The State Machine Approach
The preceding intuitive discussion implies a simple technique for implementing a fault-tolerant service in terms of a State Machine:
- Place copies of the State Machine on multiple, independent servers.
- Receive client requests, interpreted as Inputs to the State Machine.
- Choose an ordering for the Inputs.
- Execute Inputs in the chosen order on each server.
- Respond to clients with the Output from the State Machine.
- Monitor replicas for differences in State or Output.
The remainder of this article develops the details of this technique.
- Step 1 and 2 are outside the scope of this article.
- Step 3 is the critical operation, see Ordering Inputs.
- Step 4 is covered by the State Machine Definition.
- Step 5, see Ordering Outputs.
- Step 6, see Auditing and Failure Detection.
The appendix contains discussion on typical extensions used in real-world systems such as Logging, Checkpoints, Reconfiguration, and State Transfer.
Read more about this topic: State Machine Replication
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