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Distributed Snapshot Algorithm: Consistent Global States in Systems, Study notes of Electrical and Electronics Engineering
An in-depth explanation of the distributed snapshot algorithm, a method used to construct consistent global states in distributed systems without freezing participating processes. The algorithm, proposed by chandy and lamport in 1985, assumes fifo delivery through strongly connected unidirectional point-to-point communication channels. The organization of processes and channels, the algorithm steps, and the role of any process in initiating the snapshot.
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Distributed Snapshot
EECS 498– Lecture Notes
Prof. Farnam Jahanian
Univ. of Michigan
Section 5.3 Tanenbaum (informal description) Section 4.13 Mullender (formal description - recommended reading)
Remember Consistent Cuts!
a) A consistent cut
b) An inconsistent cut
Distributed Snapshot
- Goal: construct a consistent global state in a distributed fashion without freezing the participating processes
- Original DS algorithm proposed by Chandy & Lamport 1985
- Assume FIFO delivery through strongly connected uni- directional point-to-point communication channels between processes
- System State:
- Local state of a process (application dependent)
- Channel state between a pair of processes: msgs sent but not received
a) Organization of a process and channels for a distributed snapshot
Algorithm
- Any process can initiate the algorithm.
- Initiating process P starts by recording its own local state. Then it sends a marker along each of its outgoing channels.
- When a process Q receives a marker through an incoming channel C:
- If Q hasn’t already saved its local state, Q first records its local state and then sends a marker along each of its own outgoing channels.
- If Q has already recorded its state earlier, the marker on channel C is an indicator that Q should record the state of the channel. The channel state is the sequence of messages that have been received by Q since the last time Q recorded its local state and before it received the marker on C.
- A process is done when it has received a marker on each of its coming channels. The local state of the process and the state of each of its incoming channels are sent to the initiating process.
Note: any process can initiate the algorithm. So several snapshots may be in progress concurrently. How?
Distributed Snapshot Algorithm
b) Process Q receives a marker for the first time and records its local state c) Q records all incoming message d) Q receives a marker for its incoming channel and finishes recording the state of the incoming channel