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state-machine-builder

An on-chain, permission-based state machine builder.

This is a VERY EXPERIMENTAL project and comes with absolutely no security guarantees. Do NOT use this codebase in production.

Getting Started

Requires npm and Truffle

git clone https://github.com/adklempner/state-machine-builder.git
cd state-machine-builder
npm install
truffle compile
truffle test

State Library

State.Machine

The core data structure of the State library is the Machine struct, which stores a graph representation of the state machine and the state of every running process.

struct Machine {
  mapping(bytes32 => mapping(bytes32 => bytes32)) transitionGraph;
  mapping(bytes32 => bytes32) processes;
}

Transition Graph

The transitionGraph mapping defines a labeled, directed graph where each node refers to a state and each label refers to a transition.

        nodeA              nodeB      label
mapping(bytes32 => mapping(bytes32 => bytes32)) transitionGraph;

Here's how the graph below would be stored in the transitionGraph mapping (assume strings are converted to bytes32) pvqnb

transitionGraph["a"]["b"] = "1"
transitionGraph["b"]["c"] = "2"
transitionGraph["c"]["d"] = "3"
transitionGraph["d"]["a"] = "4"
transitionGraph["a"]["c"] = "5"
transitionGraph["c"]["a"] = "6"

The graph is defined by calling addTransition and removeTransition.

State.Machine machine;
machine.addTransition("a", "b", "1");
machine.addTransition("b", "c", "2");
...
machine.addTransition("c", "a", "6");

Processes

The processes mapping tracks the state of each process run on the machine. Every process starts at 0, which represents the initial state of the machine.

       ProcessID   ProcessState
mapping(bytes32 => bytes32) processes;

For the machine above, since there is no transition defined from state 0 to another state, a process can't be started. After adding a transition from the inital state:

machine.addTransition("0", "a", "0");

a user can then start a new process by calling performTransition

machine.performTransition("process1", "a")

The call above changes the state of the process with ID "process1" from 0 to "a"

StateMachineBuilder Contract

StateMachineBuilder is an example of how the State library can be implemented.

Permissions

StateMachineBuilder uses OpenZeppelin's Role Based Access Control contract for managing permissions. The deployer of StateMachineBuilder is assigned the role of administrator. Only administrators can assign and remove roles from addresses by calling the adminAddRole and adminRemoveRole functions.

Building a State Machine

Each State.Machine in a StateMachineBuilder is stored in mapping(bytes32 => State.Machine) stateMachines

Only administrators can build out the transition graph for each State.Machine by calling addStateTransition and removeStateTransition. Here's how to define the same machine as above, assigning it the id "exampleMachine"

StateMachineBuilder builder;
builder.addStateTransition("exampleMachine", "a", "b", "1");
builder.addStateTransition("exampleMachine", "b", "c", "2");
...
builder.addStateTransition("exampleMachine", "c", "a", "6");
builder.addStateTransition("exampleMachine", "0", "a", "0");

Labels

StateMachineBuilder gives state transitions meaning by defining labels.

struct Transition {
   string authorizedRole;
   bool networked;
}

mapping(bytes32 => Transition) labels;

By calling addLabel, the administrator can define which role is authorized to perform a transition with that label.

builder.addLabel("0", "BasicUser", false);

After assigning that role to an address

builder.adminAddRole(0x3f5CE5FBFe3E9af3971dD833D26bA9b5C936f0bE, "BasicUser");

that address can perform any transition with label 0 (on ANY of the machines) by calling performStateTransition

builder.performStateTransition("exampleMachine", "newProcess", "a");

Networked State Transitions

Since users are authenticated using Ethereum addresses, one StateMachineBuilder can be authorized to perform state transitions on another StateMachineBuilder.

An admin of our original builder gives the otherBuilder the role of AuthorizedBuilder

StateMachineBuilder otherBuilder;
builder.adminAddRole(otherBuilder, "AuthorizedBuilder");

they then create a new label that requires the caller to have the AuthorizedBuilder role, and adds a new transition with that label

builder.addLabel("InboundNetworkedTransition", "AuthorizedBuilder", false);
builder.addStateTransition("exampleMachine", 0, "initiatedByOtherBuilder", "InboundNetworkedTransition");

The admin of the otherBuilder creates a label where the networked flag is set to true, and creates a new transition with that label

otherBuilder.addLabel("OutboundNetworkedTransition", "admin", true);
otherBuilder.addStateTransition("otherExampleMachine", 0, "MadeOutboundTransition", "OutboundNetworkedTransition");

Setting the networked flag to true means that the transition cannot be performed by calling performStateTransition. Instead, we use performNetworkedStateTransition.

performNetworkStateTransition takes additional parameters for building a performStateTransition call to another StateMachineBuilder. If the call fails, any state changes are reverted.

In this example, otherBuilder is authorized to change the state of any process in builder from 0 to "initiatedByOtherBuilder"

otherBuilder.performNetworkedStateTransition("otherExampleMachine", "processOutbound", "MadeOutboundTransition", "exampleMachine", "processInbound", "initiatedByOtherBuilder", builder);

If the call is successfull, the following happens in one transaction:

  • in otherBuilder: the state of the process with ID processOutbound in the machine with ID otherExampleMachine transitions from 0 to MadeOutboundTransition
  • in builder: the state of the process with ID processInbound in the machine with ID exampleMachine transitions from 0 to initiatedByOtherBuilder

Notes on networked transitions

StateMachineBuilder implements the interface ProcessRunner, and uses it to call other machines.

interface ProcessRunner {
  function performStateTransition(bytes32 machineId, bytes32 processId, bytes32 toState) returns (bool);
}

Technically, any contract that implements that interface can be called via performNetworkedStateTransition, and there is currently no on-chain check that verifies anything happenned other than the call returning true.

Additionally, flagging a transition as networked does not specify the details of the transition (which builder, machine, process, and state), it only requires that the transition is successfully performed. This functionality will be added.