Summary of advertisement business bug replay

Because of the company's business adjustment , Recently took over the business of the company's advertising system , One of them encountered such a Bug, Its appearance is like this ： There is no money in the advertiser's account , But the advertisement still put it on him .

---

One 、 Basic business logic

First of all, there are several ways to charge advertisements ,CPA,CPT,CPC,CPM etc. , and CPM This billing method is per 1000 Charge for each exposure x element , That is to say, if the unit price is 10 element , Then a single exposure is 0.01 element , For each exposure, we will first deduct the corresponding money from the cache balance of the advertising system , Deduct all the money , Will send a message to the account system , And then execute local things , Drop the deduction order , Submit sth , Then the account system will also record the deduction amount in the cache , If the account fails to deduct money , It will also send a message of failed deduction , After the advertising system receives this message , It will replenish the balance into its own cache .
 

Here is a point that I don't quite understand ：

Why has the advertisement been exposed , But calling the account system to deduct money failed , It is necessary to make up the user's balance , For example, merchants have sold their goods , But the user failed to deduct money when paying , The balance is made up for the user again .

---

Two 、Bug spot

bug Just before the deduction failed , Make up the user's balance here , Because it is sending to the account system CPM A kind of advertising message , Will improve the accuracy 3, That is, the amount of a single exposure will * 1000 Send to the account system , When the account system fails to deduct money , Will be intact * 1000 Send the later unit price , And make up the user balance of the advertising system . Here's an example ：

Assume user balance 1000, Single exposure 2 element （ Expensive, huh ～）, After a single exposure , The user balance should be 998 element , But the account system failed to deduct money , Will make up the user's balance , because * 1000, promote 3 precision , The balance made up becomes 998 + 2 * 1000 = 2998

Pinduoduo doesn't bring how to play ...

That's what happened There is no money in the advertiser's account , But the advertisement still put it on him . Because the user cache balance of the advertising system is much larger than the balance of the account system , When the cache balance is less than 0 It will trigger an advertisement off the shelf , In this way, it will not be triggered , And because the account is out of money , The advertising system has been receiving the message of failed deduction , Crazy continue to fill the balance ...

---

3、 ... and 、 Remedial measures

1、 Precision processing ： Contact the students of the account system , Add a precision to the message scale Field , Such as scale = 3, Corresponding to will amout / 1000

2、 The message of deduction failure is the message that the account balance is insufficient , Do not make up the balance

   Then I found that the code has such a comment , Then I'll do it to the end , By the way

  // Pseudo code

Long setnx = redis.setnx( Business Key+ The only news id,"1");
redis.expire( Business Key+ The only news id,x);
if(setnx == 0) return;