# The Why of Jug¶

This explains the philosophy behind jug and some of its design decisions.

## Why jug? A Bit of History¶

Jug was designed to solve two intertwined problems:

1. Writing parallel code.
2. Managing intermediate files.

Up until that point, I had been writing code that saved intermediate results to files with complex filenames (e.g., r3_d2_p0_p3_p22_v9.pp for the results of stage 3, dataset 2, with parameters (0,3,22), running version 9 of the code). This becomes taxing on the mind that needs to keep track of things and extremely brittle. You constantly run the risk of having results that you are not sure how to reproduce again.

Therefore, I decided I was going to write a solution for this.

The initial idea was something like an enhanced Makefile language. This evolved into something similiar to scons. Very rapidily it became apparent that a good solution involved Tasks and saving results to files based on a hash of the inputs. This is still the basis of jug’s architecture. All of this was at the paper napkin stage, written in some off time I had before I wrote some actual code.

The motivating applications were scientific applications and some of that is probably part of jug’s DNA in ways that are most apparent to those outside of science.

## Design Criteria¶

Jug was meant to run in a queuing batch system. Therefore, it was a good idea to have the possibility of just adding processes to a running process without any explicit synchronisation. This explains why there is no central manager handing out tasks. Tasks coordinate based on a central store such as the filesystem. This also required jug to play nice with NFS and not rely on intra-memory communication.

Another goal (not one of the original goals, but it became a feature that we felt we needed to keep) is that, as much as possible, code should look normal. Many scripts can be jugified by adding @TaskGenerator to a few function declarations. Part of this involves making it unintrusive (not necessarily light-weight or low on features, but the user code should not need much work).

jug was also meant to be used in an exploratory development environment. This is why we have features such as jug shell, jug invalidate (a much better alternative than attempting to selectively update “all of the affected files” after some code change), and, to a certain extent, barrier(). Much of the optimisation work that has been put into jug has been to support interactive work better.

As for more down to earth goals, there should never be any known bugs in jug. Any bug report has a promise to fix it ASAP. Fixing bugs takes priority over new features, always. To attempt to keep quality high, when a bug is found, a regression test is always written.