Imperative configs are out; Declarative configs are in

https://www.prodvana.io/blog/declarative-configs

by ayf • 4 years ago

136 89 4 years ago

ayf• 4 years ago

A good overview of why legacy CD systems cause so much pain in cloud-native environments

andybak• 4 years ago

This is an old debate surely? My first awareness of it was probably related to Django's settings.py back around 2007 but I got the feeling from discussions then that it was a discussion that had been bouncing around for a long time before that.

The counter-argument is that declarative configs inevevitably sprout programming-like features - and if they don't someone will write code to generate them.

(disclaimer - in true HN fashion I haven't properly RTFA'd - dinner is nearly ready and I'm feeling bold)

naphatkrit6• 4 years ago

Yep the idea of declarative winning out over imperative is nothing new. Yet for some reason, most deployment systems make you go through an imperative flow if you want to orchestrate releases.

taeric8• 4 years ago

Because "burn it all down and rebuild everything" is by far the easiest orchestration to create from a declarative config. And is almost certainly not what any technician is going to want to happen to their production system.

gtirloni• 4 years ago

The whole article is an ad.

nickcw• 4 years ago

The counter-argument is that declarative configs inevevitably sprout programming-like features - and if they don't someone will write code to generate them.

Ha! See the make manual for examples.

Background reading: https://stackoverflow.com/questions/3480950/are-makefiles-tu...

taeric8• 4 years ago

I.... don't buy the initial example. Google maps is still an imperative list of directions. It just keeps track of the current pointer for you, and is able to reroute. If anything, that is an example that a dynamic action plan is more adaptable than a static one.

That said, declarative clearly has advantages when it can work. You probably still want a "break glass" way to see what the actual steps that will be taken are, but can get surprisingly far without that.

naphatkrit6• 4 years ago

Oh totally that declarative can and will often compile down to imperative. The question is what do you ask the user for? My take is Google maps is declarative in the sense that you ask for your destination, your constraints (e.g. no highway), and time to leave, and it dynamically generates the underlying imperative steps like you said (but continuously adapt if you go off course, which you cannot do unless the initial input was declarative).

taeric8• 4 years ago

That example still feels off. The "offline printout" was done in a very declarative way. It is just fixed on the execution plan. (And, quite frankly, less obnoxious to me if I decide to stop and get gas or food.)

You can even prepare alternative routes ahead of time, if you want to speculate on conditions. Is a good idea to role play some of that ahead of the time, in any case.

0xbadcafebee5• 4 years ago

"My dad switched from working with a map printout without knowing real-time conditions, the imperative flow, to Google Maps with turn-by-turn directions, the declarative flow."

There is no such thing as an imperative or declarative "flow". By its very definition, declarative does not have "flow". It is just a statement. Imperative (in programming) literally means "describing steps that change state". Declarative (in programming) literally means "describing a state which is desired".

Declarative would be "I want a cheeseburger." Imperative would be "Get me a bun, and some lettuce, and tomato, and mayo, and raw meat. Cook the meat on a grill at high heat, flipping half way. Put the mayo on the bottom of the bun, then the meat, then the lettuce, then the tomato, then put on the top of the bun. Give it to me."

It's still strange to me how people learn about "magic words" like declarative and imperative, and then try to ssstttrrreeeeettttttcccccccchhhhhhhhhhh their meaning into some new paradigm that they have just thought up.

There is no such thing as an imperative configuration file. A configuration file describes how a program should be configured. Even when the configuration file "describes a series of steps to change state", it's still declarative, because the configuration file is still declaring to the program how to operate. It's the program that is imperative or declarative, depending on how it interprets and acts on the configuration file. (This is made clearer in programs like Ansible, which, within the exact same configuration file, supports both declarative and imperative statements)

Before you say "what about template files! jinja2! go templates! hcl!", that is merely a DSL, which is no longer a configuration file; it is effectively a program in a crude programming language, interpreted by an interpreter (the program loading the file).

(edit: I agree with the author's point! But I suggest we stop using these terms 'declarative' and 'imperative', and instead say "let's write programs that are functional enough that we don't need to write configuration files that are mini-programs")

sly0102• 4 years ago

Indeed. Reading SICP should be a requirement for starting a YAML company.

phailhaus5• 4 years ago

Even when the configuration file "describes a series of steps to change state", it's still declarative, because the configuration file is still declaring to the program how to operate

Not sure I buy that logic. With that reasoning, all code is declarative because it's "declaring to the interpreter/compiler" how to operate.

I think he's making the same point you did. In one case, the config file is laying out a sequence of steps like you did for how to make a burger (imperative). In the second case, the file just defines the end state (declarative).

Sure, you could say that the first case is declarative because the "end state" being defined is pipeline itself. But the point is that we're talking about how to get a burger, not define how to make one.

naphatkrit6• 4 years ago

yep!

0xbadcafebee5• 4 years ago

With that reasoning, all code is declarative because it's "declaring to the interpreter/compiler" how to operate.

Sort of! All code is also imperative, eventually, at the machine code level. This is a perfect example of how useless the whole "imperative vs declarative" distinction is. Nearly everything in a computer is both imperative and declarative, in some fashion, at some point.

These terms were not made to be some concrete and inviolable paradigm of computing. Some academics just wanted to tell people to write programs where you didn't have to spell out every instruction to the compiler, so they made this crude distinction. Things like a function called give_me_a_temporary_file(), rather than writing out an entire function yourself to create and return a temporary file. But both are executing steps imperatively under the hood. So we shouldn't make a big deal about these terms or programs that do more of one or the other.

The differences that I'm pointing out are 1) declarative does not describe a flow, the flow is under the hood; and 2) configuration files do not actually perform steps, they merely describe arbitrary data, and only the program interpreting the configuration can determine if the result is imperative or declarative. For some programs, the exact same configuration file may result in either imperative or declarative execution.

TOGoS• 4 years ago

Right. I think the important distinction is whether an imperative language is used to describe things that could be done purely declaratively, as is the case with Gradle[1]. I think this happens all too often because the oldschool declarative system has some edge case that it can't handle, so someone reinvents it with a fake DSL which is actually a dialect of an imperative language, and now you have the worst of both worlds. I've always thought that what we needed was a way to allow both declarative and functional information to be provided, but with a clearer separation between the two. e.g. a build configuration language based on this approach might allow one to specify either 'buildArtifactId: "xyz123"' or 'buildArtifactIdExpression: (functional expression to compute a build artifact ID goes here)'.

I sort of tried to do this with the Factorio terrain generation system[2]. The first phase is to run a Lua program, which is imperative, but the result is an immutable object representing the terrain generation configuration, which in turn includes functional expressions, since a map where everything is constant would be boring.

[1] I f&!@#^ing hate Gradle; it is my go-to example of thoughtlessly mashing paradigms together because you can, resulting in something that nobody I have ever met really has really been able to work with except by trial and error.

[2] See https://factorio.com/blog/post/fff-200 and https://togos.github.io/togos-example-noise-programs/

atomicnumber3• 4 years ago

I find discussions like this ("html is/isn't a programming language" etc) kind of tiresome because eventually you end up at "a compiler is a file format converter". You have reached Truth but it ends up not being a terrifically interesting statement and everyone just kind of goes back to whatever they originally meant with whatever they said that started the argument.

lrem• 4 years ago

This is an important concept in my work. From experience, there is a pretty simple litmus test. It is pretty well understood that the word “imperative" denotes a system where the user describes a series of steps. The system executes the steps, in the end result bringing its universe to a desired state. The user does not need to describe what that is to the system, but a proof of correctness would decide whether that has been reached. Now, a declarative system is one where the user describes the desired end state and invariants on the admissible intermediate states of the universe. The system needs to figure out the current state of the universe, find the difference and find an admissible path to eliminate the difference. The litmus test to discern the two types of systems is whether the system concerns itself with the difference as a primary concept.

To give an example familiar to this forum: imagine we’re building a package. This can be set up imperatively with a shell script, or declaratively with GNU Make. Make will figure out which output files are missing or older than their input files, constituting a difference. It will figure out (topological sorting) an admissible path through computing a new up to date file from input files that are already up to date (re their own input files). Now that’s cool and all. But the same end result can be achieved way simpler if you can skip the diffing bit. If you assume you’re doing a clean build, you can order your build steps optimally in a shell script, skipping the now useless complexity. If there has been an abandoned rub that had some successful steps, your shell script will waste their results, but still end in a correct state.

marcosdumay• 4 years ago

With that reasoning, all code is declarative because it's "declaring to the interpreter/compiler" how to operate.

Well, yes. Those words are not as well defined as common usage implies. Their meaning is completely dependent on what you define as your basic operations.

People usually agree on some large spaces of incomplete definitions, so the terms aren't meaningless without context. But never everybody agree, and never most people agree on all the details.

Anyway, configuration files is one context where people have very diverse concepts of basic operations, and so most people almost never agree.

taeric8• 4 years ago

There is no such thing as an imperative configuration file

My Emacs config would like a word with you. :D

thfuran• 4 years ago

There is no such thing as an imperative configuration file.

Clearly you haven't seen the gradle files I have been subjected to.

rising-sky• 4 years ago

I had the same thought when I read that paragraph and anecdotal tortured "lost with map printout" description, immediately thought, not sure this author knows quite what they are talking about, but kept reading to see what the configuration would look like...

ssstttrrreeeeettttttcccccccchhhhhhhhhhh

apt

geuis• 4 years ago

Very good breakdown. Couldn't agree more.

Except that mayo should go on top.

0xbadcafebee5• 4 years ago

I don't disagree, but a lot of burgers end up dry, so the bottom bun stays dry and bland, which makes me sad. Sauce on the bottom solves that, and the top bun gets wet from the tomato. Optimizing for the 80% burger.

tzs• 4 years ago

Alton Brown recommends mayo on the bottom to create a barrier to keep the bun from getting soggy from burger juices.

But forget about the mayo...regardless of where one stands on proper mayo position there is a much bigger issue with the imperative cheeseburger instructions given: there was no cheese!

eurasiantiger• 4 years ago

”There is no such thing as an imperative configuration file.”

My babel.config.js would like to disagree.

WastingMyTime89• 4 years ago

You are missing the forest for the trees by confusing the end result with its implementation.

The idea between having a declarative configuration vs a traditional one is well understood in the context of a machine. In one case, you describe how you want the final machine to look like and let the system decides how to reach that states, in the other you yourself input how the state will be changed step by step to reach the final stage. Sure the declarative configuration really is a DSL and there is an interpreter somewhere turning it into a step by step list of actions but that’s invisible to the end user.

It’s not magic just abstracted. That’s the good old Wheeler aphorism: “ There is no problem in computer science that can't be solved using another level of indirection.”

ClumsyPilot• 4 years ago

It's still strange to me how people learn about "magic words" like declarative and imperative, and then try to ssstttrrreeeeettttttcccccccchhhhhhhhhhh their meaning into some new paradigm that they have just thought up.

Develipera make fun of how project managers call anything Agile without understanding what it means, then go and commit the same sin themselves.

eternityforest• 4 years ago

The common meaning of imperative means you tell a computer something step by step. If a config file does that, it's imperative.

Declarative in practice usually means that an intelligent solver works backwards from a description of a final state

Make a file is imperative. There should be a file here is delarative.

Making a file may fail depending on implementation if it already exists. "This file should exist" likely will not because it will do whatever is needed, including nothing, to ensure constraints are met.

The best configuration file in my opinion, though, is to keep it trivial, and just have one factory config with very few options.

Need to log in and think you need to set a username and password? Linux has that already, lots of apps use the user accounts instead of their own nonsense layer or even directly use SSH as their transport.

Need to enable optional modules? Can you just make them enable and disable themselves on demand?

Network settings? I hope there's at least the option to just discover everything automatically.

1letterunixname• 4 years ago

If felt as hand-waving as reading that "Clean Code" guy.

Before you say "what about template files! jinja2! go templates! hcl!", that is merely a DSL, which is no longer a configuration file; it is effectively a program in a crude programming language, interpreted by an interpreter (the program loading the file).

It will devolve into messy "imperative" code writing "declarative" "configuration"? or whatever using templates, so there's no point in being a purist about The One True Way.

wruza2• 4 years ago

Declarative would be "I want a cheeseburger." Imperative would be "Get me a bun, and some lettuce, and tomato, and mayo, and raw

Is there a reason it couldn’t be “Give me a cheeseburger”? This declarative camp narrative kills me sometimes.

thriftwy• 4 years ago

Most Java configuration files are a series of method calls laid out in XML or YAML. These are clearly imperative, the bun-and-patty imperative.

raxxorraxor• 4 years ago

let's write programs that are functional enough that we don't need to write configuration files that are mini-programs

...or procedural programs while we are at it.

I don't disagree with your argument, but the definition can be extended in this use case. If we assume the "configuration" of the software is the target destination. What you can configure imperatively perhaps is in contrast to that what roads to take or not and I believe the term fits reasonably well without it losing precision.

sly0102• 4 years ago

This just looks like a frontend over kubernetes.

naphatkrit6• 4 years ago

Not exactly? Kubernetes natively doesn’t have a solution for pushing across environments afaik.

moralestapia2• 4 years ago

My dad switched from working with a map printout without knowing real-time conditions, the imperative flow, to Google Maps with turn-by-turn directions, the declarative flow.

LOL, he got it backwards, "turn-by-turn directions" is the imperative flow. Can't expect much after reading that.

phailhaus5• 4 years ago

I was thinking the same thing, but his point is that with Google Maps you simply declare your end goal. It figures out how you should get there based on current conditions.

naphatkrit6• 4 years ago

I guess I should have said "voice-guided" turn-by-turn huh? https://www.forbes.com/sites/anthonykosner/2012/12/13/google... is what i was referring to :)

kazinator• 4 years ago

If a pipeline fails halfway, most CD systems don’t have the ability to retry. The only option is to restart a pipeline from the very beginning. Now with declarative configs, an intelligent delivery system, the system will detect that staging is already on the desired version and skip that push, allowing you to deliver value to your customers faster.

Hey, an interrupted "make" can always restarted from where it left off without ever having to do a "make clean", so what could go wrong.

zmgsabst• 4 years ago

This example is backwards:

Showing a map printout while ignoring runtime state is declarative; executing directions step by step, adapted to the present state of the system, is imperative.

I’ll set aside my feelings about stretching meaning to say — if you are going to use technical words this way, don’t use them backwards.

shkkmo3• 4 years ago

Declarative works better than imperative only when someone has already defined the imperative process that takes you from any given state to your goal and does that for you based on your declaration.

Google maps works great for a set of origins to a set of departures for certain modes of travel. Once you get outside those bounds, it can fail in significant ways.

Thus the entire premise of the article thus seems to boil down to: If someone has already done the work, don't re-do it.

The problem with declarative configuration is that as the systems they manage become more complex, inevitably you leave the bounds of the solved problem and have to start solving it for yourself imperatively.

righttoolforjob• 4 years ago

This is the correct answer.

You can declare only when someone else already has performed the imperative chore, i.e. written the source code, programmed.

Q.E.D.

Jtsummers• 4 years ago

Declarative works better than imperative only when someone has already defined the imperative process that takes you from any given state to your goal and does that for you based on your declaration. [emphasis added]

No, someone (as in a person) doesn't have to define the imperative process, that would be stupid. We have computers to compute things, including computing new processes. Oracle didn't hire a bunch of human query planners to manually construct every query plan, they made a query planner. Google didn't sit down a nation of navigators to plan out every route, they wrote a program (or likely a set of programs) that computes it on the fly.

shkkmo3• 4 years ago

they wrote a program (or likely a set of programs) that computes it on the fly.

Which is precisely the sort of imperative process I am referrencing...

pmarreck• 4 years ago

As a recent NixOS convert, I concur

bastardoperator• 4 years ago

You might end up with the nightmare config they highlighted if the person writing the config has no semblance of DRY, but it's unlikely someone versed in CD systems isn't going to use techniques afforded to them for reducing boilerplate.

tremon5• 4 years ago

The irony is that many declarative systems have very poor support for symbols, templates, and code reusability in general, so declarative configurations carry more boilerplate than imperative configurations.

otikik• 4 years ago

Hey author: your css seems off on mobile. Text reads fine, but code looks tiny in comparison

rgovostes• 4 years ago

As a university student discovering constraint solvers like Z3, I believed that the future of computing was declarative programming, with black box solvers that would obviate the need for specialized algorithms.

Dealing with Webpack cured me of the idea that declarative is strictly superior. Configuring it is just endlessly consulting the documentation, and then the underlying code, and then cargo culting StackOverflow answers to coerce the black box to create the output I want.

thenipper• 4 years ago

There is a ton of interesting work being done with solvers adjacent to machine learning. I just finished a job where I was working with an operations research team solving inventory placement and transportation

btown• 4 years ago

Declarative is great if there is always one path to get to where you're going.

But I like to use the example of a Git repository where you've renamed and also changed some parts of a file. You commit the new state - was it a deletion and a brand new file, or a migration and modification of an existing resource? It's usually a non-issue because Git is smart, and files don't really care about their identity - but what if this was a database server and you need control over what happens when, and how things drain? Do you trust the system to choose the right path vs. "delete the database and make a new one?"

At some point you'll need imperative thinking to make sure your bases are covered, even if that is "we have a 3 phase rollout plan for different versions of the declarative config." And that's perfectly fine, in many cases. But it's important to never get into a cargo-cult level of "if it can't be done with one declarative change it shouldn't be done" - because then you'll either get stuck or light things on fire.

alajmo• 4 years ago

The config has now more than doubled in line count, making it impossible to remember the original intent of the configuration.

Not really, I found it quite self-explanatory.

8jy89hui• 4 years ago

Complicated static configs are out; Scripts as configs (like Neovim) are in.

Both of these examples seem like they would be better fit with Lua-based configs. Scripts as configs give incredible amounts of power to the user without the programmer having to specify everything the user might want to do.

lamontcg• 4 years ago

If they pull in dependencies that can make them very non-portable, while that is typically where all their power comes from over normal config files.

rcarr• 4 years ago

My two cents on the declarative vs imperative debate:

Yes all code is eventually imperative if you go down low enough. But your code should be as declarative as possible for your user. For example, the business department write a ticket for me to build a new feature. They essentially declare some functionality. I then take the imperative steps to code that on the front or back end. To do this, I will likely need to use some tooling. As far as the tooling is concerned, I am now the user/business department, I declare what I want in the tooling and the tooling should make it for me. The developers of the tooling then write imperative code to handle that. Maybe they rely on tooling to do so, in which case they become the user and so on. The cycle repeats itself recursively until it reaches the machine code at the bottom.

You could even take it further if you like in that the hardware engineers behind the processors are now the developers carrying out the imperative functionality who in turn carry out their job using electronic measuring tools which makes them users etc.

Basically, everything should be declarative by default for the intended user of the application unless they have specifically requested finer grained control. The user has enough to deal with getting their particular task done, they shouldn’t have to navigate between layers building the tools that they need to get their job done. Unless he’s working on something unique that requires it, the tradesman doesn’t want to go to the DIY store and give them instructions to build a drill, he just wants to buy a drill that meets his requirements so he can get on with his job.

est• 4 years ago

Ahh yes, the old stuff is new again. Ansible.

mbrodersen• 4 years ago

Declarative: Specify the result you want. Imperative: Specify (step by step) how to get the result you want.

Examples:

Declarative: I want a PC with the following code installed: X, Y and Z. Imperative: First upgrade X, then install Y, then uninstall W, then install Y and Z.

Imperative configs are out; Declarative configs are in

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