How to estimate what an architecture will cost to run
Attach a budget to the model so that cost moves when the architecture does, and adopt a practice pack so the design starts inside the standards it will eventually be reviewed against.
When this is useful
- Two options are on the table and the cost difference is the deciding factor.
- A design is finished and finance asks a question the diagram cannot answer.
- A new team is starting and you would rather they began inside the practice than be retro-fitted to it at review.
Before you start
- A model with the components in it. Cost is estimated against components, so the estimate is only as complete as the model.
- Whichever of practice packs and budget your organisation has enabled. They are separate capabilities and either can be available without the other.
Step 1: Start inside a practice
Apply a practice pack. It carries the principles, viewpoints and standards of an established way of working, which means the model starts inside the practice instead of being measured against it for the first time at review.
Doing this first is cheaper than doing it later. A practice applied at review is a list of findings; a practice applied at the start is just how the model was built.
Arch ▸ Practice PackArch ▸ Viewpoints
Step 2: Open the budget against the model
Open budget and work through the components. Because the estimate is built against what is in the model, it is not a spreadsheet that drifts — change the architecture and the figure changes with it.
Arch ▸ Budget
Step 3: Compare the options while they are still options
Cost a second version of the design and compare. This is the whole reason to do it in the model rather than afterwards: a cost figure produced after the architecture is settled informs nothing, because nothing can change in response to it.
Step 4: Take it to review with the compliance result
Principle compliance evaluates the model against the practice, and the policy and standards report shows how it stands against the rules. Take those to review alongside the budget, so the design is assessed on cost and conformance together rather than in two separate meetings.
Simulate ▸ Principle ComplianceSimulate ▸ Policy & Standards
What happens next
The cost figure is attached to a specific version of the architecture, so when someone asks later why it went up, the answer is a diff rather than an argument.
Combined with the simulation, you have both what the design costs and whether it will hold up — which are the two questions a design review is usually trying to answer.
Example
Two options for an event pipeline: managed streaming against self-hosted brokers. Costed against the same set of components, the managed option was more expensive to run and cheaper in the first year once the operational effort was included in the comparison. The discussion took twenty minutes because both numbers were attached to models everyone could see.
Tips
- Estimate the expensive components properly and the cheap ones roughly. Precision on the small items adds effort and changes no decision.
- Re-run the estimate after the design changes, not before the review. An unrefreshed number is worse than no number, because people believe it.
- Record what the estimate assumed. Most cost disagreements turn out to be assumption disagreements about volume.
Limitations
- Practice packs, budget and principle compliance are each separately enabled capabilities.
- The estimate is as good as the model and the rates supplied. It reflects the architecture you have drawn, not the one you will build if the drawing is incomplete.