use Bitrix\Main\Application;

$session = Application::getInstance()->getSession();

if (!$session->has('cart'))
{
    $session->set('cart', ['items' => []]);
}

$session['cart']['items'][] = $productId;
$session['cart'] = $cart; // set via ArrayAccess

$session->remove('flash_message');
$session->clear();          // remove everything

$kernelSession = Application::getInstance()->getKernelSession();
$kernelSession->set('UF_LAST_LOGIN', time());

$local = Application::getInstance()->getLocalSession('cart');

if (!isset($local['productIds']))
{
    $local->set('productIds', [1, 2, 3]);
    $local->set('total', 42);
}

$ids = $local->get('productIds');

// before including prolog
define('BX_SECURITY_SESSION_READONLY', true);

require $_SERVER['DOCUMENT_ROOT'] . '/bitrix/modules/main/include/prolog_before.php';

define('BX_SECURITY_SESSION_VIRTUAL', true);

'session' => [
    'value' => [
        'mode'     => 'separated',
        'lifetime' => 14400,
        'handlers' => [
            'kernel'  => 'encrypted_cookies',
            'general' => ['type' => 'redis', 'host' => '127.0.0.1', 'port' => 6379],
        ],
    ],
],

'session' => [
    'value' => [
        'mode' => 'default',
        'handlers' => [
            'general' => [
                'type' => 'redis',
                'servers' => [
                    ['host' => '10.0.0.1', 'port' => 6379],
                    ['host' => '10.0.0.2', 'port' => 6379],
                    ['host' => '10.0.0.3', 'port' => 6379],
                ],
                'serializer' => \Redis::SERIALIZER_IGBINARY,
                'persistent' => false,
                'failover'   => \RedisCluster::FAILOVER_DISTRIBUTE,
                'timeout'     => null,
                'readTimeout' => null, // camelCase (session Redis handler)
            ],
        ],
    ],
],

'handlers' => [
    'general' => [
        'type' => 'memcache',
        'servers' => [
            ['host' => '10.0.0.1', 'port' => 11211, 'weight' => 1],
            ['host' => '10.0.0.2', 'port' => 11211],
        ],
    ],
],

'handlers' => [
    'general' => ['type' => 'database'], // b_user_session table
],

'session' => [
    'value' => [
        'lifetime'                 => 14400,  // seconds
        'mode'                     => 'default',
        'regenerateIdAfterLogin'   => true,   // recommended: fixation protection
        'ignoreSessionStartErrors' => false,  // true — hit continues even if Redis is unavailable
        'handlers' => [ ... ],
    ],
],

$session = Application::getInstance()->getSession();
$session->set('flash.success', 'Post saved');

// next request:
if ($msg = $session->get('flash.success'))
{
    $session->remove('flash.success');
    echo htmlspecialcharsbx($msg);
}