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Chapter 9 Global Snapshot

Chapter 9 Global Snapshot. The mystery of the missing dollars. Picture taken at A - $400 A sends $100 to B Picture taken at B - $400 Total is $800. Send $100. B. A. $300. $400. Global Snapshot Problem. Determine the global system state (e.g. the total money )

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Chapter 9 Global Snapshot

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  1. Chapter 9Global Snapshot

  2. The mystery of the missing dollars • Picture taken at A - $400 • A sends $100 to B • Picture taken at B - $400 • Total is $800 Send $100 B A $300 $400

  3. Global Snapshot Problem • Determine the global system state (e.g. the total money ) • Each process records its own state • No shared clock/memory • Group of photographers taking snaps of different portions and trying to combine to get the overall picture.

  4. Consistent cut • Given computation (E,!) and F µ E is a cut iff • F is a consistent cut (global snapshot) iff

  5. Consistent and inconsistent cuts

  6. Chandy and Lamport's Algorithm • Assumes • FIFO and • Unidirectional channels • A bidirectional channel is modelled as two unidirectional channels

  7. Chandy and Lamport's Algorithm • Each process has an associated color. All processes are initially white. • A process records it local state just before turning red • On turning red the process sends out a marker on all outgoing channels • On receiving a marker a white process turns red

  8. Classification of messages • w – white process • r – red process • E.g.. rw – sent by a red process, received by a white process

  9. publicclass RecvCamera extends Process implements Camera { . . . public RecvCamera(Linker initComm, CamUser app) { . . . for (int i = 0; i < N; i++) if (isNeighbor(i)) { closed[i] = false; chan[i] = new LinkedList(); } else closed[i] = true; } publicsynchronizedvoid globalState() { myColor = red; app.localState(); // record local State; sendToNeighbors("marker", myId); // send Markers } publicsynchronizedvoid handleMsg(Msg m, int src, String tag) { if (tag.equals("marker")) { if (myColor == white) globalState(); closed[src] = true; if (isDone()){ ----- Display channel state (transit messages) chan[] ---- } } else { // application message if ((myColor == red) && (!closed[src])) chan[src].add(m); app.handleMsg(m, src, tag); // give it to app } } boolean isDone() { if (myColor == white) returnfalse; for (int i = 0; i < N; i++) if (!closed[i]) returnfalse; returntrue; } } Algorithm

  10. Non FIFO channels • Include color in all outgoing messages • Each process counts the number of white messages received by it • Marker from i to j includes number of white messages sent by i to j

  11. Applications • Checkpointing • Provide fault tolerance in distributed systems • Distributed debugging

  12. Applications • Stable predicate B: Once B becomes true it remains true, i.e. B is stable iff • Stable predicate detection • Property of a global snapshot S* • If Si is the initial global state, Sf is the final global state B(S*) ) B(Sf) B(S*) ) B(Si) • Take repeated snapshots

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