Basic Workflow: Sequential Step Execution

The Problem: Business Logic Scattered Across Services

Your e-commerce system processes orders. Payment must complete before shipping. Shipping must complete before confirmation. These aren't arbitrary constraints—they reflect real business dependencies.

Without workflow orchestration:

• Tangled callback chains that obscure business logic
• Inconsistent error handling at each integration point
• No visibility into where an order is in its journey
• Manual intervention when processes fail mid-stream
• State scattered across services with no single source of truth

What you need: A workflow that:

1. Defines steps as first-class concepts
2. Executes them in a deterministic sequence
3. Survives process restarts without losing progress
4. Makes the current state visible and queryable
5. Handles failures with explicit error paths

This is the sequential workflow pattern—the foundation for all other patterns.

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Learning Objectives

After this example, you will understand:

• Workflow definitions as declarative step sequences
• Immutable state and why it matters for reliability
• Step contracts via IWorkflowStep<TState>
• State transitions using the With() pattern
• Error results for explicit failure handling
• What gets generated by the source generator

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Conceptual Foundation

Why Workflows Instead of Code?

Consider processing an order with traditional code:

// Traditional approach - problems hidden in plain sight
public async Task ProcessOrder(Order order)
{
    var valid = await _validator.Validate(order);      // What if this fails?
    var payment = await _payments.Charge(order);       // What if payment succeeds but...
    var shipment = await _fulfillment.Ship(order);     // ...shipping fails? Payment is already charged!
    await _notifications.SendConfirmation(order);      // What if notification fails?
}

Problems:

1. Atomicity illusion: Looks atomic, but isn't—partial failures leave inconsistent state
2. No visibility: Where is order #12345 right now?
3. No recovery: If the process crashes after payment, how do you resume?
4. No audit: What happened to this order and when?

Workflows solve this by making each step explicit, persisted, and recoverable.

The Saga Pattern

Sequential workflows implement the saga pattern:

Step 1 ──▶ Step 2 ──▶ Step 3 ──▶ Step 4 ──▶ Complete
   │          │          │          │
   ▼          ▼          ▼          ▼
 Event 1   Event 2    Event 3    Event 4

Each step:

• Persists its result before continuing
• Emits an event for observability
• Can be retried if the process crashes
• Has a known position in the sequence

Immutable State: Why Records?

Workflow state is a record, not a class:

[WorkflowState]
public record OrderState : IWorkflowState
{
    public Guid WorkflowId { get; init; }
    public Order Order { get; init; } = null!;
    public PaymentResult? Payment { get; init; }
    // ...
}

Why immutability?

Benefit	Explanation
Replay	Can rebuild state from events
Debugging	State at any point is reconstructable
Concurrency	No shared mutable state to corrupt
Testing	Predictable, deterministic

The With() pattern creates new state without mutation:

// Creates a NEW record with updated Payment
return state
    .With(s => s.Payment, paymentResult)
    .With(s => s.Status, OrderStatus.Paid)
    .AsResult();

Steps as First-Class Concepts

Each step is a class implementing IWorkflowStep<TState>:

public interface IWorkflowStep<TState> where TState : IWorkflowState
{
    Task<StepResult<TState>> ExecuteAsync(
        TState state,
        StepContext context,
        CancellationToken ct);
}

Why this design?

Aspect	Benefit
Single responsibility	Each step does one thing
Dependency injection	Steps get their dependencies automatically
Testable	Mock the state, assert the result
Composable	Combine steps into workflows declaratively

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Design Decisions

Decision	Why This Approach	Alternative	Trade-off
Records for state	Immutability for reliability	Classes	Requires With() pattern
DI for steps	Testability, loose coupling	Direct instantiation	More registration code
StepResult return	Explicit success/failure	Exceptions	More verbose, but clearer
Source generation	Type-safe, no reflection	Runtime reflection	Compile-time complexity

When to Use This Pattern

Good fit when:

• Operations must execute in order
• Each step depends on previous results
• You need visibility into workflow progress
• Recovery from failures is important
• Audit trails are required

Poor fit when:

• Operations are independent (use Fork/Join)
• Simple request-response with no persistence
• Real-time requirements (saga overhead)

Anti-Patterns to Avoid

Anti-Pattern	Problem	Correct Approach
Mutable state	Unpredictable behavior	Use immutable records
Side effects in steps	Can't replay or test	Make steps pure functions of state
Swallowing errors	Silent failures	Return explicit StepResult.Fail()
Giant steps	Hard to test and debug	Single responsibility per step
Shared state	Concurrency bugs	All state flows through workflow

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Building the Workflow

The Shape First

┌───────────────┐    ┌────────────────┐    ┌──────────────┐    ┌──────────────────┐
│ ValidateOrder │───▶│ ProcessPayment │───▶│ FulfillOrder │───▶│ SendConfirmation │
│               │    │                │    │              │    │                  │
│ Check items,  │    │ Charge the     │    │ Ship the     │    │ Notify the       │
│ inventory,    │    │ customer       │    │ items        │    │ customer         │
│ address       │    │                │    │              │    │                  │
└───────────────┘    └────────────────┘    └──────────────┘    └──────────────────┘
       │                    │                    │                      │
       ▼                    ▼                    ▼                      ▼
   Validated             Paid               Shipped              Completed

State: What We Track

[WorkflowState]
public record OrderState : IWorkflowState
{
    // Identity - every workflow has a unique ID
    public Guid WorkflowId { get; init; }

    // Input - what was passed to start the workflow
    public Order Order { get; init; } = null!;

    // Step outputs - each step adds its result
    public bool IsValid { get; init; }
    public PaymentResult? Payment { get; init; }
    public ShipmentInfo? Shipment { get; init; }

    // Current status - where are we in the process?
    public OrderStatus Status { get; init; }
}

Why this design?

• Order: The input, never changes after workflow starts
• IsValid, Payment, Shipment: Step outputs, set once per step
• Status: Summary of current position for queries

The Supporting Records

public record Order(
    string CustomerId,
    IReadOnlyList<OrderItem> Items,
    Address ShippingAddress);

public record OrderItem(string ProductId, int Quantity, decimal Price);

public record PaymentResult(string TransactionId, bool Success);

public record ShipmentInfo(string TrackingNumber, DateOnly EstimatedDelivery);

public enum OrderStatus { Pending, Validated, Paid, Shipped, Completed }

The Workflow Definition

var workflow = Workflow<OrderState>
    .Create("process-order")
    .StartWith<ValidateOrder>()
    .Then<ProcessPayment>()
    .Then<FulfillOrder>()
    .Finally<SendConfirmation>();

Reading this definition: "Create a process-order workflow that starts with validating the order, then processes payment, then fulfills the order, and finally sends confirmation."

This reads like a sentence because that's what business logic should be—a clear description of what happens.

Step Implementation: ValidateOrder

public class ValidateOrder : IWorkflowStep<OrderState>
{
    private readonly IOrderValidator _validator;

    // Dependencies injected automatically
    public ValidateOrder(IOrderValidator validator)
    {
        _validator = validator;
    }

    public async Task<StepResult<OrderState>> ExecuteAsync(
        OrderState state,
        StepContext context,
        CancellationToken ct)
    {
        // Do the work
        var result = await _validator.ValidateAsync(state.Order, ct);

        // Explicit failure with error details
        if (!result.IsValid)
        {
            return StepResult.Fail<OrderState>(
                Error.Create("ORDER_INVALID", result.ErrorMessage));
        }

        // Success: return new state (immutable!)
        return state
            .With(s => s.IsValid, true)
            .With(s => s.Status, OrderStatus.Validated)
            .AsResult();
    }
}

Key points:

• Step receives current state, returns new state
• Failures are explicit, not exceptions
• State is never mutated, only transformed

Step Implementation: ProcessPayment

public class ProcessPayment : IWorkflowStep<OrderState>
{
    private readonly IPaymentService _paymentService;

    public ProcessPayment(IPaymentService paymentService)
    {
        _paymentService = paymentService;
    }

    public async Task<StepResult<OrderState>> ExecuteAsync(
        OrderState state,
        StepContext context,
        CancellationToken ct)
    {
        // Calculate total from order items
        var amount = state.Order.Items.Sum(i => i.Price * i.Quantity);

        // Charge the customer
        var payment = await _paymentService.ChargeAsync(
            state.Order.CustomerId,
            amount,
            ct);

        if (!payment.Success)
        {
            return StepResult.Fail<OrderState>(
                Error.Create("PAYMENT_FAILED", "Payment processing failed"));
        }

        return state
            .With(s => s.Payment, payment)
            .With(s => s.Status, OrderStatus.Paid)
            .AsResult();
    }
}

Step Implementation: FulfillOrder

public class FulfillOrder : IWorkflowStep<OrderState>
{
    private readonly IFulfillmentService _fulfillment;

    public FulfillOrder(IFulfillmentService fulfillment)
    {
        _fulfillment = fulfillment;
    }

    public async Task<StepResult<OrderState>> ExecuteAsync(
        OrderState state,
        StepContext context,
        CancellationToken ct)
    {
        var shipment = await _fulfillment.ShipAsync(
            state.Order.Items,
            state.Order.ShippingAddress,
            ct);

        return state
            .With(s => s.Shipment, shipment)
            .With(s => s.Status, OrderStatus.Shipped)
            .AsResult();
    }
}

Step Implementation: SendConfirmation

public class SendConfirmation : IWorkflowStep<OrderState>
{
    private readonly INotificationService _notifications;

    public SendConfirmation(INotificationService notifications)
    {
        _notifications = notifications;
    }

    public async Task<StepResult<OrderState>> ExecuteAsync(
        OrderState state,
        StepContext context,
        CancellationToken ct)
    {
        await _notifications.SendOrderConfirmationAsync(
            state.Order.CustomerId,
            state.Shipment!.TrackingNumber,
            ct);

        return state
            .With(s => s.Status, OrderStatus.Completed)
            .AsResult();
    }
}

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Registration and Starting

Service Registration

services.AddStrategos()
    .AddWorkflow<ProcessOrderWorkflow>();

// Register step dependencies
services.AddScoped<IOrderValidator, OrderValidator>();
services.AddScoped<IPaymentService, StripePaymentService>();
services.AddScoped<IFulfillmentService, WarehouseFulfillmentService>();
services.AddScoped<INotificationService, EmailNotificationService>();

Starting the Workflow

public class OrderController : ControllerBase
{
    private readonly IWorkflowStarter _workflowStarter;

    public OrderController(IWorkflowStarter workflowStarter)
    {
        _workflowStarter = workflowStarter;
    }

    [HttpPost]
    public async Task<IActionResult> CreateOrder(CreateOrderRequest request)
    {
        var workflowId = Guid.NewGuid();
        var initialState = new OrderState
        {
            WorkflowId = workflowId,
            Order = new Order(
                request.CustomerId,
                request.Items,
                request.ShippingAddress)
        };

        await _workflowStarter.StartAsync("process-order", initialState);

        return Accepted(new { WorkflowId = workflowId });
    }
}

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Generated Artifacts

The source generator produces:

Phase Enum

public enum ProcessOrderPhase
{
    NotStarted,
    ValidateOrder,
    ProcessPayment,
    FulfillOrder,
    SendConfirmation,
    Completed,
    Failed
}

Saga with Handlers

public partial class ProcessOrderSaga : Saga<OrderState>
{
    public async Task<object> Handle(
        ExecuteValidateOrderCommand command,
        ValidateOrder step,
        CancellationToken ct)
    {
        // Execute step
        var result = await step.ExecuteAsync(State, context, ct);

        // Apply state update
        State = OrderStateReducer.Reduce(State, result.StateUpdate);

        // Emit event
        // ...

        // Return next command
        return new ExecuteProcessPaymentCommand(WorkflowId);
    }

    // Similar handlers for each step...
}

Commands and Events

// Commands
public record StartProcessOrderCommand(Guid WorkflowId, OrderState InitialState);
public record ExecuteValidateOrderCommand(Guid WorkflowId);
public record ExecuteProcessPaymentCommand(Guid WorkflowId);
// ...

// Events
public record ProcessOrderStarted(Guid WorkflowId, DateTimeOffset StartedAt);
public record ProcessOrderPhaseChanged(Guid WorkflowId, ProcessOrderPhase Phase);
public record ProcessOrderCompleted(Guid WorkflowId, DateTimeOffset CompletedAt);

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The "Aha Moment"

A workflow definition is a contract with your future self.

When this code runs 6 months from now and fails at step 3, you'll know exactly what succeeded, what failed, and where to resume. The workflow state tells you: "Order #12345 is at ProcessPayment, payment failed with error INSUFFICIENT_FUNDS."

That's not debugging—that's operational visibility by design.

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Extension Exercises

Exercise 1: Add Error Handling

When payment fails, refund and notify the customer:

1. Configure error handling for ProcessPayment
2. Add RefundPayment compensation step
3. Add SendFailureNotification step
4. Route failures through compensation path

Exercise 2: Add Inventory Check

Before shipping, verify inventory is available:

1. Add InventoryReservation to state
2. Create ReserveInventory step after payment
3. Handle "out of stock" scenario
4. Add compensation to release reservation

Exercise 3: Add Status Query

Enable querying workflow status:

1. Create OrderStatusReadModel projection
2. Subscribe to ProcessOrderPhaseChanged events
3. Build query endpoint returning current status
4. Include estimated completion time

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Key Takeaways

1. Workflows make business logic explicit—steps are named, ordered, visible
2. Immutable state enables reliability—replay, debugging, concurrency safety
3. Steps are testable units—mock state in, assert state out
4. Failures are explicit—StepResult.Fail() not exceptions
5. Source generation provides type safety—compile-time errors, not runtime surprises
6. This pattern is the foundation—branching, loops, forks all build on this

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Related

• Branching Pattern - Conditional routing based on state
• Iterative Refinement Pattern - Loops until quality achieved
• Fork/Join Pattern - Parallel execution with synchronization