Step 3: Process AJAX Submissions Securely

Handle the AJAX request, sanitize inputs, validate email syntax, and write the record to the custom database table while preventing duplicate subscriptions for the same product and email combination.

add_action('wp_ajax_sitesuraksha_subscribe_stock', 'sitesuraksha_handle_stock_subscription');
add_action('wp_ajax_nopriv_sitesuraksha_subscribe_stock', 'sitesuraksha_handle_stock_subscription');

function sitesuraksha_handle_stock_subscription() {
    check_ajax_referer('sitesuraksha_stock_nonce', 'security');

    $email = sanitize_email($_POST['stock_email']);
    absint($_POST['product_id']);
    absint($_POST['variation_id']);

    if (!is_email($email)) {
        wp_send_json_error(array('message' => 'Invalid email address provided.'));
    }

    global $wpdb;
    $table_name = $wpdb->prefix . 'woocommerce_stock_subscriptions';

    // Check for existing active subscription
    $existing = $wpdb->get_var($wpdb->prepare(
        "SELECT id FROM $table_name WHERE product_id = %d AND variation_id = %d AND customer_email = %s AND status = 'pending'",
        $product_id, $variation_id, $email
    ));

    if ($existing) {
        wp_send_json_success(array('message' => 'You are already subscribed to updates for this product.'));
    }

    $inserted = $wpdb->insert(
        $table_name,
        array(
            'product_id' => $product_id,
            'variation_id' => $variation_id,
            'customer_email' => $email,
            'subscribed_at' => current_time('mysql'),
            'status' => 'pending'
        ),
        array('%d', '%d', '%s', '%s', '%s')
    );

    if ($inserted === false) {
        wp_send_json_error(array('message' => 'Database error occurred. Please try again later.'));
    }

    wp_send_json_success(array('message' => 'You will be notified as soon as this product is back in stock.'));
}

Step 4: Trigger Alerts via Action Scheduler on Inventory Restock

Listen for stock quantity updates in WooCommerce. When stock changes from zero to a positive integer, query pending subscribers and queue background email jobs using Action Scheduler.

add_action('woocommerce_product_set_stock', 'sitesuraksha_check_restock_trigger');
add_action('woocommerce_variation_set_stock', 'sitesuraksha_check_restock_trigger');

function sitesuraksha_check_restock_trigger($product) {
    if (!$product->is_in_stock()) {
        return;
    }

    $product_id = $product->get_id();
    $variation_id = ($product->is_type('variation')) ? $product->get_id() : 0;
    
    // If it's a parent variable product restock, check variations too or handle accordingly
    global $wpdb;
    $table_name = $wpdb->prefix . 'woocommerce_stock_subscriptions';

    $subscribers = $wpdb->get_results($wpdb->prepare(
        "SELECT id, customer_email FROM $table_name WHERE product_id = %d AND status = 'pending'",
        $product_id
    ));

    if (!empty($subscribers)) {
        foreach ($subscribers as $subscriber) {
            // Schedule individual actions via Action Scheduler to prevent memory exhaustion
            as_enqueue_async_action('sitesuraksha_send_restock_email', array(
                'subscription_id' => $subscriber->id,
                'product_id' => $product_id,
                'email' => $subscriber->customer_email
            ));
        }
    }
}

// Handler for the scheduled action
add_action('sitesuraksha_send_restock_email', 'sitesuraksha_process_restock_email', 10, 3);

function sitesuraksha_process_restock_email($subscription_id, $product_id, $email) {
    $product = wc_get_product($product_id);
    if (!$product || !$product->is_in_stock()) {
        return;
    }

    $subject = 'Good news! ' . $product->get_name() . ' is back in stock.';
    $product_url = esc_url($product->get_permalink());
    $message = "Hello,\n\nThe item you requested is now back in stock: " . $product->get_name() . "\n\nPurchase it here: " . $product_url . "\n\nThank you for shopping with us!";
    
    $headers = array('Content-Type: text/plain; charset=UTF-8');

    $sent = wp_mail($email, $subject, $message, $headers);

    if ($sent) {
        global $wpdb;
        $table_name = $wpdb->prefix . 'woocommerce_stock_subscriptions';
        $wpdb->update(
            $table_name,
            array('status' => 'sent'),
            array('id' => $subscription_id),
            array('%s'),
            array('%d')
        );
    }
}

Production Hardening & Edge Cases

Deploying automated stock notification systems in high-traffic production environments introduces several technical complexities that require explicit hardening strategies.

Database Table Optimization and Retention Policies

Over time, the `wp_woocommerce_stock_subscriptions` table will accumulate thousands of rows with a `sent` status. Unbounded table growth degrades query performance on checkout and product pages if joins are introduced. Implement a WP-Cron event to purge records older than 90 days or records marked as `sent` exceeding a specific threshold.

if (!wp_next_scheduled('sitesuraksha_purge_old_stock_subscriptions')) {
    wp_schedule_event(time(), 'weekly', 'sitesuraksha_purge_old_stock_subscriptions');
}

add_action('sitesuraksha_purge_old_stock_subscriptions', 'sitesuraksha_execute_subscription_purge');

function sitesuraksha_execute_subscription_purge() {
    global $wpdb;
    $table_name = $wpdb->prefix . 'woocommerce_stock_subscriptions';
    
    // Delete sent subscriptions older than 30 days to keep database lean
    $wpdb->query(
        "DELETE FROM $table_name WHERE status = 'sent' AND subscribed_at < NOW() - INTERVAL 30 DAY"
    );
}

Rate Limiting and Abuse Mitigation

Public-facing AJAX endpoints that accept email inputs are prime targets for credential stuffing, spam list verification, and denial-of-service vectors via database write operations. Implement IP-based rate limiting using transients or Redis cache object storage to restrict subscription submissions.

$client_ip = $_SERVER['REMOTE_ADDR'];
$rate_limit_key = 'stock_sub_limit_' . md5($client_ip);
$attempts = get_transient($rate_limit_key);

if ($attempts !== false && $attempts > 5) {
    wp_send_json_error(array('message' => 'Too many requests. Please try again later.'));
}

set_transient($rate_limit_key, ($attempts ? $attempts + 1 : 1), 15 * MINUTE_IN_SECONDS);

MTA Throttling and Deliverability

When restocking a high-demand item with thousands of subscribers, firing `wp_mail` concurrently can trigger SMTP rate limits imposed by your hosting provider or transactional email vendor (SendGrid, Mailgun, Amazon SES). Relying on Action Scheduler natively mitigates this by pacing job execution through WP-Cron or system-level cron runners. Ensure your system cron calls `wp-cron.php` every minute rather than relying on visitor-triggered page loads:

# Add to crontab to execute WordPress cron reliably every minute
* * * * * /usr/local/bin/php /var/www/html/wp-cron.php >/dev/null 2>&1

Diagnostic Verification & Checklist

Verify your back-in-stock notification system using the following operational checks and monitoring protocols.

Monitor error logs continuously via your server's PHP error log or WP_DEBUG_LOG to capture unhandled database exceptions during high-concurrency inventory updates. Maintain strict security postures around input sanitization and database indexing to ensure your WooCommerce store remains stable, performant, and secure under peak operational loads.