Volatile Software
The following message came from The Listserv (http://thelistserve.com/):

I want to use my fifteen minutes of fame on The Listserve to rant about
something that's close to my heart: the stability of the software I use.

NOTE: This is written for people who create software. If you don't do that you
probably won't find this very interesting. Sorry! Maybe you could read Text
from Dog if you haven't seen it already? Either way, have a nice
morning/afternoon/evening!

The Situation
-------------

Every time I get a new computer, I go through the same song and dance:

1. Look at what programs and packages I have installed on the old computer.
2. Install these programs on the new computer.
3. Copy over my configuration files from the old computer to the new one.
4. Spend the rest of my day fixing all the things that broke because I'm using
a newer version of program X.

Step 4 is always the most painful part of getting a new machine. Always.

Without fail I spend several hours tweaking configuration files, adjusting my
workflow, and so on because I've upgraded to a new version of foo which doesn't
support option X any more or requires library Y version N+1 now.

Getting a new computer should be a pleasant experience! The unboxing from the
sleek packaging, that "new laptop" smell, the nostalgia of the default desktop
image. Why does this horrible step 4 have to exist and how can we get rid of
it?

The Divide
----------

I've noticed something interesting lately: I can categorize almost all of the
software I use into two distinct groups:

* Software that breaks pretty much every time I update it (e.g. weechat,
offlineimap, Clojure, many Python packages, Skype).
* Software that almost never breaks when I update it (e.g. Mercurial, git,
tmux, Python, ack, zsh, Vim, Dropbox).

Software that falls in between these two extremes is surprisingly rare. There
seems to be a pretty clean divide between the two groups.

This makes me think that there's some special attribute or quality of the
second group (or its authors) which the first one lacks.

Brokenness
----------

I think it's important that I nail down what I mean by "breaks" or "is broken".
I don't necessarily just mean the introduction of "new bugs".

When I say that a program "breaks", I mean:

* When I update from version X to version Y of a program, library, or language...
* Without changing my configuration files, source code, etc...
* The resulting combination doesn't work properly

In effect, I'm saying that "breaking backwards compatibility" means "the program
is broken"!

This may be a strong statement, but I stand by it in most cases.

Backwards compatibility matters! Every time someone makes a backwards
incompatible change in a program or library, they cost the world the following
amount of time:

Number of people Time it takes each person
using that part of X to figure out what changed
the program and how to fix it

Often this can be a significant amount of time!

The Process of Updating
-----------------------

When pointing out a backwards incompatible change to someone, you'll often get
a response similar to this:

"Well, I mentioned that backwards incompatibility in the changelog, so what
the hell, man!"

This is not a satisfactory answer.

When I'm updating a piece of software there's a good chance it's not because I'm
specifically updating that program. I might be:

* Moving to a new computer.
* Running a "$PACKAGE_MANAGER update" command.
* Moving a website to a bigger VPS and reinstalling all the libraries.

In those cases (and many others) I'm not reading the release notes for
a specific program or library. I'm not going to find out about the brokenness
until I try to use the program the next time.

If I'm lucky the program will have a "this feature is now deprecated, read the
docs" error message. That's still a pain, but at least it's less confusing than
just getting a traceback, or worst of all: silently changing the behavior of
a feature.

Progress
--------

I completely understand that when moving backwards to an older version
I should expect problems. The older version hasn't had the benefit of the extra
work done on the new version, so of course it should be less stable.

But when I'm updating to a higher version number the software should be
better and more stable! It has had more work done on it, and I assume no
one is actively trying to make software worse, so why does something that
previously worked no longer work?

We're supposed to be making progress as we move forward. The software has had
more work done on it, why does it not function correctly now when it
functioned correctly before?

Yes, this means developers will need to add extra code to handle old
input/configuration. Yes, this is a pain in the ass, but the entire point of
most software is to save people time by automating things. Again, every
backwards-incompatible change costs the world an amount of time:

Number of people Time it takes each person
using that part of X to figure out what changed
the program and how to fix it

If our goal is to save time then we should not make changes that cost time.
Or at least we should not make such changes lightly.

A Culture of Sadness
--------------------

Proof that this is a real issue can be found in the tools we use every day. As
programmers we've invented elaborate dependency systems to deal with it.

We say "pip install django==1.3" or put "[clojure "1.2"]" in our Leiningen
project.clj files to avoid using the newest versions because they'll break.

Step back and look at this for a second.

What the hell?

What the hell?

We have invented software with features designed to help us use old versions
of other software!

We have written code to avoid using the "latest and greatest" software!

Obviously this is not entirely bad, but the fact that manually specifying
version numbers to avoid running newer code is commonplace, expected, and
a "best practice" horrifies me.

I would love to be able to say something like this in my requirements.txt and
project.clj files:

Of *course* I want the latest version of library X! I want *all* the
newest bug fixes and improvements!

Unfortunately I can't do that right now because so many projects make backwards
incompatible changes all the time.

The moment I try to build the project at some point in the future I'll be sent
on a wild goose chase to figure out what function moved into what other
namespace and what other function was split into its own library and dammit the
documentation on the project's website is autogenerated from the tip of its git
repo and so it doesn't apply to the latest actual version and jesus christ
I think I'll just quit programming and teach dance full time instead even though
I'll go hungry.

The Tradeoff
------------

One could argue that sometimes backwards incompatible changes cost time up front
but save time in the long run by making the software more "elegant" and "lean".

While I'm sure there are cases where this is true, I feel like it's a cop out
most of the time. Allow me to illustrate this with a helpful Venn diagram:

-------
/3333333\
|333333333|
\3333333/
-------

11 -> People who give a shit what a program's codebase
11 looks like.

22 -> The authors of said program.
22

For libraries where the author is the only user, none of this rant applies.
You're free! Break as much as you like!

For the majority of libraries, however, there are probably vastly more "users"
than "authors". Saving a few hours of the authors' own time has to be weighed
against the 10 minutes each that the hundreds of users will have to spend
figuring out what happened and working around it.

I want to be clear: being backwards compatible doesn't mean sacrificing new
features! New features can still be added! Refactoring can still happen!

In most cases keeping backwards compatibility simply means maintaining a bit of
wrapper code to support people using the previous version.

For example: in Python, if we moved the public foo() function to a new module,
we'd put the following line in the original module:

from newmodule import new_foo as foo

Is it pretty? Hell no! But this single line of code will probably save more
people more time than most of the other lines in the project!

This may just be an artifact of how my brain is wired, but I actually get
a sense of satisfaction from writing code that bridges the gap between older
versions and new.

I can almost hear a little voice in my head saying:

"Mwahaha, I'll slip this refactoring past them and they'll never even know
it happened!"

Maybe it's just me, but I think that "glue" code can be clever and beautiful in
its own right.

It may not bring a smile to anyone's face like a shiny new feature, but it
prevents many frowns instead, and preventing a frown makes the world a happier
place just as much as creating a smile!

Exceptions
----------

One case where I feel the backwards incompatibility tradeoff is worth it is
security.

A good example of this is Django's change which made AJAX requests no longer be
exempt from CSRF checks. It was backwards incompatible and I'm sure it broke
some people's projects, but I think it was the right thing to do because it
improved security.

I also think it's unreasonable to expect all software to be perfectly ready from
its first day.

Sometimes software needs to get poked and prodded in the real world before it's
fully baked, and until then requiring strict backwards compatibility will do
more harm than good.

By all means, backwards compatibility should be thrown to the wind in the first
stage of a project's life. At the beginning it needs to find its legs, like
a baby gazelle on the Serengeti. But at some point the project needs to get its
balance, grow up, and start concerning itself with backwards compatibility.

But when should that happen?

A Solution
----------

I think there's a simple, intuitive way to mark the transition of a piece of
software from "volatile" to "stable":

Version 1.0

Before version 1, software can change and evolve rapidly with no regards for
breaking, but once that first number becomes "greater than or equal to 1" it's
time to be a responsible member of the software community and start thinking
about the real humans whose time gets wasted for every breaking change.

This is the approach semantic versioning takes, and I think it's the right one.

I know a lot of people dislike semantic versioning. They hate how requires
incrementing the major version number every time a breaking change is made.

I consider it to be a good thing.

You should pause and carefully consider making a change that will break
people's current code.

You should be ashamed if your project is at version 43.0.0 because you've made
42 breaking changes. That's 43 times you've disregarded your users' time!
That's a bad thing!

As programmers we need to start caring about the people we write software for.

Before making a change that's going to cause other people pain, we should ask
ourselves if it's really worth the cost. Sometimes it is, but many times it's
not, and we can wrap the change up so it doesn't hurt anyone.

So please, before you make that backwards incompatible change, think of the
other human beings who are going to smack their monitors when your software
breaks.

Further Reading
---------------

I'm certainly not the only person to notice this problem. Many smarter people
than me have talked about it. If you want to read more you might want to look
up some or all of the following (Google is your friend):

* The Semantic Versioning spec (the specific numbering details don't matter as
much as the philosophy).
* Anything Matt Mackall has written on the Mercurial mailing list (especially
the mails where he sounds especially grouchy).
* Anything about "software rot" or "code rot".