Scrum Master (PSM-I) Certified

The Scrum.org Professional Scrum Master I assessment is a 60 minute time boxed test where you answer 80 multiple choice type questions. The passing score is 85%.

Last time, for my PSD I certification, I studied all by myself.

This time, I participated in a 2 day crash course given by agile coach Pawel Mysliwiec of Pyxis. The course was in French. It was much better than studying alone. It was fun to meet like minded Scrum practitioners.

I took the test April 9, 2017 and passed. My Score was 74 points (or 92.5%)

I now have two pieces of scrum.org flair:

WordPress as a Development Platform

Many people dislike WordPress code. It’s no secret that for contemporary PHP developers WordPress feels antiquated. The founder of WordPress was even once-upon-a-time vocal about not keeping up to date with the PHP eco-system because reasons.

Times changed. So did PHP. So did WordPress.

To give credit where credit is due, the reasoning behind WordPress’ conservative change management is sound. They don’t want to mess with their insanely huge user base.

WordPress powers 27.8% of all websites on the internet.

That’s a lot of users. By choosing WordPress as your development platform you get massive traction for free.

But… that code. Ugh!

The good news is that when you develop for WordPress you don’t ever touch WordPress code. Instead you write a Plugin. [1] I put forward that in 2017 nothing is stopping you from writing a good, clean Plugin other than yourself.

Environment

WordPress is PHP 7 compatible. WordPress is also HHVM compatible. Running on either vastly increases performance.

It follows that if your environment is PHP 7 then you get the syntax.

Syntax

WordPress Plugins can have PSR compatible namespaces.

WordPress’ answer to Event Dispatcher (and/or Observer) are the add_action() and add_filter() functions. These functions are compatible with closures.

Meaning you can write code like:

add_action('init', function() use ($v) {
    (new \Acme\Foo\Bar\SomeClass($v))->someMethod();
});

add_action('init', '\Acme\some_function');

add_action('init', ['\Acme\Foo\Bar\SomeOtherClass', 'someStaticMethod']);

add_action('init', [$this, 'someOtherMethod']);

Or *any* standards compliant PHP 7 code you want to write.

Tools

PHPStorm supports WordPress Plugin development out-of-the-box.

WP-CLI is a set of command-line tools for managing WordPress installations. It simplifies many developer and deployment related tasks and makes unit testing your plugin possible.

WordPress Plugins have PHP CodeSniffer rules ready to go.

WordPress Plugins can be installed using Composer.

Bedrock and Trellis for the win.

Open

WordPress is licenced under the GPL. This still matters.

Getting Things Done

Is there room for improvement? Of course! Just like PHP, WordPress is always improving with the caveat that just like PHP, WordPress strives to keep backwards compatibly.

Developers rejoice, WordPress is moving forward, kicking and screaming as we drag it into the future.

[1] WordPress also has a REST API if you’re into that kind of thing…

Using MySQL Deadlocks To Avoid Overselling

When developing an e-commerce application, unless you work at United Airlines, you generally want to avoid overselling.

Instead of punching your customers in the face why not use MySQL Deadlocks? (Turns out this is a feature not a bug!)

First attempt, creating deadlocks

MySQL has 4 transaction isolation levels: SERIALIZABLE, REPEATABLE READ, READ UNCOMMITTED, READ COMMITTED.

In the following proof of concept, where we have 50 of the same product in stock, and we run seige to represent concurrent customers buying the same product at the same time, we expect 50 “Success!” messages in our log files.

When we use any of REPEATABLE READ, READ UNCOMMITTED, or READ COMMITTED we oversell. (boo!)

When we use SERIALIZABLE we do not oversell (yay!) but some users get deadlock errors while others do not. (SQLSTATE[40001]: Serialization failure: 1213 Deadlock found when trying to get lock; try restarting transaction)

<?php

error_reporting(E_ALL | E_STRICT); // Development

/*
SQL:
CREATE DATABASE `deadlocktest` COLLATE 'utf8_general_ci';
CREATE TABLE `products` ( `id` int NOT NULL AUTO_INCREMENT PRIMARY KEY, `inventory` int NOT NULL );
INSERT INTO `products` (`id`, `inventory`) VALUES ('123', '50');

USEFUL LINUX COMMANDS:
$ rm log.txt; touch log.txt; chmod 777 log.txt
$ seige http://host/file.php
*/

// ------------------------------------------------------------------
// Config
// ------------------------------------------------------------------

$mysqlIsolation = 'SERIALIZABLE'; // ( SERIALIZABLE, REPEATABLE READ, READ UNCOMMITTED, READ COMMITTED )
$productId = 123;
$logFile = __DIR__ . '/log.txt';

$host = '127.0.0.1';
$db = 'deadlocktest';
$user = 'root';
$pass = '';
$charset = 'utf8';
$opt = [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::ATTR_EMULATE_PREPARES => false,
];

// ------------------------------------------------------------------
// Functions
// ------------------------------------------------------------------

/**
 * Simulate time it takes to call the payment gateway and do stuff
 */
function doPaymentGatewayStuff()
{
    usleep(500000); // Wait for 0.5 seconds
}

/**
 * Simulate buying a product from our inventory
 *
 * @param PDO $pdo
 * @param int $productId
 * @return int
 * @throws Exception
 */
function buyProduct(PDO $pdo, int $productId): int
{
    $pdo->beginTransaction();

    $selectStmt = $pdo->prepare('SELECT inventory FROM products WHERE id = :id ');
    $selectStmt->execute(['id' => $productId]);
    $res = $selectStmt->fetch();
    if ($res['inventory'] <= 0) {
        throw new Exception("Oh no! Sorry we're out inventory!");
    }

    $newInventory = $res['inventory'] - 1;
    $updateStmt = $pdo->prepare('UPDATE products SET inventory = :inventory WHERE id = :id ');
    $updateStmt->execute(['inventory' => $newInventory, 'id' => $productId]);

    doPaymentGatewayStuff();

    $pdo->commit();

    return $newInventory;
}

// ------------------------------------------------------------------
// Procedure
// ------------------------------------------------------------------

$uniqueUser = uniqid();
try {
    // Set up DB driver
    $pdo = new PDO("mysql:host={$host};dbname={$db};charset={$charset}", $user, $pass, $opt);
    $pdo->query("SET TRANSACTION ISOLATION LEVEL {$mysqlIsolation} ");

    // Simulate buying a product and decreasing inventory
    $newInventory = buyProduct($pdo, $productId);

    // No exceptions were thrown, we consider this successful
    $successMsg = "{$uniqueUser} - Success! Product {$productId} inventory has been decreased to {$newInventory}" . PHP_EOL;
    file_put_contents($logFile, $successMsg, FILE_APPEND);
    echo "$successMsg";
}
catch (Exception $e) {
    if (isset($pdo) && $pdo->inTransaction()) {
        $pdo->rollBack();
    }
    $errorMsg = "{$uniqueUser} - Error! " . $e->getMessage() . PHP_EOL;
    file_put_contents($logFile, $errorMsg, FILE_APPEND);
    echo "$errorMsg";
}

Second attempt, handling deadlocks

The above code has good intentions but many users get the dreaded deadlock message.

Turns out deadlocks are OK! You just have to handle them somehow.

Here’s a fixed proof of concept:

<?php

// ------------------------------------------------------------------
// Config
// ------------------------------------------------------------------

$mysqlIsolation = 'SERIALIZABLE';
$productId = 123;
$logFile = __DIR__ . '/log.txt';

$host = '127.0.0.1';
$db = 'deadlocktest';
$user = 'root';
$pass = '';
$charset = 'utf8';
$opt = [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::ATTR_EMULATE_PREPARES => false,
];

// ------------------------------------------------------------------
// Functions
// ------------------------------------------------------------------

/**
 * Check if $e is of type MySQL deadlock
 *
 * @param PDO $pdo
 * @param mixed $e
 * @return bool
 */
function isDeadlock(PDO $pdo, $e): bool
{
    return (
        $e instanceof PDOException &&
        $pdo->getAttribute(PDO::ATTR_DRIVER_NAME) == 'mysql' &&
        $e->errorInfo[0] == 40001 &&
        $e->errorInfo[1] == 1213
    );
}

/**
 * Simulate time it takes to call the payment gateway and do stuff
 */
function doPaymentGatewayStuff()
{
    usleep(500000); // Wait for 0.5 seconds
}

/**
 * Simulate buying a product from our inventory
 *
 * @param PDO $pdo
 * @param int $productId
 * @return int
 * @throws Exception
 */
function buyProduct(PDO $pdo, int $productId): int
{
    $pdo->beginTransaction();

    $selectStmt = $pdo->prepare('SELECT inventory FROM products WHERE id = :id ');
    $selectStmt->execute(['id' => $productId]);
    $res = $selectStmt->fetch();
    if ($res['inventory'] <= 0) {
        throw new Exception("Oh no! Sorry we're out inventory!");
    }

    $newInventory = $res['inventory'] - 1;
    $updateStmt = $pdo->prepare('UPDATE products SET inventory = :inventory WHERE id = :id ');
    $updateStmt->execute(['inventory' => $newInventory, 'id' => $productId]);

    doPaymentGatewayStuff();

    $pdo->commit();

    return $newInventory;
}

// ------------------------------------------------------------------
// Procedure
// ------------------------------------------------------------------

$uniqueUser = uniqid();
$retry = true;
while ($retry)
{
    try {
        // Set up DB driver
        $pdo = new PDO("mysql:host={$host};dbname={$db};charset={$charset}", $user, $pass, $opt);
        $pdo->query("SET TRANSACTION ISOLATION LEVEL {$mysqlIsolation} ");

        // Simulate buying a product and decreasing inventory
        $newInventory = buyProduct($pdo, $productId);

        // No exceptions were thrown, we consider this successful
        $successMsg = "{$uniqueUser} - Success! Product {$productId} inventory has been decreased to {$newInventory}" . PHP_EOL;
        file_put_contents($logFile, $successMsg, FILE_APPEND);
        echo "$successMsg";
        $retry = false;
    }
    catch (Exception $e) {
        if (isset($pdo) && isDeadlock($pdo, $e)) {
            $retry = true;
        } else {
            $retry = false;
            if (isset($pdo) && $pdo->inTransaction()) {
                $pdo->rollBack();
            }
            $errorMsg = "{$uniqueUser} - Error! " . $e->getMessage() . PHP_EOL;
            file_put_contents($logFile, $errorMsg, FILE_APPEND);
            echo "$errorMsg";
        }
    }
}

Huge gaping caveat: With 15 concurrent users the 15th user would be waiting for a long time. Patches welcome.

PHP Composer for Developers

Ever wanted to make a bugfix to a Composer package? You can!

Get a local git clone of the dependency by requiring it with the –prefer-source option.

composer require kizu514/package --prefer-source

But wait that’s not all! If you have your own GitHub namespace you can set things up so that your own code is always installed from source. For example, In the following composer.json snippet all the packages from kizu514 are installed from source, and everything else is dist.

{
    "config": {
        "preferred-install": {
            "kizu514/*": "source",
            "*": "dist"
        }
    }
}

Ever wanted to use a git branch instead of a specific version? You can!

Use inline aliases. To declare an inline alias you must:

  • Prefix branch names with: dev
  • No wildcards (*), must be unambigous.

For example, if my composer.json file had this in it:

"kizu514/package": "1.*",

Then to use a branch I would simply change it to:

"kizu514/package": "dev-BRANCH_NAME as 1.0.9",

Where BRANCH_NAME is a branch that exists on GitHub and 1.0.9 is unambiguous. If you want to check out a branch instead of a tag then simply do:

"kizu514/package": "dev-BRANCH_NAME",

What about private repos?

Use Private Packagist or add to your repositories configuration:

{
  "type": "vcs",
  "no-api": true,
  "url":  "git@github.com:kizu514/secret-project.git"
}

But wait that’s not all! Oh wait, yes, it is.