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❓ Rapid Fire - 1-Line Drills

One-line definitions for warm-up or last-minute review. Cover the term, then check. For depth, jump to the matching Fundamentals note or the topic Q&A file. Format legend: Overview.

Each entry: term - a one-sentence answer you could say in an interview.

Systems & OS

  1. Process vs thread - process = isolated memory; thread = shares its process's memory, cheaper to switch.
  2. Context switch - save one thread's CPU state, load another's; pure overhead that also thrashes caches.
  3. Deadlock - threads wait on each other forever; needs all four Coffman conditions.
  4. Race condition - result depends on interleaving of operations on shared state.
  5. Mutex vs semaphore - mutex allows one holder; semaphore allows up to N.
  6. Virtual memory - per-process address space mapped to RAM in pages; gives isolation + overcommit.

Concurrency

  1. Concurrency vs parallelism - structuring overlapping tasks vs actually running them at once.
  2. Idempotent - doing it twice has the same effect as doing it once.
  3. Amdahl's law - speedup is capped at 1/(serial fraction).
  4. GIL - Python's lock allowing one thread of bytecode at a time (kills CPU-bound threading).

Networking

  1. TCP vs UDP - reliable ordered stream vs fast best-effort datagrams.
  2. 3-way handshake - SYN, SYN-ACK, ACK to open a TCP connection.
  3. TLS handshake - asymmetric crypto to authenticate + exchange a key, then symmetric for data.
  4. DNS - resolves a hostname to an IP by walking root → TLD → authoritative, cached by TTL.
  5. Head-of-line blocking - one stalled item blocks everything queued behind it.

Databases

  1. ACID - Atomicity, Consistency, Isolation, Durability.
  2. CAP - under a partition, choose Consistency or Availability, not both.
  3. Index - secondary sorted structure (usually B-tree) that avoids full scans; slows writes.
  4. Sharding vs replication - split different data across nodes vs copy the same data to many.
  5. MVCC - versioned rows so readers and writers don't block each other.

Hashing, caching, security

  1. Consistent hashing - ring placement so a node change remaps only ~K/N keys.
  2. Bloom filter - probabilistic set: "definitely not" or "maybe"; no false negatives.
  3. Cache-aside - check cache, on miss read DB and populate it.
  4. AuthN vs authZ - who you are vs what you're allowed to do.
  5. Idempotency key - client-supplied token letting the server dedup retried requests.

Related: QA-Bank index · Fundamentals · System Design Guide