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DBMS
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Operating Systems
Virtual Memory & Paging
How the OS gives every process the illusion of its own contiguous address space.
System Design
Consistent Hashing
Distributing keys across nodes so adding or removing one reshuffles as little as possible.
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Operating Systems
Virtual Memory & Paging
- Every memory access goes virtual address → page table lookup → physical address. TLB caches this.
- Each process has its own page table — virtual address 0x1000 in process A maps to different RAM than in process B.
- OS only loads pages into RAM when accessed — a page fault triggers the load; "not present" pages live on disk.
DBMS
Database Normalization
- Unnormalized tables cause update anomalies — change one fact in one place, break it in three others.
- Each cell holds one atomic value — no arrays, no comma-separated lists inside a column.
- Every non-key column depends on the whole composite key, not just part of it.
System Design
Consistent Hashing
- hash(key) % N breaks when N changes — a server add/remove remaps ~all keys, causing a thundering herd.
- Map both servers and keys onto a circular hash space — each key is owned by the next clockwise server.
- Each physical server gets K virtual ring positions so load spreads evenly even with few servers.
Networks
TCP Three-Way Handshake
- Two packets can open a channel one way — you need three to prove both sides can send AND receive.
- SYN (client opens) → SYN-ACK (server acknowledges and opens) → ACK (client acknowledges server).
- Random ISNs prevent attackers from injecting data into old connections or forging packets.
60-second cards
Quick revise
TCP vs UDP in one line?
TCP is reliable and ordered. UDP is fast and fire-and-forget.
REVIEW 60s
What is the purpose of the TCP three-way handshake?
Synchronize sequence numbers in both directions before data transfer
REVIEW 60s
What are the four Coffman conditions for deadlock?
Mutual exclusion, hold-and-wait, no preemption, circular wait
REVIEW 60s