GitSuperRepo

(draft)

Background:

• Einstein toolkit built from many different components living in their own repositories

• End user must check out each component and compile them together into an executable which is then run to produce output

• End user is often also a developer of some of the components (public or private)

• GetComponents (URL) is a tool to simplify this process by collecting component repository information into a single "CRL" file (CRL = Component Retrieval Language).

• GetComponents allows you to check out the latest versions from a CRL file, or to update an existing set of checkouts to the latest version

Problems:

• Upstream projects use different version control systems (SVN, Git, Mercurial, ...) leading to a nonuniform experience for the end user/developer. Multiple tools must be learned for merging/branching/committing etc.

• It is not easy to see at a glance exactly what version of the code is in use. One could iterate over all the different repositories, of different types, and print the revision information, and any local differences. This could be added to GetComponents, but this has not been done yet and we argue that this is not the best solution to the problem.

• Knowing what version of the code has been used to produce a given scientific result is essential for the scientific process, where results must be repeatable. The current best solution to this problem is the Formaline thorn which stores a complete copy of the source code of all thorns in the simulation output directory. We argue that this is only a partial solution to the problem. While all the source code is present, the version control metadata has been entirely stripped. When comparing different simulations, at best one obtains a large diff of all the source changes between them, without information about why they were made or who made them. There is also no method for conveniently using the formaline output for a new simulation.

• Updating a Cactus source tree is currently an irreversible and dangerous process. There is no guarantee that the "current" trunk branch of all the components will function correctly, and there is no way, short of a manual backup beforehand, of reverting to the previous state if they don't. It is not possible to see, at a glance, exactly what will be updated when you run "GetComponents -u".

• It is desirable for different members of a scientific research group to be using the same version of the code for production simulations, or at least for this to be possible/easy. In the current setup, each user is responsible for managing their own Cactus tree and will likely have completely different versions of the code, depending on when they last updated. It is not even guaranteed that the code can be described by a single "checkout date", since different components could be checked out at different times. Users may also have applied patches or altered behaviour, fixing bugs or adding features, to any of the components.

• SVN does not allow distributed version control. Many components of the ET are in SVN, which means that users cannot use version control locally. They cannot commit locally frequently and go back to previous versions when there are problems, then bundle up the changes as a coherent commit to upstream.

• Managing branches is difficult. Suppose a user wants to implement a new feature. First, a new branch is created in the corresponding repository (we ignore here the fact that most of the components are in SVN, which does not encourage this mode of development), then the feature is committed bit by bit to that branch. In the course of development, the user might want to run a production simulation. So they would switch that repository back to the production branch temporarily. However, there is no global record of which branch each repository is currently on, and it might be that some repositories are not on "production" branches. The best solution at the moment is to have a separate production tree, but users rarely have the discipline to do this.