Heaviside

Heaviside is a domain specific language (DSL) and Python compiler / support libraries for working with AWS StepFunctions.

Why

The reason for a StepFunctions DSL is that the state machine language, while flexible, is hard to write and maintain. The DSL provides a simplied format for writing StepFunction state machines and serveral of common flow control statements.

DSL

The StepFunctionDSL document describes the Heaviside DSL.

Getting Started

In this document .hsd will be used to denote a StepFunction file written in the Heaviside DSL. The extension .sfn will be used to denote a StepFunction file written in the AWS state machine language.

Installing

pip install heaviside

CLI Script

The Heaviside package installs a script called heaviside that provides a CLI to the library. Running the command without arguments or with the --help or -h flag will provide detailed help.

AWS Credentials

All sub-commands (except compile) connect to AWS to manipulate StepFunctions. There are multiple ways to define the AWS credentials, listed in the order of precedence.

• Explicitly pass the values as command line arguments or environmental variables
• Pass a file path to a JSON file with the arguments
• Letting Boto3 try to resolve secret / access keys
• Looking at EC2 meta data for current AWS region
• Looking at current IAM user for AWS account_id

The compile sub-command doesn't connect to AWS, but does use the region and account_id values when resolving Task ARNs. If the Heaviside DSL file has full Task ARNs or the compiled file will not be uploaded to AWS these values can be blank.

Note: Since compile doesn't connect to AWS, only the first two options in the list above are valid for passing the region and account_id value.

Compiling

To compile a Heaviside file into a AWS StepFunction file use the compile sub-command.

$ heaviside compile state_machine.hsd -o state_machine.sfn 

Creating a StepFunction

The heaviside script can compile and upload the resulting file to AWS.

$ heaviside create state_machine.hsd AwsIamStepFunctionRole

Arguments:

• state_machine.hsd: The path to the Step Function definition written in the Heaviside DSL.
• AwsIamStepFunctionRole: The AWS IAM Role that the StepFunction will use when executing. Most often this will be used to control which Lambdas and Activities the StepFunction has permission to execute.

Updating a StepFunction

The heaviside script can update an existing Step Function definition and / or IAM role in AWS.

$ heaviside update state_machine --file  state_machine.hsd --role AwsIamStepFunctionRole

Arguments:

• state_machine: Name of the state machine to update
• state_machine.hsd: The path to the Step Function definition written in the Heaviside DSL.
• AwsIamStepFunctionRole: The AWS IAM Role that the StepFunction will use when executing. Most often this will be used to control which Lambdas and Activities the StepFunction has permission to execute.

Deleting a StepFunction

The delete sub-command can be used to delete an existing StepFunction

$ heaviside delete state_machine

Executing a StepFunction

The start sub-command can be used to start executing a StepFunction.

$ heaviside start --json "{}" state_machine

Note: By default the start sub-command will wait until the execution has finished and will print the output of the StepFunction.

Python Library

The Heaviside package installs the Python library heaviside. The public API is documented in the Library API file.

Compatibility

Currently, Heaviside has only been tested with Python 3.8.

Related Projects

• statelint: A Ruby project that verifies a AWS StepFunction definition file. Includes checks like making sure that all states are reachable. Helpful when developing a new StepFunction to ensure everything is correct. It will catch structural problems that Heaviside doesn't check for.

Legal

Use or redistribution of the Boss system in source and/or binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code or binary forms must adhere to the terms and conditions of any applicable software licenses.
2. End-user documentation or notices, whether included as part of a redistribution or disseminated as part of a legal or scientific disclosure (e.g. publication) or advertisement, must include the following acknowledgement: The Boss software system was designed and developed by the Johns Hopkins University Applied Physics Laboratory (JHU/APL).
3. The names "The Boss", "JHU/APL", "Johns Hopkins University", "Applied Physics Laboratory", "MICrONS", or "IARPA" must not be used to endorse or promote products derived from this software without prior written permission. For written permission, please contact BossAdmin@jhuapl.edu.
4. This source code and library is distributed in the hope that it will be useful, but is provided without any warranty of any kind.