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internals/overview.md
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internals/overview.md
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# Hobbes Overview
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Hobbes is a key/value database with a service architecture inspired by FoundationDB.
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The database is unbundled into a number of independent roles which communicate and interoperate.
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These roles can be organized into three main subsystems:
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- The **Control Plane**, which performs distributed consensus (i.e. Paxos)
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- The **Transaction Plane**, which provides transactional support to the database
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- The **Storage Plane**, which stores key/value data for the cluster and serves multiversion reads
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Each plane is responsible for managing the next.
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The Control Plane consists of a consensus cluster of Coordinator servers.
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The Coordinators have known, static names and serve as the entry point of the cluster.
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They perform fault-tolerant distributed consensus, specifically Viewstamped Replication (similar to MultiPaxos or Raft).
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The Transaction Plane consists of several unique roles which work together to provide transactional guarantees like consistency and atomicity.
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The Transaction Plane is generational:
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if *any* member fails or is unresponsive, the entire system is destroyed and a new generation is created to replace it.
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The creation of a new generation is kicked off by the Control Plane;
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as the Control Plane is implicitly fault-tolerant (distributed consensus), it therefore confers that fault-tolerance onto the Transaction Plane.
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The Storage Plane consists of a number of Storage servers which store the actual key/value data.
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Data is split into contiguous (ordered by key) shards and distributed (and replicated) amongst Storage servers.
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If a Storage server fails or becomes unresponsive,
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the Transaction Plane will issue commands to re-replicate its shards to restore fault-tolerance.
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Because the Transaction Plane is fault-tolerant as explained above, it therefore confers that fault-tolerance into the Storage Plane.