The document summarizes the Chandy-Lamport algorithm, which is used to record a consistent global state of an asynchronous distributed system. It works by having an observer process initiate a snapshot by saving its state and sending snapshot request messages bearing a token to other processes. Processes receiving the token for the first time save their state and attach the token to outgoing messages to propagate it. The observer receives states and messages to construct a consistent global snapshot.

1. ARBA MINCH UNIVERSITY
INSTITUTE OF TECHNOLOGY
FACULTY OF COMPUTING AND SOFTWARE ENGNEERING
DEPARTMENT OF COMPUTER SCIENCE
Presentation on :The Chandy–Lamport algorithm
Prepared by
1.Tekle Dargaso ------------------PRAMIT/094/13

2. The Chandy–Lamport algorithm
 The Chandy–Lamport algorithm is a snapshot algorithm that is
used in distributed systems for recording a consistent global
state of an asynchronous system. It was developed by and
named after Leslie Lamport and K. Mani Chandy.
• The assumptions of the algorithm are as follows:
 There are no failures and all messages arrive intact and only once
 The communication channels are unidirectional and FIFO ordered
 There is a communication path between any two processes in the
system
 Any process may initiate the snapshot algorithm
 The snapshot algorithm does not interfere with the normal execution
of the processes
 Each process in the system records its local state and the state of its
incoming channels

3. How Algorithm Works
• The Chandy–Lamport algorithm works like this:
The observer process (the process taking a snapshot):
Saves its own local state
Sends a snapshot request message bearing a snapshot token to all other
processes
A process receiving the snapshot token for the first time on any message:
Sends the observer process its own saved state
Attaches the snapshot token to all subsequent messages (to help propagate the
snapshot token)
 When a process that has already received the snapshot token receives a message
that does not bear the snapshot token, this process will forward that message to
the observer process.