Using Hints for Read/Write Splitting
In addition to the weight assignment rules of read/write splitting, hints serve as a complementary SQL syntax to specify whether a SQL statement is executed on a primary node or read replica.
This section describes how to use hints to route read and write requests to a primary node or read replica.
Precautions
Hints are only used as routing suggestions. In non-read-only SQL and non-transaction scenarios, SQL statements cannot be routed to read replicas.
Usage
You can add the following hints at the beginning of a SQL statement as needed.
/*FORCE_MASTER*/: The SQL statement is executed on the primary node.
/*FORCE_SLAVE*/: The SQL statement is executed on read replicas.
For example, if you run select * from table1, the SQL statement will be executed on a read replica by default. If you change it to /*FORCE_MASTER*/ select * from table1, the SQL statement will be executed on the primary node.
/*FORCE_MASTER*/ only works for read/write addresses. If your primary node is read-only, adding /*FORCE_MASTER*/ will not help route the SQL statement to the primary node.
Hints are case insensitive. For example, /*FORCE_MASTER*/ and /*force_master*/ work the same.
Hints cannot contain spaces. Incorrect example: /* FORCE_MASTER */; correct example: /*FORCE_MASTER*/
Hints must be placed at the beginning of a SQL statement.
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