Bpmn Workflow

BPMN 2.0 & Workflow Engine Standards

This skill defines patterns for workflow engine implementation following BPMN 2.0 standards.

BPMN 2.0 Core Concepts

Element Types

Gateway Semantics (CRITICAL)

Exclusive Gateway (Decision — XOR)

• Evaluates conditions on outgoing edges in order
• Takes the first matching path
• Must have a default path (fallback)
• Conditions live on edges, not the gateway node
• Gateway ConfigPanel shows: question label + outgoing path summary
• User clicks an edge to open the ConditionBuilder

Parallel Gateway (Split & Wait — AND)

• Splits: ALL outgoing paths fire unconditionally
• Joins: Waits for ALL incoming paths before continuing
• NO conditions — ever. ConditionBuilder must NEVER appear for parallel edges
• Use for concurrent operations (e.g., "send email AND update inventory AND notify Slack")

Inclusive Gateway (Match — OR)

• Evaluates conditions on all outgoing edges
• Takes ALL matching paths (not just the first)
• Must have a default path for when nothing matches
• Join waits only for the paths that were actually taken

Anti-Patterns

• Conditions on parallel gateway edges (BPMN violation)
• Missing default paths on exclusive/inclusive gateways (creates dead-end workflows)
• Deeply nested gateways (>3 levels) — refactor into sub-processes
• Gateway nodes trying to hold business logic — gateways ROUTE, they don't COMPUTE

Workflow Engine Architecture

Instance Lifecycle

Created → Running → [Suspended | WaitingForHuman | Completed | Failed]
                         ↓                ↓
                      Running          Running (on resume)

State Machine Rules

public enum WorkflowStatus
{
    Pending,            // Created but not started
    Running,            // Currently executing
    Suspended,          // Waiting on timer/retry
    WaitingForHuman,    // Human task pending
    Completed,          // Successfully finished
    Failed              // Unrecoverable error
}

Valid transitions:

• Pending → Running (trigger fires)
• Running → Completed (reached end node)
• Running → Failed (unrecoverable error)
• Running → Suspended (delay, retry backoff)
• Running → WaitingForHuman (human task created)
• Suspended → Running (timer/retry fires)
• WaitingForHuman → Running (human approves/rejects)

Execution Model

public async Task ExecuteInstanceAsync(WorkflowInstance instance, CancellationToken ct)
{
    instance.Status = WorkflowStatus.Running;
    instance.ErrorCode = null;      // Clear stale errors from retry
    instance.ErrorMessage = null;

    while (instance.CurrentNodeId != null)
    {
        var node = graph.GetNode(instance.CurrentNodeId);
        var executor = _registry.Resolve(node.Type);

        var result = await executor.ExecuteAsync(context);

        // Handle result: advance to next node, suspend, complete, or fail
    }
}

Node System

Node Contract

Every node type has:

1. Descriptor (backend): Declares params, default outputs, category
2. Executor (backend): Implements ExecuteAsync, produces output DataBag
3. Definition (frontend): Mirrors descriptor — params, outputs, display info

Output Contract

// Descriptor declares what the executor produces
public OutputFieldDef[] DefaultOutputs => new[]
{
    new OutputFieldDef("order_id", "Order ID", OutputType.String),
    new OutputFieldDef("total", "Total Amount", OutputType.Number),
};

// Executor MUST set exactly these keys
output.Set("order_id", orderId);
output.Set("total", totalAmount);

CRITICAL: DefaultOutputs keys must EXACTLY match executor DataBag keys AND frontend nodeDefinitions.ts keys. All three locations must stay in sync.

Expression Evaluation

• Nodes reference upstream outputs via {{ nodes.node_1.field_name }}
• Expression evaluator resolves at runtime from WorkflowVariables
• After JSON round-trip (suspend/resume), values are JsonElement — evaluator handles both CLR types and JsonElement

Test Mode

public async Task ExecuteAsync(NodeContext ctx)
{
    if (ctx.IsTest)
    {
        ctx.Output.Set("result", "[TEST] Simulated output");
        return;
    }
    // Real execution...
}

Side-effecting executors MUST check ctx.IsTest. Read-only executors don't need guards.

Human Tasks

Flow

1. Workflow reaches Human Task node → status becomes WaitingForHuman
2. Task appears in user's task list (SSE notification sent)
3. User reviews data, makes decision (approve/reject/custom)
4. POST /api/workflows/{id}/resume with decision payload
5. Engine resumes, stores payload under vars["__resume"] (namespaced)
6. Downstream nodes access via {{ __resume.decision }}

Resume Payload Safety

CRITICAL: Never merge resume payloads into top-level workflow variables. Namespace under __resume to prevent variable collision with downstream node configs.

Timer & Scheduling

Delay Node

// Suspend instance with ResumeAfter timestamp
instance.Status = WorkflowStatus.Suspended;
instance.ResumeAfter = DateTimeOffset.UtcNow.Add(delay);

Cron Triggers

• Use Cronos library for cron expression evaluation
• Scheduler polls every 30s for due triggers
• Track _lastFired per definition to prevent double-firing

Retry with Backoff

instance.Status = WorkflowStatus.Suspended;
instance.ErrorCode = "TRANSIENT_FAILURE";  // MUST set for scheduler to distinguish retry from delay
instance.ResumeAfter = DateTimeOffset.UtcNow.AddSeconds(Math.Pow(2, instance.RetryCount));

Error Handling

Transient vs Permanent Errors

Error Propagation

try
{
    await executor.ExecuteAsync(context);
}
catch (TransientException ex)
{
    instance.RetryCount++;
    if (instance.RetryCount >= MaxRetries)
    {
        instance.Status = WorkflowStatus.Failed;
        instance.ErrorMessage = $"Max retries exceeded: {ex.Message}";
    }
    else
    {
        instance.Status = WorkflowStatus.Suspended;
        instance.ErrorCode = "TRANSIENT_FAILURE";
        instance.ResumeAfter = CalculateBackoff(instance.RetryCount);
    }
}
catch (Exception ex)
{
    instance.Status = WorkflowStatus.Failed;
    instance.ErrorMessage = ex.Message;
}

Graph Persistence

• Workflow definitions stored as JSON graph in database
• Graph deserialization MUST use JsonStringEnumConverter(JsonNamingPolicy.SnakeCaseLower) to match global config
• Node positions, connections, and configs all serialized
• On load: advance nextNodeId past all existing IDs to prevent collisions

UX Principles (The Non-Technical User Test)

"Can a non-technical user who uses basic office tools daily but has never written a formula, build this workflow in under 5 minutes without help?"

• Zero syntax — no brackets, dots, or template expressions visible to users
• Human-readable labels — "Split & Wait" not "Parallel Split"
• Click-to-configure — data pills for expressions, not text input
• Conditions on edges — not in gateway config panels
• Visual scope zones — For Each loops show a tinted region