Fail-If, Warn-If

Basic validation can be achieved using metadata settings such as type and range. The fail-if and warn-if metadata settings are scriptable enabling more advanced validation. They evaluate logical expressions, flagging warnings if they return false.

fail-if and warn-if can be run on the command line using rose macro or on-demand in the rose config-edit GUI.


Simple metadata settings such as range can be evaluated on-the-fly when a value changes. As fail-if and warn-if can take longer to evaluate they must be done on-demand in the rose config-edit GUI or on the command line.


The syntax is Pythonic, and relies on Jinja2 to actually evaluate relationships between values, after some initial pre-processing.

You can reference setting values by using their IDs - for example:

fail-if=namelist:coffee=cup_volume < namelist:coffee=machine_output_volume;

You can also use this as a shorthand for the current (metadata section) ID - e.g.:

fail-if=this < namelist:coffee=daily_min or this >= namelist:coffee=daily_max;

There is also shorthand for arrays, which we’ll demonstrate later.

Note that the ; at the end is optional when we only have one expression (it’s a delimiter), but it’s better style to keep it.


We’ll use the example of a rocket launch.

Create a new application called failif-warnif:

mkdir -p ~/rose-tutorial
rose tutorial failif-warnif ~/rose-tutorial/failif-warnif
cd ~/rose-tutorial/failif-warnif

You will now have a new Rose app with a rose-app.conf that looks like this:




battery_levels=80, 60
Apollo 11 Lunar Module, returning from the surface of the Moon

This app configuration controls the liftoff of a particular rocket - in our case, the Lunar Module (Apollo Program spacecraft).

There is also metadata in the meta/rose-meta.conf file which provides the application inputs with descriptions, help text and type information.

Try running rose config-edit in the app directory. You should be able to navigate between the pages and view the help and description for the settings.


If the ratio of rocket fuel to total weight is too high, or the efficiency of the rocket (specific impulse) is too low, the Lunar Module will never make it off the Moon.

We want to be able to flag an error based on a combination of the rocket settings and the necessary orbital velocity (env=ORBITAL_VELOCITY_MS). We need to set some fail-if metadata on one of these settings - as it’s evaluated on-demand, it doesn’t matter which one we choose.

Open the meta/rose-meta.conf file in a text editor.

Add the following line to the metadata section [namelist:rocket=total_weight_kg]:

fail-if=this < namelist:rocket=fuelless_weight_kg * 2.7183**(env=ORBITAL_SPEED_MS / (9.8 * namelist:rocket=specific_impulse_s));

This states the relationship between these settings (a rearrangement of the Tsiolkovsky rocket equation). The rocket must have a sufficient ratio of fuel to rocket mass, with a sufficiently fast exhaust velocity (=9.8 * namelist:rocket=specific_impulse_s) to get to the orbital speed env=ORBITAL_SPEED_MS.

Save the metadata file and then reload the config editor metadata (Metadata -> Refresh Metadata).

You now need to ask Rose to evaluate the fail-if condition, as it’s an on-demand process.

Either press the toolbar button Check fail-if … or click the menu item Metadata ‣ Check fail-if, warn-if.

Hopefully, this should not flag any errors, as these are the Apollo mission parameters! A success message will appear in the bottom right-hand corner of the window.

Try adding a few more moonrocks. Add 1000 to the values of total_weight_kg and fuelless_weight_kg.

Re-run the check by clicking Metadata ‣ Check fail-if, warn-if. An error dialog will appear, and the total_weight_kg setting will have an error flag.

However, neither of these are very informative, other than quoting the metadata.

Change the fail-if line to:

fail-if=this < namelist:rocket=fuelless_weight_kg * 2.7183**(env=ORBITAL_SPEED_MS / (9.8 * namelist:rocket=specific_impulse_s));  # Fuel mass ratio or specific impulse too low to achieve orbit.

If you reload the metadata and run the check again, the error message will include the helpful text.

You can also check the fail-if metadata by running rose macro --validate or rose macro -V in a terminal, inside the app directory. Try saving the configuration in a failed state, and then run the command.


The warn-if metadata setting is exactly the same as fail-if, but is used to report non-critical concerns.

Let’s try adding something for namelist:rocket=battery_levels.

Open the metadata file meta/rose-meta.conf in a text editor, and add this line to the [namelist:rocket=battery_levels] section:

warn-if=namelist:rocket=battery_levels(1) < 75 or namelist:rocket=battery_levels(2) < 75;

This uses a special syntax for referencing the individual array elements in battery_levels.

If the first array element value and/or the second array element value of battery_levels is less than 75% full, a warning will be produced when the check is run.

We already know the shorthand syntax this, so rephrase the metadata to:

warn-if=this(1) < 75 or this(2) < 75;

Save the metadata file and then reload the config editor metadata. Click Metadata ‣ Check fail-if, warn-if - a warning should now appear for the battery_levels option.

For large arrays, it can sometimes be convenient to use whole-array operations - the fail-if and warn-if syntax includes any() and all().

We can change the warn-if setting to:

warn-if=any(this < 75);

which will flag a warning if any battery_levels array element values are less than 75.

Multiple Expressions

In both fail-if and warn-if, expressions can be chained using the Python operator or, or you can separate them to give clearer error/warning messages. Using our battery_levels example again, change the setting to:

warn-if=any(this < 75);
       =all(this > 95);

This will produce a warning if any elements are less than 75, and a separate warning if all elements are greater than 95 (we don’t want to cook the batteries!).

You can add separate helper messages for each expression:

warn-if=any(this < 75);   # Battery level low
       =all(this > 95);   # Don't over-charge!

Try adding the above lines to the metadata, saving and playing about with the array numbers in the config editor and re-running the fail-if/warn-if check.


For more information, see Configuration Metadata.