Why OTP? Why “pure” and not “raw” Erlang?

I’ve been working on my little instructional project for the last few days and today finally got around to putting a very minimal, but working, chat system into the ErlMUD “scaffolding” code. (The commit with original comment is here. Note the date. By the time this post makes its way into Google things will probably be a lot different.)

I commented the commit on GitHub, but felt it was significant enough to reproduce here (lightly edited and linked). The state of the “raw Erlang” ErlMUD codebase as of this commit is significant because it clearly demonstrates the need for many Erlang community conventions, and even more significantly why OTP was written in the first place. Not only does it demonstrate the need for them, the non-trivial nature of the problem being handled has incidentally given rise to some very clear patterns which are already recognizable as proto-OTP usage patterns (without the important detail of having written any behaviors just yet). Here is the commit comment:

Originally chanman had been written to monitor, but not link or trap exits of channel processes [example]. At first glance this appears acceptable, after all the chanman doesn’t have any need to restart channels since they are supposed to die when they hit zero participants, and upon death the participant count winds up being zero.

But this assumes that the chanman itself will never die. This is always a faulty assumption. As a user it might be mildly inconvenient to suddenly be kicked from all channels, but it isn’t unusual for chat services to hiccup and it is easy to re-join whatever died. Resource exhaustion and an inconsistent channel registry is worse. If orphaned channels are left lying about the output of \list can never match reality, and identically named ones can be created in ways that don’t make sense. Even a trivial chat service with a tiny codebase like this can wind up with system partitions and inconsistent states (oh no!).

All channels crashing with the chanman might suck a little, but letting the server get to a corrupted state is unrecoverable without a restart. That requires taking the game and everything else down with it just because the chat service had a hiccup. This is totally unacceptable. Here we have one of the most important examples of why supervision trees matter: they create a direct chain of command, and enforce a no-orphan policy by annihilation. Notice that I have been writing “managers” not “supervisors” so far. This is to force me to (re)discover the utility of separating the concepts of process supervision and resource management (they are not the same thing, as we will see later).

Now that most of the “scaffolding” bits have been written in raw Erlang it is a good time to sit back and check out just how much repetitive code has been popping up all over the place. The repetitions aren’t resulting from some mandatory framework or environment boilerplate — I’m deliberately making an effort to write really “low level” Erlang, so low that there are no system or framework imposed patterns — they are resulting from the basic, natural fact that service workers form constellations of similarly defined processes and supervision trees provide one of the only known ways to guarantee fallback to a known state throughout the entire system without resorting to global restarts.

Another very important thing to notice is how inconsistent my off-the-cuff implementation of several of these patterns has been. Sometimes a loop has a single State variable that wraps the state of a service, sometimes bits are split out, sometimes it was one way to begin with and switched a few commits ago (especially once the argument list grew long enough to annoy me when typing). Some code_change/N functions have flipped back and forth along with this, and that required hand tweaking code that really could have been easier had every loop accepted a single wrapped State (or at least some standard structure that didn’t change every time I added something to the main loop without messing with code_change). Some places I start with a monitor and wind up with a link or vice versa, etc.

While the proper selection of OTP elements is more an art than a science in many cases, having commonly used components of a known utility already grouped together avoids the need for all this dancing about in code to figure out just what I want to do. I suppose the most damning point about all this is that none of the code I’ve been flip-flopping on has been essential to the actual problem I’m trying to solve. I didn’t set out to write a bunch of monitor or link or registry management code. The only message handler I care about is the one that sends a chat message to the right people. Very little of my code has been about solving that particular problem, and instead I consumed a few hours thinking through how I want the system to support itself, and spent very little time actually dealing with the problem I wanted to treat. Of course, writing this sort of thing without the help of any external libraries in any other language or environment I can think of would have been much more difficult, but the commit history today is a very strong case for making an effort to extract the common patterns used and isolate them from the actual problem solving bits.

The final thing to note is something I commented on a few commits ago, which is just how confusing tracing message passage can be when not using module interface functions. The send and receive locations are distant in the code, so checking for where things are sent from and where they are going to is a bit of a trick in the more complex cases (and fortunately none of this has been particularly complex, or I probably would have needed to write interface functions just to get anything done). One of the best things about using interface functions is the ability to glance at them for type information while working on other modules, use tools like Dialyzer (which we won’t get into we get into “pure Erlang” in v0.2), and easily grep or let Emacs or an IDE find calling sites for you. This is nearly impossible with pure ad hoc messaging. Ad hoc messaging is fine when writing a stub or two to test a concept, but anything beyond that starts getting very hard to keep track of, because the locations significant to the message protocol are both scattered about the code (seemingly at random) and can’t be defined by any typing tools.

I think this code proves three things:

  • Raw Erlang is amazingly quick for hacking things together that are more difficult to get right in other languages, even when writing the “robust” bits and scaffolding without the assistance of external libraries or applications. I wrote a robust chat system this afternoon that can be “hot” updated, from scratch, all by hand, with no framework code — that’s sort of amazing. But doing it sucked more than it needed to since I deliberately avoided adhering to most coding standards, but it was still possible and relatively quick. I wouldn’t want to have to maintain this two months from now, though — and that’s the real sticking point if you want to write production code.
  • Code convention recommendations from folks like Joe Armstrong (who actually does a good bit of by-hand, pure Erlang server writing — but is usually rather specific about how he does it), and standard set utilities like OTP exists for an obvious reason. Just look at the mess I’ve created!
  • Deployment clearly requires a better solution than this. We won’t touch on this issue for a while yet, but seriously, how in the hell would you automate deployment of a scattering of files like this?

The economy of good intentions

This is a story about four people: Everybody, Somebody, Anybody and Nobody.
There was an important job to be done and Everybody was asked to do it.
Everybody was sure that Somebody would do it.
Anybody could have done it, but Nobody did.
Somebody got angry because it was Everybody’s job.
Everybody knew that Anybody could do it, but Nobody realised that Somebody wouldn’t do it.
And Everybody blamed Somebody when Nobody did what Anybody could have done.

Finally working on an intermediate Erlang instructional

After contemplating the state of the tiny constellation of Erlang resources available, both in-depth instructionals and books, I’ve finally decided that what is most lacking is an intermediate resource. So I’ve started working on one. The goal is to demonstrate a non-trivial system (a MUD, in this case) as it would be written in raw, beginner-style Erlang and show how a project can evolve to a for-real OTP system by providing complete project examples and code commentary throughout the intermediate stages of evolution.

Hopefully this will force me to cover many of the giant, undocumented gaps Erlangers tend to encounter when trying to make the transition from Erlang beginner to OTP master, and I’ll undoubtedly learn a lot in the process myself. That last bit about self education is, of course, my own selfish motive for doing this. I have yet to find an aggregate body of Erlang best practices or collection of “common things people do by hand in Erlang and the idiomatic way to handle them in OTP”, and I really wish there was such a thing. I especially wish there were such a thing based on a single, non-trivial project instead of toy examples so that learners could get a grasp on why OTP matters by reading a real project instead of trying to assemble a mental model of the grander sculpture by examining bits of dust cast off by the chisel.

I chose a MUD as an example for ease of understanding, non-triviality (too many example systems are unrealistically small), freedom from graphical distractions, and the striking number of parallels between many MUD subsystems and real-world business, game, and social software. Hopefully work and real life don’t interfere to the point that I have to stop this mid-way through!

An Observation on Economic Motivation

When we want to discourage people from smoking we levy taxes on cigarettes. When we want to discourage people from drinking we levy taxes on alcohol. When we want to discourage the purchase of a product from a particular country we levy taxes on their imports.

What should we interpret as the motivation behind taxes on businesses? Businesses of a particular size? Income? Income of a particular size?

None of this has a happy ending.

Speaking of police states, whatever happened to StackExchange?

StackOverflow, StackExchange, etc. and its whole community rather saddens me these days. It seems the assburgers who dump their angst on Wikipedia have found a new home making StackExchange sites as unfun as possible. The situation isn’t helped at all by the recent rabbit-like proliferation of SE sites which exhibit so much subject overlap as to make determining the “proper” place of most posts nearly impossible — all that has done is given the Post Police reason, in every case, to force re-posting, closing, pausing, deletion, or otherwise official vandalism of valid posts and answers.

Let’s say I have a question about travelling to Austria to see historical sites. Let’s put it in travel. But if the question is phrased in such a way as to reference a specific historical event, like say a little battle in which Polish king demonstrated how much he appreciated the Ottoman visitors, then it is an almost certain thing that some twerp who knows nothing about history or travelling in Austria will promptly vote to close or move the question on the grounds that it belongs on the SE site for history and not travel. The post will face the same fate once it arrives there, of course, and the asker will be helpless.

This sort of thing goes on all the time between StackOverflow, “Programmers”, and “Workplace” as many questions professional programmers have cut sharply across all three lines.

Another annoyance is unnecessary Post Police comments like the one left on this post. Sure, I was being silly in the way I did it, but my response represents the general consensus of the Erlang community on this particular subject and is, in essence, a fairly standard response. Obviously not good enough for 19-year-old Alexy Schmalko, Protector of the Realm.

Whatever happened to the sense of community I used to get from, you know, the community? Did that all die when usenet got retarded in the early 90’s? Did it evaporate with the coming of the cool web kids? I’ve probably got kids somewhere near his age…

The Lightweight Nature of Erlang Processes

Understanding the difference between Erlang processes and OS processes can be a bit confusing at first, partly because the term “process” means something different in each case, and partly because the semantics of programming terms have become polluted by marketing, political and religious wars. A post to the Erlang questions mailing list asking why Erlang processes are so fast and OS processes are so slow reminded me of this today.

Erlang processes are more similar to the “objects” found in most OOP languages than the “processes” managed by an OS kernel, but have a proper message passing semantics added on in a way that abstracts the OS network, pipe and socket mechanisms. We wouldn’t be surprised if the Python runtime handled its objects with less overhead than the OS kernel handles a process, of course, and it should come as no surprise that the Erlang runtime handles its processes with less overhead than the OS kernel. After all, a Python “object” and an Erlang “process” are very nearly the same thing underneath.

Most OOP runtimes implement “objects” as a special syntactical form of a higher order function, one that forms a closure around its state, includes pointers to methods as a part of that state (usually with their own special syntax that abstracts the difference between a label, a pointer and a variable) and returns a dispatch function which manages access to its internal methods. Once you get down to assembly, this is the only way things work anyhow (and on von Neuman architectures there is exactly zero difference between pointers to data, pointers to data, instructions and pointers to a next instruction). If you strip that special syntax away there is no practical difference between directly writing a higher order function that does this and using the special class definition syntax.

Even in a higher language the higher-order functional nature of an “object”s class definition can be illustrated. For example, the following Python class and function definitions are equivalent.

class Point():
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

    def set_x(self, x):
        self.x = x

    def set_y(self, y):
        self.y = y

    def get_x(self):
        return self.x

    def get_y(self):
        return self.y


def gen_point(x=0, y=0):
    coords = {"x": x, "y": y}

    def set_x(x):
        coords["x"] = x

    def set_y(y):
        coords["y"] = y

    def get_x():
        return coords["x"]

    def get_y():
        return coords["y"]

    def dispatch(message, value=0):
        if message == "set x":
            set_x(value)
        elif message == "set y":
            set_y(value)
        elif message == "get x":
            return get_x()
        elif message == "get y":
            return get_y()
        else:
            return "Bad message"

    return dispatch

We would be utterly unsurprised that both the class definition and the function definition return entities that are lighter weight than OS processes. This is not so far from being the difference between Erlang processes and OS processes.

Of course, the above code is ridiculous to do in Python either way. The whole point of the language is to let you avoid dealing with this exact sort of code. Also, Python has certain scoping rules which are designed to minimize the confusion surrounding variable masking in dynamic languages — and the use of a dictionary to hold the (X, Y) state is a hack to get around this. (A more complete example that uses explicit returns and reassignment is available here.)

For a more direct example, consider how this can be done in Guile/Scheme:

(define (point x y)
  (define (setter coord value)
    (cond ((eq? coord 'x) (set! x value))
          ((eq? coord 'y) (set! y value))))
  (define (getter coord)
    (cond ((eq? coord 'x) x)
          ((eq? coord 'y) y)))
  (define (dispatch m)
    (cond ((eq? m 'set) setter)
          ((eq? m 'get) getter)
          (else (error "point: Unknown request"))))
  dispatch)

OOP packages for Lisps wrap this technique in a way that abstracts away the boilerplate and makes it less messy, but its the same idea. This can be done in assembler or C directly as well. Equivalent examples are a bit longer, so you’ll have to take my word for it. (A commented version of the Guile example above can be found here.)

While OOP languages typically focus on access to state and access to methods as state, Erlang focuses like a laser on the idea of message passing. Easy, universal access to state in OOP languages makes it natural to do things like share state, usually by doing something innocent like declaring a name in an internal scope that points to an independent object from somewhere outside.

Erlang forbids this, and forces all data to either be a part of a the definitions that describe the process (things declared in functions or their arguments), or go through messages. Combined with recursive returns and assignment in a fresh scope (akin to the last Python example in the extra code file) this means state is effectively mutable and side effects can occur without violating single assignment, but that everything that changes must change in an explicit way.

This restriction comes at the cost of requiring a sophisticated routing and filtering system. Erlang has an unusually complete message concept, going far beyond the “signals and slots” style found in some of the more interesting OOP systems. In fact, Erlang goes so far with the idea that it abstracts message, filters, a process scheduler and the entire network layer with it. And hence we have a very safe environment for concurrent processing — using “processes” that certainly feel like OS type processes, but are actually named locations Erlang’s runtime keeps track of in the same way an OOP runtime does objects, functions and other declared thingies. They feel like OS processes because of the way Erlang handles access to them in the same way that Java objects feel like my mother-in-law’s purse because of the way the JVM handles access to them — but underneath they are much more alike each other than either are to OS processes.

In the end, all this stuff is just long lines of bits standing in memory. The special thing is the rules we invent for ourselves that tell us how to interpret those bits. Within those rules we have various ways of declaring our semantics, but in the end the lines of bits don’t care if you think of them as “objects”, as “processes”, as “closures”, as “structs with pointers to code and data” or as “lists of lists with their own embedded processing rules”. OSes have particularly heavy sets of rules regarding how bits are accessed and moved around. Runtimes tend not to. Erlang “processes” are of a kind with Python “objects”, so we shouldn’t be surprised that they are significantly lighter weight than the “processes” found in the OS.

Republication of GNU’s Guile 2.0 Manual

I’ve republished the “one page per node” html version of the GNU Guile 2.0 Manual here. I’ve been referencing the manual quite a bit lately and noticed that, at least from my location, the site responds quite slowly at times. So I’m mirroring the manual where I know its fast and I can always find it.

I hope that the reason the GNU servers are responding slowly is legitimate load and not something more sinister. Attacks on community sites are one of the more stupid forms of tragedy that these commons experience.

On the Meaning of “Carbon Neutral”

I noticed that a few products in my house have begun to proudly proclaim themselves as being “carbon neutral” over the last few months. Apparently this is among the new set of empty phrases marketing people feel are necessary to distinguish their otherwise ordinary commodity products from identical products of comparable quality. It used to be “Made in U.S.A.” or “日本製” (depending on the neighborhood), then it was “low sodium”, then “waterproof”, then “low fat” then “low transfat” then “cholesterol free” then “omega-3″ then something else I probably forgot.

The problem isn’t that any of these things aren’t potentially good selling points, its that they usually don’t apply to the things I see the labels on. For example, I remember seeing an electric wok that said “Made in U.S.A.” on the bottom. I’m not so sure that’s the best thing to concern one’s self with when buying a cooking apparatus that originated in another hemisphere. That’s like buying a tuna steak because the sticker on the package marks it as being “a peanut-free product” or believing that a piece of software is high quality because its written in Java (or even more uselessly, “utilizes Java technology!”).

This reminds me of my sister’s enlightening tale of the truth behind the now heavily regulated terms “organic” and “all natural” as applied to food labels. She did her undergraduate study in genetics and graduate work in nutrition, worked in colon cancer research for a while, started a dietary medicine program at a hospital in Virginia a few years back, and now (after reaching some disillusionment with the state of government-funded research) raises “range fed Angus beef” as a side interest. She is therefore directly governed by some of the more hilarious regulations the FDA has come up with.

Needless to say, her opinion on the value of these buzzwords has much more influence to me than whatever a “medicinal cannabis expert” has to tell me about the benefits of toking up or the local yoga girl at the gym has to tell me about the benefits of yogurt shakes or almond oil or peanut-butter enemas or whatever it happens to be this week (of course, she’s just right about the benefits of sex in exciting places). In short, the regulations governing terms such as “organic” and “natural flavor” (or even the way the term “X% fat free” is permitted to be used) are both economically draining legally apply due to the administrative overhead of regulatory compliance and yet so full of loopholes that there is really no clear distinction between a head of lettuce that is “organic” and one that isn’t so labeled. Essentially the only difference is the price of the market package.

Of course, the real difference is that the lettuce sporting an “organic” sticker on it is almost undoubtedly produced by a large agribusiness firm that can afford the overhead of doing all the pencil-drills necessary to proclaim their lettuce to be “organic”. Either that, or it is quite pricey lettuce only rich folks who feel the need to spend more to sate their moral thirst can afford, grown at an “organic” farm run by one savvy businessman and a flock of altruist peons bent on saving humanity from itself one vegetable at a time. I’m certainly not saying that large agribusiness is bad — ultimately its the only way we’re going to survive over the long-term (and here I’m including post colonization of space) — but that the terms used on packaging are enormously deceptive in nature.

But that’s food. It is a specific example where it is relatively easy to trace both the regulatory documentation and the research literature. Of course, very few people actually track all that down — other than unusual people like my sister who happen to be trained in genetics, directly involved in agriculture, and so habituated to both scientific and regulatory research that they find nothing daunting about navigating the semantic labyrinth the FDA has let agricultural regulation become in the US (and the phrase “let…become” could easily be replaced with “deliberately made of…”). I suppose the problem isn’t that few people track all that down, really; its more a problem that even if my sister were to go to the trouble of explaining what she knows to the average consumer they wouldn’t have the faintest clue what she was getting at. The average consumer is instead faced with an essentially religious (or at least dogmatic) choice of whether to trust someone that has a stack of official paper backing up her credibility, or a government agency and a huge food industry which are both populated by thousands of people who each have every bit as much officious documentation backing up their reputations.

And that brings me back to “carbon neutral”. We still chase the purported value of demonstrably empty terms such as “cloud computing”, demonstrably failed vehicles such as “social networking”, and demonstrably flimsy labels such as “organic” and “all natural”. But we don’t stop there. We are jumping head-first onto the “carbon neutral” bandwagon as well. The point isn’t that we shouldn’t be concerned with the terrestrial environment, but rather that we must at all times guard against political forces that constantly seek to invent new social mores and foist them on us by conjuring meaning into empty phrases like “carbon neutral”. It tricks you not just into buying ordinary thing A over ordinary-but-cheaper-thing B, but also into feeling morally superior. In this it is indistinguishable from other dogmatic rhetoric that engenders an unfounded sense of moral certainty. If we thought convincing people that a man in the sky doesn’t want them to fly airplanes into office buildings was hard, consider how much more difficult it is to convince average people who genuinely want to “do good” that reasonablish sciency words are nothing more than unfounded political siren songs trying to open one more door for the tax man.

So back to the reasonablish sciency phrase “carbon neutral”… what does it mean? This is where I have some semantic issues, mainly because nobody really knows.

Let’s say, for example, that we start a paper mill. We’ll make paper, but only from recycled paper and only using wind energy. This could probably qualify as being entirely “carbon neutral”. But so could the same paper mill if it planted its own trees. But what about the wind generators? They have to come from somewhere. What about the diesel-powered trucks that carry the old paper stock to the recycling mill? What about the initial material itself? Are we being carbon neutral if we don’t go replace as many trees as our recycled stock represents? How about the electricity used by the paper-compactors run by other companies we have no control over? What about our employees’ cars they use to get to work? What about all the flatulence the invite by eating pure vegan meals?

The initial production itself would almost certainly not qualify as being “carbon neutral” — which demonstrates that we have to draw an arbitrary line somewhere from which we can derive a meaning for the term “carbon neutral”. It is almost certain that something, whether directly or indirectly, involved an increase in carbon emissions (and the meaning of “direct” and “indirect” really should be their own battlegrounds here, considering what people think the term “carbon neutral” means) somewhere at some point, otherwise there wouldn’t be people to buy our recycled earth-friendly paper to begin with.

But what are “carbon emissions”? This is, apparently, intended to only refer to releasing carbon into the air. Consider for a moment how monumentally arbitrary that is. There are currently some well-intended, but enormously misguided efforts to “sequester” carbon by burying it in the crust of the Earth. This, of course, represents an enormously heavy emission of carbon into the environment, but we are calling this a “good” emission (actually, we refrain from using the word “emission” because we intend that to be a “bad” word) because it is going into the ground and not the air. Incidentally, it is also not going into something useful like diamond-edge tools or nano insulators or any other beneficial process that is desperate for carbon (which our planet happens to be poor in by cosmological standards).

So where did all this “bad” carbon come from? If you believe the press, its coming from our SUV exhaust, coal-burning plants, Lady GaGa (well, she might be a Democrat, in which case she can’t be bad), and pretty much anything else that humans use to modify local order at the expense of a probable increase in universal entropy.

Where did the carbon come from for the coal, crude, natural gas and bovine flatulence? Probably from the atmosphere and the sea. At least that’s what a biologist will tell you.

And here is a problem for me. Nobody has explained (not just to my satisfaction, but explained at all) where all the billions of tons of carbon necessary to create the forests that created the coal (and possibly crude oil) came from in the first place.

Well, that’s not quite true. In the first place it came from a long-dead stellar formation, some crumbs of which clumped together to form our solar system. That’s the first place. So the second place. Where did the carbon for all this organic activity come from in the second place? Was it distributed evenly in the early Earth? Has it always been a fixed quantity in the atmosphere? Does it boil out of the molten terrestrial substrate and gradually accumulate in the atmosphere and ocean?

If the forests grew in the first place then the carbon was in the air, at least at one point. If it is a fixed amount of atmospheric carbon then the growth of the forests and their subsequent demise and burial beneath sediment represents an absolutely massive sequestration of atmospheric carbon. If it is indeed a fixed amount, then the absolutely huge amounts of flora and fauna represented by these forests were not prevented from thriving in an atmosphere which contained a huge amount more carbon than the atmosphere contains today. If that is true, then either climate change is not affected much by the carbon content of the atmosphere, or a changed climate does not pose much of a threat to organic life on Earth.

Some parts of the fixed atmospheric quantity scenario don’t really add up. Despite a bit of effort I’ve only managed to scratch the surface of the ice core research literature, but a static amount of available atmospheric carbon doesn’t seem to be the story research efforts so far tell. This area of research has been made amazingly difficult to get a straight tack on ever since environmental sciences got overheated with politically-driven grants looking for results that validate political rhetoric instead of grants seeking research into what is still largely an observational field, but it seems fairly clear that there have been fluctuations in atmospheric carbon content that do not directly coincide with either the timing of ice-ages or the timing of mass terrestrial forestation. (The record is much less clear with regard to life in the ocean — and this could obviously be a key element, but it doesn’t seem that many people are looking there, perhaps because the current rhetoric is full of fear of rising sea levels, not full of hope for a marine component to the puzzle of eternal human salvation). That said, there must be some pretty massive non-human sources of atmospheric carbon which have been in operation millions of years before we evolved (as for where trillions of tons of carbon may have gone, I think the huge coal formations may be an indication).

While the idea that a carbon-rich atmosphere providing adequate conditions for thriving terrestrial life might seem odd (at least when compared with the “Your SUV is killing the Earth!” dialog), the idea that the Earth itself has both mechanisms to gradually ratchet up the total amount of carbon in the atmosphere over the eons and to drastically change the climate in spans measured in mere years (not decades, not centuries or millenia) without human or even atmospheric input is pretty scary.

A lot more scary than the idea that driving my kids to school might be damaging in some small way.

But this isn’t the way we are thinking. We are letting marketers and politicians — two groups infamous for being as destructively self-serving as possible — sell us a new buzzword stew, and we, the consumers, are ready to confidently parrot phrases such as “carbon neutral” about as if they mean something. “Oh, Irene, that salad dressing is 100% organic and carbon neutral — you’re such a gourmet!”

We’re clearly having the wool pulled over our eyes, except this time it doesn’t just play to our ego-maniacal craving to live forever (“If you eat gluten-free yogurt and drink positive-ion water you’ll live forever — and have huge tits/a thicker penis/ungraying hair/a tiny waist!”), it engenders a dangerous sense of moral superiority (“I’m doing this for the planet/global socialism/God/The Great Leader!”) which tends to eliminate all possibility of rational thought on a subject which could indeed affect us all.

What if, for example, the Atlantic currents are just panting their last, barely keeping us away from a global mass cooling event? We won’t just be blind to the threat because we’ve blown our research money on politically driven quests to generate the academic support necessary to pursue whatever new pork-barrel projects we come up with over the next decade or two — we will deny the very idea that a threat other than carbon emissions could possibly exist on moral grounds because we’ve already identified the “real enemy” (wealthy people in SUVs who come with the added benefit of being fun to hate). That’s dangerous.

Words mean things. We should remember that.

Using the alternatives command

A quick demonstration of using alternatives:

[root@taco ~]# cat /opt/foo/this
#! /bin/bash
echo "I am this."
[root@taco ~]# cat /opt/foo/that
#! /bin/bash
echo "I am that."
[root@taco ~]# cat /opt/foo/somethingelse 
#! /bin/bash
echo "I am somethingelse."
[root@taco ~]# which thisorthat
/usr/bin/which: no thisorthat in (/usr/lib64/qt-3.3/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin)
[root@taco ~]# alternatives --install /usr/bin/thisorthat thisorthat /opt/foo/this 1
[root@taco ~]# alternatives --install /usr/bin/thisorthat thisorthat /opt/foo/that 2
[root@taco ~]# which thisorthat
/usr/bin/thisorthat
[root@taco ~]# ls -l /usr/bin/thisorthat 
lrwxrwxrwx. 1 root root 28 Mar 23 10:38 /usr/bin/thisorthat -> /etc/alternatives/thisorthat
[root@taco ~]# ls -l /etc/alternatives/thisorthat 
lrwxrwxrwx. 1 root root 13 Mar 23 10:38 /etc/alternatives/thisorthat -> /opt/foo/that
[root@taco ~]# thisorthat
I am that.
[root@taco ~]# alternatives --set thisorthat /opt/foo/that 
[root@taco ~]# thisorthat
I am that.
[root@taco ~]# alternatives --set thisorthat /opt/foo/somethingelse 
/opt/foo/somethingelse has not been configured as an alternative for thisorthat
[root@taco ~]# alternatives --display thisorthat
thisorthat - status is manual.
 link currently points to /opt/foo/that
/opt/foo/this - priority 1
/opt/foo/that - priority 2
Current `best' version is /opt/foo/that.

Once upon a time, in a time long forgotten, the alternatives command on Fedora-descended systems (RHEL, Scientific Linux, CentOS, etc.) seemed a magical thing. It permitted two versions of the same utility to exist on a system at the same time by automatically switching links from a canonical path to the actual versioned path when necessary. Very cool. This was so cool, in fact, that other system utilities came to rely on it, namely, the post-install section on quite a few RPMs calls alternatives to set up parallel versions of things like language runtimes.

The downside to this automation is use of alternatives itself seems to have become a lost art (or possibly its that the popularity of this distro family has simply diluted the knowledge pool as the user ranks have swelled with newcomers who simply take the automation for granted).

It should be noted that alternatives is a system-wide command, so when root sets an alternative, it affects everyone’s view of the system. It should be further noted that this problem has other, but very similar, solutions on other systems like Gentoo and Debian. Gentoo’s eselect system is a bit more sophisticated and can manage families of alternatives at once (links to a number of disparate language utilities which have to change in arbitrary ways based on which underlying runtime is selected, for example). Fortunately its not very difficult to write a wrapper for alternatives that can provide a similar experience, but unfortunately I don’t have the time this morning to get into that here.