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State Machine

This smart contract set implements a state machine. State machines are usually used to represent a system where an entity goes through several sequential states.

Each state has different functions and different roles associated with it. You can call certain functions only if the state you are in is associated with that function. The different roles associated with a state are the roles who are allowed to perform transition to the next state from the given state.

Our templateset provides a powerful, highly customisable way to create a statemachine for your use case.


Representation of a state

Each state in the statemachine is represented using a State struct. A struct is a data type in solidity used to represent an object containing different attributes. The State struct is declared in the StateMachine.sol file as follows:

struct State {
// a boolean to check if the state is actually created
bool hasBeenCreated;
// a mapping of functions that can be executed when in this state
mapping(bytes4 => bool) allowedFunctions;
// a mapping of all roles that have been configured for this state
mapping(bytes32 => bool) allAllowedRoles;
// a list of all the roles that have been configured for this state
bytes32[] allowedRoles;
// a list of all the preconditions that have been configured for this state
function(bytes32, bytes32) internal view[] preConditions;
// a list of callbacks to execute before the state transition completes
function(bytes32, bytes32) internal[] callbacks;
// a list of states that can be transitioned to
bytes32[] nextStates;
// function that executes logic and then does a StateTransition
bytes4 preFunction;

To create a state, you can call the createState function which our templateset provides out of the box. It is defined in the StateMachine.sol contract.

Defining a state

To comprehensively represent your state, you need to define:

  1. The next states that you can transition to from the given state (nextStates field)
  2. The functions you can access when you are in the given state (allowedFunctions field)
  3. The roles who can perform the state transition to the next state (allowedRoles field)

Our templateset allows you to do this very easily through the functions addNextStateForState, addAllowedFunctionForState and addRoleForState. These functions are defined in the StateMachine.sol file.

One thing to note is that these three functions can be only called by the admin. The admin is set to the address from which the contract creation transaction is sent.

Transitioning from a state

1. Pre-conditions

There are usually some conditions that need to be satisfed before transitioning to the next state. Our templateset allows you to effortlessly add these conditions using the preConditions field. It is important to note that while defining pre-condition functions, you need to ensure they throw an exception when the condition fails. Add a pre-condition for a state using addPreConditionForState function.

2. Callbacks

Before you transition to another state, there may be several actions you need to perform first. To accomodate for this, our templateset provides support for callbacks. callbacks for a given state are functions which are called before moving to the next state from the given state. Add a callback for a state using addCallbackForState function.

3. State transition

Once pre-conditions are satisfied and the callbacks have been executed, we are ready to perform our state transition.

Generally in a state transition, there is some business logic to be executed, followed by the actual change of state. Both these steps are bundled in a function, let's call such a function a transition function for the state. This transition function should be called when you want to transition from the given state.

To build your transition function - all you need to do is define the business logic you want to execute before the state transition. To perform the actual state transition, you can simply call the transitionState function.

The transitionState function we provide does all the heavy lifting of a state transition for you (for example: checking the preconditions, execution of callbacks, etc.). It also verifies all the edge conditions associated with state transitions so you don't need to worry about them.

After you have defined your transition function for a state, you can calculate its function signature and set the preFunction field on the state to the calculated function signature. You can do this by calling setPreFunctionForState defined in the StateMachine.sol contract.

History of state transitions

To have more transparency, our templateset also records the history of state transtions.

We use a StateTransition struct to store information on a state transition. It is defined in the StateMachine.sol contract as:

struct StateTransition {
bytes32 fromState;
bytes32 toState;
address actor;
uint256 timestamp;

The history is stored as an array of StateTransitions. To view the transition at a particular index in the history array, you can use the getHistory function defined in the StateMachine.sol contract.

You can also get the length of the history array using getHistoryLength function.

To view the while history, you could get the length of the history array using getHistoryLength and call getHistory for each index in the array.


1. Setting up the state machine states and flow

For your convenience the Generic.sol template contract has all the necessary boilerplate code set up. So modify the different states and their relationships at your leisure.

Creating states

// Create the variable
bytes32 public constant STATE_START = "STATE_START";

// use the createState helper inside the setupStateMachine function
function setupStateMachine(address adminAddress) internal override {
// set the correct state as the starting state

Defining flow and roles

// Add next states to existing states to define flow
// Add Roles to state
function setupStateMachine(address adminAddress) internal override {
addNextStateForState(STATE_START, STATE_END);
addRoleForState(STATE_START, ROLE_ADMIN, adminAddress);


2. Understanding the deployment process and deploying the contract

Before we dive in you should know that the Generic.sol contract extends from the StateMachineMetadata.sol contract. It's an interface on top of the base StateMachine.sol contract that allows linking metadata to a contract. The setup for this takes place during the deployment of the contract. So let's do just that.

Now when deploying the contract you may want to bind some IPFS data to it. A potential use case for this could be lifecycle tracing of a vehicle for example. So you could define a state machine that describes the various states a vehicle goes through (manufacturing, maintenance, etc...), but the vehicle itself has several unchanging attributes that don't need to be stored on-chain (f.e. a repair manual). IPFS is perfect for that.

Let's look at the constructor of our Generic.sol contract to get an idea of the inputs it will need for deployment:

uint256 entityId,
string memory ipfsHash,
string memory baseURI
) {
address adminAddress = msg.sender;
_grantRole(DEFAULT_ADMIN_ROLE, adminAddress);
_entityId = entityId;
_baseURI = baseURI;
_setEntityURI(_entityId, ipfsHash);

Since the binding of the metadata is key-value based, you can see the entityId being the key and the value being the url/ipfshah or any other identifier to retrieve your metadata from the web (for example the unique suffix of a wetransfer link). The baseURI is an optional prefix that will be attached to the passed identifier (here ipfsHash). This baseURI will be ipfs:// here, indicating the character of the metadata.

The reason we want to use an entityId as key to retrieve the bound value is to provide flexibility to the end user. F.e., if only the deployer knows the key, then only he can retrieve the value. Or maybe you want to extend the functionalities of the StateMachineMetadata.sol contract, allowing users to add metadata later on etc...

Your entityId can be any slug you want. But we recommend using a crypto library to generate a unique 32 bytes long one at least, for obvious reasons.

Now to deploy our contract you define an entityId and the data you want to upload to IPFS:

// This  identifier is used as a key to attach metadata to the smart contract
// See StateMachineMetadata.sol for more info
// This value is hardcoded here to make graph indexing of the metadata possible
export const entityId = 3073193977; // crypto.randomBytes(32).readUInt32LE()

// Let's define the metadata for our entity that we want to upload to IPFS
const metadata = {
param1: 'param1',
param2: 'param2',

// using our hardhat task to upload the data to IPFS
const jsonCid: string = await run('ipfs-upload-string', {
data: JSON.stringify(metadata),
ipfspath: `/generic-statemachine/metadata/metadata-${entityId}.json`,

// Then we deploy
const statemachine = await factory.deploy(BigNumber.from(entityId), jsonCid, 'ipfs://');

3. Indexing on-chain data

statemachinemetadata Indexing Module

The statemachinemetadata indexing module has 3 main and 1 helper file:

  1. subgraph/datasource/statemachinemetadata.gql.json - Schema definition file
  2. subgraph/datasource/statemachinemetadata.yaml - Subgraph manifest template file
  3. subgraph/datasource/statemachinemetadata.ts - Mapping functions file

And a helper file at subgraph/fetch/statemachinemetadata.ts

1. statemachinemetadata Schema

We define 2 entities in the schema:

  1. StateMachineMetadataContract

    This is the entity modelling the Generic.sol statemachine contract.

    • The field currentState holds the current state the entity represented by the statemachine is in.
    • The stateTransitions field hold a list of transitions that the entity has gone through.
    • The two fields param1 and param2 seem confusing, since we don't see them as state variables on the Generic.sol contract or any of the contracts that Generic.sol inherits. This is because they are not state variables on the contract, but metadata for the entity that we have uploaded on IPFS.

You can see them being set in the deploy script at deploy > 00_deploy_StateMachine.ts.

If you wish to change the name of the parameters in the metadata from param1, param2 to your custom field name in the deploy script, please be sure to propagate the changes in:

a. schema definition at subgraph/datasource/statemachinemetadata.gql.json

b. handler at subgraph/datasource/statemachinemetadata.ts

  1. StateTransition

This is the entity representing the Transition event emitted by the Generic.sol statemachine contract. Its fields represent the information emitted by the event.

2. statemachinemetadata Subgraph Manifest Template

The field of interest to us in the subgraph manifest template at subgraph/datasource/statemachinemetadata.yaml is the eventHandlers field.

Here, we list the events we want to listen to, as given here:

- event: Transition(address,bytes32,bytes32)
handler: handleTransitions

We listen to the Transition event emitted by the Generic statemachine contract. When that event is emitted, we call the handleTransitions mapping function defined in subgraph/datasource/statemachinemetadata.ts

3. statemachinemetadata Mapping function

The mapping functions for the statemachinemetadata indexing module are defined in subgraph/datasource/statemachinemetadata.ts

It is advisable to run graph:config, graph:compile, and graph:codegen tasks before playing around with this file to generate types and classes.**

Now that we have our types and classes, let's see how they are used.

The handleTransitions handler takes in the Transition event. Then, it performs three main tasks:

  • fetches the StateMachineMetadataContract entity which emitted the Transition event

To do this, a custom fetcher was written (./subgraph/fetch/statemachinemetadata.ts).

Inside the fetcher, you will see the hard coded entityId from the deploy script deploy > 00_deploy_StateMachine again:

const try_entityURI = sm.try_entityURI(BigInt.fromString(`3073193977`));

We fetch the metadata from IPFS using this entity ID and populate the fields on the StateMachineMetadataContract entity accordingly:

const try_entityURI = sm.try_entityURI(BigInt.fromString(`3073193977`));
const metadataURI = try_entityURI.reverted ? '' : try_entityURI.value;

if (metadataURI.includes('ipfs://')) {
const ipfsHash = metadataURI.replace('ipfs://', '');
const metadataURIBytes =;
if (metadataURIBytes) {
const metadataURIContent = json.try_fromBytes(metadataURIBytes);
if (metadataURIContent.isOk && metadataURIContent.value.kind == JSONValueKind.OBJECT) {
const entityMetadata = metadataURIContent.value.toObject();

const param1 = entityMetadata.get('param1');
const param2 = entityMetadata.get('param2');

contract.param1 = param1 ? param1.toString() : null;
contract.param2 = param2 ? param2.toString() : null;;

It's really important that this entityId matches with the one defined in the deployment script. Also we want to point out that general indexing logic can be re-used based on the protocol prefix we defined in the deployment as well (ipfs://).

  • Then, we create a new StateTransition entity to keep a track of the events emitted by the contract
  • Finally, we save the changes in the storage


** Before using this file, it is recommended to run the tasks graph:config, graph:compile and graph:codegen.

The graph:codegen task is where the types/classes are generated based on the entities defined in the schema (at subgraphs > x.gql.json). These types/classes are imported and used in the mapping functions.

Without running this task, you will run into several Cannot find module.. linter errors while trying to use this file.