Distributed systems fundamentals

Kosa kata

English	Makna
distributed system	Independent computers coordinating through messages to provide a service or perform work.
partial failure	Failure of some components while other components continue operating.
idempotence	The property that repeating an operation has the same effect as performing it once.
Lamport clock	A logical counter scheme assigning timestamps consistent with the happens-before relation.
happens-before	A causal ordering induced by local event order and message transmission, including transitive consequences.

English

Physics of cooperation

A distributed system shares state across machines with partial failure: one machine crashing must not corrupt the rest. Messages reorder and duplicate; idempotence makes re-execution safe.
A timeout does not mean failed — it means unknown: the third outcome every protocol must handle.

{exercise:uni-cs0907-physics}

Latihan

A timeout means —

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English

Two machines' clocks skew without bound, so timestamps cannot safely order cross-machine events. The Lamport clock respects causality: send attaches $C+1$; receive sets $C = \max(C, \text{received}) + 1$.
P0 sends $m_1$ at $C = 1$; P1 receives at $C = 2$; P1 sends $m_2$ at $3$; P0 receives at $4$. If $a$ happens-before $b$ then $C(a) < C(b)$ — one direction only.

{exercise:uni-cs0907-lamport}

Latihan

P1 sends at C = 3 to P0 at C = 1: on receive, P0 becomes —

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English

deduct(user, 25, id=xyz) executed twice must deduct once: the operation is idempotent by key — the ledger records xyz as done and the retry is a no-op.
Retry-safe systems are built from idempotent operations plus a dedupe ledger, not from hoping the network stops duplicating.

{exercise:uni-cs0907-idempotent}

Latihan

deduct(user, 25, id=xyz) twice deducts once because —

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