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Interactions between Components in an FSM App

In this section we present sequence diagrams in order to help understand the transmitted messages and method calls between the software components that are part of an FSM App.

The ABCIApp - AbstractRoundBehaviour interaction

The interaction between AbstractRoundBehaviour and the ABCIApp occurs via the consensus engine, the setup of which proceeds on the Period. and the shared state among the skills. A typical workflow looks as follows:

  1. At setup time, the consensus engine node creates connections with the associated AEA and sync together. In the AEA process, the ABCIApp starts from the first round, waiting for transactions to update its state. The AbstractRoundBehaviour schedules the initial state behaviour for execution of the act() method.

  2. The current state behaviour sends transactions to update the state of the ABCIApp, and once its job is done waits until it transitions to the next round, by checking read-only attributes of the ABCIApp accessible through the AEAs' skill context. Concrete implementations of AbstractRoundBehaviour cannot update the state the of ABCIApp directly, only through an agreed upon update with other agents via the consensus engine node.

  3. Once the transaction gets added to a block accepted by the consensus engine, the consensus engine node delivers the block and transactions contained in it to the ABCIApp using the AEAs ABCIConnection and ABCIHandler. The ABCIApp processes the transaction and enacts the encoded state transition logic.

  4. The transition to the next round is detected and all scheduled behaviour and other tasks get cancelled before transitioning to the next round.

  5. Cycles of such rounds may, either entirely or in part, be repeated until a final state is reached, implemented as a

AbstractRoundBehaviour:

Concrete implementations of AbstractRoundBehaviour can be seen as a client of the ABCIApp (in that sense it encapsulates the "user" of a normal blockchain).

AbstractRoundBehaviour diagrams

The following diagram shows how the FSM behaviour operates in concert with the AEA event loop. (Not all components are included in the diagram.)

sequenceDiagram participant EventLoop participant AbsRoundBehaviour participant Behaviour1 participant Behaviour2 participant Outbox note over AbsRoundBehaviour,Behaviour2: Let the FSM App start with Behaviour1
it will schedule Behaviour2 on event e. loop while round does not change EventLoop->>AbsRoundBehaviour: act() alt if Behaviour1 not done note over AbsRoundBehaviour: Behaviour1 act() is called. AbsRoundBehaviour->>Behaviour1: act() activate Behaviour1 note over Behaviour1: During the execution,
the current round may
(or may not) change. Behaviour1->>Outbox: put_message("GET (...) /tx_sync=0x(...)") Behaviour1->>Outbox: [Wait until tx delivered] put_message("GET (...) /tx?hash=0x(...)") note over Outbox: Multiplexer will route the transactions
to the corresponding connection. Behaviour1->>AbsRoundBehaviour: return deactivate Behaviour1 else AbsRoundBehaviour->>Behaviour1: clean_up() end note over EventLoop: Other routines. end note over AbsRoundBehaviour: Read current AbciApp round and pick matching Behaviour
in this example, Behaviour2. loop while round does not change EventLoop->>AbsRoundBehaviour: act() note over AbsRoundBehaviour: Now Behaviour2 act() is called. AbsRoundBehaviour->>Behaviour2: act() activate Behaviour2 Behaviour2->>AbsRoundBehaviour: return deactivate Behaviour2 note over EventLoop: Other routines. end

The following diagram describes the addition of transactions to the transaction pool. (Not all components are included in the diagram.)

sequenceDiagram participant ConsensusEngine participant ABCIServerConnection as ABCIServerConnection
TendermintDecoder participant ABCIRoundHandler note over ConsensusEngine,ABCIServerConnection: client submits transaction tx ConsensusEngine->>ABCIServerConnection: [Request] CheckTx(tx) ABCIServerConnection->>ABCIRoundHandler: check_tx(tx) ABCIRoundHandler->>RoundSequence: check_is_finished() RoundSequence->>ABCIRoundHandler: false ABCIRoundHandler->>ABCIServerConnection: OK ABCIServerConnection->>ConsensusEngine: [Response] CheckTx(tx) note over ConsensusEngine,ABCIServerConnection: tx is added to tx pool ConsensusEngine->>ABCIServerConnection: [Request] DeliverTx(tx) ABCIServerConnection->>ABCIRoundHandler: deliver_tx(tx) ABCIRoundHandler->>RoundSequence: check_is_finished() RoundSequence->>ABCIRoundHandler: false ABCIServerConnection->>RoundSequence: deliver_tx(tx) RoundSequence->>ABCIApp: check_transaction(tx) ABCIApp->>Round: check_transaction(tx) Round->>ABCIApp: OK ABCIApp->>RoundSequence: OK RoundSequence->>ABCIRoundHandler: OK ABCIRoundHandler->>ABCIServerConnection: OK ABCIServerConnection->>ConsensusEngine: [Response] CheckTx(tx)

The following diagram describes the delivery of transactions in a block:

sequenceDiagram participant ConsensusEngine participant ABCIServerConnection as ABCIServerConnection
TendermintDecoder participant ABCIRoundHandler participant RoundSequence participant ABCIApp participant Round1 participant Round2 activate Round1 note over Round1,Round2: Round1 is the active round,
Round2 is the next round. note over ConsensusEngine,ABCIRoundHandler: Validated block ready to
be submitted to the FSM App. ConsensusEngine->>ABCIServerConnection: [Request] BeginBlock() ABCIServerConnection->>ABCIRoundHandler: begin_block() ABCIRoundHandler->>RoundSequence: begin_block() ABCIRoundHandler->>ABCIServerConnection: OK ABCIServerConnection->>ConsensusEngine: [Response] BeginBlock(OK) loop for tx_i in block ConsensusEngine->>ABCIServerConnection: [Request] DeliverTx(tx) ABCIServerConnection->>ABCIRoundHandler: deliver_tx(tx) ABCIRoundHandler->>RoundSequence: check_is_finished() RoundSequence->>ABCIRoundHandler: false ABCIServerConnection->>RoundSequence: deliver_tx(tx) RoundSequence->>ABCIApp: check_transaction(tx) ABCIApp->>Round1: check_transaction(tx) Round1->>ABCIApp: OK ABCIApp->>RoundSequence: OK RoundSequence->>ABCIRoundHandler: OK ABCIRoundHandler->>ABCIServerConnection: OK ABCIServerConnection->>ConsensusEngine: [Response] CheckTx(tx) end ConsensusEngine->>ABCIServerConnection: [Request] EndBlock() ABCIServerConnection->>ABCIRoundHandler: end_block() ABCIRoundHandler->>RoundSequence: end_block() alt if condition is true note over Round1, Round2: Replace Round1 with Round2. deactivate Round1 RoundSequence->>Round2: schedule (*) activate Round2 end RoundSequence->>ABCIRoundHandler: OK ABCIRoundHandler->>ABCIServerConnection: OK ABCIServerConnection->>ConsensusEngine: [Response] EndBlock(OK) deactivate Round2
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