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46 lines
2.3 KiB
Markdown
46 lines
2.3 KiB
Markdown
<!--[metadata]>
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+++
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title = "Raft consensus in swarm mode"
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description = "Raft consensus algorithm in swarm mode"
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keywords = ["docker, container, cluster, swarm, raft"]
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[menu.main]
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identifier="raft"
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parent="engine_swarm"
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weight="21"
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+++
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<![end-metadata]-->
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## Raft consensus algorithm
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When the Docker Engine runs in swarm mode, manager nodes implement the
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[Raft Consensus Algorithm](http://thesecretlivesofdata.com/raft/) to manage the global cluster state.
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The reason why *Docker swarm mode* is using a consensus algorithm is to make sure that
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all the manager nodes that are in charge of managing and scheduling tasks in the cluster,
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are storing the same consistent state.
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Having the same consistent state across the cluster means that in case of a failure,
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any Manager node can pick up the tasks and restore the services to a stable state.
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For example, if the *Leader Manager* which is responsible for scheduling tasks in the
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cluster dies unexpectedly, any other Manager can pick up the task of scheduling and
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re-balance tasks to match the desired state.
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Systems using consensus algorithms to replicate logs in a distributed systems
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do require special care. They ensure that the cluster state stays consistent
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in the presence of failures by requiring a majority of nodes to agree on values.
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Raft tolerates up to `(N-1)/2` failures and requires a majority or quorum of
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`(N/2)+1` members to agree on values proposed to the cluster. This means that in
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a cluster of 5 Managers running Raft, if 3 nodes are unavailable, the system
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will not process any more requests to schedule additional tasks. The existing
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tasks will keep running but the scheduler will not be able to rebalance tasks to
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cope with failures if when the manager set is not healthy.
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The implementation of the consensus algorithm in swarm mode means it features
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the properties inherent to distributed systems:
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- *agreement on values* in a fault tolerant system. (Refer to [FLP impossibility theorem](http://the-paper-trail.org/blog/a-brief-tour-of-flp-impossibility/)
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and the [Raft Consensus Algorithm paper](https://www.usenix.org/system/files/conference/atc14/atc14-paper-ongaro.pdf))
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- *mutual exclusion* through the leader election process
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- *cluster membership* management
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- *globally consistent object sequencing* and CAS (compare-and-swap) primitives
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