This document discusses distributed computing and global snapshots. It explains that a global snapshot captures the instantaneous state of each process and communication channel in a distributed system. It is useful for checkpointing to restart applications after failures, garbage collection, deadlock detection, and termination of distributed computations. The document also summarizes two election algorithms used to select a new coordinator process when the current one fails - one for fully connected systems and one for ring-connected systems.

1. PRESENTED BY
A.KEERTHIKA
M.SC(IT)
DISTRIBUTED COMPUTING

2.  Distributed applications/services execute
concurrently on multiple machines. • A
Snapshot of the distributed application, i.e. a
global picture is useful ○ Checkpointing: can
restart distributed application on failure ○
Garbage collection of objects: objects at
servers that don’t have any other objects (at
any servers) with pointers to them ○ Deadlock
detection: Useful in database transaction
systems ○ Termination of computation: Useful
in batch computing systems like Folding@Home,
SETI@Home

3.  Global Snapshot = Global State Individual
state of each process in the distributed
system + Individual state of each
communication channel in the distributed
system
 Capture the instantaneous state of each
process
 Capture the instantaneous state of each
communication channel, i.e., messages in
transit on the channels

4. o The notions of global time and global state are
closely related.
o But, merely synchronizing clocks and taking local
snapshots is not enough
o Need to account for messages in transit
o A process can (without freezing the whole
computation) compute the best possible
approximation of a global state [Chandy & Lamport
85]
o A global state that could have occurred
o No process in the system can decide whether the
state did really occur
o Guarantee stable properties (i.e. once they become
true, they remain true

8. Election algorithms choose a process from group
of processors to act as a coordinator. If the
coordinator process crashes due to some
reasons, then a new coordinator is elected on
other processor. Election algorithm basically
determines where a new copy of coordinator
should be restarted.
 Election algorithm assumes that every active
process in the system has a unique priority
number. The process with highest priority will be
chosen as a new coordinator. Hence, when a
coordinator fails, this algorithm elects that
active process which has highest priority
number.Then this number is send to every active
process in the distributed system.

9.  This algorithm applies to system where every process
can send a message to every other process in the
system.
 Algorithm – Suppose process P sends a message to
the coordinator.
 If coordinator does not respond to it within a time
interval T, then it is assumed that coordinator has
failed.
 Now process P sends election message to every
process with high priority number.
 It waits for responses, if no one responds for time
interval T then process P elects itself as a
coordinator.
 Then it sends a message to all lower priority number
processes that it is elected as their new coordinator.

11.  This algorithm applies to systems organized
as a ring(logically or physically). In this
algorithm we assume that the link between
the process are unidirectional and every
process can message to the process on its
right only. Data structure that this algorithm
uses is active list, a list that has priority
number of all active processes in the system.

12.  If process P1 detects a coordinator failure, it
creates new active list which is empty
initially. It sends election message to its
neighbour on right and adds number 1 to its
active list.
 If Process P1 receives its own election
message 1 then active list for P1 now
contains numbers of all the active processes
in the system. Now Process P1 detects
highest priority number from list and elects
it as the new coordinator.