Module splinter::consensus::two_phase

A simple n-party, two-phase commit (2PC) consensus algorithm implemented as a ConsensusEngine. This is a bully algorithm where the coordinator for a proposal is determined as the node with the lowest ID in the set of verifiers. Only one proposal is considered at a time. A proposal manager can define its own set of required verifiers by setting this information in the consensus data.

Known limitations of this 2PC implementation

There is a potential race condition in two-phase commit where two different proposals are in flight:

• The two proposals have different coordinators
• Both proposals have two or more verifiers in common
• One of the common verifiers evaluates the 1st proposal; the other evaluates the 2nd proposal
• Neither proposal will be completed, since only a single proposal can be evaluated by a verifier at a time

The solution to this limitation would require 2PC to have more sophisticated knowledge about the proposals available to it, and be able to process multiple non-overlapping proposals at the same time.

Another limitation of this implementation is that it is not fully resilient to crashes; for instance, if the coordinator commits a proposal but crashes before it is able to send the APPLY message to the other nodes, the network will be out of sync because the coordinator does not know to send the message when it restarts. This limitation will be solved by re-implementing 2PC as a stateless algorithm.

Structs

TwoPhaseEngine