---
title: Deadlock (Process Model Behavioural Anomaly)
type: concept
tags: [bpm, modelling, soundness, anomaly, behavioural-correctness, verification]
sources:
  - "[[sources/2018-dumas-fundamentals-of-bpm]]"
  - "[[sources/2018-dumas-fundamentals-of-bpm-ch5-discovery]]"
  - "[[sources/1998-vanderaalst-verification-of-workflow-nets]]"
created: 2026-05-04
updated: 2026-05-04
---

# Deadlock

A **behavioural anomaly** in which a running process instance reaches a state from which it cannot progress further. A token gets stuck. The instance can never complete.

Definition ([[sources/2018-dumas-fundamentals-of-bpm]] §5.4.1):

> *A deadlock occurs when a running process instance is not able to progress any further once a given state is reached, i.e., a token gets stuck at that state.*

## How deadlocks arise in BPMN

The most common cause is a **mismatched split-join pair**:

| Pattern | Why it deadlocks |
|---|---|
| **XOR-split → AND-join** | XOR routes a token down one branch only; the AND-join waits for tokens on *all* incoming branches → wait forever |
| **OR-split (incomplete coverage) → AND-join** | If conditions don't activate all branches the AND-join expects, the same wait-forever |
| **Branch injected into AND-block** | Token returning from the injection arrives at the AND-join but the symmetrically-expected token never appears (Fig 5.12a) |
| **Loop with always-false exit + AND-join entry** | Internal AND-join in a loop body never gets the second token |

A model with a deadlock **violates option to complete** (the first soundness clause — see [[concepts/soundness]]).

## Detection

**Mechanical**: any standard Petri-net liveness checker flags deadlocks. See [[sources/1998-vanderaalst-verification-of-workflow-nets]] Theorem 11 — soundness reduces to live + bounded on the extended net; deadlocks violate liveness.

**Practical tools**:
- **WOFLAN** (workflow-specific verifier).
- **ProM** (process-mining suite, multiple soundness plug-ins).
- **Camunda Modeler** (basic structural validation; not full deadlock detection).
- **bpmn-js-token-simulation** — visual token-flow simulator; deadlocks become observable as stuck tokens.

**By inspection**: walk every gateway and check that for every split there is a matching join of the same type. This catches most deadlocks at design time.

## Prevention

Three modes (cf. [[concepts/soundness]] §"achieving correctness"):

1. **Verification after the fact** — build, then check.
2. **Construction by design** — use a tool that prevents soundness-violating edits.
3. **[[concepts/block-structure|Block-structuring]]** — every split paired with a same-type join; sound by construction.

[[concepts/7pmg]] G3 (single start/end) and G4 (model as structured) reduce deadlock incidence empirically.

## Example (Dumas Fig 5.12a)

A model with an apparent AND-block has an extra branch injected such that activity G's path bypasses one half of the AND-block. If G is executed, a token arrives at the AND-join — but the AND-join expects two tokens (one from each branch of the original block). The second token never arrives → deadlock.

## Distinction from related anomalies

| Anomaly | Symptom |
|---|---|
| **Deadlock** | Token unable to progress; instance never completes |
| [[concepts/livelock]] | Token cycles in a loop forever; instance never completes |
| [[concepts/lack-of-synchronization]] | Multiple tokens on same flow; instance "completes" with extra tokens |
| [[concepts/dead-activity]] | Activity never executable in any instance |

Deadlocks and livelocks both violate **option to complete** but for different reasons — stuck vs. spinning.

## Related
[[concepts/soundness]] · [[concepts/workflow-net]] · [[concepts/block-structure]] · [[concepts/livelock]] · [[concepts/lack-of-synchronization]] · [[concepts/dead-activity]] · [[concepts/process-model-quality]] · [[concepts/7pmg]] · [[frameworks/bpmn]]

## Sources
[[sources/2018-dumas-fundamentals-of-bpm]] §5.4.1 (textbook treatment) · [[sources/2018-dumas-fundamentals-of-bpm-ch5-discovery]] · [[sources/1998-vanderaalst-verification-of-workflow-nets]] (formal Petri-net foundation)