Many articles have been written about AWS Step Functions since it was first introduced in 2016. Most of them create the impression that the service is simply an extension of the Lambda function that allows us to stitch together multiple Lambda functions to call each other.

But actually, it is much more than that. Step Functions allows us to design and build the flow of execution of modules in our application in a simplified manner. This enables a developer to focus solely on ensuring that each module performs its intended task, without having to worry about connecting each module with others.

In this article, we will explore the what, the how and the why of Step Functions, before walking through some use cases, limitations and best practices around using the service.

What is AWS Step Functions?

AWS Step Functions is an orchestrator which helps to design and implement the complex workflows. When we need to build a workflow, or have multiple tasks that need orchestration, Step Functions coordinates between those tasks. This makes it simple to build multi-step systems.

Step Functions is built on two main concepts Tasks and State Machine. 

All work in the state machine is done by tasks. A task performs work by using an activity or an AWS Lambda function, or passing parameters to the API actions of other services.

A state machine is defined using the JSON-based Amazon States Language. When an AWS Step Functions state machine is created, it stitches the components together and shows the developers their system and how it is being configured. Have a look at a simple example:

Can you imagine if you had to do it yourself using a Messaging Queue, Istio or App Mesh? It would be a big task, and that’s without considering the overhead of actually maintaining that component.

It’s really great to see what features it provides out of the box. However, it would have been even better if AWS had added the ability to design it visually rather than through JSON.

How Step Functions works

As discussed earlier, the state machine is a core component of the AWS Step Functions service. It defines communication between states and how data is passed from one state to another.

State

A state is referred to by its name, which can be any string, but must be unique within the scope of the entire state machine. It does the following functions:

• Performs some work in the state machine (a Task state).
• Makes a choice between branches of execution (a Choice state).
• Stops execution with failure or success (a Fail or Succeed state).
• Simply passes its input to its output or injects some fixed data (a Pass state).
• Provides a delay for a certain amount of time or until a specified time/date (a Wait state).
• Begins parallel branches of execution (a Parallel state).

Here is an example of a state definition for Task type:

"States": {
"FirstState": {
"Type": "Task",
"Resource": "arn:aws:lambda:ap-southeast-2:710187714096:function:DivideNumbers",
"Next": "ChoiceState"
}

Input and Output Processing

For Step Functions, input is always passed as a JSON file to the first state. However, it has to pass through InputPath, ResultPath and OutputPath before the final output is generated. JSON output is then passed to the next state.

InputPath – selects which parts of the JSON input to pass to the task of the Task state (for example, an AWS Lambda function).

ResultPath then selects what combination of the state input and the task result to pass to the output.

OutputPath can filter the JSON output to further limit the information that’s passed to the output.

Let’s take a look at an example to better understand this in detail:

For Lambda execution, Input is described as JSON like above. That input is bound to the symbol $ and passed on as the input to the first state in the state machine.

By default, the output of each state would be bound to $ and becomes the input of the next state. In Each state, we have InputPath, ResultPath and OutputPath attributes which filters the input and provide the final output. In the above scenario, “ExamResults” state is filtering “lambda” node, appending the result of a state execution to “results” node and final output is just “result” node rather than the whole JSON object:

Hence, the final output will be:

{

  "math": 80,

  "eng": 93,

  "total": 173

  },

Step Functions can be triggered in four ways :

• API Gateway
• CloudWatch Events
• S3 events
• Step Functions API – StartExecution

As I mentioned earlier, Step Functions is not only about Lambda Functions. It has support for several other Integration Patterns like SQS, DynamoDB, SNS, ECS and many others.

AWS Step Functions use cases

There are many use cases that can be resolved using Step Functions. However, we’ll restrict ourselves to a few major ones here:

1. Sequence Batch Processing Job

If you have many Batch Jobs to be processed sequentially and need to coordinate the data between those, this is the best solution. For example, an e-commerce website can first read the product data and the next job will find out which products are running out of stock soon and then, the third job can send a notification to all the vendors to expedite the supply process.

2. Easy Integration with Manual Tasks

If a workflow needs a manual approval/intervention, Step Function would be the best solution to coordinate it. For example, the Employee promotion process – It needs approval from the manager. So the Step function can send the email using AWS SES service with Approve or Reject link and once receives it, can trigger the next action using lambda or ECS jobs.

3. Coordinate Container Task in Serverless Workflow

Step function can help to make a decision about how best to process data. Based on the file size, you can decide to use either lambda, ECS or on-premise activity to optimize the cost and runtime both.

Step Functions benefits

• Retry: Before Step Functions, there was no easy way to retry in the event of a timeout error or runtime error or any other type of failure. It also provides an exponential backoff feature.

"Retry": [ {

  "ErrorEquals": [ "States.Timeout" ],

  "IntervalSeconds": 3,

  "MaxAttempts": 2,

  "BackoffRate": 1.5

} ]

• Error Handling: It provides an easy way of error handling at each state. It can handle several types of errors a state can throw out, like:
  • States.Timeout – When a Task state cannot finish the job within the TimeoutSeconds or does not send heartbeat using SendTaskHeartbeat within HeartbeatSeconds value. 
  • States.TaskFailed – When a Task state fails for any reason.
  • States.Permissions – When a Task state does not have sufficient privileges to call the Lambda/Activity code.
  • States.All – It captures any known error name.

It can also catch Lambda service exceptions(Lambda.ServiceException) and even the unhandled errors (Lambda.Unknown). A typical example for an error handling:

"Catch": [ {

  "ErrorEquals": [ "States.TaskFailed", “States.Permission” ],

  "Next": “state x”

} ]

You can bet that it was never this easy to implement error handling like this with any other workflow solution.

• Parallelization: You can parallelize the work declaratively. A step machine can have a state calling multiple states in parallel. This will make the workflow complete faster.
• High Execution Time: Step Functions has one year as max execution time so if some of the tasks of the workflow are high (more than 15 minutes), they can be run on ECS or EC2 or as an Activity hosted outside of AWS.
• Pricing: Step Functions counts a state transition each time a step of a workflow is executed. You are charged for the total number of state transitions across all your state machines. Charges are a little on the higher side, but it would almost certainly be costlier to come up with your own solution for orchestrating the different services, Error Handling, Retries and the many other features Step Function provides.

Limitations

Despite all the powerful features Step Functions offers, there are still a few things missing:

• Shorter Execution History: The maximum limit for keeping execution history logs is 90 days. It cannot be extended and that may preclude the use of Step Functions for businesses that have longer retention requirements.
• Missing Triggers: Some Event Sources and Triggers are still missing, such as DynamoDB and Kinesis.
• State machine Execution name: Each Execution name for a state machine has to be unique (not used in the last 90 days). This can be very tricky.
• AWS Step Functions does not horizontally scale to dynamically launch AWS Lambda functions in parallel. For example, if my state 1 generates 10 messages, it cannot spin up 10 AWS Lambda invocations at state 2 to process those messages individually (This feature is available if you use Lambda function directly with concurrent execution). State 2 is statically defined in the state machine and provisions the same number of task executors each time. Therefore, it cannot scale dynamically. Hopefully, this feature may be added in the future.

AWS Step Functions Best Practices

• In a workflow sometimes, we would like to resume the process from the fail state as opposed to re-running it from the beginning. This isn’t provided as a built-in feature, but there is a workaround to achieve this.

• Beware: State Machine can run infinitely. It has a max execution time of one year. That itself is too high. On top of that, it provides a feature “Continue as new Execution”. This allows you to start a new execution before terminating your current running execution. This opens up the possibility of it running infinitely by mistake. Monitoring Execution metrics is a good way to identify and fix those mistakes.

• AWS Step Functions has a hard limit of 25,000 event entries in the execution history (with a retention of 90 days), which is going to fill very quickly for a long-running execution. To overcome this limitation, we can implement “Continue as new Execution” pattern, where we can spin up the new execution from an existing running execution. For example, if a long running execution has 10 steps and you’re expecting to have 40,000 event entries in your execution history, you may configure to start a new execution at step 5. And it will create a total of two executions of the state machine. Therefore, it will distribute the entries between those with step 1-5 and step 6-10.

• By default, the Amazon State Language doesn’t set timeouts in state machine definitions. In a scenario where a Lambda Function or Activity has a problem and keeps running without responding back to Step Functions, it will keep waiting for a year (max timeout) at least. To prevent this, set the timeout using TimeoutSeconds like this:

"ExamResults": {

   "Type": "Task",

   "Resource": "arn:aws:lambda:us-east-1:123456789012:function:HelloFunction",

   "TimeoutSeconds": 200,

   "HeartbeatSeconds": 30,

   "End": true

  }

• Using TimeoutSeconds & HeartbeatSeconds, we can design a long-running workflow alive. HeartbeatSeconds value should be less than TimeoutSeconds. And, we need to use SendTaskHeartbeat periodically within the time we set in HeartbeatSeconds in our state machine task definition to keep the task from timing out.

Let’s take an example where HeartbeatSeconds value is 30 seconds and TimeoutSeconds is 400 seconds for a long activity worker process.

When the state machine and activity worker process starts, the execution pauses at the activity task state and waits for your activity worker to poll for a task. Once a taskToken is provided to your activity worker, your workflow will wait for SendTaskSuccess or SendTaskFailure to provide a status. If the execution doesn’t receive either of these or a SendTaskHeartbeat call before the time configured in TimeoutSeconds, the execution will fail and the execution history will contain an ExecutionTimedOut event. So, by configuring these, we can design a long running workflow effectively.

• Some developers use Step Functions for their API calls and want to send the response of the state machine output to a UI or other system. Firstly, developers should avoid using Step Functions for Microservice API calls. Microservices are supposed to be small and respond back in 2-3 seconds, if not milliseconds.

• For longer workflows, keep in mind that API Gateway calls the state machine in an asynchronous way and responds back only the execution ARN. You can define another method in API Gateway to call “DescribeExecution” API. This method will have to call this API periodically (poll) to get the output until the returned status is no longer “RUNNING”.

• Like any other application code, handle the exception. Use the Retry feature to handle occasional transient service errors.

Logging and Monitoring Step Functions

Similar to Lambda functions, Step Functions also sends logs to CloudWatch and it generates several metrics around it. For example, Execution metrics, Activity metrics, Lambda metrics, etc. Below is an example of Execution Metrics:

Visual Workflow panel shows the current status of the execution. Look at the right side of the panel (below picture). We can see the details of any state by clicking on the state in the workflow diagram. It shows the input, output, and an exception (if any) for the state.

It also logs the execution history and status for each state. AWS Console does provide a nice visual of the states from start to end. We can also click on CloudWatch Logs to go to LogGroups and see detail logs.

One recommendation is to create a Unique Trace ID which should be passed to all the integration services these states connect to. It will help to track the transactions easily.

It also has integration with CloudTrail to log the events.

Conclusion

In this article, we explored the basic concepts of Step Functions and how it works. We also talked about how with the Visualization panel, Error Handling and Retry features, it makes the workflow creation process much smoother. Step Functions should properly be described as state-as-a-service. Without it, we would not be able to maintain the state of each execution having multiple lambda functions/activities.

Just keep in mind that you need to keep a watch on your bills as it can burn a hole in your pocket very fast. And the best way to do that is to ensure that proper monitoring and metrics are in place.