L4.sp11 - Distributed Systems Distributed Systems CS 425 /...

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Distributed Systems CS 425 / CSE 424 / ECE 428 Distributed Systems CS 425 / CSE 424 / ECE 428 Global Snapshots Reading: Sections 11.5 010, I. Gupta, K. Nahrtstedt, S. Mitra, N. Vaidya, M. T. Harandi, J. Hou
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Last Lecture Last Lecture Time synchronization Berkeley algorithm Cristian’s algorithm NTP Is it possible to synchronize two servers’ clocks with error=0? Lamport’s timestamps Logical timestamps Do the clock values of two servers need to be the same? What are “concurrent” events? Vector Timestamps
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Example of a Global State Example of a Global State http://www.geog.ucsb.edu/img/news/2009/bird%20flock%20wiki.jpg Birds in the sky
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The distributed version is challenging and important The distributed version is challenging and important How would you take this photograph if the camera couldn’t capture the breadth of the view all at once? In a cloud: multiple servers handling multiple concurrent events and interacting with each other Without the ability to obtain a global snapshot of the system, it would be a chaotic system (with potentially lots of inconsistencies)
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Detecting Global Properties Detecting Global Properties p 2 p 1 message garbage object object reference a. Garbage collection p 2 p 1 wait-for wait-for b. Deadlock p 2 p 1 activate passive passive c. Termination
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Algorithms to Find Global States Algorithms to Find Global States Why? (Distributed) garbage collection (Distributed) deadlock detection, termination Two clients buy the last flight ticket at around the same time What? Global state = state of all processes + state of all communication channels Capture the instantaneous state of each process And the instantaneous state of each communication channel, i.e., messages in transit on the channels How? We’ll see this lecture!
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Obvious First Solution… Obvious First Solution… Synchronize clocks of all processes Ask all processes to record their states at some time t Time synchronization possible only approximately What about messages in transit? Synchronization not required – causality is enough!
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Two Processes and Their Initial States Two Processes and Their Initial States p 1 p 2 c 2 c 1 account widgets $1000 (none) account widgets $50 2000
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Execution of the Processes p 1 p 2 (empty) <$1000, 0> <$50, 2000> (empty) c 2 c 1 1. Global state S 0 2. Global state S 1 3. Global state S 2 4. Global state S 3 p 1 p 2 (Order 10, $100) <$900, 0> <$50, 2000> (empty) c 2 c 1
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L4.sp11 - Distributed Systems Distributed Systems CS 425 /...

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