Senior & Lead Engineer
Interview Mastery Hub

Handling \Throwable (Exception + Error)

In PHP 7+, all exceptions and fatal runtime errors implement \Throwable:

set_exception_handler(function (\Throwable $e) {
    Log::error($e->getMessage(), ['trace' => $e->getTraceAsString()]);

    http_response_code(500);
    header('Content-Type: application/json');
    echo json_encode([
        'error' => 'Internal Server Error',
        'code' => 500,
    ]);
    exit;
});

Recursive CTE Example

WITH RECURSIVE OrgChart AS (
    SELECT id, name, manager_id, 1 as level
    FROM employees WHERE manager_id IS NULL
    UNION ALL
    SELECT e.id, e.name, e.manager_id, o.level + 1
    FROM employees e
    JOIN OrgChart o ON e.manager_id = o.id
)
SELECT * FROM OrgChart ORDER BY level;

Connection Pooling with ProxySQL / PgBouncer

• Without connection pooling: $400 \text{ pods} \times 20 \text{ connections} = 8,000 \text{ open DB connections}$, overwhelming database thread management.
• With Multiplexing Proxy: 8,000 application client connections multiplex over 50 persistent, high-speed connections to MySQL, keeping DB CPU in the sweet spot.

Utilizing selectSub and addSelect with Window Functions

$rankedStudents = User::query()
    ->select('id', 'name', 'email')
    ->addSelect([
        'last_quiz_score' => QuizAttempt::select('score')
            ->whereColumn('user_id', 'users.id')
            ->latest('completed_at')
            ->limit(1),
        'rank' => DB::table('quiz_attempts')
            ->selectRaw('DENSE_RANK() OVER (ORDER BY SUM(score) DESC)')
            ->whereColumn('user_id', 'users.id')
    ])
    ->orderByDesc('rank')
    ->take(100)
    ->get();

Avoids loading intermediate Eloquent collections in memory.

Shallow vs Deep Clone

Standard $b = clone $a copies primitive properties by value, but nested objects remain references to the original instance.

class Invoice
{
    public Customer $customer;

    public function __clone()
    {
        // Deep copy nested object
        $this->customer = clone $this->customer;
    }
}

MySQL FULLTEXT Index with N-Gram Parser

ALTER TABLE products ADD FULLTEXT INDEX ft_title_desc (title, description) WITH PARSER ngram;

-- High performance full text search
SELECT * FROM products
WHERE MATCH(title, description) AGAINST('+laptop +pro' IN BOOLEAN MODE);

Caching Strategies

1. Cache-Aside (Lazy Loading): Application reads cache; on miss, reads DB and writes to cache. Standard for most web apps.
2. Write-Through: Application writes to cache, and cache synchronously writes to DB before returning. Consistency guaranteed, higher write latency.
3. Write-Back (Write-Behind): Application writes to cache immediately; cache asynchronously flushes batch writes to DB in the background. High write performance, small risk of data loss if cache crashes before sync.

Implementing Invokable Rule with Circuit Breaker & Timeout

class ValidEnterpriseTaxNumber implements ValidationRule
{
    public function validate(string $attribute, mixed $value, Closure $fail): void
    {
        try {
            $response = Http::timeout(2)
                ->retry(2, 100)
                ->get("https://api.taxservice.com/validate/{$value}");

            if (! $response->successful() || ! $response->json('is_valid')) {
                $fail("The {$attribute} is not a verified business tax entity.");
            }
        } catch (\Throwable $e) {
            // Log warning and fail gracefully or flag for manual review
            Log::warning("Tax API unavailable: " . $e->getMessage());
        }
    }
}

Calculating Optimal pm = static Configuration

On dedicated production servers, pm = static avoids the CPU overhead of constantly spawning and destroying worker processes.

Formula:

max_children = (Total RAM - OS/Services RAM) / Average Worker Memory (e.g. 50MB)
Example: (60,000 MB) / 50 MB = 1200 workers

pm = static
pm.max_children = 1200
pm.max_requests = 1000 ; Recycles workers to prevent slow memory leaks

MySQL 8 Invisible Indexes & Online Drop

1. Make Index Invisible First:

   ALTER TABLE orders ALTER INDEX idx_old_status INVISIBLE;
   

   Verifies if optimizer or queries degrade without that index.
2. Drop Online:

   ALTER TABLE orders DROP INDEX idx_old_status, ALGORITHM=INPLACE, LOCK=NONE;
   

HMAC-Signed URLs with Expiration & IP Binding

1. Backend generates an HMAC-signed URL containing: path, expires_timestamp, and optional client_ip.
2. Cloudflare / CloudFront edge worker validates the signature in $O(1)$ at the edge node:

   https://cdn.example.com/video/master.m3u8?token=a8f9c1...&exp=1772189400
   

3. If token expired or tampered, edge rejects with HTTP 403 without touching origin servers.

TOTP Implementation Workflow

1. Secret Generation: Generate 32-character base32 secret and store encrypted in database (Crypt::encryptString($secret)).
2. QR Code Provisioning: Generate otpauth://totp/App:user@email?secret=...&issuer=App.
3. Verification: Validate code within drift window (+/- 30 seconds).
4. Hashed Recovery Codes: Generate 8 random recovery codes, hash them with Hash::make() before saving, and burn upon single use.

PHP 8.2 Readonly Classes

Marking a class readonly automatically enforces that all declared properties are readonly and prevents dynamic property creation:

readonly class TransferLedgerDTO
{
    public function __construct(
        public string $transactionId,
        public int $amountCents,
        public string $currency,
        public DateTimeImmutable $timestamp
    ) {}
}

MySQL InnoDB Cluster Architecture

1. Group Replication: Uses a Paxos-based consensus protocol where transactions are validated across majority nodes (quorum) before commit.
2. MySQL Router: Automatically routes incoming read/write traffic to the primary node and read traffic across secondaries.
3. Automatic Failover: If the primary crashes, the cluster elects a new primary in seconds without human intervention.

Change Data Capture (CDC) via Debezium & Kafka

Instead of dual-writing in application code (which risks desynchronization if search fails):

1. Database writes to transaction WAL (Binlog).
2. Debezium CDC connector tails the Binlog and streams row change events to Kafka.
3. An indexing consumer batch writes updates into Elasticsearch / Meilisearch. Guarantees 100% data consistency with sub-second synchronization latency.

The Golden Rule: Never call external network APIs inside a database transaction

Use DB::afterCommit() callbacks:

DB::transaction(function () use ($order, $paymentData) {
    $order->update(['status' => 'paid']);
    PaymentTransaction::create([...]);

    // Dispatch side-effects ONLY after database commit succeeds
    DB::afterCommit(function () use ($order) {
        SendInvoiceEmailJob::dispatch($order);
        NotifyThirdPartyFulfillment::dispatch($order);
    });
});

Implementing CastsAttributes

class MoneyCast implements CastsAttributes
{
    public function get(Model $model, string $key, mixed $value, array $attributes): ?Money
    {
        return $value !== null ? new Money((int) $value, $attributes['currency'] ?? 'USD') : null;
    }

    public function set(Model $model, string $key, mixed $value, array $attributes): array
    {
        if ($value instanceof Money) {
            return [$key => $value->getAmountInCents(), 'currency' => $value->getCurrency()];
        }
        return [$key => (int) $value];
    }
}

Using DB::whenQueryingForLongerThan

In AppServiceProvider::boot():

DB::whenQueryingForLongerThan(500, function (Connection $connection) {
    Log::warning("Database query exceeded 500ms on [{$connection->getName()}]");
    // Send alert to Sentry or Slack webhook
});

Dynamic Rate Limiter in AppServiceProvider

RateLimiter::for('api', function (Request $request) {
    $user = $request->user();
    if (! $user) {
        return Limit::perMinute(30)->by($request->ip());
    }

    return match ($user->subscription_tier) {
        'enterprise' => Limit::perMinute(10000)->by($user->id),
        'pro' => Limit::perMinute(1000)->by($user->id),
        default => Limit::perMinute(60)->by($user->id),
    };
});

Graceful Termination Workflow

1. Catch SIGTERM in worker: In Kubernetes, set terminationGracePeriodSeconds: 120.
2. Supervisor Configuration: Set stopwaitsecs = 120 so Supervisor gives workers up to 2 minutes to finish processing current job before force killing (SIGKILL).
3. Queue Worker Command:

   php artisan queue:work --stop-when-empty --max-time=3600 --timeout=90