Read-write Separation

Last updated: 2019-12-06 16:19:19

    Overview

    Feature overview

    When the read requests lead to high pressure and demand during processing of high volume of data, read-write separation can be used to distribute the read requests to slave nodes.
    TDSQL supports read-write separation by default. Each slave in the master-slave architecture can be read-only. If multiple slaves are configured, read requests will be automatically allocated to low-load slaves by the gateway cluster (TProxy) to support the read traffic of large applications.

    How it works

    Read-write separation is to separate read and write capabilities by letting the master node handle transactional operations (INSERT, UPDATE, and DELETE) and the slave nodes perform query operations (SELECT).

    Using Read-Write Separation

    Read-write separation based on read-only account

    A read-only account is a type of account only with read permission to read data from a slave (or read-only instance) in a database cluster by default.
    A read-only account can be used to implement automatic distribution of read requests to slaves and return results.

    Read-write separation policy

    In the settings of a read-only account, you can set the "read-only request allocation policy" to define the "read" policy when a slave fails (or lags for a long time).

    • Select Master to read from the master when the delay of the slave exceeds the limit.
    • Select Report Errors to report an error when the delay of the slave exceeds the limit.
    • Select Read Only from Slave Server to ignore the delay parameter and always read from the slave (this is generally used to pull binlog for sync).
    • Set the Read-only slave server delay parameter to define the data sync delay time, which is to be used together with Master Server and Report Errors under the Read-only request allocation policy.

    Read-write separation based on comment

    Add a /*slave*/ field before each SQL that needs to "read" from the slave, and add the -c parameter after mysql to parse the comment. For example, mysql -c -e "/*slave*/sql" can automatically allocate "read" requests to the slave. Below is the sample code:

    //Read from master//
    select * from emp order by sal, deptno desc;
    //Read from slave//
    /*slave*/ select * from emp order by sal, deptno desc;
    • Read from slave is supported only for the SELECT operation. Non-SELECT statements will fail.
    • The -c parameter needs to be added after mysql to parse the comment.
    • /*slave*/ must be in lowercase, and no spaces are needed before and after the statement.
    • If the MAR mechanism (strong sync) is affected by slave exception, read from slave will be automatically switched to read from master.

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