---
title: The nine axes
description: Nine questions the library asks of the rules you declare, each a different kind of bug. One page each.
url: https://docs-v3-preview.elastic.dev/axes
---

# The nine axes
An **axis** is one kind of question you can ask of a rule. Nine of them cover the behaviour a type cannot state. You declare a rule; the library asks every question that applies to its shape.

| Axis                                                                        | The question                            |
|-----------------------------------------------------------------------------|-----------------------------------------|
| [01 Invariant](https://docs-v3-preview.elastic.dev/axes/01-invariant)       | Is it true in every state?              |
| [02 Idempotency](https://docs-v3-preview.elastic.dev/axes/02-idempotency)   | Does doing it twice change anything?    |
| [03 Monotonicity](https://docs-v3-preview.elastic.dev/axes/03-monotonicity) | Does it ever go backwards?              |
| [04 Conservation](https://docs-v3-preview.elastic.dev/axes/04-conservation) | Did anything appear or vanish?          |
| [05 Absence](https://docs-v3-preview.elastic.dev/axes/05-absence)           | Can this bad sequence ever happen?      |
| [06 Temporal](https://docs-v3-preview.elastic.dev/axes/06-temporal)         | Does B follow A within a deadline?      |
| [07 Liveness](https://docs-v3-preview.elastic.dev/axes/07-liveness)         | Does it eventually finish?              |
| [08 Confluence](https://docs-v3-preview.elastic.dev/axes/08-confluence)     | Does the order inputs arrive in matter? |
| [09 Vacancy](https://docs-v3-preview.elastic.dev/axes/09-vacancy)           | Did the test even reach the rule?       |

Each page is built the same way: the question in plain words, the bug you would ship, the rule you declare, what the library does, and the real failure you read. Start with whichever sounds like a bug you have had.

## Which questions are about what


| The rule is about                 | Axes                                             |
|-----------------------------------|--------------------------------------------------|
| One state                         | 01 Invariant                                     |
| A step from one state to the next | 02 Idempotency, 03 Monotonicity, 04 Conservation |
| Events over time                  | 05 Absence, 06 Temporal, 07 Liveness             |
| The order inputs arrive in        | 08 Confluence                                    |
| The test itself                   | 09 Vacancy                                       |


## Running them

```csharp
await AutoTest.For(OrderConstraints.Instance, orders).RunAsync();    
await EventAutoTest.For(rules, script, time => new Service(time), s => s.Log).RunAsync();  
```

`RunAsync()` throws one exception listing every axis that failed. To run one: `InvariantAsync()`, `IdempotencyAsync()`, `MonotonicityAsync()`, `ConservationAsync()`, `ConfluenceAsync()`, `VacancyAsync()`; for events `AbsenceAsync()`, `TemporalAsync()`, `LivenessAsync()`. For your test framework, `AsDataSource()` gives one test per axis the rules declare, and every case is awaited with `RunAsync()`.
An axis the type has no rule for is reported as *not applicable*; it is never a silent pass. `RequireAxes(...)` turns "not applicable" into a failure when you expect a rule there.

## How to read a failure

Every failure has the same labels, so you learn them once.

| Label                           | Meaning                                                                |
|---------------------------------|------------------------------------------------------------------------|
| **Rule broken**                 | The rule, by name, and what it required                                |
| **Operation** / **Calls made**  | What was done to cause it                                              |
| **Started from** / **Produced** | The value before and after                                             |
| **Events recorded**             | For the event axes: what the service did, with times                   |
| **Law**                         | The question this axis asks                                            |
| **Consequence**                 | What goes wrong in production if it fails                              |
| **Smallest case found**         | The library tried smaller cases and kept the smallest that still fails |
| **.Seed("…")**                  | Pass it back to repeat exactly this case while you fix the bug         |

The [concepts](https://docs-v3-preview.elastic.dev/concepts) page explains smallest case and seed.