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--- skills:interview [2026/06/13 07:00] – [Golang] phong2018
+++ skills:interview [2026/06/15 10:07] (current) – [Transactions] phong2018
@@ Line 260: / Line 260: @@
 . What are deadlocks?
+<code>
+- Deadlock: A situation where two or more transactions hold locks and wait for each other indefinitely.
+- How it occurs: Transactions access the same resources in different orders. Long-running transactions hold locks too long. Missing indexes cause unnecessary locking.
+- How to detect it: The database detects deadlocks automatically. MySQL returns: ERROR 1213: Deadlock found when trying to get lock. Investigate with SHOW ENGINE INNODB STATUS.
+- How to prevent it: Keep transactions short. Always access tables/rows in a consistent order. Add proper indexes. Avoid large batch operations in a single transaction.
+- How to recover: The database rolls back one transaction automatically. Catch the error in the application and retry the transaction with backoff.
+</code>
 ==== Large Scale Databases ====
@@ Line 491: / Line 497: @@
 . What backend technology trends are you currently following?
+====== System Design Interview Questions ======
+===== Interview Framework =====
+Always follow this order:
+  - Clarify requirements
+  - Estimate scale
+  - Define APIs
+  - Design data model
+  - Draw high-level architecture
+  - Explain data flow
+  - Identify bottlenecks
+  - Discuss scaling
+  - Discuss trade-offs
+  - Explain failure handling
+===== Questions =====
+==== Beginner to Intermediate ====
+  - Design a URL shortener
+  - Design a file storage service
+  - Design a chat application
+  - Design a notification system
+  - Design a rate limiter
+  - Design a distributed cache
+  - Design a search autocomplete service
+  - Design an API gateway
+  - Design a job queue system
+  - Design a payment processing system
+==== Intermediate to Advanced ====
+  - Design a ride-sharing platform
+  - Design a food delivery system
+  - Design a social media news feed
+  - Design a video streaming platform
+  - Design an e-commerce platform
+  - Design a real-time collaboration tool
+  - Design a monitoring and logging platform
+  - Design a recommendation engine
+  - Design a distributed lock service
+  - Design a multi-tenant SaaS platform
+==== Senior Backend Engineer Topics ====
+  - Design an order management system
+  - Design an inventory system that prevents overselling
+  - Design a coupon and promotion engine
+  - Design a loyalty points system
+  - Design an invoice generation system
+  - Design a webhook processing platform
+  - Design an OAuth2 / SSO authentication system
+  - Design an event-driven microservices architecture
+  - Design a distributed scheduler service
+  - Design a monolith-to-microservices migration strategy
+===== Drawing Template =====
+For every question, draw the following components:
+<code>
+Users / Mobile App / Web Browser
+                |
+                v
+        CDN / Load Balancer
+                |
+                v
+            API Gateway
+                |
+                v
+         Application Services
+          /        |        \
+         /         |         \
+        v          v          v
+     Cache      Database    Queue
+      |             |          |
+      |             |          |
+      v             v          v
+    Redis      MySQL/NoSQL   Workers
+                |
+                v
+      Monitoring / Logging
+</code>
+===== What Interviewers Expect =====
+For each component, explain:
+  * Why it exists
+  * How it scales
+  * Single points of failure
+  * Data consistency requirements
+  * Availability requirements
+  * Security considerations
+  * Monitoring and alerting
+===== Non-Functional Requirements Checklist =====
+  * Availability (99.9%, 99.99%, etc.)
+  * Scalability
+  * Reliability
+  * Latency
+  * Throughput
+  * Durability
+  * Consistency
+  * Security
+  * Cost
+===== Estimation Checklist =====
+Estimate before designing:
+  * Daily active users
+  * Requests per second (RPS)
+  * Peak traffic
+  * Storage requirements
+  * Read/write ratio
+  * Bandwidth requirements
+===== Deep Dive Topics =====
+Discuss when prompted:
+  * Database sharding
+  * Caching strategy
+  * Queue design
+  * Event-driven architecture
+  * Replication
+  * Multi-region deployment
+  * Disaster recovery
+  * Rate limiting
+  * Idempotency
+  * Distributed locking
+  * CAP theorem
+  * Eventual consistency
+===== Common Trade-offs =====
+^ Choice ^ Pros ^ Cons ^
+| SQL | Strong consistency | Harder to scale horizontally |
+| NoSQL | High scalability | Eventual consistency |
+| Sync communication | Simple | Tight coupling |
+| Async communication | Resilient | Increased complexity |
+| Cache aside | Simple | Stale data risk |
+| Write through cache | Consistent cache | Higher write latency |
+===== Example: URL Shortener =====
+==== Requirements ====
+  * Shorten long URLs
+  * Redirect users quickly
+  * High read traffic
+  * Custom aliases (optional)
+  * Analytics (optional)
+==== APIs ====
+<code>
+POST /api/v1/shorten
+Request:
+{
+  "url": "https://example.com/very/long/url"
+}
+Response:
+{
+  "shortUrl": "https://short.ly/abc123"
+}
+</code>
+<code>
+GET /abc123
+</code>
+==== High-Level Design ====
+<code>
+Client
+  |
+  v
+Load Balancer
+  |
+  v
+URL Service
+  |       \
+  |        \
+  v         v
+Redis     MySQL
+</code>
+==== Data Model ====
+<code>
+urls
+----
+id
+short_code
+long_url
+created_at
+expires_at
+</code>
+==== Scaling ====
+  * Cache popular URLs in Redis
+  * Read replicas for MySQL
+  * Shard by short_code
+  * CDN for global traffic
+==== Failure Handling ====
+  * Retry failed writes
+  * Circuit breaker for dependencies
+  * Database replication
+  * Multi-region backup
+===== Whiteboard Tips =====
+  * Start simple
+  * Draw before explaining
+  * Label every component
+  * State assumptions clearly
+  * Ask clarifying questions
+  * Explain trade-offs
+  * Think out loud
+  * Optimize only after the basic design works
+===== Golden Rule =====
+Do not jump directly into technology choices.
+Always follow:
+Requirements -> Scale -> APIs -> Data Model -> Architecture -> Bottlenecks -> Trade-offs
 ====== More detail versions ======
@@ Line 511: / Line 760: @@
 . What is Swoole or RoadRunner and how do they differ from the traditional PHP-FPM request model?
 . What do the three numbers in Semantic Versioning (MAJOR.MINOR.PATCH) mean, and when should each be incremented?
 ===== Laravel & Symfony =====