[Qgis-developer] Processing algorithms and API breaks

[Nyall Dawson]

On 7 March 2016 at 19:13, Alexander Bruy <alexander.bruy at gmail.com> wrote:

> 1. Postpone such changes for next major QGIS release when API break is
> allowed. In this case all existing scripts/models will work. But we
> should live
> with bugs and without some features, even if fix already available and keep the
> code of this fixes in sync with master until next major release.

I honestly think 1 is the only valid approach here.

I'll preface this opinion by saying that I HATE frozen API with a
passion. It makes development harder, slower and more expensive, and
prevents new features landing and bugs from being fixed [1]. With my
totally selfish developer hat on I'd love nothing more than for us to
adopt a non-frozen, do-whatever-the-heck-you-want API and with no care
for compatibility with old projects ;)

BUT

The decision has been made for the project that QGIS has a frozen API
between major releases, and now that processing is part of the core
QGIS install it also needs to abide by that rule! Yes, it's a freaking
horrible PITA, but the alternative (breaking user's scripts and
models) is far worse for the project. And with my non-developer, QGIS
user hat on I would hate for scripts and models which I've invested
time into creating to get broken between releases.

I'd suggest following the same approach as is taken by the c++ code:
make a versioned copy of the algorithm so that existing scripts/models
can still use the old version BUT it's not exposed anywhere in the GUI
and new models will transparently just use the new version. That's
what we've done for labelling (heck... even the old label engine is
STILL in QGIS on that extremely rare case that someone opens a pre 2.0
project!), symbology, various composer items, etc. It results in
duplicate code and fills the codebase with a lot of deprecated cruft,
but that's the price of a frozen API :)

Just my 2c! ;)
Nyall

1. see https://github.com/qgis/qgis3.0_api/issues for a list of stuff
we CAN'T implement/fix until API is unfrozen