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Linearizability

Distributed/networked systems employ data replication to achieve availability and scalability. Consistency is concerned with the question of what should happen if a client modifies some data items and concurrently another client reads or modifies the same items possibly at a different replica. Linearizability is a strong form of consistency. (That is why it is also called as strong-consistency.) For a system to satisfy linearizability,  each operation must appear (from client perspective) to occur at an instantaneous point between its start time (when the client submits it) and finish time (when the client receives the response), and  execution at these instantaneous points should form a valid sequential execution (i.e., it should be as if operations are executed one at a time ---without concurrency, like they are being executed by a single node)  Let's simplify things further. In practice, consistency is often defined for systems that have two very specific operations: read and w

Using Lightweight Formal Methods to Validate a Key-Value Storage Node in Amazon S3 (SOSP21)

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This paper comes from my colleagues at AWS S3 Automated Reasoning Group, detailing their experience applying lightweight formal methods to a new class of storage node developed for S3 storage backend. Lightweight formal methods emphasize automation and usability. In this case, the approach involves three prongs: developing executable reference models as specifications, checking implementation conformance to those models, and building infrastructure to ensure the models remain accurate in the future. ShardStore ShardStore is a new append-only key-value storage node developed for AWS S3 backend. It is over 40K lines of Rust code. Shardstore is a log-structured merge tree (LSM tree) but with shard data stored outside the tree to reduce write amplification. ShardStore employs soft updates for avoiding the cost of redirecting writes through a write-ahead log while still being crash consistent. A soft updates implementation ensures that only writes whose dependencies are persisted are sent

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