Proof of concept for git governed project wide configuration

The aim is to have project wide configuration managed and applied by git. To have a unique central git repository of configuration settings defaults per project, present as a git submodule in every other repository belonging to the project, irrespective of the context where the code included in the repoitory has its meaning. The idea is that the files versioned by git only keep the names of the configuration settings, in the places where their values should be. When the project is checked out by git, all the config setting names are replaced by their default, or, possibly, locally customized, values, as given by the mentioned unique central git repository; and when changes are commited to the repository, the config setting values are replaced by their names.

This is achieved with a mechanism like the filters explained in the documentation for git attributes.

The possible advantages of these scheme would be two fold:

All configurable files can be kept under git; all environments can be managed by files kept in git. Except git itself, Or...

There is no danger of accidentally commiting local settings during development; In this sense, this idea can be seen as a very fine grained gitignore scheme.

Testing the POC

The first thing we need is this code:

$ git clone https://github.com/enriquepablo/eduid-conf-poc.git /path/to/eduid-conf-poc

make a git repo:

$ mkdir test-poc
$ cd test-poc
$ git init

We also need to activate a virtualenv with the diff_match_patch library:

$ virtualenv venv
$ source venv/bin/activate
$ pip install diff_match_patch

We now copy a few files from eduid-conf-poc over to our newly created repo. First, we copy the .gitattributes file. In it, we indicate that files that match *.test will use the filter eduid-config:

$ cp /path/to/eduid-conf-poc/.gitattributes .

Now we copy over eduidconf-gitconfig, where the filter is defined, with references to the scripts that will be used to check the code in and out:

$ cp /path/to/eduid-conf-poc/eduidconf-gitconfig .

We get those scripts:

$ cp /path/to/eduid-conf-poc/eduidconf-gitfilter-in .
$ cp /path/to/eduid-conf-poc/eduidconf-gitfilter-out .

Finally we get the file with the default settings, used by the scripts:

$ cp /path/to/eduid-conf-poc/eduidconf_defaults.py .

In this file there is already a single setting, to wit:

EDUIDCONF_TEST="testing"

All settings for this POC must start with EDUIDCONF.

We now need to activate the git configuration in eduidconf-gitconfig. This can be achieved with this command:

$ git config --local include.path ../eduidconf-gitconfig

We can now start playing with this setup. Let's first create and add a file that uses our setting:

$ echo "This is a test with a EDUIDCONF_TEST setting" >> one.test
$ git add one.test
$ git commit -m 'testing settings'

We now have the file with the same contents that git keeps internally:

$ cat one.test
This is a test with a EDUIDCONF_TEST setting

To use this POC, we need to force the file to be checked out. Of course, if all this were to be implemented as git commands, this would be automatic:

$ git checkout -b test-branch
$ echo "more text" >> one.test
$ git add one.test
$ git commit -m 'testing settings'

We can now check out our master branch, and see that the name of our config setting has been replaced by its value, but what has been committed is the name, and git status does not see it as an addable change:

$ git checkout -
$ cat one.test
This is a test with a testing setting
$ git diff HEAD~1
diff --git a/one.test b/one.test
new file mode 100644
index 0000000..8f6f4ef
--- /dev/null
+++ b/one.test
@@ -0,0 +1,2 @@
+This is a test with a EDUIDCONF_TEST setting
+
$ git status

If we modify the file again, and we commit the changes, only the new changes are commited, with our local copy showing the value of the setting, but the copy held by git having the name of the setting:

$ echo "and a bit more text" >> one.test
$ git add one.test
$ git commit -m 'testing settings'
$ cat one.test
This is a test with a testing setting
and a bit more text

If we check the last commit, we can see that git is indeed holding the setting name:

$ git diff HEAD~1
diff --git a/one.test b/one.test
index 8f6f4ef..e96f4bf 100644
--- a/one.test
+++ b/one.test
@@ -1,2 +1,3 @@
 This is a test with a EDUIDCONF_TEST setting
+and a bit more text

To customize the settings, we have to use a eduidconf_custom.py module, with the same contents as in eduidconf_defaults.py, but customizing the values therein:

$ cp eduidconf_defaults.py eduidconf_custom.py
$ vim eduidconf_custom.py
$ git checkout -
$ git checkout -
$ cat one.test
This is a test with a testong setting
and a bit more text

Note that testing has become testong.