In distributed computing, a shared snapshot object is a type of data structure, which is shared between several threads or processes. For many tasks, it is important to have a data structure, that can provide a consistent view of the state of the memory. In practice, it turns out that it is not possible to get such a consistent state of the memory by just accessing one shared register after another, since the values stored in individual registers can be changed at any time during this process. To solve this problem, snapshot objects store a vector of n components and provide the following two atomic operations: update(i,v) changes the value in the ith component to v, and scan() returns the values stored in all n components.[1][2]
Snapshot objects can be constructed using atomic single-writer multi-reader shared registers.
In general, one distinguishes between single-writer multi-reader (swmr) snapshot objects and multi-writer multi-reader (mwmr) snapshot objects. In a swmr snapshot object, the number of components matches the number of processes and only one process Pi is allowed to write to the memory position i and all the other processes are allowed to read the memory. In contrast, in a mwmr snapshot object all processes are allowed to write to all positions of the memory and are allowed to read the memory as well.
^Fich, Faith Ellen (2005). "How Hard is It to Take a Snapshot?". SOFSEM 2005: Theory and Practice of Computer Science. Lecture Notes in Computer Science. Vol. 3381 (SOFSEM 2005: Theory and Practice of Computer Science ed.). Springer Berlin Heidelberg. pp. 28–37. doi:10.1007/978-3-540-30577-4_3. ISBN 978-3-540-24302-1.
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