MySQL write lock does not take effect

1. background

   Need to verify ： Program query mylsq Whether the execution process after failure meets expectations .
   Procedures to be tested ：1. From time to time mysql Read data from ;2. The program has configured read-write timeout .

Verification process :
   After the program starts , Add write lock , Next query mysql It should have failed , Will execute the failed process . After adding write lock , It can also read data correctly .

2. Lock / Unlock

Lock ：

• locak table ‘table’ [read|write]

Unlock ：

• unlock tables

3. Cause of failure

Performance after locking ：

1. Regular reading of data in the program is normal
2. Current window terminal , Can query data
3. Newly opened terminal , Query data blocking

In terms of performance 1 and 2 Before locking, there are from db Read data in ,3 Start reading data after locking . Speculation is related to caching .

Query found open cache ：

MariaDB> show variables like "%query_cache%";
+------------------------------+----------+
| Variable_name                | Value    |
+------------------------------+----------+
| have_query_cache             | YES      |
| query_cache_limit            | 1048576  |
| query_cache_min_res_unit     | 4096     |
| query_cache_size             | 33554432 |
| query_cache_strip_comments   | OFF      |
| query_cache_type             | ON       |
| query_cache_wlock_invalidate | OFF      |
+------------------------------+----------+

The cache will expire after the data is updated , Update the table data , Program timing query failed , In line with expectations .

4. summary

   When the table needs to be locked for verification , If the data can be read after locking , The reason for the invalidation may be related to opening the cache . terms of settlement , perform update or insert Update the table .