Configuring SQL Statement Concurrency Control

Scenarios

SQL statement concurrency control aims to keep TaurusDB instances running stably even if there is a sudden increase in concurrent SQL statements.

Constraints

• This function is only available to TaurusDB instances that meet the following requirements. For details about how to check the kernel version, see How Can I Check the Version of a TaurusDB Instance?
  • 2.0.28.40 > kernel version ≥ 2.0.28.15
  • Kernel version ≥ 2.0.29.1
• Concurrency control rules for INSERT statements are only supported when the kernel version of your TaurusDB instance is 2.0.54.240600 or later.
• A single concurrency control rule can contain a maximum of 128 keywords.
• Single backslashes (\) or single null characters (' ') cannot be used as keywords.
• Spaces at the beginning and end of a keyword and special null characters (such as \'t', \'r', and \'n') will be ignored.
• A concurrency control rule cannot end with a tilde (~).
• Keywords in a concurrency control rule are sorted in a specific order, and the system will match them from first to last. For example, if one rule contains the keyword a~and~b, the system only matches xxx a>1 and b>2.
• Each SQL concurrency control rule applies to only the SQL statements that your database received after the rule is created.
• If different rules are created for the primary node and read replicas of a DB instance, the rules still apply to the primary node and read replicas after their roles are switched over.
• If an SQL statement matches multiple concurrency control rules, only the most recently created rule is applied.
• SQL statements that have been executed before a concurrency control rule is added are not counted.
• If the TaurusDB kernel version is 2.0.54.240600 or later, the total length of all rules for SELECT, UPDATE, DELETE, or INSERT statements and the Concurrency value in each rule cannot exceed 4,000 bytes. The length of a single rule cannot exceed 1,000 bytes.
• If the TaurusDB kernel version is earlier than 2.0.54.240600, the total length of all rules for SELECT, UPDATE, or DELETE statements and the Concurrency value in each rule cannot exceed 1,024 bytes.
• If there are too many concurrency control rules, performance of SELECT, UPDATE, DELETE, and INSERT statements will deteriorate.
• SQL concurrency control rules are applied based on prefix match. For example, if the concurrency control rule is SELECT~COUNT~t1, SQL statements SELECT COUNT(*) FROM t1 and SELECT COUNT(*) FROM t1 LIMIT 1 will both be intercepted.
• After concurrency control is triggered, an execution error is reported on the service side, indicating that query execution was interrupted. The error code is ERROR 1317 (70100).
• This function controls how many statements can run at the same time. However, it does not limit concurrency for:
  • System tables
  • Queries where no database data is involved, such as select sleep(xxx)
  • Account root
  • SQL statements in stored procedures, triggers, and functions

Procedure

1. Log in to the management console.
2. Click in the upper left corner and select a region and project.
3. Click in the upper left corner of the page and choose Databases > TaurusDB.
4. On the Instances page, click the instance name.
5. In the navigation pane, choose DBA Assistant > Historical Diagnosis.
6. Choose SQL Explorer > Concurrency Control.
7. On the displayed page, enable Concurrency Control.
   Figure 1 Enabling SQL statement concurrency control
   

8. Click Add Rule. In the displayed dialog box, specify SQL Type, Keyword, and Max. Concurrency.
   Figure 2 Adding a concurrency control rule
   
   • Keyword: You can enter keywords or copy an existing SQL statement to the text box and click Generate Keyword.

     Keyword: Take select~a as an example. select and a are two keywords contained in a concurrency control rule. The keywords are separated by a tilde (~). In this example, the rule restricts the execution of only the SQL statements containing keywords select and a.

   • Max. Concurrency: SQL statements that meet the specified SQL type and keyword and exceed the value of Max. Concurrency will not be executed.
   • If you select Kill existing sessions that match this rule, the sessions that match the rule will be killed.
   • If you select Synchronize rules to other nodes, the new rules can be synchronized to other nodes in the same instance.

9. Confirm the settings and click OK.
10. If a concurrency control rule is not required, select the rule and click Delete above the rule list. In the displayed dialog box, click OK.