# Writing great assertions

[Writing assertions](https://www.youtube-nocookie.com/embed/55McQ8-NAg4?rel=0&showinfo=0&modestbranding=1&autoplay=0)

At this stage, you should hopefully have a `.blackfire.yaml` file containing your very first test. If not, you can [create one in minutes](https://blog.blackfire.io/getting-started-with-the-blackfire-test-suite-part-1-of-series.html).

You can define as many tests as you need, each containing as many assertions as required. But, **what makes a good assertion?**

Assertions are based on the available metrics and, as Blackfire provides over 700 metrics and allows you to create your own, there's a lot of choice and testing available.

To write an assertion on a specific dimension, append the dimension name to the metric name, separated with a `.`. For example, if you want to base your assertion on the number of SQL requests made, append `count` to `metrics.sql.queries`: `metrics.sql.queries.count`.

The available dimensions are `count`, `wall_time`, `cpu_time`, `memory`, `peak_memory`, `network_in`, `network_in`, `io`. Then use main to target the profile itself, using the same dimensions.

Assertions can also use functions to account for the context. The `is_dev()`function returns false in a production environment, and the `var()` function returns the value of a user-defined variable.

```yaml
tests:
    "Pages shouldn't use too much memory":
        path: "/.*"
        assertions:
            - "main.peak_memory < 10mb * var('memory_coeff', 1)"
```

> **Note**
>
> [Dig deeper into the possibilities.](../../testing-cookbooks/assertions)