Thursday, July 7, 2011

Keeping the discipline of not changing Moodle core

We have said in the past that at the OU we made too many changes to core code in our Moodle 1.9 system, and that as we moved to Moodle 2, we would make far fewer. The release of Moodle 2.1 provides a good opportunity to stop and reflect on how we are doing.

Exactly how many core changes we had made in 1.9 seems to depend on who you ask. It was something of the order of one or two thousand depending on how you count. As a result, every time there is a new Moodle 1.9.x release, someone (Derek) has to do a couple of days painstaking merging to upgrade to the new version.

Moodle 2.1 was released on Friday. On Monday afternoon we decided to try upgrading our development branch to it. The merge (literally git merge MOODLE_21_STABLE) only took a few hours, and that was most mostly a matter of thinking before typing git checkout --theirs to resolve most of the conflicts in favour of the Moodle 2.1 code. Then we had to test test install, upgrade, and basic functionality before pushing the merge to our central repository.

But, how many OU-specific changes do we have in core code right now? Well, the answer appears to be eight. Let me explain that number.

To control the core code changes, we use a simple approval procedure. Each change must be proposed by one of the leading developers. They do this by opening a special sort of ticket in our issue tracking system. That serves as a permanent record of the change, and is also a place to log any discussion. The other leading developers then review the proposal. For the change to be approved, at least one other leading developer must endorse it with a +1 vote, and there must not be any -1 votes. Votes are normally accompanied by an explanation of why that developer is voting that way.

After a suitable time for votes, the issue is declared either accepted or rejected. OU-specific changes can be rejected for two reasons.We may decide that it is not acceptable to change core to implement this feature, so we drop the feature; or we think of some devious way to achieve the feature without changing core code.

If a change is approved, then the code is written. Well, in some cases the code will already have been written, because you can have a much more informed debate about whether a certain change is a good idea if you can see exactly what the proposed change is. Once the code is written and approved, it is committed to out git repository and the issue moves into state 'Code committed'. Finally, we may find a way to get rid of the OU-specific change in future. The most common way that happens is if we contribute the change upstream to For example the new Moodle question engine was an ou-specific change as long as we were using it in Moodle 2.0, but now we have upgraded to Moodle 2.1, it is standard code. Therefore, that issue has now changed status to 'No longer required'.

Overall, our we, have 22 ou-specific change issues in our bug tracker. The break-down is:

Rejected: 2
New (under discussion): 4
Approved (but not yet implemented): 1
Code committed: 8
No longer required: 7

Most of the 'Code committed' changes are pretty boring. For example three of them are bug-fixes to the questionnaire module that we have submitted upstream, but which have not been reviewed and accepted by the questionnaire maintainers yet. Therefore, those three will almost certainly end up as 'No longer required' in due course. Another example is that we want to customise the "Database connection failed / It is possible that the database is overloaded or otherwise not running properly" page that you get when Moodle fails to connect to the database. If Moodle can't connect to the database, then it cannot load the configuration, and so cannot determine which theme to use to display the error. Therefore, the only way customise that page is to edit lib/setup.php.

The one 'serious' ou-specific change we have is some hacking around in course/lib.php to support one of our custom modules called 'subpage' (not released yet, but we hope to share it eventually). Given more time, we might be able to find a more elegant way to handle these changes, but we don't have that sort of time at the moment.

While we have controlled the core code changes, we do have written a lot of custom plugins. Those range from big things like forumng and ouwiki, to small things like a local plugin that just implements a single web-service function. I'm afraid I don't have a complete list, but we must have more than 50 plugins by now. As far as I am aware, the upgrade from 2.0 to 2.1 did not break any of them.

1 comment:

  1. Thank you for sharing your experience.

    I also read your superb chapter "Moodle" in "The architecture of Open Source Applications".