BPMN with simulation: what the notation can compute — and what it cannot
BPMN is a notation for structure — steps, gateways, roles. To compute, it lacks five things: the arrival process, the variability of processing times, capacity per resource, branching probabilities and working calendars. The BPSim standard exists precisely to attach these to a BPMN model. So a BPMN tool with a simulation module is only as good as its ability to take in those five kinds of data — and whether it reports results as a range rather than a single value.
One sentence regularly leads tool selections astray: “our BPMN tool does simulation too.” It is usually true and almost always useless, because it can mean anything from a plausibility check to a full discrete-event engine.
The reason lies in the notation itself.
What BPMN describes — and what it does not
BPMN 2.0 is an OMG standard for the structure of a process: tasks, events, gateways, lanes, message flows. It answers “what happens in which order, and who does it?” completely and precisely.
To compute, it lacks five inputs that the notation simply does not provide for:
- The arrival process. How many cases arrive — and distributed how? Twenty applications a day arriving evenly is a completely different picture from twenty arriving in a Monday-morning burst.
- Variability of processing times. Not “10 minutes” but “5 to 20 minutes, usually 8”. Variability drives the queue, not the mean.
- Capacity per resource. How many people at what allocation, and which roles share a pool?
- Branching probabilities. An XOR gateway shows that the flow branches. How often each path is taken appears nowhere in the diagram.
- Calendars. Eight hours a day, holidays, shifts. A case arriving at 16:50 on a Friday does not wait ten minutes; it waits three days.
BPSim was created for exactly this problem — a WfMC standard that attaches simulation parameters to a BPMN model without changing the notation. That such a standard is needed is the best evidence that BPMN alone cannot compute.
What simulation modules in BPMN tools typically provide
The common modelling suites — Bizagi Modeler, SAP Signavio, ARIS and others — offer simulation in graded levels. The usual structure, independent of vendor:
- Structure validation. Does a token even flow through the model, or are there dead ends and unreachable branches? Useful, but not yet simulation in the narrow sense.
- Time analysis. Durations per task, hence lead time — initially without capacity limits, therefore without queues.
- Resource analysis. Now it gets interesting: people with capacity, and therefore queues and utilisation.
- Calendar analysis. Working hours and shifts, which is what makes waiting times realistic.
Camunda deliberately does not belong in this list: it is an execution engine, not a simulation environment. It runs real cases through the process and measures them. That is more valuable than any simulation — but only once the process is already automated. Before the decision, it does not help.
Five questions for your tool
Open your modelling tool and check within ten minutes:
1. Can I enter a duration as a range? If the field accepts only one number, the tool computes with means. The result will then be systematically optimistic — queues arise from variability, and without variability none arise.
2. Do I get a result with a range? A lead time of “6.4 days” is a number, not a statement. “P10 4.1 — P90 11.8 days” is a statement. Tools that output only point values either computed deterministically or suppressed the range.
3. Do roles share capacity? In practice, the same person performs three different steps. Tools that track capacity per task rather than per pool will show each step comfortably loaded — and the person doing all three loaded nowhere.
4. Am I seeing the peak day or the monthly average? On a monthly average almost every role is below 80 % utilised. On the peak day, that is exactly where the process collapsed. If a utilisation figure does not state what it refers to, it is worthless.
5. What does the improvement cost — and what does it return? Almost no modelling tool answers this, because cost rates and investments are not part of the model. Yet that number is what decides the project.
Where even good modules stop
Suppose your tool passes all five checks. One question remains, and it is where most analyses fail — not from technical inability, but because nobody asks it:
Where does the constraint move once I relieve it?
An example everyone recognises: review takes too long, so it gets automated. The simulation confirms review time drops by 80 %. What it often does not show: the approval step behind it used to receive a case every 26 minutes, because review did not let more through. Now it receives one every 10 minutes — and is overloaded itself. Throughput does not rise as promised, and the time saved reappears as a new queue.
This is not an exotic edge case, it is the rule. A constraint does not disappear, it migrates. A model that cannot play that movement through in seconds produces business cases that do not materialise in operations.
Practical recommendation
- Already have a modelling suite with a simulation module? Use it. A second tool is worth it only once you fail one of the five checks.
- Just starting to model? Do not start with BPMN. The notation is built for documentation; for “where does it break and what is a change worth?” it is a detour through a language you must learn first.
- Have to justify an investment? Then you need ranges, the peak day, shared resource pools and a before/after case in money. Check which of your existing tools delivers that before buying a new one.
FlowVisual deliberately skips BPMN: six building blocks instead of a notation, durations as ranges, utilisation on the peak day, shared pools — and a before/after in money with a P10–P90 band. If you need BPMN for documentation, this does not replace it. It answers a different question.
Frequently asked
What is BPSim?
BPSim is a Workflow Management Coalition standard that attaches simulation parameters — arrival rates, distributions, costs, resources, calendars — to a BPMN model without changing the notation itself. It exists because BPMN describes structure and simply does not provide for those inputs.
Can Camunda simulate processes?
Camunda is an execution engine: it runs real process instances and measures their actual durations. That is more defensible than any simulation — but it presupposes that the process is already automated. To decide whether a change is worth making, you need a model before you build.
Is the simulation in Bizagi or Signavio enough for a business case?
For structure and timing questions, often yes. Whether it suffices for the business case depends on three things: whether you can enter durations as ranges, whether results come out with a band, and whether shared resource pools are represented. Check those three before writing a number into a decision paper.
Why is the mean processing time not enough?
Because queues arise from variability. Two processes with identical mean processing times have completely different lead times if one of them varies strongly. Computing with means systematically understates waiting time — and the closer utilisation gets to capacity, the larger the error becomes.
Run the numbers on your own process
FlowVisual turns the figures in this article into a model that runs — with your volumes, your capacities, your range.
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