This document discusses snapshots in distributed systems. It begins by defining a snapshot as recording the simultaneous local states of all processes and communication channels. Snapshots can be used for deadlock detection, monitoring systems, and checkpointing distributed databases. Determining a global state is difficult due to the distributed nature of systems with no shared memory or clocks. Consistent cuts that do not cross message orderings can accurately capture a global state. The document then discusses several snapshot algorithms, including Chandy-Lamport for FIFO systems using markers, and Lai-Yang for non-FIFO systems using message coloring.

1. Guided by Presented by
Shri. R.S. Sharma Ashutosh Jaiswal
Associate Prof, CSE M.tech 2nd Yr

2. What is a snapshot

 A snapshot of a distributed system is a global state
where the local states of all processes and of all
communication channels are recorded
simultaneously

3. Where is a snapshot
used?

 Detection of deadlock of a distributed system
 Compute monotonic functions of the global state
such as lower bounds on the simulation time.
 Check pointing and recovery of distributed data
bases
 Monitoring and debugging of distributed systems.

4. Determining Global
States

 Global State
“The global state of a distributed system is the set of local
states of all individual processes involved in the
computation plus the state of the communication
channels.”

5. Why global state determination is
difficult in Distributed Systems?

 Distributed State : Have to collect information
that is spread across several machines!!
 Only Local knowledge : A process in the
computation does not know the state of other
processes.
 The lack of globally shared memory, global clock
and unpredictable message delays in a
distributed system make this problem non-
trivial.

6. Consistent and
Inconsistent cuts
 arrow starts in
 A cut is consistent if no message
future and ends in past. (e.g. ) AB
 Otherwise it is inconsistent ( e.g.) CD C
A
1
2
3
4
B D

7. Global States of Consistent Cuts

 A global state computed along a consistent cut is
correct
 The global state of a consistent cut comprises the local
state of each process at the time the cut event
happens and the set of all messages sent but not yet
received
 The snapshot problem consists in designing an efficient
protocol which yields only consistent cuts and to
collect the local state information

8. Issues in recording a global state

 How to distinguish between the messages to be
recorded in the snapshot from those not to be
recorded.
 -Any message that is sent by a process before
recording its snapshot, must be recorded in the global
snapshot
 -Any message that is sent by a process after recording
its snapshot , must not be recorded in the global
snapshot

9. How to determine the instant
when a process takes its snapshot

 A process pj must record its snapshot before
processing a message mij that was sent by process pi
after recording its snapshot.

10. Models of
communication

There are two models of communication : FIFO, non-
FIFO
 In FIFO model, each channel acts as a first-in first-
out message queue and thus, message ordering is
preserved by a channel.
 In non-FIFO model, a channel acts like a set in which
the sender process adds messages and the receiver
process removes messages from it in a random order.

11. System Model for Global Snapshots

 The system consists of a collection of n processes
p1, p2, ..., pn that are connected by channels.
 There are no globally shared memory and physical
global clock and processes communicate by passing
messages through communication channels.
 Cij denotes the channel from process pi to process pj
and its state is denoted by SCij .
 The actions performed by a process are modeled as
three types of events:
 Internal events,the message send event and the message
receive event.
 For a message mij that is sent by process pi to process pj , let
send(mij ) and rec(mij ) denote its send and receive events.

12. Process States and Messages in transit

 At any instant, the state of process pi , denoted by
LSi , is a result of the sequence of all the events
executed by pi till that instant.
 For an event e and a process state LSi , e∈LSi iff e
belongs to the sequence of events that have taken
process pi to state LSi .
 For an event e and a process state LSi , e (not in) LSi
iff e does not belong to the sequence of events that
have taken process pi to state LSi .

13. Snapshot algorithms
for FIFO channels

Chandy-Lamport algorithm
 The Chandy-Lamport algorithm uses a control
message, called a marker whose role in a FIFO system is
to separate messages in the channels.
 After a site has recorded its snapshot, it sends a
marker, along all of its outgoing channels before sending
out any more messages.
 A marker separates the messages in the channel into those
to be included in the snapshot from those not to be
recorded in the snapshot.
 A process must record its snapshot no later than when it
receives a marker on any of its incoming channels.

14. 

15. Snapshot algorithms for non-
FIFO channels

 In a non-FIFO system, a marker cannot be used to
delineate messages into those to be recorded in the
global state from those not to be recorded in the
global state.

16. Lai-Yang algorithm

 The Lai-Yang algorithm fulfills this role of a marker in a non-
FIFO system by using a coloring scheme on computation
messages that works as follows:
 Every process is initially white and turns red while taking a
snapshot. The equivalent of the “Marker Sending Rule” is
executed when a process turns red.
 Every message sent by a white (red) process is colored
white (red) indicating if it was sent before(after) snapshot.
 Each process (which is initially white) becomes red as soon
as it receives a red message for the first time and starts a
virtual broadcast algorithm to ensure that all processes will
eventually become red.

17. Determining Messages
in transit

 White process records history of white msgs
sent/received on each channel.
 When a process turns red, it sends these histories along
with its snapshot to the initiator process that collects the
global snapshot.
 Initiator process evaluates transit(LSi , LSj ) to compute
state of a channel Cij :
 SCij = white messages sent by pi on Cij − white messages
received by pj on Cij =
{send(mij )|send(mij ) ∈ LSi } − {rec(mij )|rec(mij ) ∈ LSj }.

18. The Proposed Algorithm

Assumptions:
 Author assumes a pre-established spanning tree on the
distributed system for an initiator to perform a one-to-all
broadcast to inform all other processes the initiation of a
new global snapshot.
 In doing so, it can be ensured that even without receiving
any red message, a white process will learn that one
snapshot execution has been initiated and then will
execute the global-snapshot algorithm to construct the
global snapshot cooperatively with all other processes

19. 
 There are three phases in the algorithm
 Phase 1: This is the phase of snapshot initiation. The
initiator performs a one-to-all broadcast of INIT
control messages along the pre-established spanning
tree to inform all other processes .

20. 
 Phase 2: This is the phase of accumulating numbers of
white messages. Upon the receipt of an INIT control
message or a red computation message, a white process
turns red and records its local state.
 Subsequently, the algorithm directs the whole processes
to calculate in a symmetrical manner the total number of
white messages that each process is supposed to receive.
 In particular, every process pi maintains a vector of size
N, wmsg senti[], to count in wmsg senti[j] the number of
white messages that it has sent to another process pj .

21. 


22. 
 Phase 3: The is the phase of recording channel states.
When a red process pi receives a white computation
message along a channel, such a message is added to
the state of the channel. In addition, when the second
phase is done and all white messages supposed to be
received by pi have been received by pi, i.e. wmsg
receivedi = sum wmsgi[i], pi turns white and then
terminates the algorithm locally.

23. Algorithm
 Initiator - 1
3
2 4
1

24.  Initiator - 1
3
2 4
1 Control Messages & a
Red message sent

25.  Initiator - 1
3
2 4
Red Message
Received at 2 1

26.  Initiator - 1
3 sends white 3 Ctrl Mesg from
Mesg to 1 1 reaches 3
2 4
1

27.  Initiator - 1
3
2 4
Red Ctrl
Mesg
1 received
White Mesg
received at 1

28. Summary

 Global State detection difficult in Distributed
Systems
 Snapshot algorithm may not give an actual state but
is very helpful in detecting Stable Properties

29. References

[1] Jichiang Tsai, “Flexible Symmetrical Global-Snapshot Algorithms for
Large-Scale Distributed Systems,” IEEE Transactions On Parallel And
Distributed Systems, vol. Xx, no. Y, apr. 2012
[2] R. Garg, V. Garg, and Y. Sabharwal, “Efficient algorithms for global
snapshots in large distributed systems,” IEEE Trans. Parallel and Distributed
Systems, vol. 21, no. 5, pp. 620–630, May 2010.
[3] A. D. Kshemkalyani, “Fast and message-efficient global snapshot
algorithms for large-scale distributed systems,” IEEE Trans. Parallel and
Distributed Systems, vol. 21, no. 9, pp. 1281–1209, Sept. 2010.
[4]F. Mattern, “Efficient Algorithms for Distributed Snapshots and Global
Virtual Time Approximation,” J. Parallel and Distributed Computing, pp.
423-434, Aug. 1993.
[5] K.M. Chandy and L. Lamport, “Distributed Snapshots: Determining
Global States of Distributed Systems,” ACM Trans. Computer Systems,
vol. 3, no. 1, pp. 63-75, Feb. 1985.

30. 
Thank You !