Consensus Loop

The Loop is the most high-level component of the SBA* consensus implementation. It simulates a state-machine like architecture, with a functional flavor, where consensus phases are sequenced one after the other, returning functions as they complete, based on their outcomes.

Architecture

For any caller to be able to start the consensus, they will need to have access to a fully initialized Loop. It is initialized by giving a couple of pre-requisite items:

• A public key, to which block generator rewards can be attributed
• An Emitter, responsible for routing communications
• A Requestor, which is capable of retrieving candidate blocks on request from the network
• A callback to be used for candidate block verification
• A database instance
• The initial timeout

This Loop can then be re-used for the lifetime of the node to start and maintain consensus loops.

Communications

The Loop is responsible for establishing the communications bridge between the consensus and the rest of the node. This is done by creating an Emitter and passing it to the Loop on startup. The Loop will then set up message channels for each of the messages we expect to receive for the consensus, and subscribe them to the event bus with the relevant topics. From this point, all consensus messages will be directly routed to the correct components as they start coming in.

Running

To start the consensus loop, simply create the state machine by calling CreateStateMachine on the Loop instance. This will return two consensus phases - the score phase, and the agreement phase. There is a key distinction between the two, which is that the score phase will return another function as it completes, and so will it's successors until they reach the score phase again. This ensures the sequential, state-machine-like progression of the synchronous consensus phases. The agreement phase is intended to run concurrently, as it can receive messages at any time, and is not bound to a timer or anything of the sort.

With these two phases, all we have left to do to start the consensus loop, is to formulate a RoundUpdate. This contains all the stateful information needed by the consensus to do its job. Finally, with all of these items in place, call loop.Spin, passing these items, in order to launch the consensus loop. Once this is called, the consensus will progress until an error is encountered, or until it is cancelled through a context cancellation.