DBMS Transactions, Redis Caching, Concurrency Drills, Summer Plan Lock, HLD Begins

Solve 4 new problems from NeetCode 150 — 1-D Dynamic Programming; time-box each at 25 min before any hint (2.5h includes editorial on misses), then log pattern and miss reason in the Journal. Never repeat a problem already in the ledger.

Codeforces Div.2/3 (fortnightly) + LeetCode Weekly Contest + mandatory upsolving. Ratings are tracked metrics, never gates.

Add Redis to PlacementPilot: cache-aside with @Cacheable, TTL and eviction policy, cache-stampede protection (lock or jittered TTL), measure hit ratio. Why: caching is the most common HLD follow-up.

Implement a complete object-oriented system in compiled Java on LLDCoding.com / Workat.tech under a strict 45-minute timer (e.g. Design a Cache with LRU Eviction or Tic-Tac-Toe Game Engine). Include at least one thread-safety mechanism (synchronized, ConcurrentHashMap, or ReentrantLock). Architecture §6.1

Vertical vs horizontal scaling, load balancers (L4 vs L7, round robin, least connections, consistent hashing), caching strategies (read-through, write-through, write-back, cache invalidation), CAP Theorem, PACELC Theorem. Produce a standardized 1-page HLD interview blueprint template (clarify requirements, estimation, API, data model, architecture, deep dive) to be reused in all subsequent HLD drills and mocks. The template MUST include a worked back-of-envelope estimation example (DAU → QPS → storage → bandwidth → number of servers → cache hit ratio from Zipf distribution). Every subsequent HLD case study should start with 5 minutes of quantitative estimation before drawing any boxes. Architecture §6.2 Also cover the standard follow-ups: CDN and edge caching, consistent hashing with virtual nodes, retries with exponential backoff and jitter, circuit breakers and bulkheads, and rate-limit failure behaviour (429 + Retry-After). Primary sources: Alex Xu Vol 1 chapters 1–5 and Hello Interview core concepts.

Re-solve 3 tagged graph problems.

Decide the summer 2027 plan with evidence from the Recruiting Pipeline: internship offer vs GSoC acceptance vs capstone sprint [VERIFY GSoC dates]; record the decision, fallback and the Week 35–38 consequences.

Calendar check against the Opportunity Calendar (Architecture §16); send ≥ 5 personalised outreach messages (alumni, seniors, recruiters) and log each in the Tracker; follow up open threads; ask for a referral 1–2 days after any OA invite; apply within 72 h of any new posting on the Target-40.

ACID, isolation levels (Read Committed, Repeatable Read, Serializable) and the anomalies each prevents (dirty read, non-repeatable read, phantom, write skew), MVCC basics, optimistic vs pessimistic locking, database deadlocks; reproduce two anomalies with two psql sessions. Primary source: the PostgreSQL documentation chapter on transaction isolation; reproduce a lost update and a write-skew anomaly in two psql sessions, then fix each with the right isolation level or SELECT ... FOR UPDATE.

Implement from memory, with tests: (1) bounded blocking queue with ReentrantLock + two Conditions, (2) thread-safe LRU cache, (3) token-bucket rate limiter guarded by ReentrantLock, (4) Print in Order (LC #1114) and Print FooBar Alternately (LC #1115) with semaphores. Resource: Java Concurrency in Practice chapters 1–5 and 14. Why: every LLD rubric checks thread safety.