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States Language on Cadence

This is an implementation of States Language on top of the Cadence Workflow engine.

Project Status

⚠️ Note: This project is not production ready! ⚠️

This project is still under development and isn't ready for use in production. Feedback is welcome from the Cadence community. The goal is to provide a plugable way to execute State Language workflows on top of Cadence.

There a few States Language features still missing:

  • Error handling
  • Retry logic
  • JSON paths
  • Map task type

About States Language

Amazon States Language (ASL) is a JSON specification describing state machines and workflows. It is the description that powers AWS Step Functions. Cadence is a robust workflow engine created and open sourced by Uber. This project allows you to mimic Step Functions outside of AWS infrastructure on inside a Cadence workflow.

Running tests

make test

Updating vendors

make mod-vendor

Example

An example command is provided in cmd/workflow. It should demenstrate how to use the framework and register workflows.

Related projects

Thanks to the folks at Amazon for designing the state machine spec, the folks at Coinbase for building an implementation in Golang and the folks at Uber for building Cadence

What is States Language?

The operation of a state machine is specified by states, which are represented by JSON objects, fields in the top-level "States" object. In this example, there is one state named "FirstState".

When this state machine is launched, the interpreter begins execution by identifying the Start State ("StartAt"). It executes that state, and then checks to see if the state is marked as an End State. If it is, the machine terminates and returns a result. If the state is not an End State, the interpreter looks for a "Next" field to determine what state to run next; it repeats this process until it reaches a Terminal State (Succeed, Fail, or an End State) or a runtime error occurs.

Example

{
  "Comment": "An example of the Amazon States Language using a choice state.",
  "StartAt": "FirstState",
  "States": {
    "FirstState": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:REGION:ACCOUNT_ID:function:FUNCTION_NAME",
      "Next": "ChoiceState"
    },
    "ChoiceState": {
      "Type": "Choice",
      "Choices": [
        {
          "Variable": "$.foo",
          "NumericEquals": 1,
          "Next": "FirstMatchState"
        },
        {
          "Variable": "$.foo",
          "NumericEquals": 2,
          "Next": "SecondMatchState"
        }
      ],
      "Default": "DefaultState"
    },
    "FirstMatchState": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:REGION:ACCOUNT_ID:function:OnFirstMatch",
      "Next": "NextState"
    },
    "SecondMatchState": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:REGION:ACCOUNT_ID:function:OnSecondMatch",
      "Next": "NextState"
    },
    "DefaultState": {
      "Type": "Fail",
      "Error": "DefaultStateError",
      "Cause": "No Matches!"
    },
    "NextState": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:REGION:ACCOUNT_ID:function:FUNCTION_NAME",
      "End": true
    }
  }
}