DDL Fast Timeout

Introduction

In MySQL Community Edition, all DDL operations require an MDL lock. If a DDL operation keeps waiting for such a lock, subsequent DML operations will be blocked.

TaurusDB lets you set an MDL wait time (DDL timeout) for ALTER TABLE, CREATE INDEX, and DROP INDEX operations. This allows these operations to quickly time out if the lock is unavailable, avoiding blocking subsequent DML operations.

Constraints

The kernel version must be 2.0.45.230900 or later.

For details about how to check the kernel version, see How Can I Check the Version of a TaurusDB Instance?

Enabling DDL Fast Timeout

**Table 1** Parameter description
Parameter	Level	Description
rds_ddl_lock_wait_timeout	Global, Session	Defines how long that a DDL operation waits for a lock in the current session or global sessions. Value range: 1 to 31536000 (s). Default value: 31536000, indicating that the function is disabled. The actual lock wait timeout for DDL operations is the smaller value between lock_wait_timeout and this parameter value. The actual table lock timeout during DDL execution at the InnoDB layer is the minimum value of innodb_lock_wait_timeout and this parameter value. Row locks are not considered.

Example

Start a client and add a lock for tables.
Figure 1 Adding a lock
Run the following command to check the status of the DDL fast timeout function
show variables like "%rds_ddl_lock_wait_timeout%";

Figure 2 Querying the status of the DDL fast timeout function

As shown in the preceding figure, the value of rds_ddl_lock_wait_timeout is 31536000 (default value). The function is disabled. The subsequent operations will wait for a long time.

To enable the function, go to 3.
Run the following command to set rds_ddl_lock_wait_timeout.
set rds_ddl_lock_wait_timeout=1;

Figure 3 Configuring parameters
Run the following command to create an index. It is found that the DDL operation times out quickly.
alter table lzk.t_lzk drop index indexa;
Figure 4 Creating an index