB: (test_rep3,test_rep)
                 Channel_Name: channel_3308
           Master_TLS_Version: 
       Master_public_key_path: 
        Get_master_public_key: 0
            Network_Namespace: 
3 rows in set, 1 warning (0.00 sec)

在上面的輸出中，可以重點關注如下字段信息，說明db轉換映射成功

$ MYSQL_PWD=greatsql mysql -ugreatsql -h127.0.0.1 -P3309 -e 'show replica status G'| grep -wE 'Replica_IO_Running|Replica_SQL_Running|Replicate_Rewrite_DB|Channel_Name'
           Replica_IO_Running: Yes
          Replica_SQL_Running: Yes
         Replicate_Rewrite_DB: (test_rep1,test_rep)
                 Channel_Name: channel_3306
           Replica_IO_Running: Yes
          Replica_SQL_Running: Yes
         Replicate_Rewrite_DB: (test_rep2,test_rep)
                 Channel_Name: channel_3307
           Replica_IO_Running: Yes
          Replica_SQL_Running: Yes
         Replicate_Rewrite_DB: (test_rep3,test_rep)
                 Channel_Name: channel_3308

在3309實例中，查詢replica_skip_errors，確認復制異常跳過的錯誤碼，設置為ddl_exist_errors會自動轉換為如下錯誤碼

greatsql> select @@replica_skip_errors;
+---------------------------------------------------+
| @@replica_skip_errors                             |
+---------------------------------------------------+
| 1007,1008,1050,1051,1054,1060,1061,1068,1091,1146 |
+---------------------------------------------------+
1 row in set (0.00 sec)

5.數據同步驗證

在3309庫中，創建database test_rep

這個庫需要手動創建，是測試發現映射關系只對庫下面的表生效，庫不會自動轉換創建。

在3306庫中，創建database test_rep1，并且創建tab1表

$ mysql -ugreatsql -h127.0.0.1 -p -P3306 -s

greatsql> create database test_rep1;
greatsql> use test_rep1;
greatsql> create table tab1(id int primary key auto_increment, cname varchar(10), age int);

在3307庫中，創建database test_rep2，并且創建tab1表

$ mysql -ugreatsql -h127.0.0.1 -p -P3307 -s

greatsql> create database test_rep2;
greatsql> use test_rep2;
greatsql> create table tab1(id int primary key auto_increment, cname varchar(10), age int);

在3308庫中，創建database test_rep3，并且創建tab1表

$ mysql -ugreatsql -h127.0.0.1 -p -P3308 -s

greatsql> create database test_rep3;
greatsql> use test_rep3;
greatsql> create table tab1(id int primary key auto_increment, cname varchar(10), age int);

在3309中確認database及表的同步

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -s

greatsql> show databases;
Database
information_schema
mysql
performance_schema
sys
test_db
test_rep
test_rep1
test_rep2
test_rep3

greatsql> show tables from test_rep;
Tables_in_test_rep
tab1
greatsql> show tables from test_rep1;
greatsql> show tables from test_rep2;
greatsql> show tables from test_rep3;

從上面的信息可以看出，在3309中，3306、3307、3308中創建的庫均按照原有的名字進行了同步，但是表只同步在了3309映射的庫test_rep中。

分別在3306、3307、3308中插入一條記錄

3306 : insert into test_rep1.tab1 values(1,'a',10);
3307 : insert into test_rep2.tab1 values(2,'b',20);
3308 : insert into test_rep3.tab1 values(3,'c',30);

然后在各自節點查詢數據插入情況

$ mysql -ugreatsql -h127.0.0.1 -p -P3306 -s test_rep1 -e 'select * from tab1'
id       cname    age
1        a        10

$ mysql -ugreatsql -h127.0.0.1 -p -P3307 -s test_rep2 -e 'select * from tab1'
id       cname    age
2        b        20

$ mysql -ugreatsql -h127.0.0.1 -p -P3308 -s test_rep3 -e 'select * from tab1'
id       cname    age
3        c        30

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -s test_rep -e 'select * from tab1'
id       cname    age
1        a        10
2        b        20
3        c        30

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -s test_rep1 -e 'select * from tab1'
ERROR 1146 (42S02) at line 1: Table 'test_rep1.tab1' doesn't exist

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -s test_rep2 -e 'select * from tab1'
ERROR 1146 (42S02) at line 1: Table 'test_rep2.tab1' doesn't exist

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -s test_rep3 -e 'select * from tab1'
ERROR 1146 (42S02) at line 1: Table 'test_rep3.tab1' doesn't exist

從上面的查詢情況可以看出，3306、3307、3308節點中只有一條記錄，并且記錄都被同步到了3309的test_rep.tab1表中，而且在3309的test_rep1、test_rep2、test_rep3中是沒有表存在的。

分別在3306、3307、3308給表tab創建一個索引

greatsql> alter table tab1 add index idx_cname(cname);

觀察3309中表的索引情況，可以看到索引idx_cname被同步過來了

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -s

greatsql> use test_rep
greatsql> show create table tab1 G
*************************** 1. row ***************************
       Table: tab1
Create Table: CREATE TABLE `tab1` (
  `id` int NOT NULL AUTO_INCREMENT,
  `cname` varchar(10) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  `age` int DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_cname` (`cname`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
1 row in set (0.00 sec)

分別在3306、3307、3308做update、delete操作

greatsql> update test_rep1.tab1 set age=110 where id=1;
greatsql> update test_rep2.tab1 set age=120 where id=2;
greatsql> update test_rep3.tab1 set age=130 where id=3;

greatsql> delete from test_rep1.tab1 where id=1;
greatsql> delete from test_rep2.tab1 where id=1;
greatsql> delete from test_rep3.tab1 where id=1;

查看3309的數據同步情況，確認數據被清理

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -s
Welcome to the MySQL monitor.  Commands end with ; or g.
greatsql> select * from test_rep.tab1;

觀察3個channel的同步情況，可以確認三個復制同步均正常

$ mysql -ugreatsql -h127.0.0.1 -p -P3309 -e 'show replica status G'| grep -E 'Replica_IO_Running|Replica_SQL_Running|Channel_Name'
greatsql: [Warning] Using a password on the command line interface can be insecure.
           Replica_IO_Running: Yes
          Replica_SQL_Running: Yes
    Replica_SQL_Running_State: Replica has read all relay log; waiting for more updates
                 Channel_Name: channel_3306
           Replica_IO_Running: Yes
          Replica_SQL_Running: Yes
    Replica_SQL_Running_State: Replica has read all relay log; waiting for more updates
                 Channel_Name: channel_3307
           Replica_IO_Running: Yes
          Replica_SQL_Running: Yes
    Replica_SQL_Running_State: Replica has read all relay log; waiting for more updates
                 Channel_Name: channel_3308

至此dml、ddl同步均驗證。

方案缺陷

本方案中，業務訪問MyCat的表名字，和server后端的表名字完全一致，只是庫名字不相同，然后MyCat代理表名和實際server的表名字可以不相同，這種情況下，暫時無法映射處理
MyCat代理的實際上是多個單獨的庫，如果這些庫之前沒有做自增主鍵步長處理，或者其他一些主鍵不重復策略，同步過程中，會存在主鍵沖突導致數據同步中斷的情況，需要提前準備處理方案。

最后附上參考資料

https://dev.mysql.com/doc/refman/8.0/en/replication-options-replica.html#option_mysqld_replicate-rewrite-db
https://dev.mysql.com/doc/refman/8.0/en/replication-options-replica.html#option_mysqld_slave-skip-errors