CS8603:Distributed Systems Department of CSE
2020 – 2021 2.
18
Jeppiaar Institute of Technology
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The snapshot of channel C
ij
includes all the messages that process p
j
receives
after taking the snapshot and whose timestamp is smaller than the time of the
snapshot.
• However, a global physical clock is not available in a distributed system. Hence the
following two issues need to be addressed to record a consistent global snapshot.
•
I1: How to distinguish between the messages to be recorded in the snapshot from those
not to be recorded?
• Any message i.e., sent by a process
before recording its snapshot, must be recorded in
the global snapshot. (from
C1).
• Any message that is sent by a process after recording its snapshot, must not be recorded
in the global snapshot (from
C2).
•
I2: How to determine the instant when a process takes its snapshot.
•
A process p
j
must record its snapshot before processing a message m
ij
that was sent by
process p
i
after recording its snapshot.
• These algorithms use two types of messages: computation
messages and control
messages. The former are exchanged by the underlying application and the latter are
exchanged by the snapshot algorithm.
2.9 Snapshot algorithms for FIFO channels
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