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Overview

In transformer service, we are doing data transformation from customer events to destination events, and we want organize the transformation process into logically separated steps for better understanding and maintainability so we have created a workflow engine to address the following requirements.

• Validation
  • Event validation
  • Destination config validation
• Source to destination data mapping
• Enriching data with destination API calls
• Handling different types of events
  • Track, Identify, Page, etc.
  • Custom categories:
    • Product Viewed
    • Product Purchased
• Multiplexing
• Batching
• Response building.

Currently, most steps are implemented using Javascript code, which provides the most flexibility. Still, it is getting difficult to maintain, understand, debug, test, and develop in a standardized way. To bring standardization, we are building a workflow engine that is config driven to provide improved readability, testability, reusability, and speed of development.

Since we want to express the transformation of the logic using easy to read and write template based language. We support following template languages:

• JSONata
• JsonTemplate.
• Easily extendable to more template languages.
  Workflow Example using Jsonata:

templateType: Jsonata
steps:
  - name: unsupported
    condition: $not(op in ["+", "-", "*", "/"])
    template: |
      $doThrow("unsupported operation")
  - name: add
    description: Do addition
    condition: op = "+"
    template: |
      ( a + b )
  - name: subtract
    description: Do subtraction
    condition: op = "-"
    template: |
      ( a - b )
  - name: multiply
    description: Do multiplication
    condition: op = "*"
    template: |
      ( a * b )
  - name: divide
    description: Do division
    condition: op = "/"
    template: |
      ( 
        $assert( b != 0, "division by zero is not allowed");
        a / b 
      )

Getting started

• npm install rudder-workflow-engine

const workflowEngine = WorkflowEngineFactory.createFromFilePath("workflow.yaml", options);
workflowEngine.execute(input);

Features

Config Driven

Users should be able to express the destination transformation logic as a series of steps in a YAML file as a workflow. Steps can be written as template base languages.

Bindings

Supports importing of external functions and data using bindings.

Workflow Bindings

• Bindings are similar to imports, which allow importing of externally defined functions and data to the workflow.
• Types
  • Type 1: Import a specific field from a file.

    name: EventType 
    path: ./config

    • EvenType is defined in ./config file then it will be imported as $EventType
  • Type 2: Import everything from a file as something.

    name: MappingData 
    path: ./mapping
    exportAll: true

    • Everything from ./mapping file will be imported to the variable MappingData
    • If something1 and something2 are defined in ./mapping then we need to access them using $MappingData.something1 and $MappingData.something2
  • Type 3: Import everything as it is defined in the file
    • Everything from ./utils file will be imported with the same names.
    • If something1 and something2 are defined in ./utils then we need to access them using $something1 and $something2
• Full example:

  bindings:
    - name: EventType 
      path: ./config
    - name: MappingData 
      path: ./mapping
      exportAll: true
    - path: ./utils 

• These are user-specified bindings while defining the workflow.

Platform bindings

• The platform provides these bindings, which can be used directly without defining them in the bindings block.
• We will soon release detailed documentation on these bindings.

Execution bindings

• $outputs: Provides access to the outputs of the previous steps executed before the current step.

  steps:
    - name: step1
      template: |
        {
          "a": something
        }
    - name: step2
      template: |
      {
        "b": $doSomething($outputs.step1.a)
      }

  • Step2 uses the output of step1.
  • Workflow Engine automatically bindings step outputs to the $outputs variable.
• $setContext: It is a function to store any data in $context and use it later. $outputs are read-only variables for users to refer to the previous step outputs, so we can’t use them to pass a modifiable result. So if we want to update the same variable in multiple steps, then $setContext should be used.
  • Example:

    steps:
      - name: setAForCase1
        condition: $isCase1(message)
        template: |
          $setContext("a", something1)
      - name: setAForCase2
        condition: $isCase2(message)
        template: |
          $setContext("a", something2)
      - name: updateA
        template: |
          $setContext("a", $updateA($context.a))
      - name: useA
        template: |
          $doSomething($context.a)

    • In this example, we update the variable repeatedly in several steps, so it is impossible to use $outputs.
    • A practical scenario for this feature is: that we want to populate an object differently based on some conditions and later use it.
  • $context: To access variables set using $setContext function. Please refer to the above example for clarity.

Steps

• Steps are the main execution blocks of the workflow.
• Steps must contain a name to track outputs.
• Steps can contain an optional description field to describe the details.
• The step can contain an optional condition field to execute only if the condition is satisfied.
• The step can contain an optional inputTemplate field to customize the input, which will be passed while executing the step.
• There are two different types of steps supported:
  • SimpleStep
  • WorkflowStep

Conditions

• A step in a workflow can mention an optional condition so that it gets executed only when the condition is satisfied.

• Condition is also a Jsonata code.

  steps:
    - name: commonValidation
      template: |
        ( common validations for events )
    - name: ValidateInputOfTrackEvent
      condition: message.type = EventType.Track
      template: |
        ( some validations specific to track events)

InputTemplate**

• By default, all steps receive the same input as the workflow input, but when we want to modify the input before executing the step, we can use this feature.

  steps:
    - name: step1
      (some logic ...)
    - name: step2
      inputTemplate: |
        (customize the input)
    - name: step3
      (some logic ...)

  • In the above example: step1 and step3 will be executed with the workflow’s input, but the step2 receives custom input as defined in the inputTemplate

ContextTemplate

• By default, all steps receive the current context, but we can use this feature when we want to modify the context before executing the step. This is useful when using external workflows, workflow steps, or template paths.

  steps:
    - name: step1
      (some logic to prepareContext)
    - name: step2
      contextTemplate: |
        (customize the context for step2)
      (some logic ...)
    - name: step3
      (some logic ...)

  • In the above example: step 3 will execute with the context prepared in step 1, but step 2 receives custom context as defined in the contextTemplate.

LoopOverInput

• We can use this feature when the input is an array, and we want to execute the step logic for each element independently.
• This is mainly used for batch processing, and we report failed and successful executions without failing the step if an error occurs while processing a particular step.

  name: executeForEach
  loopOverInput: true
  template: |
    ( do something )

  • If the input for the step is [e1, e2, e3], then the step will be executed for all elements independently, and imagine that it failed for e1 and succeeded for e2 and e3 then, the overall step output will be the following:

    [
      {
        "error": someErrorForE1
      },
      {
        "output": someOutputForE2
      },
      {
        "output": someOutputForE3
      }
    ]

OnComplete

• When the step is completed, the next step will be executed, but if we want to exit the workflow with the output of a particular step, then we can use this.

• This feature should be used only in a conditional step.

• Example 1: Avoid reprocessing, so return without modifying the input message.

    steps:
      - name: checkIfProcessed
        condition: message.processed = true
        template: |
          message
        **onComplete: return**
      - name: processMessage
        template: |
          (...)

  • In the above example, we don’t want to reprocess messages, so we need to return them immediately if they are already processed.

• Example 2: Return early after processing the input message.

  steps:
    - name: step1
      template: |
        (doSomeProcessing)
    - name: **step2**
      condition: someCondition
      template: |
        (doSomeProcessing)
      onComplete: return
    - name: step3
      template: |
        (doSomeProcessing)

  • In this example, we want to return early after successfully processing the message in step2 since this step is conditional, and if the condition is not satisfied, then step3 will be executed.

OnError

• By default, if any step fails, then the entire workflow fails but if the step uses OnError: continue setting, then the workflow will ignore the error and continue with execution.

  steps:
    - name: step1
      template: |
        (doSomeProcessing)
    - name: **step2**
      template: |
        (doSomeProcessing)
      onError: continue
    - name: step3
      template: |
        (doSomeProcessing)

  • In the above example, if any error occurs in either step1 or step3, the workflow will exit immediately, but when step2 fails, the workflow ignores the error and continues to execute step3.

Steps

Simple Step

• Simple step is the basic unit of execution in the workflow.

• A simple step can be a function that is defined in the Bindings.

  bindings:
    - name: **processTrackEvent
      path: ./transform # actual file name is transform.js**
  steps:
    - name: processTrackEvent
      functionName: processTrackEvent

  • We can omit .js extension while defining the bindings.
  • processTrackEvent must have the following definition.
  A simple step can be a JSONata template.

  name: processTrackEvent
  template: |
    (JSONata template to process track events)**

  • The template also can be imported from the file path.

    name: processTrackEvent
    templatePath: ./trackTemplate.yaml

• We can use an external workflow in a simple step.

  steps:
    - name: prepareContext
      template: $setContext("batchMode", true)
    - name: transform
      **externalWorkflow:
        path: ./pinterest_tag_single_workflow.yaml**
      loopOverInput: true

  • We are reusing the single event workflow in the batch events transformation workflow.
  • The external workflow will be executed as a black box, so we can only access the final output of the workflow but not the individual outputs of steps.
  • The external workflow is executed with step input and context of the original workflow.
  • The context of the parent workflow is passed to the child workflow (externalWorkflow) but not vice-versa. This is helpful to customize the child workflow execution based on where it is used.
  • The external workflow doesn’t have access to the parent workflow outputs.

Workflow Step

• Series of sim