Handling Exceptions and Boundary Events

When a process fails to proceed because of an unexpected event, it’s rarely the user’s fault—it’s the model that’s missing a safety net. I’ve seen too many diagrams collapse under real-world conditions because they assumed everything would go as planned.

That’s where BPMN boundary events come in. They don’t just catch errors—they provide a structured, visual way to respond when things go sideways, allowing you to model exceptions, timeouts, and alternate flows with precision.

This chapter is your guide to mastering BPMN exception handling through boundary events. You’ll learn how to integrate error events BPMN into your process flows, create alternate flows that adapt to failure, and ensure your models reflect real operational behavior—not idealized theory.

Understanding BPMN Boundary Events

Boundary events are attached to activities and are triggered when a specific condition occurs during the execution of that activity. Unlike standard events that sit on their own, boundary events are tethered to a task or subprocess and react to interruptions.

They’re the most visible form of reactive logic in BPMN. Think of them as alarms that go off when something unexpected happens—like a payment failing, a document expiring, or a system timeout.

There are two main types of boundary events:

Interrupting boundary events—Cancel the parent activity and trigger an alternate flow.

Non-interrupting boundary events—Run in parallel without stopping the main flow.

For error handling, interrupting boundary events are usually the right choice. They allow you to pause the process and act when something goes wrong.

When to Use Boundary Events

Not every exception needs a boundary event. Ask yourself: does this error change the flow of the process? If yes, a boundary event is likely appropriate.

Common use cases include:

Timeouts on long-running tasks (e.g., customer onboarding)

Validation failures (e.g., missing documents)

System errors (e.g., API failure, database timeout)

External events (e.g., a customer cancels the request)

These aren’t just exceptions—they’re real-world decisions that impact the process. Boundary events give you the control to respond.

Implementing Error Events BPMN

Let’s walk through a real-world example: a loan application process.

The main path assumes the applicant submits all documents on time. But what if they don’t?

You can model this using an interrupting boundary event on the “Review Documents” task, linked to an error event symbol.

Here’s how to set it up:

1. Place a boundary event on the “Review Documents” task.
2. Set the event type to Error Event.
3. Define the error code, such as “MISSING_DOCUMENT” or “INVALID_FILE.”
4. Connect the boundary event to an “Request Additional Documents” task.

This creates a clean, visual path for handling a critical failure. The main flow continues only when the error is resolved.

Key insight: error events BPMN are not just about catching bugs. They’re about modeling business rules—what happens when a condition is met, and who should act.

Choosing the Right Error Type

Not all errors are the same. Use BPMN’s error codes to distinguish between:

Recoverable (e.g., temporary system outage)

Non-recoverable (e.g., invalid ID submitted)

External (e.g., customer withdraws application)

Assigning specific error codes prevents ambiguity and helps tools or automation systems respond correctly.

For example:

Error Code	Meaning	Recommended Flow
DOCUMENT_EXPIRED	Submitted document is outdated	Request renewal
API_TIMEOUT	Service didn’t respond	Retry or escalate
INVALID_CREDENTIALS	User authentication failed	Block or redirect

These codes become part of the model’s logic—no more guesswork.

Modeling BPMN Alternate Flows

When you introduce error events, you’re not just adding a failure path—you’re building a full decision tree.

BPMN alternate flows are not just for exceptions. They’re for any situation where the process branches based on conditions.

Here’s how to structure them:

• Use boundary events on tasks for reactive error handling.
• Use gateways for conditional decisions based on data or status.
• Keep the main flow clean and focused on success.

Example: A customer service ticket process.

The “Assign Agent” task has a boundary event for “Agent Unavailable” (error event). When triggered, it branches to “Reassign to Backup Team.”

This is a classic case of BPMN alternate flows. The model reflects that not every ticket goes to the same agent—and that’s okay. It’s how real teams work.

Best Practices for Alternate Flows

Don’t let alternate flows clutter your diagram. Here’s how to keep them effective:

• Limit to 1–2 error paths per task—More than that, and complexity overwhelms clarity.
• Use descriptive labels—E.g., “Payment Failed” not just “Error.”
• Group similar errors—Use a single boundary event for multiple similar failures.
• Document error codes—Add a note or legend for stakeholders who may not know the model.

Remember: the goal is communication, not perfection. A clean, readable model is more valuable than one that’s technically correct but impossible to follow.

Advanced Tips: Non-Interrupting Events and Timers

Not all boundary events stop the main flow. Non-interrupting events run in parallel, which is useful for monitoring or logging.

For example, a “Monitor Processing Time” non-interrupting boundary event can track how long a task takes. If it exceeds 30 minutes, it logs an alert—but the task still completes.

Timer events are another powerful tool for exceptions. A timer boundary event can trigger a retry or escalation after a set period.

Use case: “Verify Identity” task with a 5-minute timer. If no response, the system escalates to a supervisor.

This kind of logic is standard in customer service, finance, and logistics—where time is a factor, and delays must be managed.

Common Pitfalls and How to Avoid Them

Even experienced modelers make mistakes. Here are the most common issues with BPMN boundary events—and how to fix them:

• Overusing boundary events—Only attach them when you genuinely need to respond. Most tasks don’t need error handling.
• Using generic error labels—Avoid “Error” or “Error Event.” Be specific: “API Connection Failed.”
• Forgetting to handle the recovery flow—An error path must lead to a resolution. Don’t leave it open-ended.
• Confusing interrupting vs. non-interrupting—Interrupting stops the task. Non-interrupting runs alongside. Use the right one.

As you grow in confidence, revisit your models and ask: does this event add value? If not, remove it. Simplicity is power.

Frequently Asked Questions

What is a BPMN boundary event?

A boundary event is an event attached to a task or subprocess that triggers when a specific condition occurs during its execution—such as an error, timeout, or external signal.

How do I use error events BPMN in a real workflow?

Place an interrupting boundary event on a task, set it to an error event, define the error code, and link it to a recovery task. For example, use it to request missing documents in a loan approval process.

Can I use multiple boundary events on the same task?

Yes, but keep it simple. Use one for critical errors and another for monitoring (e.g., timeout). Avoid more than two to prevent confusion.

What’s the difference between a boundary event and a gateway?

A gateway determines the next step based on a condition. A boundary event reacts to an external trigger *during* task execution. They serve different purposes: decision vs. interruption.

When should I use non-interrupting boundary events?

Use them when you want to monitor or log an event without stopping the main task—like tracking processing time or sending a reminder.

How do I handle BPMN alternate flows without cluttering the diagram?

Limit to 1–2 key alternate flows per task. Use clear labels, keep the main path simple, and document error codes in a legend for clarity.