# Components of an FSM App

There are a number of important concepts that will be introduced in this section so that the developer understands how FSM Apps work. Namely:

Core classes that instrument the FSM App:

• AbciApp class: The base class that defines the FSM transitions in an FSM App.
• AbstractRoundBehaviour class: The base class defining the overall FSM Apps behaviour.

Classes that define the operations per state of the FSM:

• AbstractRound class: The base class to define the FSM state rounds of the FSM App.
• AsyncBehaviour class: The base class to define FSM state behaviours to be executed during each round in the FSM App.

Other classes related to the FSM App FSM global state:

• SynchronizedData class: The class providing access to the shared state.

## How FSM Apps work

The AbciApp class defines the constituent FSM of the FSM App. That is, it defines the set of start and final states (rounds) and the transition function.

The state of the FSM App is updated through transactions committed to the temporary blockchain. This ledger is local with respect to the Period, i.e., its existence is controlled by, and only relevant in context of, the Period. It is distributed and decentralized with respect to the AEAs in the system, all of whom run a local consensus node. The underlying consensus engine (currently Tendermint) allows decentralized state replication among different processes.

The transitions in the FSM are triggered by the delivery of blocks from the consensus engine through the ABCI. The transactions that are contained in these blocks are submitted by AEAs Behaviours, and can lead to updates of the shared SynchronizedData.

Each round has its own business logic, which specifies how the participants' transactions are validated or the conditions that trigger a transition to another round (e.g., enough votes received for the same value). The transitions are triggered by means of events, i.e., values that are set by the round implementation and are processed by the framework in order to compute the round successor.

There are two kinds of events: normal events and timeout events:

• Normal events are set by the round according to certain conditions that are checked during the execution of a round. These conditions can be defined arbitrarily according to the business logic of the FSM App.

• Timeout events are automatically triggered in case the timeout associated to the event has expired. The timeout is checked against the "global clock", which is a side effect of block validation: each block header carries the timestamp of the event of block proposal.

Example

A normal event can

On the other hand, consider a block $$B_1$$ with timestamp $$t_1$$, and block $$B_2$$ with timestamp $$t_2 = t_1 + 10$$ seconds. Assume that after $$B_1$$ is processed, the FSM App current state round is defined to trigger a timeout event set at $$T=5$$ seconds. When $$B_2$$ is delivered, the timeout event had already been triggered (because $$T<t_2-t_1$$), and the associated transition had already happened in the FSM, regardless of the content of $$B_2$$.

The ABCIHandler, is in charge of receiving the transactions through the DeliverTx ABCI requests, and forwards them to the current active round, which produces the response for DeliverTx. The end_block abstract method is called on the ABCI request EndBlock, and determines the round successor by setting the event to take the transition to the next state, as defined by the transition function in the AbciApp.

Important

We remark that the FSM App, which resides inside the AEA, is updated by the consensus engine through the ABCI requests handled by the ABCIHandler, and NOT by the AEA behaviours. The AEA behaviours can only send updates through the process of sending transactions to the consensus engine.

The Period class keeps track of an execution of the FSM App. It is instantiated in the state of the Skill and accessible through the SkillContext. It is a concrete class, so the developer should not need to extend it.