# Either Or Parallel Workflow

This workflow shows a pattern where there are multiple processes running in paralle, but the state machine can proceed as soon as one of the processes completes.

Learn more about this workflow at Step Functions workflows collection: << Add the live URL here >>

Important: this application uses various AWS services and there are costs associated with these services after the Free Tier usage - please see the [AWS Pricing page](https://aws.amazon.com/pricing/) for details. You are responsible for any AWS costs incurred. No warranty is implied in this example.

## Requirements

* [Create an AWS account](https://portal.aws.amazon.com/gp/aws/developer/registration/index.html) if you do not already have one and log in. The IAM user that you use must have sufficient permissions to make necessary AWS service calls and manage AWS resources.
* [AWS CLI](https://docs.aws.amazon.com/cli/latest/userguide/install-cliv2.html) installed and configured
* [Git Installed](https://git-scm.com/book/en/v2/Getting-Started-Installing-Git)
* [AWS Serverless Application Model](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/serverless-sam-cli-install.html) (AWS SAM) installed

## Deployment Instructions

1. Create a new directory, navigate to that directory in a terminal and clone the GitHub repository:
    ``` 
    git clone https://github.com/aws-samples/step-functions-workflows-collection
    ```
1. Change directory to the pattern directory:
    ```
    cd either-or-parallel-pattern
    ```
1. From the command line, use AWS SAM to deploy the AWS resources for the workflow as specified in the template.yaml file:
    ```
    sam deploy --guided
    ```
1. During the prompts:
    * Enter a stack name
    * Enter the desired AWS Region
    * Allow SAM CLI to create IAM roles with the required permissions.

    Once you have run `sam deploy --guided` mode once and saved arguments to a configuration file (samconfig.toml), you can use `sam deploy` in future to use these defaults.

1. Note the outputs from the SAM deployment process. These contain the resource names and/or ARNs which are used for testing.

## How it works

The pattern shows how to implement a parallel task in a way that if any one of the parallel flows finish, the execution will move out of the parallel step. It works in the following way.

- In this step functions workflow pattern there are 3 parallel flows inside the Parallel state.
- One of them is a time out, while the other two are actual flows. For the purposes of this pattern I have added two but it can be any number.
- The time out flow waits for 15 seconds and completes the execution of the Parallel state is none of the other flows finish by then.
- The other two flows use the following logic.
    - Check for any process to see if it completed (this is a Pass state in this pattern, but it could be any call to check, say, for RDS backup completion, or completion of an AWS Batch job triggered earlier, etc). If it is not completed, wait for 5 seconds and check again.
    - If the process completed, it will raise an "error", which is needed to break out of this parallel flow. For your business need this might not be an actual error scenario, but it is still implemented as an error, so that the flow can move out of the Parallel step
- The Parallel state has 3 catch statements in its error handling, one to receive the time out error, and the other two receive the "error"s thrown by the 2 flows. To reiterate, these might not be actual errors in terms of your business logic but is just used to move control to outside the Parallel step.

## Image
![image](./resources/statemachine.png)

## Testing

1. After deployment navigate to the AWS Step Functions console and select the `EitherOrParallel` workflow. If you don't see it, make sure you are in the correct Region.

1. Select `Start Execution`, provide the input as
    ```
    {
        "Process1ToComplete" : true
    }
    ```
and then click `Start Execution`.

3. You will see that the task "Next Step If Process 1 Completes" gets executed.

1. Select `Start Execution` again, and this time provide the input as 
    ```
    {
        "Process2ToComplete" : true
    }
    ```  and then click `Start Execution`.  

1. You will see that the task "Next Step If Process 2 Completes" gets executed.  
1. Select `Start Execution` again, and this time use the default json and then click `Start Execution`.  
1. You will see that the task "Next Step If Process Ran out of time" gets executed.


## Cleanup
 
1. Delete the stack
    ```bash
    aws cloudformation delete-stack --stack-name STACK_NAME
    ```
1. Confirm the stack has been deleted
    ```bash
    aws cloudformation list-stacks --query "StackSummaries[?contains(StackName,'STACK_NAME')].StackStatus"
    ```
----
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