The design of `validphys 2`

Introduction

The objective of this page is to explain the high level considerations underpinning the design of validphys 2, and of reportengine [Kas19], the code it is based on. It should be useful for anyone aiming to understand the general philosophy and design goals of the project or take in in qualitatively different directions. More concrete information on how to use the code can be found in the sections under Using validphys.

Some specific issues of scientific code

There are some characteristics of typical scientific programs that are rather specific to them. A program aiming to optimize the workflow of a scientific collaboration should be designed with those in mind. Some of these characteristics make work easier compared to other types of programs like text editors or games. In general the runtime execution can be made simpler because user interactivity is not required. On the other hand we need easy access to all parts of the code, since we never know which parts might be subject to changes. That’s the whole point of research!

We now proceed to list these characteristics in more detail.

Lack of interactivity

Typically programs such as report generators, fitting codes, or simulations are entirely determined by the initial user input together with perhaps some random source and environment events like which replicas failed. We typically don’t make network connections in the middle of fits, don’t wait for the user to be pushing buttons. In many instances it is possible to know about the amount of resources such as memory that a scientific program is going to require shortly after running it.

Changing structure

It happens often that the hardcoded parameter you are optimizing becomes the next object of research. Scientific code should make as little assumptions as possible on how it is supposed to run, while managing to get by and produce some results. This somewhat upsets the balance of trade-offs compared to other kinds of programs. We are willing to trade some increased complexity for the ability to look under the hood.

Complexity

Non trivial scientific programs can rarely be assembled exclusively by putting together external components without adding anything of our own. That typically consists on unspecified, or at best, underspecified algorithms that are difficult to test in isolation.

Design considerations

Look before you leap

A scientific program usually requires a large set of complex input parameters and can take a very long time to produce results. Results tend to be frequently unexpected even if everything is working correctly. It is essential to check that the inputs are correct and make sense. Thus good error reporting capabilities are an important design consideration.

Also, the inputs of a given function should be checked as early as possible, which is not necessarily at the point of the program where the function is to be called. For example, something like this:

def check_parameter_is_correct(parameter):
    ...

def plot(complex_calculation, parameter):
    check_parameter_is_correct(parameter)
    #make plot
    ...

has the disadvantage that in order to get to the check, we presumably need to compute complex_calculation first, and that could be a waste of time if it turns out that parameter is not correct for some reason and we can’t do the plot anyway. Instead we should arrange to call check_parameter_is_correct(parameter) as early as possible (outside the plotting function) and show the user an informative error message.

All inputs should be checked as early as possible. If the checks pass, the program should not fail.

Of course it is impossible to guarantee that this is always going to happen. For example it is possible that we check that a folder exists and we have permissions to write to it, but by the time we actually need to write, the folder has been deleted. However in such cases the state of the program can be considered broken and it’s OK to make it fail completely.

There is an API for early checks in reportengine. We would write something like:

@make_argcheck
def check_parameter_is_correct(parameter):
    ...

@check_parameter_is_correct
def plot(complex_calculation, parameter):
    #make plot
    ...

The checking function will now be called as soon as the program realizes that the plot function will be required eventually (at “compile time”). The user would be shown immediately an error message explaining why the parameter is wrong.

The fancy term for this style of coding is Contract Programming.

Declarative input

It is convenient to be able to specify the what the program should do without any regard of knowledge of how that is achieved by the underlying implementation. The primary input of validphys are YAML run cards. A very simple one looks like this:

 pdfs:
     - NNPDF40_nlo_as_01180
     - NNPDF40_nnlo_as_01180
     - NNPDF40_nnlo_as_01180_hessian

norm:
    normalize_to: NNPDF40_nlo_as_01180

first:
    Q: 1
    flavours: [up, down, gluon]

second:
    Q: 100
    scale: linear

actions_:
    - first::norm plot_pdfreplicas
    - first plot_pdfs
    - second plot_pdfreplicas

This has a number of advantages:

Correct by definition

A declarative input specifies what you want. It is up to the underlying code to try to provide it (or fail with an informative message).

Clear meaning

It is easy for a human to verify that the input is indeed what it was intended. Even without any explanation it should be easy enough to guess what the runcard above does.

Implementation independent

The input is very loosely coupled with the underlying implementation, and therefore it is likely to remain valid even after big changes in the code are made. For example, in the runcard above, we didn’t have to concern ourselves with how LHAPDF grids are loaded, and how the values of the PDFs are reused to produce the different plots. Therefore the underlying mechanism could change easily without breaking the runcard.

Usage as a programmatic API

While the goal of reportengine is to allow simple and easily repeatable bach actions, sometimes it is far simpler to get the work done with a raw (Python) script, or it is needed to explore the outcomes using something like an Jupyter notebook. It would be good to be able to use all the tools that already exist in validphys for that, without needing to reinvent the wheel or to alter functions so that for example they don’t write data to some preconfigured path.

Therefore The various computing and plotting tools should work well when included in a normal script that doesn’t use the reportengine graph compiler.

This is implemented by making sure that as much as possible all the validphys functions are pure. That is, the output is a deterministic function of the inputs, and the function has no side effects (e.g. no global state of the program is altered, nothing is written to disk). There are some exceptions to this though. For example the function that produces a reweighted PDF set needs to write the result to disk. The paths and side effects for other more common results like figures are managed by reportengine. For example, the @figure decorator applied to a function that returns a Python (matplotlib) figure will make sure that the figure is saved in the output path, with a nice filename, while having no effect at all outside the reportengine loop. The same goes for the check functions described above.

Easy to loop

Very frequently there is the need to compare. It is easy enough to write simple scripts that loop over the required configurations, but that cannot scale well when requirements change rapidly (and is also easy to make trivial mistakes). Therefore reportengine allows configurations to easily loop over different sets of inputs. For example the following runcard:

pdfs:
    - id:  NNPDF40_nlo_as_01180
      label: NLO

    - id: NNPDF40_nnlo_as_01180
      label: NNLO


theoryids:
    - 208
    - 200
use_cuts : nocuts

experiments:
  - experiment: LHCb
    datasets:
      - { dataset: LHCBWZMU7TEV, cfac: [NRM] }
      - { dataset: LHCBWZMU8TEV, cfac: [NRM] }

  - experiment: ATLAS
    datasets:
      - { dataset: ATLASWZRAP36PB}

actions_:
 - theoryids::pdfs::experiments::experiment plot_fancy

Will produce a separate plot for each combination of the two theories (200 and 208), the two PDFs at the top, and each dataset in the two experiments (so 28 plots in total). This syntax is discussed in more detail in the [Usage] section.

It should be trivial to repeat an action for different sets of inputs.

The design of validphys 2