Concurrency Control

Scenarios

Concurrency Control keeps TaurusDB instances stable regardless of how many SQL statements are concurrently submitted.

Constraints

• This function is only available to TaurusDB instances that meet the following requirements:
  • 2.0.28.40 > kernel version ≥ 2.0.28.15
  • Kernel version ≥ 2.0.29.1
• Each SQL concurrency control rule can contain up to 128 keywords.
• The keywords in a rule cannot contain \t, \r, and \n, and cannot be a backslash (\) or a single null character ('').
• Spaces at the start, end of or in the middle of a keyword are ignored.
• The SQL 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 a 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.
• The total length of all rules for SELECT, UPDATE, or DELETE statements and the Concurrency value in each rule cannot exceed 1024 bytes.
• If you add too many SQL concurrency control rules for your instance, the execution of SELECT, UPDATE, or DELETE statements will slow down.
• 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 catalog
  • 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, choose Database > TaurusDB.
4. On the Instances page, click the instance name.
5. In the navigation pane, choose DBA Assistant > Historical Diagnosis.
6. Click SQL Explorer and then Concurrency Control.
7. On the displayed page, enable Concurrency Control.
8. Click Add Rule. In the displayed dialog box, specify SQL Type, Keyword, and Max. Concurrency.
   • 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 meet this rule, the sessions that meet 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.