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This article introduces the relevant knowledge of "problem description of php multi-process simulation concurrent transactions". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Preface

Through the example code, this paper introduces some problems caused by php multi-process simulation concurrent transactions, which are shared for your reference and study. I won't say much below, let's take a look at the detailed introduction.

Table

Drop table if exists `test`; create table if not exists `test` (id int not null auto_increment, count int default 0, primary key `id` (`id`)) engine=innodb character set utf8mb4 collate = utf8mb4_bin comment 'test table'; insert into test (`count`) values

Php code

/ / the number of processes $pro_count = 100 per capita pids = []; for ($I = 0 per $I

0) {/ / main process $pids [] = $pid;} else {/ / Child process try {$pdo = new PDO (...); $pdo- > beginTransaction (); $stmt = $pdo- > query ('select `count` from test'); $count = $stmt- > fetch (PDO::FETCH_ASSOC) [' count']; $count = intval ($count); if ($count > 0) {$count--; $pdo- > query ('update test set `count` ='. $count. ' Where id = 2');} $pdo- > commit ();} catch (Exception $e) {$pdo- > rollBack (); throw $e;} / / exit child process exit;}

Expected result

It is expected that the amount of reduction in the count field is more than 100 and becomes negative! That is to say, more reduction!

Actual result

In the case of 200 parallel, the results of multiple runs are as follows:

1. Count = 65

2. Count = 75

3. Count = 55

4. Count = 84

...

It is a far cry from the expected result! Why is there such a phenomenon?

explain

First of all, be clear about the current program running environment, concurrent scenarios. What is concurrency, executed almost at the same time, is called concurrency. The specific explanation is as follows:

The process gets updates

1-40 create and run 100 99 at the same time

41-80 simultaneously create and run 99 98

81-100 simultaneously create and run 98 97

To explain the first line above, the 1st-40th subprocesses are created and run almost at the same time:

Process 1 gets count = 100, update 99

Process 2 gets count = 100, update 99

...

Process 40 gets count = 100, update 99

So, in fact, these processes have done consistent operations, not as expected: process 1 gets the count=100 and updates 99; process 2 gets the updated result count=99 of process 1, updates 98. Process 99 gets the updated count=1 of process 98, and updates 0.

The phenomenon is reduced!

Conclusion

Using the above approach to achieve the program, inventory is always > = 0.

Doubt

So how to design the program to simulate the scenario of overstock?

Still using the above code, put the following code:

If ($count > 0) {$count--; $pdo- > query ('update test set `count` ='. $count. ' Where id = 2');}

Modify it to look like this:

If ($count > 0) {$pdo- > query ('update test set `count` = `count`-1 where id = 2');}

As a result, there will be excess inventory!

Inventory 100, concurrently 200, the final inventory is reduced to-63. Why did this happen? The following describes the specific process of running the program

Process 1 acquires inventory 100, updates 99

Process 2 acquires inventory 100, updates 98 (99-1)

Process 3 acquires inventory 100, updates 97 (98-1)

....

Process 168 acquires inventory 1, updates 0 (1-1)

Process 169 get inventory 1, update-1 (0-1)

Process 170 acquires inventory 1, updates-2 (- 1-1)

....

Process 200 acquires inventory 1, updates-63 (- 62-1)

Now it seems to be very confused, but it is actually caused by the following sentence:

$pdo- > query ('update test set `count` = `count`-1 where id = 2')

Here, process 1 is described in detail, referred to as a; process 2, referred to as b, their specific execution order:

1. A query to inventory 100

2. B query to inventory 100

3. A update inventory to 99 (100-1), this should be understood in seconds

4. B Update inventory to 98 (99-1)

-b gets a updated inventory when performing the update operation!

-Why is that? Because the update statement is `update test set count = count-1 where id = 2`

This is the end of the description of the problems caused by php multi-process simulation of concurrent transactions. Thank you for your reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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