Chandy–Lamport algorithm: Difference between revisions

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{{one source |date=May 2024}}

The '''snapshotChandy–Lamport algorithm''' is ana [[snapshot algorithm]] that is used in [[distributed systems]] for recording a consistent global state of an [[asynchronous communication|asynchronous]] system. TheIt algorithmwas discusseddeveloped hereby isand also known as the '''Chandy–Lamport algorithm''',named after [[Leslie Lamport]] and [[K. Mani Chandy]]. <ref>

Leslie Lamport, K. Mani Chandy: [https://research.microsoft.com/users/lamport/pubs/pubs.html#chandy ''Distributed Snapshots: Determining Global States of a Distributed System'']. In: ''ACM Transactions on Computer Systems 3''. Nr. 1, February 1985. ([http://lamport.azurewebsites.net/pubs/chandy.pdf PDF; 1&nbsp;MB])</ref>

==History==

According to [http://research.microsoft.com/users/lamport/pubs/pubs.html#chandy Leslie Lamport's website], “The distributedthe snapshot algorithm was described here came about when Ihe visited Chandy, who was then at the [[University of Texas at Austin|University of Texas in (Austin)]]. HeChandy posed the problem to me over dinner, but wethey had both had too much wine to think about it right then. The next morning, while Lamport was in the shower, Ihe came up with the solution. When Ihe arrived at Chandy's office, he was waiting for mehim with the same solution.” Lamport considers the algorithm to be a straightforward application of the basic ideas in his article ''Time, Clocks and the Ordering of Events in a Distributed System''. <ref>{{Cite web |title=The Writings of Leslie Lamport |url=https://lamport.azurewebsites.net/pubs/pubs.html?from=https://research.microsoft.com/users/lamport/pubs/pubs.html&type=path#time-clocks |access-date=2024-08-24 |website=lamport.azurewebsites.net}}</ref>

It was defined in a paper titled “[http://research.microsoft.com/users/lamport/pubs/chandy.pdf Distributed Snapshots: Determining Global States of a Distributed System].”

==Definition==

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* 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

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The algorithm works using marker messages. Each process that wants to initiate a snapshot records its local state and sends a marker on each of its outgoing channels. All the other processes, upon receiving a marker, record their local state, the state of the channel from which the marker just came as empty, and send marker messages on all of their outgoing channels. If a process receives a marker after having recorded its local state, it records the state of the incoming channel from which the marker came as carrying all the messages received since it first recorded its local state.

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==Algorithm==

The snapshotChandy–Lamport algorithm works like this:

# The observer process (the process taking a snapshot):

## Saves its own local state

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## Sends the observer process its own saved state

## Attaches the snapshot token to all subsequent messages (to help propagate the snapshot token)

# ShouldWhen a process that has already received the snapshot token receivereceives a message that does not bear the snapshot token, this process will forward that message to the observer process. This message was obviously sent before the snapshot “cut off” (as it does not bear a snapshot token and thus must have come from before the snapshot token was sent out) and needs to be included in the snapshot.

From this, the observer builds up a complete snapshot: a saved state for each process and all messages “in the ether” are saved.

==References==

{{Reflist}}

{{DEFAULTSORT:Chandy-Lamport algorithm}}

[[Category:Distributed algorithms]]