如何防止数据库更新的并发问题？ [英] How to prevent concurrency issue with database update?

问题描述

我有以下序列，可以在任何时候由我的程序的多个实例执行：

1：检查帐单是否未付款

步骤2：如果帐单未付， p>

我担心我的程序的实例1和实例2将几乎完全同时执行步骤1，因此继续执行步骤2.如何防止这种情况？

解决方案

 
 
 code> $ invoice_status = call_database（
 select INVOICE_STATUS 
从INVOICES 
其中INVOICE_ID = $ invoice_id）
 
 if（$ invoice_status = UNPAID）{
 call_database（
 UPDATE INVOICES 
 SET INVoICE_STATUS = PAID 
 WHERE INVOICE_ID = $ invoice_id）
} 
  

您将执行

  call_database（
 update invoices 
 set invoice_status = PAID 
其中invoice_id = $ invoice_id 
和invoice_status = UNPAID）
  

I have the following sequence which may be executed by more than one instance of my program at any time:

Step 1: Check to see whether bill is unpaid

Step 2: If bill is unpaid, then mark bill as paid

I am worried that Instance 1 and Instance 2 of my program will both do Step 1 at almost exactly the same time and therefore proceed to both do Step 2. How can I prevent this happening?

解决方案

It's hard to be concrete about this without seeing code, but the obvious way to do this is to combine steps 1 and 2.

So, instead of:

$invoice_status = call_database(
select INVOICE_STATUS
from   INVOICES
where  INVOICE_ID = $invoice_id)

if ($invoice_status = UNPAID){
   call_database (
      UPDATE INVOICES 
      SET INVOICE_STATUS = PAID 
      WHERE INVOICE_ID = $invoice_id)
}

You would do something like

call_database(
   update invoices
   set    invoice_status = PAID
   where  invoice_id = $invoice_id
   and    invoice_status = UNPAID)

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