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Two criteria that a hardware clock should meet
Correctness: it drift rate falls within known bound p
Monotonicity (is implied by correctness) : clock never runs backwards
How fault tolerant are mutual exclusion algos?
dist. mutual exclusion :
centralized vs. distr.
- message loss: neither
- process failure:
Lamport Logical Clock - Algorithm
Local:
Li is incremeneted before each event is processed (local)
Messages:
LC2(a) when pi sends message m, it carries time of sending event
LC2(b) recievieng (m,t) a process pj computes Li=max(Lj, t) and then applies +1 for event recieve
Vector clock algorithm
How to compare vector clocks?
Two Vectors V and V'
Bully Algorithm steps:
How to interpret vector values in vector clocks
happened-before relation by Lamport
HB1: If ∃ 𝑝𝑟𝑜𝑐𝑒𝑠𝑠 𝑝𝑖:𝑒 →𝑖𝑒´,𝑡ℎ𝑒𝑛 𝑒 → 𝑒´ (local history)
HB2: For any message 𝑚:
𝑠𝑒𝑛𝑑 𝑚 → 𝑟𝑒𝑐𝑒𝑖𝑣𝑒 𝑚
HB3: If e, e´ and e´´ are events such that 𝑒 → 𝑒´ and 𝑒´ → 𝑒´´ , then 𝑒 → 𝑒´´
How to get total ordering from lamport clocks? Usage?
Add unique identifier per process (eg proces number).
Usage: entering critical section
Vector clocks components
Each process pi keeps its own vector Vi used to timestamp local events − Array of N integers for N processes − Carried with each message
NTP operational modes
Multicast: lowest accuracy, multicast time to other server (for LANs)
Procedure call: similar to cristians', server accepts requests and replies with timestapm. higher accuracy
symmetric: highers accuracy, used in low startum, servers keep timing data as part of association for accuracy
Lower level is not aware of application needs and semantics! -> consider end-to-end principle when designing systems (no questions, just remember it)
im tired sorry
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