Introduction to FSM Apps
Definition of an FSM App
An FSM App is a replicated application which uses an underlying consensus engine implementing the ABCI. Its internal state takes the form of an FSM, and it exhibits proactive behaviours in each of such states. FSM Apps constitute a core part in the Open Autonomy framework to implement agent services.
The figure below depicts an sketch of the internal workings of an FSM App, composed of six states (A-F) and six events (1-6):
Note that in an FSM App, the responsibility of a state is distributed across two components, as it can be seen in the "zoomed" State C above:
A round is the component that defines the rules to transition across different states. It is a concrete implementation of the
AbstractRoundclass. It usually involves interactions between participants, although this is not enforced at this level of abstraction. A round can validate, store and aggregate data coming from different agents by means of transactions. The actual meaning of the data depends on the implementation of the specific round. This component produces the
Eventsthat will enable the FSM transit from one state to another, that is, capturing the role of the transition function.
Each round has an associated Behaviour, which is the component that defines the proactive actions to be executed at a particular state. It is a concrete implementation of the
BaseBehaviourclass, and contains the application logic for each state. It is scheduled for execution by the agents.
We will sometimes use indistinctly the terms "state" or "round" in the context of the Open Autonomy framework. We also define the concept of synchronized data, which is the component that stores persistent data and is accessible from any state. The consensus mechanism ensures that the synchronized data is consistently shared by all the agents. It is updated at the end of each round, where its contents can be updated. It is a concrete implementation of the
The framework defines a concept of "period" for the
SynchronizedData. When a new period starts, the
SynchronizedData allocates a new slot to store new values for the variables stored within it. This way, the developer has access to the historic values that the FSM App has stored (which can be pruned out). New periods are automatically created for the
SynchronizedData when an FSM App traverses the
ResetAndPauseRound inside Reset and pause.
Due to the variety of FSMs that can be defined, the framework does not enforce that an FSM App use the period concept, however, it is expected that any production worthy FSM App defines it, as it ensures the
SynchronizedData does not grow in an unbounded fashion.
A round/state might just be a stage in the overall flow of the application (e.g., waiting that a sufficient number of participants commit their observations to a temporary blockchain), or a voting round (e.g., waiting until at least an observed value has reached \lceil(2N + 1) / 3\rceil of the votes).
The developer could define the concept of "period" for their FSM as the sequence of stages in the FSM state flow that achieve a specific objective defined by the FSM App. Consider the price oracle demo, which aggregates asset prices from different data sources and submits the aggregated result to an L1/L2 blockchain. In this example, the developer could define a period as follows:
- Collect observations from external APIs or prior rounds.
- Reach consensus on the set of collected observations (i.e., 2/3 of the agents must agree).
- Compute a function (e.g., mean) on the set of collected observations, and reach consensus on the computed result.
- Construct a transaction that contains the aggregated result.
- Sign the transaction (2/3 of the agents must agree to sign).
- A single agent, randomly nominated as the keeper, sends the transaction to the chain. If it does not do this before a timeout event occurs, another agent is selected to be the keeper.
- Go to Step 1.
Composition of FSM Apps
One of the key features of the Open Autonomy framework is the generation of FSM Apps whose internal FSM is a composition of different FSMs. The composition mechanism facilitates rapid development of complex applications, by reusing FSMs already developed for other FSM Apps, obtaining the aggregated functionality.
Departing from a collection of FSMs, say FSM1, FSM2, ..., FSMn, a composed FSM can be constructed following certain rules:
- when composing FSMs, an FSMi can only transit from a final state to a start state of another FSMj, and
- a given FSM can only be used once in a composition.
We call FSM composition mapping the mapping that indicates how to move from one FSM to another. For every entry in the FSM composition mapping, e.g., from a final state Z from FSM1 to a start state A from FSM2, the composition mechanism will enforce that all the transitions ending in the final state Z of FSM1 be redirected to the start state A of FSM2. See an example below.
The figure above depicts an excerpt of a composition of three FSMs into a single one. Note how the finish states of FSM1 are linked to start states of FSM2 and FSM3. We remark that the transitions indicated by the FSM composition mapping are not regular transitions that respond to events, rather they are merely a construct to indicate how the states in the composed FSM are connected.
The result of a composition of a collection of FSMs is an FSM whose set of spaces is a subset of the union of state spaces of the constituent FSMs. For this reason is it not possible to "reuse" a given FSM twice in a composition. All the final states of the constituent FSMs that are defined in the transition mapping will be removed, as exemplified in the figure above.
Therefore, althought it might be useful and intuitive thinking of a composed FSM in terms of its constituent FSMs, the structure is not retained internally by the FSM App. For example, consider the following setting:
Note that in the composed FSM, when transitioning to A3, the FSM loses track of what was the FSM from which it transitioned (either FSM1 or FSM2). Nevertheless, if the business logic in state A3 requires knowledge of what was the history of visited states before reaching it (e.g., in order to execute a different action), the developer has access to that history through the field
_previous_rounds from the class
AbciApp which will be discussed in a separate section.
Considerations to Develop FSM Apps
- agent services and their associated FSM Apps must be designed with determinism at their core. For this reason, in each round, agents must agree upon their execution outputs in order to make progress. Any source of "uncontrolled" randomness or code inconsistency that leads to non-determinism will undermine the FSM App's ability to achieve its goal. Sources of non-determinism include but not limited to:
- Non-verifiable randomness libraries.
- Non deterministic libraries, like ML frameworks that do not guarantee exact same results under the same initial conditions.
- Using the agent's local time to make decisions along the execution flow. Agents synchronize using the consensus engine, and that is the only reliable source of "time" that should be used. Sometimes, block timestamp can be a handy property common to all agents and can be used as a
- Using local resources that are not synchronized among all agents to make decisions.
- Using data serialization, e.g.,
json.dumpwithout sorting the keys and inputs. Try to work with sorted data structures whenever possible (e.g., sorted lists instead of arbitrary lists).
- Using block metrics in a
Behaviour, other than those determining round transitions. All other blocks read from behaviours might involve race conditions.
- Sometimes it is required to retry interactions with external services. There are generally two ways retries can be implemented: On the behaviour level and on the round level. The default retry should be implemented via round transitions. For instance, when agents cannot finish their behaviour level activities because of failure of an external service they can signal this via the payload (e.g., sending an explicit "None"). Then at the round level the agents can coordinate transition to the next state: either retrying the current state or transitioning to another state. Behaviour level retries should be limited when making calls to external services where the agent is the only one retrieving such information. For example, in an oracle service, each agent interacts with a different external API and the retries are naturally timed out by the round timeout.
- It is desirable that agent
Skillshave their own unit tests. The Open AEA framework offers tools to make easier for developers to write tests for their Skills. Beware that the AEA skill testing framework might have some limitations with respect to idiomatic Python testing. For example, it is a known issue that patching skill module-level components does not work, but only instance-based patching is effective (e.g. using
- When developing agent applications raise exceptions for all branches which are currently unhandled and would cause inconsistencies in the app. These exceptions can then be surgically addressed in separate PRs. Example: on the happy path a file is loaded properly, on the unhappy path an exception is raised by the file not being present. In production handling this can get involved. During development it is best to "park" this issue and focus on the happy case. But the unhappy case should raise so we can capture its prevalence in tests. Logging is the worst approach as it hides this issue!