Upgrading from Drupal 6 to Drupal 7: best programmer's practices?

Question

Although I am using drupal since the D4 series, I only started developing professionally for it with D6, so - despite I did various site upgrades - I was never faced by the task of having to port my own code to a new version.

I know the Drupal community will come up with lot of technical support about changed API's and architectural changes (see the deadwood module for D5-D6 or even these stubs of D6-D7 how-to's for modules and themes).

However what I am looking for with my question is more in the line of strategy thinking, or in other words, I am looking for inputs and advice on how to plan / implement / review the process of porting my own code, in the light of what colleague developers learned by previous experience. Some example:

1. Would you advice to begin to port my modules as soon as I have time for doing it, and to maintain a concurrent D7 for some time (so I am "prepared" for the D-day) or would you advice to rather wait for the day in which the port will be actually imminent and then upgrade the modules to D7 and drop the D6 version?
2. Only some of my modules have full test coverage. Would you advice to complete test coverage for the D6 version so to have all tests working to check the D7 port, or would you advice to write my test directing at porting time, to test the D7 version?
3. Did you find that being an early adopter gives you an edge in terms of new features and better API's or did you rather find that is more convenient to delay the conversion so as to leverage the larger amount of readily available contrib modules?
4. Did you set for yourself quality standards / evaluation criteria or did you just set the bar to "if it works, I'm happy"? Why? If you set certain standards or goals, what did they where / what will they be? How did they help you?
5. Are there common pitfalls that you experienced in the past and that you think are applicable to the D6-D7 porting process?
6. Is porting a good moment to do some refactoring or it is just going to make everything more complex to be put back together?
7. ...

These questions are not an exhaustive list, but I hope they give an idea of what kind of information I am looking for. I would rather say: whatever you think is relevant and I did not list above gets a "plus"! :)

If I did not manage to express myself clearly enough, please post a comment with the info you think I should add in the question. Thank you in advance for your time!

PS: Yes I know... D7 is not yet out and it will take months before important contrib modules will be upgraded... but it's never too early to start thinking! :)

Answer 1

Good questions, so let's see:

1. (when to start porting)
   This certainly depends on the complexity of the modules to port. If there are really complex/large ones, it might be useful to start early in order to find tricky spots while not being under pressure. For smaller/standard ones, I'd try to find a bigger time slot later on where I can port many of them in a row in order to get the routine stuff memorized quickly (and benefit from the probably improved documentation).

2. (test coverage)
   Normally I'd say that having a good test coverage before starting refactoring/porting would certainly be advisable. But given that Drupal-7 introduces a major change concerning the testing framework by moving it to core, I'd expect the need to rewrite a significant amount of tests anyway. So if there is no need to maintain the Drupal-6 versions after the migration, I'd save the time/trouble and aim for increased coverage after the porting.

3. (early adopter vs. wait and see)
   Using Drupal since the 4.7 version, we have always waited for at least the first official release of a new major version before even thinking about porting. With Drupal 6, we waited for the views module before porting our first site, and we still have some smaller projects on Drupal-5, as they are working just fine and it would be hard to justify the extra bill for our clients as long as there are still maintenance/security fixes for it. There is just so much time in a day and there is always this backlog of bugs to fix, features to add, etc., so no use playing with unfinished technology while there are more imminent things to do that would immediately benefit our clients. Now this would certainly be different if we'd have to maintain one or more 'official' contributed modules, as offering an early port would be a good thing.
   I'm a bit in a bind here - being an early adopter certainly benefits the community, as someone has to find that bugs before they can get fixed, but on the other hand, it makes little business sense to fight hour after hour with bugs others might have found/fixed if you'd just waited a bit longer. As long as I have to do this for a living, I need to watch my resources, trying to strike an acceptable balance between serving the community and benefiting from it :-/

4. (quality standards)
   "If it works, I'm happy" just doesn't cut it, as I don't want to be happy momentarily only, but tomorrow as well. So one of my quality standards is that I need to be (somewhat) certain that I 'grokked' new concepts well enough in order to not just makes things work, but make them work like they should. Now this is hard to define more precisely, as it is obviously impossible to know if one 'got it' before 'getting it', so it boils down to a gut feeling/distinction of 'yeah, it kinda works' vs. 'yup, that looks right', and one has to accept that he will quite regularly be wrong about this.
   That said, one particular point I'm looking out for is 'intervene as early as possible'. As a beginner, I often tweaked stuff 'after the fact' during the theming stage, while it would have been much easier to apply the 'fix' earlier in the processing chain by means of one hook or the other. So right now, whenever I'm about to 'adjust' something in the theme layer, I deliberately take a small time out to check if this can not be done more cleanly/compatible within a hook earlier on. As I expect Drupal-7 to add even more hooking options, this is something I will pay extra attention to, as it usually reduces conflicts and sudden 'breaking of stuff' when adding new modules.

5. (common pitfalls)
   Well - mainly porting to early, finding out afterwards/in between that one or more needed modules were not available for the new version at all, or only in dev/alpha/early beta state. So I'd make sure to compile a complete list of used/needed modules first, listing their porting state, along with a quick inspection of their issue queues.
   Besides that, I have so far always been very pleased with the new versions and their improvements, and I'm looking forward for Drupal-7 again.

6. (refactoring while porting)
   One could say that porting is a rather large refactoring in itself, so there is no need to add to the complexity by restructuring non porting related stuff. On the other hand, if you already have to shred your modules to pieces anyway, why not use the opportunity to make it a major overhaul? I'd try to draw a line based on complexity - for big/complex modules, I'd do the port as straight as possible, and refactor more later on, if need be. For smaller modules, it shouldn't really matter, as the likelihood of introducing subtle bugs should be rather small.

7. (other stuff)
   ... need to think about it ...

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Ok, other stuff:

• Resource needs - given some of the Drupal-7 threads, it looks like they are likely to go up, so this should be evaluated before porting smaller sites that sit on a shared/restricted hosting account.

• Latest versions first - This one is rather obvious and always stressed in the upgrade guides, but nevertheless worth mentioning: Upgrade core and all modules to their latest current version first before doing a major upgrade, as the upgrade code is highly likely to depend on the latest table/data structures to work correctly. Given Drupals 'piecemeal', one step at a time update strategy, it would be very hard to implement upgrade code that would detect different pre-upgrade states and acted accordingly.