Chapter 9 Global Snapshot

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Chapter 9Global Snapshot
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Global state

• Global state
• A set of local states that are concurrent with
  each other
• Concurrent states no two states have a happened
  before relation with each other

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The mystery of the missing dollars
Send 100
B
A
300
400

• Picture taken at A - 400
• A sends 100 to B
• Picture taken at B - 400
• Total is 800

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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.

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Consistent cut

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

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Consistent and inconsistent cuts
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Chandy and Lamport's Algorithm

• Assumes
• FIFO and
• Unidirectional channels
• A bidirectional channel is modelled as two
  unidirectional channels

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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

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Classification of messages

• w white process
• r red process
• E.g.. rw sent by a red process, received by a
  white process

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Algorithm
public class RecvCamera extends Process
implements Camera . . . public
RecvCamera(Linker initComm, CamUser app)
. . . for (int i 0 i lt N i)
if (isNeighbor(i))
closedi false chani new
LinkedList() else closedi
true public synchronized void
globalState() myColor red
app.localState() // record local State
sendToNeighbors("marker", myId) // send
Markers public synchronized void
handleMsg(Msg m, int src, String tag)
if (tag.equals("marker")) if
(myColor white) globalState()
closedsrc true if (isDone())
----- Display channel state
(transit messages) chan ----
else // application message
if ((myColor red)
(!closedsrc))
chansrc.add(m) app.handleMsg(m,
src, tag) // give it to app
boolean isDone() if (myColor white)
return false for (int i 0 i lt N
i) if (!closedi) return false
return true
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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

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Applications

• Checkpointing
• Provide fault tolerance in distributed systems
• Distributed debugging

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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