Custom session not consistent

Question

I have a simple custom session storage where I want to save how many pages user has visited. I make an Ajax request to controller action on every page load which updates the custom foo session. Environment is local (Magento 2.4.3-p2), sessions are stored in DB, caches disabled: block_html, full_page.

The issue is that the session update is very incosistent:

• sometimes the initial foo session data is not being set on page load (sessions cleared)
• when initial foo data is set it then sometimes doesn't get updated on next page load (it should increment the pages visited)

Foo/Bar/etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Foo\Bar\Model\Session">
        <arguments>
            <argument name="storage" xsi:type="object">Foo\Bar\Model\Session\Storage</argument>
        </arguments>
    </type>
</config>

Foo/Bar/Model/Session.php

<?php

namespace Foo\Bar\Model;

class Session extends \Magento\Framework\Session\SessionManager
{
    /**
     * @var \Magento\Framework\Event\ManagerInterface
     */
    protected \Magento\Framework\Event\ManagerInterface $_eventManager;
    
    /**
     * @param \Magento\Framework\App\Request\Http                    $request
     * @param \Magento\Framework\Session\SidResolverInterface        $sidResolver
     * @param \Magento\Framework\Session\Config\ConfigInterface      $sessionConfig
     * @param \Magento\Framework\Session\SaveHandlerInterface        $saveHandler
     * @param \Magento\Framework\Session\ValidatorInterface          $validator
     * @param \Magento\Framework\Session\StorageInterface            $storage
     * @param \Magento\Framework\Stdlib\CookieManagerInterface       $cookieManager
     * @param \Magento\Framework\Stdlib\Cookie\CookieMetadataFactory $cookieMetadataFactory
     * @param \Magento\Framework\App\State                           $appState
     * @param \Magento\Framework\Event\ManagerInterface              $eventManager
     * @param \Magento\Framework\Session\SessionStartChecker|null    $sessionStartChecker
     */
    public function __construct(
        \Magento\Framework\App\Request\Http                    $request,
        \Magento\Framework\Session\SidResolverInterface        $sidResolver,
        \Magento\Framework\Session\Config\ConfigInterface      $sessionConfig,
        \Magento\Framework\Session\SaveHandlerInterface        $saveHandler,
        \Magento\Framework\Session\ValidatorInterface          $validator,
        \Magento\Framework\Session\StorageInterface            $storage,
        \Magento\Framework\Stdlib\CookieManagerInterface       $cookieManager,
        \Magento\Framework\Stdlib\Cookie\CookieMetadataFactory $cookieMetadataFactory,
        \Magento\Framework\App\State                           $appState,
        \Magento\Framework\Event\ManagerInterface              $eventManager,
        \Magento\Framework\Session\SessionStartChecker         $sessionStartChecker = null
    ) {
        $this->_eventManager = $eventManager;
        
        parent::__construct(
            $request,
            $sidResolver,
            $sessionConfig,
            $saveHandler,
            $validator,
            $storage,
            $cookieManager,
            $cookieMetadataFactory,
            $appState,
            $sessionStartChecker
        );
        
        $this->_eventManager->dispatch(
            'foo_session_init',
            ['foo_session' => $this]
        );
    }
    
    /**
     * @return void
     */
    public function setPagesVisitedData()
    {
        $pagesVisited = $this->getPagesVisited();
        
        if (empty($pagesVisited)) {
            $pagesVisited = 1;
        } else {
            $pagesVisited = (int) $pagesVisited + 1;
        }
        
        $this->setPagesVisited($pagesVisited);
    }
}

Foo/Bar/Model/Session/Storage.php

<?php

namespace Foo\Bar\Model\Session;

class Storage extends \Magento\Framework\Session\Storage
{
    /**
     * @param \Magento\Customer\Model\Config\Share       $configShare
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param string                                     $namespace
     * @param array                                      $data
     *
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function __construct(
        \Magento\Customer\Model\Config\Share       $configShare,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        string                                     $namespace = 'foo',
        array                                      $data = []
    ) {
        if ($configShare->isWebsiteScope()) {
            $namespace .= '_' . $storeManager->getWebsite()->getCode();
        }
        
        parent::__construct($namespace, $data);
    }
}

Foo/Bar/Model/Controller/Index.php

<?php

namespace Foo\Bar\Controller;

class Index extends \Magento\Framework\App\Action\Action implements \Magento\Framework\App\Action\HttpPostActionInterface
{
    /**
     * @var \Foo\Bar\Model\Session
     */
    protected \Foo\Bar\Model\Session $_fooSession;
    
    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Foo\Bar\Model\Session                $fooSession
     */
    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Foo\Bar\Model\Session                $fooSession
    
    ) {
        $this->_fooSession = $fooSession;
        
        parent::__construct($context);
    }
    
    /**
     * @return void
     */
    public function execute()
    {
        $this->_fooSession->setPagesVisitedData();
    }
}

Edit: Similar issue reported but I'm updating the session from Ajax so it's not related: Magento 2 custom sessions not consistent

Answer 1

It turns out the issue is within race conditions which occur if disable_locking is set to 1 for Redis session cache. Solution for me is to set it to 0 within app/etc/env.php:

'session' => [
    'save' => 'redis',
    'redis' => [
        ...
        'disable_locking' => 0 // Enables session locking
    ]
]

This will have some impact on performance but will prevent data loss or inconsistency when reading/writing session values.

This is one of many great explanations that describes what happens with multiple clients session interactions:

As you add more threads/clients/apps, things can go merrily and terribly wrong when multiple writers attempt modifying the data without coordination (i.e.g. locking)