Solid Principles: A Software Developer’s Framework to Robust, Maintainable Code

https://khalilstemmler.com/articles/solid-principles/solid-typescript/

by stemmlerjs • 7 years ago

300 167 7 years ago

maximus19833• 7 years ago

The author uses the more complicated definitions of the SOLID principles.

These are much simpler to remember:

* Single responsibility principle - A class should have only a single responsibility, that is, only changes to one part of the software's specification should be able to affect the specification of the class.

* Open–closed principle - Software entities should be open for extension, but closed for modification.

* Liskov substitution principle - Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.

* Interface segregation principle - Many client-specific interfaces are better than one general-purpose interface.

* Dependency inversion principle - One should depend upon abstractions, not concretions.

A lot of these principles also tie into other principles such as DRY (Do not repeat yourself).

CodiePetersen2• 7 years ago

That is the cleanest clearest collection of solid definitions I have seen.

hasseio• 7 years ago

They come directly from Robert Martin's 2000 paper, 'Design Principles and Design Patterns'[1]

[1] https://web.archive.org/web/20150906155800/http://www.object...

hideo• 7 years ago

My 2c to teams attempting to write SOLID code: Please make sure that the entire team that touches the code base understands what makes it SOLID-compliant (Don't like the term compliant, but it'll have to do). SOLID is more a framework for communication between engineers than anything else.

For example, if one engineer thinks class X has a single responsibility, but the name can be potentially interpreted differently, then unless she's on the job and reviewing every commit the principle will soon get violated. However if the team is open to having a healthy debate and iterating until they mostly agree, then SOLID raises the team's understanding of their code and THUS the quality of the codebase.

cgrealy5• 7 years ago

Some constructive criticism:

    abstract class Employee {
    // This needs to be implemented
    abstract calculatePay (): number;
    // This needs to be implemented
    abstract reportHours (): number;
    // let's assume THIS is going to be the 
    // same algorithm for each employee- it can
    // be shared here.
    protected save (): Promise<any> {
        // common save algorithm
    }
    }

    class HR extends Employee {
    calculatePay (): number {
        // implement own algorithm
    }
    reportHours (): number {
        // implement own algorithm
    }
    }

    class Accounting extends Employee {
    calculatePay (): number {
        // implement own algorithm
    }
    reportHours (): number {
        // implement own algorithm
    }

    }

    class IT extends Employee {
    ...
    }

This is still a violation of SRP. IT does not care about the employees pay and shouldn't be forced to implement it.

Modern OO adds an additional rule to SOLID: favour composition over inheritance.

HR and IT should not extend from Employee. HR should be more like

    class HR {
    calculatePay (Employee e): number {
        // implement own algorithm
    }
    reportHours (Employee e): number {
        // implement own algorithm
    }
    }

The first question you should ask when deciding to use inheritance is: do I really need inheritance here? Then, ask yourself again: Is <childClass> a <parentClass>?

In most businesses, HR is a Function or maybe a Department. HR might have Employees but HR itself is not an Employee.

jpitz• 7 years ago

This is a big part of Liskov substitutability.

Supermancho3• 7 years ago

Every employee needs to be paid, even if that's 0. The original class was correct (a class is a namespace, then functionally dependent on language implementation).

The implementation of calculatePay, is extensible as it stands.

    public Number calculatePay (Interface department)

You delegate to the class (even if the interface is optional)

    Number val = defaultPayscale();
    if(department) {
        val = department.getPay();
    }
    return val;

Because EVERYONE expects Employee to have a calculatePay of some sort, for accounting software. The idea that you want to smear a concept across domains for a misguided utopian design is over-engineering. Programmers have an average intelligence, so aim for supporting a simpler mentality, regardless of your beliefs.

cgrealy5• 7 years ago

My point is that IT doesn't (and shouldn't) need to know about employee pay. Extending the Employee class to HR, Accounting and IT is a bad design.

You're violating ISP here. The Employee class can implement IThingThatGetsPaid, but IThingThatHasNetworkAccess should know nothing about pay.

The Employee itself should have the minimal amount of information it needs. Prefer interacting with an object through a small API surface. It's not "smearing a concept across domains", it's the exact opposite. It's confining a concept to the appropriate domain.

The original design pollutes the Employee interface with multiple concerns.

flukus• 7 years ago

I don't know where you work but I generally find that employees do not calculate their own pay, apart from maybe reporting hours they tend to not be involved in the transaction at all, HR does the work and deposits it to the employees bank account.

If we subscribe to the idea that OO should model the real world (I don't) then the HR class is definitely where the work should be done.

jillesvangurp• 7 years ago

I would augment that to "ALWAYS favor composition over inheritance"

IMHO Solid is nice but I always struggle to remember what the acronym stands for. I mentally translate this to just optimizing two easy to remember metrics: classes should have low coupling and high cohesiveness. This also applies to functions and not just classes. E.g. functions with lots of parameters are a problem.

UK-AL• 7 years ago

Some of these code examples are not great.

Using inheritance when it's really not needed.

maxxxxx• 7 years ago

This looks to me like the usual textbook examples of inheritance that usually fall apart quickly when they get in touch with reality.

risaacs99• 7 years ago

The longer I've worked in software development, the less I'm convinced that any of this matters, at all.

What has mattered more to me is choosing the right architecture, technologies and dependencies. Understanding the fundamentals, especially at least a vague understanding of what the computer is doing with the code I am writing. Basic understanding of algorithms and techniques like state machines.

I've grown almost disdainful of things like SOLID principles, after taking them more seriously in the past.

ctvo• 7 years ago

I agree, mostly.

The more experience I gain, the less I’m concerned about refactorable, local issues. Distributed systems is hard. Scaling is hard. Data consistency is hard. Security is hard. API design is hard. Refactoring a local code base to extract out methods and implement interfaces, much less so.

These days if the biggest problem we have is a monolithic service is badly coded (but is correct and works), it’s something I’ll take.

Swizec3• 7 years ago

Seconding this. I used to really care about code maintainability, cleanliness, and other topics super important to us engineers.

4 years in The Valley have quickly dissuaded me of such foolish naive notions. Your code (outside some common sense basics) is completely irrelevant. Whatever you can stick together with duct tape and chewing gum is almost always the best solution. All the business wants is speed. By the time "bugs" is the highest reason for churn ... well you're likely going to be on your 5th employer by the time your first employer reaches that point.

Absolutely nobody in the real world cares about the quality of your code. They just want to do their job with as little interaction with your software as possible and move on.

It's kinda sad really how much time we waste talking about the best way to hold a hammer, the best hammer you can use, the best nails etc. when all our customers want is the picture to be up on the wall already.

Edit: One important addition -> If you're senior and you see juniors writing "bad code" on your team, your job isn't to teach them why it's bad. Your job is to create systems that make good code easier and more obvious to write than bad code. Be the force multiplier.

Silhouette• 7 years ago

The customer does generally prefer it if the picture doesn't fall off the wall the moment you leave, though.

cgrealy5• 7 years ago

Not everyone works in "The Valley".

The vast majority of software is not written with that amount of churn.

The end user doesn't care about the quality of the code, but you can be damn sure they care about the quality of your software.

Also, talking about hammers is overly reductive. Software is far more complex than hitting something with a hammer. Builders might not talk about the best way to hold a hammer, but they constantly talk about the best way to build a house.

sanderjd2• 7 years ago

This seems related to the problem of how we have no idea how to hire people. I don't want to hire or work with people who are on to their fifth job by the time their crappy work starts causing major problems for the company that paid them to make it. But asking little whiteboard coding questions gives me zero signal to determine that.

avip• 7 years ago

It has null to do with "The Valley". A young startup should probably take velocity and "just ship" over any other consideration. But new startups are not a very big or important part of the IT industry.

collyw3• 7 years ago

Have you ever worked on the same project for more than a year?

That's the sort of timescale where you notice quick hacky solutions come back to haunt you later. Sure you will deliver things quickly at first, but when you do have to modify things, or fix your own bugs it becomes a nightmare.

GordonS4• 7 years ago

What a terrible attitude! You're happy to write shit code because it'll be someone else's problem to deal with by that point?!

I can understand not being dogmatic about SOLID, design patterns and the like, but doing any old shit because you can get away with it is pretty disgraceful IMO. That kind of behaviour certainly wouldn't fly outside of "The Valley".

dudul• 7 years ago

Even using the "right" architecture or tech stack doesn't matter that much. All business problems can be solved with any language, with a monolith or a fleet of microservices, it doesn't matter.

What matters is to ship code that brings a solution to the client's problem.

ljm• 7 years ago

I think I agree too. Maybe not disdainful, but certainly skeptical.

I put things like SOLID and design patterns in the same bucket as scalable agile, and similar prescriptive practices. To me, they seem more concerned with organisation as opposed to exposing real business problems so that you can solve them, and as a result you're wrestling with the abstraction and the glue code more than the business logic.

A finite state machine is my favourite example, as I've seen all kinds of 'well-organised' setups that could have been massively simplified as an FSM.

Whatever the right architecture and tech is, I've become more convinced over time that the easier it is to delete bits of your code, the better. And that doesn't always mean you go full-on OOP. More to the point, 'maintainable' is massively subjective. Many of the metrics we use to measure that are utterly shallow.

kibibu• 7 years ago

I wonder if this is because you used them, learnt from them, internalized whatever design intuitions you could from them, and have moved onto higher levels of understanding.

Expert bike riders disdain using training wheels, but only after they've learned how to ride using them.

---

(please hn don't derail with "well actually"s about training wheels)

keithnz• 7 years ago

I was active in the XP mailing group when things like SOLID got popularized, mainly by consultants. Of course the principles had been around for much longer in various forms. To me, there seemed to be a distinct difference between what some of the consultants focused on, and the books they ended up writing compared to what people who were using XP techniques and trying to leverage (determine) the best software design approaches to write actual production software. Some of the book writers / consultants were much better at actually observing (or having direct experience of) real issues and how to deal with them. But actual software design did not center around things like SOLID, those things sort of just fell out of approaching design in a particular way. I find the language too strong ( laws / principle ). Instead as you find ways to construct modular and composable software, you'll notice that things tend to have a single responsibility, you'll notice that "modules" tend to become stable and extendable through other "modules" of software. They are like side effects of good design, not things to focus on other than if a design is not really working well, it might provide an insight into why its suffering.

Which is all a polite way of saying it is mostly crap and the insight of people with real problems and who dealt with those problems is mostly ignored.

projectileboy• 7 years ago

I find myself somewhat reluctantly agreeing. While I think the SOLID principles and similar things are helpful tools for thinking, I think they should in general take a back seat to the simple principle of Less Code. I’ve seen so many over-complicated code nightmares that were created in the name of “separation of concerns”, or some such, that my spine tingles anytime someone starts talking about some abstract design principles.

lowercased4• 7 years ago

were created in the name of “separation of concerns”

when I hear "separation of concerns", I now immediately ask (internally or externally) "do we need to be concerned about this at all? right now or even years in the future?"

Recent project: untangling something that was written 2 years ago as "best enterprise engineering practices". That dude is gone, no one else on the team understands it because it's insanely over-engineered. This is literally "send a notification email", and there's about 38 core files, another 25 'helper' files, ~20 templates and a custom template parser/nested template thing, and a few dozen node/npm dependencies. This is a 'microservice' running on lambda. literally, someone needs to be able to send a notification that "X is ready", and it's this behemoth that no one understands. They do understand that all the templates and logic were taken away from each caller's context - so all the various teams that used to be in control of their own notification messaging now have to wait for someone to dig in to this other codebase which is basically unmaintained now.

It's got its own custom date parser, because... why not? And there have been random "wrong dates" being sent out for months because... no one can quite trace down the root problem. I finally did, and I'm not saying "no one ever can", but it's under no one's direct ownership, and the people that have tried to take it over have just been overwhelmed with trying to trace through it, make it testable, etc.

And last week I heard rumblings that another team is "rebuilding notifications".

It's as much a failure of management/oversight as it is "overengineering" in general, but at each review step (so I was told), things "looked good", in isolation. No one quite pieced together that this was an unwieldy behemoth until after this guy was gone. :/ (no doubt an extremely typical story, but one which, by now, I would have hoped would be 'less typical').

barbs• 7 years ago

I agree to some extent. I think in general they're good principles, but I think people tend to take them as gospel and forget about code readability and complexity in order to adhere to them. It's important to understand the trade-offs involved, and to understand that different problems will require different solutions.

foxylad• 7 years ago

After thirty years of software development, the one principle that keeps recurring is Donald Knuth's "Premature optimization is the root of all evil".

It applies on so many levels, including whether to go all in on any software engineering methodology. For instance, applying SOLID principles to your personal side project is counter-productive; but if it grows into a multi-developer project you might well gain efficiencies by rationalising your code structure.

zmmmmm3• 7 years ago

Yep. As another old-timer, pretty much the only principles I have seen that consistently pay off are KISS and YAGNI. And the problem is nearly all of this SOLID type stuff is directly contrary to that.

13rac1• 7 years ago

I've found it important to include the context of that quote:

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil. Yet we should not pass up our opportunities in that critical 3%.

That 3% can be significant.

Jupe3• 7 years ago

So glad to see this stated publicly, and actually getting some positive responses. 25+ years of software development and the best "pattern" I can identify after dozens of projects is just one: simplicity.

If you can't follow the logic by just reading the code, then maintenance will be slow and cumbersome. New development will eventually taper off as more and more time is spent fixing issues. I've written production systems, still in operation today decades later, that hold true to the simplicity principle. I wouldn't do it any other way. But, the battle I frequently face is with upstart devs who want to use some new pattern or principle because they read that it was "better" than straight-up code. And yes, they can implement some new feature quick, but 2 years down the road all hell breaks loose.

Some of my pet-peeve anti-patterns:

* "Plugin" systems only used by the primary development team. There's power in being able to follow the logic for any call stack by just reading the code; plugins make the flow much more challenging, and provide little value.

* Large (and complicated) run-time configuration IOC systems which make any debug session a dreadful process.

* Rules Engines. 'nuff said.

* Piles of abstractions, one atop the other, because teams had no time to find commonalities and/or sensible use-cases.

* 25+ new source code files to respond to a /heartbeat endpoint.

* Fixing a problem in 3 lines of code is not fixing anything if it complicates the debug process, forces a re-write of the unit test drivers, enables mysterious build failures or taxes the DevOps team.

And some of my favorite simplifying patterns:

* AOP for logging; but NOT (dear god) for business logic.

* 3 levels of abstraction, and no more. If you can't figure out a way to do it in 3 levels of abstraction, then something else is wrong.

* 3 systems of state: config (global), current request and persistence (database). If you need more than that, then something is wrong with the architecture. (Ok, compilers may not be able to get away with this, but most applications can, in my experience.)

* If you use a pattern (MVC, for example) apply it to the whole system. E.g. doing IOC only in the upstream API system when the rest of the code doesn't, just makes a mess.

jlj3• 7 years ago

Rules engines come up frequently in my domain area. What are some good resources for managing either high volume or high complexity business rules?

Usually these start out as hard-coding, then evolve into a rules engine or framework. Sometimes after the original devs leave, the rules engine turns into a shiny black box that remaining devs are afraid to touch.

What are some anti-anti-patterns?

_57jb• 7 years ago

Had a knee jerk response to this around SOLID really advocating for simplicity in a somewhat rigorous definition.

Completely agree with your statements, a good set of guidelines.

A challenge we face is that there will always be those who either don't know what they don't know so will continue to reinvent the wheel, or those that don't feel at ease unless they are doing something clever and difficult to maintain.

I have seen many talented folks from both sides but they are ultimately a cancer to any product.

Forcing them to maintain their mess has helped but most times it is pawned off.

Transparency, traceability, ownership...that I believe is a path of least resistance that could improve things and lead us towards simplicity.

arvinsim• 7 years ago

So glad to see this stated publicly, and actually getting some positive responses. 25+ years of software development and the best "pattern" I can identify after dozens of projects is just one: simplicity.

Yup. So much so that if it takes me several reads to understand code, it's a code smell for me.

hinkley6• 7 years ago

Anyone writing a library should be forced to single step through it in a debugger in front of a group of people and listen to all the grumbling.

The thing is if you have twenty devs all making their code as absolutely simple as possible, the overall system is still gonna be pretty complicated. That’ll be challenging enough. Don’t add your own accidental complexity on. Even if you can fit it in your brain, you’ll be “misunderstood” and “have to” fix things for others all the time instead of working on what you want to work on.

adrianratnapala2• 7 years ago

* AOP for logging; ...

Can you expand on how this works in practice. Or link to an explanation? (I assume AOP means "Aspect Oriented Programming").

collyw3• 7 years ago

I can identify after dozens of projects is just one: simplicity.

Isn't that the point in SOLID though? To try and have some guiding principles for achieving simplicity. Certainly I have noticed that my own code hasn't been easy to understand a few months after writing it, one of the reasons used to be that I was trying to do too many things in one function / method.

Of course having said that, everything is usually a trade off and overemphasizing things will probably be counterproductive.

The other problem with ideas like SOLID, is that they tend to be so abstract that its difficult for them to be meaningful until you have enough experience to not really need them as guidelines.

kstenerud• 7 years ago

SOLID is useful as a rule-of-thumb, but there's always one over-arching principle that should trump all others: YAGNI.

It's easy to build something simple (and perhaps naive) and make it more complex over time as your needs become selectively more complicated and your grasp of the domain more nuanced, but it's impossible to start complex and pare down to a more simple implementation. Even worse, your bad design decisions due to your less-than-complete grasp of the domain become impossible to remove due to all the layers.

There is no One Right Answer (tm) to system architecture; it comes with experience. All of these principles should be considered guidelines to help in your design, not ironclad rules.

verinus• 7 years ago

This so much!

simplicity is underrated and unfortunately esp. young devs are proud when they are capable to build complex systems.

After working extensively with plugin architectures like OSGi an learning about functional programming I am deeply skeptical when somebody mentions plugin systems and IoC containers.

ozim• 7 years ago

I found IOC mostly usefull in my projects. Mostly problem is that when business is saying "we need to have this thing generic", dev team goes ape-shit like public WhateverGoesInAnythingCanComeOut List<T>(List<T> love), where "generic" for buisness people is "copy-shit-paste" and change 2 things.

JamesBarney• 7 years ago

Yeah

write code that's easily understood write code where things are where they're expected to be write code that can be adjusted and extended quickly write code that can be adjusted and extended quickly without producing bugs write code that allows for implementations to be swapped out (think swapping out Email APIs, ORMs or web server frameworks)

The problem is to further these goals they introduce abstraction and complexity which make the code harder to change, harder to reason about, harder to test. And a lot of times you end up taking one step forward but two steps back.

namelosw2• 7 years ago

I too disdainful SOLID. I appreciate this finally has been pointed out. Principles are in fact moral high grounds, without telling you what to do.

For example, you need to resolve the expression problem[0] in Java, with Open-Closed principle compliant. Then the answer could be Java code like in this article[1] (the technique is Object Algebra).

However the moral for me is, if I'm using Haskell or Ruby, when expressing Open-Closed idea is simple, I would use it. If I'm using Java, just forget the principle. I would just express the business, and change the code if I need to add new types or operations. Because the principle is written the code is not understandable to other people, just like design pattern abused codebases.

Another thing is Liskov Substitution Principle. I found most of the books cover this idea don't even mention how to judge if an object is LSP compliant. Every people I know talk about the principle haven't heard of covariant and contravariant, and didn't realize almost all of the mutable data type inheritance are in fact not LSP compliant.

[0] https://en.wikipedia.org/wiki/Expression_problem [1] https://koerbitz.me/posts/Solving-the-Expression-Problem-in-...

goto11• 7 years ago

The most important rule is understanding the context of any design guideline, rule-of-thumb, "pattern" or "best practice".

Something like the Open/Closed-principle is a really great principle for the kinds of projects if was created for. It is just that for 90% of software development it is not appropriate and therefore leads to needless complexity.

Discussing whether SOLID is good or bad in itself does not make any sense IMHO.

chungleong• 7 years ago

Programming paradigms are like diet plans from weight-loss companies: most of the time they're just the original problem repackaged as a solution--with a snazzy name and aspirational slogans. The difficulty of the endeavor basically get hidden in the contrapositive. "Eating meals that we provide" sounds easy. Not eating what you want to eat--that's the hard part. Writing code that meet certain standards is easy. Not writing code that does not is incredibly hard when have hard requirements and delivery dates.

maximus19833• 7 years ago

Totally disagree and I am rather dismayed by people conflating their bad experiences with people over-engineering with these principles.

They are like anything else a good they are good rules of thumb when scaffolding your projects.

Understanding IoC with DI has cleaned up my code soo much and I know it works as I can Unit Test easily.

dirkg2• 7 years ago

Agreed. Adherence to dogma like this is cancer.

i was rejected rom a pretty big company after clearing other stages because I wasn't able to recite SOLID. Its like asking people if they know 'design patterns' and expecting them to come up with the textbook answer from the GoF book.

Telling people that you need to use OOP or DI or any fixed software methodology is just stupid.

None of this crap is used in real companies, the good ones anyway. You should be flexible, open, and strive for maintainable, consistent, and simple design.

revskill6• 7 years ago

Code is not that matter. It's the database design. The understanding of domain that reflects in your database design.

Another way speaking, given me a well designed SQL database, i could produce a working software in one day, with bad code even.

Most of broken code is because of wrongly designed database, not the code itself.

whycombinater• 7 years ago

I second this. I seventy this. The bottom layer, designed inconsistently, will percolate through until some wrapper code renormalizes any inconsistencies. Layers and layers of junk built on junk.

BeetleB• 7 years ago

Not to take away from your comment, but you do realize a huge amount of SW doesn't have any SQL database?

revskill6• 7 years ago

SQL is just one example of Database.

My comment is generic about storage implementation though. The same story applies to NoSQL or any other kinds of database as well.

Supermancho3• 7 years ago

To summarize Linus (sigh)...programming is about data structures. Regardless of where they or the data comes from, data transforms are what we PRIMARILY do.

barbecue_sauce2• 7 years ago

For green field projects, specifically for commercial products, I've always felt you should design the application interface first (wireframes, use cases, dashboards, etc.), then design the ERD of a database that will support that design, then do everything in between. Everything else is just moving data around.

revskill6• 7 years ago

My experience is absolutely opposite.

Often, coders receive a design of Database from leaders, with SQL store procedures.

Only then they will follow the mock-up and then start to code.

dsego2• 7 years ago

Haha, good luck, usually the project is "agile" and experimenting and pivoting and "we'll know it when we see it" and only 7% of people are actually clicking a button so let's redesign those screens and so on.

mrkeen2• 7 years ago

Absolutely not!

The point of Dependency Inversion is that your business code doesn't know it's talking to a database.

revskill6• 7 years ago

Even with Rails, i query by writing directly SQL code, instead of using ActiveRecord.

Code is clean, no more layered abstraction for no purpose.

collyw3• 7 years ago

“Bad programmers worry about the code. Good programmers worry about data structures and their relationships.”

Apparently a quote from Linus, but I think it holds true for almost every level of the stack.

redact2072• 7 years ago

These principles are the first thing thrown out the window for new apps. I've been developing for around 15 years and I've noticed the excitement of new devs on new projects who'll rush out and just throw something together quickly to solve the immediate problem. I've done this a lot too.

This gets you a long way very quickly, which is proof that none of these principles really matter. Development continues along this path at a cracking pace for 6 months until complexity spikes and your productivity is at a crossroads. Do you take a step back, refactor, restructure so the code can be built on with any degree of velocity?

Hell no! Everyone's expecting the same output as when you started, so now you have to start copy/pasting everywhere trying to keep up, but your bug tickets have started ticking in so you have to address those. But you don't have tests, so now each bug fix raises two more bug tickets. Uh oh.

Better get more devs. Now you have people who don't understand the system making changes. There's no real structure so they just squeeze code in where they can, but progress is slow because it takes them five times the amount of time to reason about what the system does and bugs are still going out. Must be bad developers. Better hire a QA team to stop all that.

QA is reporting defects with each release candidate, and puts it back in the dev queue. After a year each feature is taking months to go out. The business just wants some simple changes, why is that so hard?

Enter the hotshot dev. They whip up a quick MVP with the latest language/framework plus it's serverless! No SOLID principles but that doesn't matter because the latest technology will fix everything. Better fire the old team and start fresh.

alexvaut• 7 years ago

Agreed 100%, I worked with legacy code at different stages: 1 to 2 years: very little of structure but still velocity matters a lot, complexity is not very high; 2 to 5 years: first customers. copy/paste designs and complexity arise; 5 to 10 years: I observed 2 different trends, software where refactoring happened, product is still not in a very good shape but it is still ok and the others, the garbage product with millions of lines: global variables, If/then/else copy/paste design, doesn't scale, broken everytime for no obvious reasons. It's going to take years to stabilise without too much churn. So everytime I'm working on a project, I pledge managers, devs to allocate time to refactor and follow those patterns, implement unit tests etc... everytime at whatever stage, after sometime the team realizes the benefit. So yes, without a doubt, good design patterns are keys to make our dev lifes as easy as possible. And the SOLID ones are definitely my favourites.

p0nce3• 7 years ago

Thanks for saying this, it's especially strange to see so much people disparage SOLID as if they have outgrown the concept because it was invented some years ago.

Traubenfuchs• 7 years ago

You have missed an important step: Once the productivity goes down because complexity spikes, the original developers flee the company, if their cv allows them.

They are then replaced by new hires that got fed lies at their job interviews. The managers had to lie to get new developers for their shitty codebase.

That's where development time and costs double while quality is bottom of the barrel.

p0nce3• 7 years ago

Surely we can do without the SOLID principles this time, right? right?

Izmaki• 7 years ago

I believe you got the D wrong. Dependency Inversion is not just about slapping an interface in between two components. It's about inverting the dependency. Let me explain:

Picture that we have two components, a main component "Master" and a subcomponent "Slave". What you seem to be saying is that we should have an interface between the two so that Master uses an "ISlave" interface that the Slave maintains (the developers of the subcomponent) instead of using Slave directly. Dependency Inversion however dictates that you should invert this dependency so that Master dictates which functionality Slave should implement. That is, Master should have an ISlave interface (or an "IDelegation" or similar new name) that Slave (!) is required to fulfil. There is a small but very important difference between the two directions of the dependency arrow. One of the main points is that with Dependency Inversion it is the main component that is in charge of functionality instead of the main component having to work with the functionality exposed by the subcomponent.

s1883• 7 years ago

Nice. That's a really useful way of looking it it. I'll definitely commit that to memory.

CodiePetersen2• 7 years ago

I must confess I am really bad at sticking to this. Sometimes I find myself writing a clunky class or function that is doing too many things and I say to my self "You're a dumb ass you are going to have to rewrite this." I just need to practice holding my feet to the fire and doing it. There are some things that I do regularly as habit, but occasionally I find myself in golden hour mode ignoring all the good practices. But I suppose as long as the refactor/rewrite follows good practice its probably not too much of an issue. Although my one complaint is the very unhelpful acronym at times.

zmmmmm3• 7 years ago

I have to confess, I have never been able to figure out what the Open Closed principle means. I've never seen an explanation that made sense. Extension vs modification? What's the difference? Is it just a very confusing way of trying to express the idea of encapsulation?

I guess I am plagued by the knowledge that I have virtually never been able to predict accurately what "extensions" will want to be done to my code in advance. So much so that I consider attempting to do it now a fairly pernicious form of over-engineering.

namelosw2• 7 years ago

About the Extension:

Think about the concept of '+', you can '+' integers, rational numbers, matrix, etc. There possibly thousands of kinds of things that could be '+'-ed. (Actually, this concept is called monoid)

Your job is to implement the abstract idea of '+' (the operator itself without specific meaning) and the integer implementation for '+'.

Then you can compile publish your package to a jar, dll or any other form of package other people can download. Everyone, possibly thousands of people can download this package and extend and use the exact same '+' from you, for their own datatype without modifying your code or recompiling your package.

At last, 99 out of 100 this could be over-engineering for most of the languages like Java. But for language like Haskell have a feature named type classes, make this almost no-brainer, so it would become 5 out 10 this could be over-engineering in Haskell because the cost and mental overhead are reduced.

charlieflowers• 7 years ago

Essentially, it means that you can extend something by writing _new_ code, in a _new_ file, rather than modifying some code.

For example, a case statement cannot be modified without adding a new case. But something that finds all objects which implement IFoo and uses them can be modified by implementing a new implementer of IFoo. (That's not the only and probably not the best example, but it is an example).

A lot of people don't realize that most (maybe all) of these principles originated on C++ code bases way back, where changing one file in one tiny way could cause massive number of files to need recompiling -- which at the time was very painful due to slow compilers.

They have some good applicability, but I think they're massively over-sold.

zmmmmm3• 7 years ago

Thanks. That makes "open for extension" clear, but it still leaves me wondering about "closed for modification". It is certainly a nice aspiration that code doesn't need to be modified to customize it if one can achieve that. But failing that, code should be as modifiable as we can possibly make it. Making it "closed for modification" sounds like a bad thing, like it will be very hard to maintain.

mannberg• 7 years ago

One alternative to learning these principles for solving problems that arise when you wrap all your logic in objects, is to actually not wrap all your logic in objects.

s1883• 7 years ago

Amen

jondubois• 7 years ago

For OOP, I aim for:

- Separation of concerns between classes/modules. The responsibilities of a class should be easy to explain to a non-developer. E.g. Ideally , every class/module abstraction should be an entity discovered in the specification (or easily inferred from it) and not invented from the developer's own mind.

- Be very careful about how you name things. Try to stick to the spec/domain terminology.

hyperpallium• 7 years ago

A program is a theory.

A theory can be evaluated: firstly, does it describe/predict the data (i.e. does it work)? Secondly, is it parsimonious?

But the key issue with a program-as-theory is that you must understand the theory to understand the program. Because of this, it may be much more effective to shoehorn a problem into an ill-fitting theory, if that theory is already well-known and well-understood.

A new, unfamiliar and strange theory must be REAL GOOD - and that improvement must also be valuable in the applied context - for it to be worth using. If so, it becomes a new standard theory.

SergeAx5• 7 years ago

I am surprised by a number of people here with a lot of relevant experience and opinions like "to hell principles and patterns, just make your software simple". Most of the software just cannot be made simple. I'll go further and say: you cannot make life with simple software, because there are a lot of players around who will take your simple software and make it into more profitable complex software.

SOLID is essentially a way to make complex software simple to understand. Yes, you have to write 5 classes to introduce new type of persisting entity, but they are simple classes, mostly boilerplate. Complexity is not in number of LOC or files, it's in business processes, their interdependencies, "when this then that, except those" and so on.

With SOLID (coupled with DDD) your hiring process becomes quite simple: you check basic knowledge of language, performance tricks and caveats (like n+1 problem), understanding of principles with examples and voila, you have new team member. On the first day you will show him or her your domain dictionary, give access to git and ticket system, and you will get good enough code to fix bugs or introduce features in a matter of days. And it doesn't matter if your code base is 10 years old, if you followed those principles yourself for all that time.

redact2072• 7 years ago

I'm a DDD convert, but boy do I still have trouble selling it. "why do we need a ubiquitous language?" "why can't I jam everything into a single domain?" "why can't everything be an aggregate root?" "It's too complicated, why can't I just PATCH my data directly into the database from API?"

Honestly for me it's these abstract principles and practices that outweigh any flavour of the month technology. DDD encourages code to model the business domain, meaning the complexity should equal that of the business. A lot of problems I see with apps are due to reducing complex domains into CRUD ops, then trying to build the complexity on top as an afterthought.

Also, a DDD helper package for node - https://github.com/node-ts/ddd

dep_b2• 7 years ago

I really can't write code other than a) experimental (when testing a new library that encompasses a totally new concept) or b) absolutely readable and SOLID. a) code never makes it to production before it became b) since when I understand the problem and my solution I can't leave dead or unreadable code.

I've seen people that write nice code by day check in horrible entangled vomit because they wanted to make a deadline. So the next time I pull it, "it has just a few issues" and it takes me two days to sort out their mess where just writing it cleanly myself might have cost me only one day.

My code just looks the same no matter what pressure. I just don't believe I go faster by making code harder to understand and maintain.

SamuelAdams• 7 years ago

I've seen people that write nice code by day check in horrible entangled vomit because they wanted to make a deadline. So the next time I pull it, "it has just a few issues" and it takes me two days to sort out their mess where just writing it cleanly myself might have cost me only one day.

I've checked in "bad" code before. Maybe I can shed some light on the other side of this.

The business needed a feature a week ago, because all their marketing material had a hard date on the go-live date for this feature. They could not miss that launch date.

Unfortunately the dev team was overloaded and they couldn't deliver all the features on time. So I, a consultant, was tasked with a few last-minute things.

Fortunately this new feature was very similar to an existing feature. It used a lot of the same code as something already in the system. So I copied that into a new class (C#), made some (very minor) adjustments, and checked it in.

We were able to launch the product on time. But now there was quite a lot of code duplication.

Yes, I could have just "written it cleanly myself". But this was a tremendously complex product. We had unit tests, integration tests, and more. But we still had to perform hours of manual testing on any code that changed prior to a release. Refactoring the old code would have introduced more testing requirements, which slows down the entire release cycle - the risk was too great for our timeline.

It's important to consider the total cost of your work, not just the amount of time a certain task takes you, individually.

It's also worth noting, we made a new Jira ticket about the code duplication right away, and we were able to fix the code duplication in the next sprint, after the product release. The next iteration, 1.0.1, had the "correct / ideal" code.

benschulz• 7 years ago

you cannot make life with simple software, because there are a lot of players around who will take your simple software and make it into more profitable complex software.

I believe you are conflating simplicity and value. Someone may take one's solution and add value, and with it complexity inherent to that value, thereby increasing revenue. Adding (incidental) complexity, without adding value, will make your solution less profitable.

edit: formatting

SergeAx5• 7 years ago

Maybe you are right, but I honestly don't see how one can add value without adding complexity. Take any real life example: pizza vs plain bread, car vs bike, piano vs harp. There are still demand and markets for simple things, but it has much less profit margin vs complex goods.

danesparza• 7 years ago

I can't tell if you're being sarcastic or not.

There is irony in that if you're not being sarcastic.

SergeAx5• 7 years ago

Sorry if my text brings mixed message to you; alas, English is not my native language. No, I wasn't being sarcastic at all, I truly believe in what I wrote and I'm practicing it on a daily basis in my code and during code review.

mannykannot2• 7 years ago

This is how SOLID, or rather its most zealous proponents, go from reasonable but limited capabilities to claims that cannot be sustained.

While small classes and methods can be (though are not necessarily) at least superficially easy to understand, this does not guarantee that the system as a whole is understandable. This is because a lot of the complexity is in how the classes interact with one another. There are contracts and constraints that have to be observed, that are inherently about more than one thing, and so cannot be localized in one method.

This approach, applied to writing, would be to allow only kernel sentences using common words. This is not going to be enough to make a complex topic simple to understand, and, if applied zealously, is likely to have the opposite effect.

Unnecessarily complex code is not a consequence of not following simple rules; it is usually a consequence of trying to write simple code but not understanding all the issues. There is nothing about SOLID that guarantees that unnecessary complexity will be avoided. Your idea of turning programming into something like painting-by-the-numbers will not work.

SergeAx5• 7 years ago

In SOLID there is S for "single responsibility", which in turn gives us simplicity on a component (class/object/file, depends on language) level but lead to complexity due to number of components. 100 tasks gives us 100 components, then software grows to 1000 and even 10000 tasks.

But then comes letter D for dependency inversion. This principle gives us back simplicity due to abstracting away any set of those components as meta-components like repositories, queues, services, factories and so on. This is about integfaces, e.g. contracts, as you call them.

My zealotry comes from about 30 years of writing and SUPPORTING code. SOLID code is harder to write, but much more easier to maintain. All those war stories about "business needs that feature week ago" comes from unmaintainable code, when you just cannot add new features by any other means except spaghetti with epoxy glue and nuts and bolts all around.

Business wants to move fast and break things. Our mission as software engineers is to create and maintain codebases solid enough (pun intended) to sustain years of development, pivots, M&As and whatnot.

craftinator2• 7 years ago

I agree; it seems that SOLID does a good job of marshalling the complexity out of individual elements into the system as a whole. This can be a good thing, if the system design itself was created with SOLID in mind, and is well implemented. However if the system design didn't account for this complexity shift, it will likely cause a lot of mayhem.

GordonS4• 7 years ago

SOLID is... well, "solid" advice. But it absolutely should not be applied dogmatically - the same principle holds true for design patterns, agile processes, and the like.

It seems like most devs have some kind of epiphany when they learn about concepts such as SOLID, and apply them with zeal. It's almost like a stepping stone to learning what SOLID is really about - simple, clean, maintainable code.

At some point this realisation dawns, and the dev reaches a zen-like state, where the tenants of SOLID are obvious, without the need to consciously think about them - it's at this point that the dev realises the error of their ways.

tempodox• 7 years ago

tenets

cjfd2• 7 years ago

I am not a big fan of 'Solid'. Some of them are true but of the category 'water is wet'. I.e., the SRP principle. Others don't seem useful. The open-closed principle, for instance. All software should be adaptable and we generally don't know what requests are going to come from the field so we cannot write parts of software to never be changed. Perhaps you think that

   bool is_prime(int n)

is never going to change but someday somebody is going to request that it can report progress during its calculation. So really, you can never know.

Also, these things with dependency injection and interfaces. Please, only introduce these complications when you can prove that you need them. The software will be complicated enough when it is satisfying all its requirements. It is unnecessary to invent complications for no reason.

croo• 7 years ago

If SRP is akin to water is wet I'm working with newborns.

What is so trivial about what to do in a class? Deciding where to put what is one of the hardest part of the job...

cjfd2• 7 years ago

What to do in a class may not be entirely trivial but also not that complicated. What is trivial is noticing that a class is doing too many things. The problem is actually doing something about it. This seems in many case to be more of a discipline problem than it actually being that hard.

I also have seen that people are trying to do too much. When one sees a problem many have the inclination to want to refactor everything at once and also redesign an entire program. A more productive way is to do one small refactoring that solves just one code quality issue at a time. Before you know it you have done 5 or 10 small refactorings and things are starting to look much better. The big refactoring is very likely to make as many things worse as it makes better. The small refactoring has much more chance to have only or mainly positive effects.

Sometimes one also has to take a step back and think about in what direction to go with an application. That can be a bit hard, indeed. Even in that case it is much better to do small steps in the right direction that one has envisioned rather than some bing bang rewrite.

juliangamble• 7 years ago

There is a fantastic talk by Kevlin Henney where he deconstructs the SOLID principles as each being either misunderstood when they were written - or frequently misunderstood after. It was done here:

https://vimeo.com/157708450

I won't spoil it by writing all the points here - just some pearls.

* The Open/Closed principle is redundant because it is already covered by the Liskov substitution principle.

* Bob Martin's view of the Dependency Inversion principle is in fact the Single Responsibility principle.

He suggests 5 more principles - but I won't spoil it by listing them here. Go see it!

codr7• 7 years ago

There's just no way to win this game, whatever good ideas are thrown in will be twisted, turned and corrupted into worse than what came before.

I once worked on the software side of a 500-employee electronics company that had managed to ISO-certify their so called Agile development process.

Everything, and I do mean everything; was performed according to some ISO-script. No deviation, no matter how meaningless the task. And we had more useless meetings than anything I've seen before, it's just that they were called stand ups and reviews.

Thanks, but no thanks.

p0nce3• 7 years ago

This article completely changed my mind about SOLID and I realized how underrated this simple mnemonic still is: https://www.gamedev.net/blogs/entry/2265481-oop-is-dead-long...

mxcrossb• 7 years ago

It seems like 90% of this can be achieved for free using a language with duck typing and named parameters. But of course we know codes written in those languages are often not robust and inmaintanable. What is missing? Is it the static typing? Or that all of these goals are implemented mindfully? Maybe the key is in how you document the principles.

planxty• 7 years ago

I suspect the majority of people who claim to disdain SOLID principles are in fact using them every day without realizing it.

Seems pretty disturbing that so many engineers are piling on to recommend abandoning adherence to some really basic (and easy to understand and apply) ideas about design. Glad I don't work with you! :-P

reallydontask• 7 years ago

I think a lot of people have seen that SOLID principles sometimes actually have the opposite effect of their intention, i.e. they make the code hard to reason about and thus less maintainable and harder to extend.

There are quite a lot of articles out there with valid criticism of SOLID, you might find that you don't disagree that much after all (or not). At the end of the day, blind adherence to dogma will likely result in negative effects.

If strict adherence to SOLID principles works for you, then great. Not been the case for me.

Izkata• 7 years ago

I think a lot of people have seen that SOLID principles sometimes actually have the opposite effect of their intention, i.e. they make the code hard to reason about and thus less maintainable and harder to extend.

For me, it's half that and half that the people most likely to promote it - like in this blog post - are doing it badly.

For example, their very first one, the Single Responsibility Principle, isn't. It's blind adherence to a code smell at the syntactic level, resulting in subclasses that are closer to god objects than single-responsibility.

IMO, extracting "Payments" into its own class/module is the "right" day to do it, whether or not the switch statement is kept or a method is created for each Employee type.

ideal_stingray• 7 years ago

In the first example, isn’t it kind of a problem to have a bunch of different functions for calculating pay? Seems like it would be easy for the accounting department to end up with a different version than the HR department that actually disburses the money, which would cause all kinds of hell when trying to audit your accounts. If one team or department owns the function, you end up with more interdepartmental communication if another team needs to change it, but you won’t end up with Accounting and HR disagreeing on how much employees should be paid.

jonathanstrange• 7 years ago

Here are two concrete problems that these general design articles never answer:

1. How to deal with global dependencies like logging, internationalization, application "constants" (not always constants in the programming language), preferences, ...? If you inject these as dependencies into every module or package, then you end up with a convoluted mess of initialization parameters and dependencies. If you make them globally accessible, re-usability is lost almost totally. In both cases, the code depends on the preference system and the particular values of preferences. (In a real-world application almost every "constant" has to be parametrized as a preference.)

2. Closely related to this, how do you deal with miscellaneous data that any real-world application needs, but that pollute and "impurify" your objects and classes. Typical examples of this second category are binary data from rich text and other potentially volatile and framework-dependent data---often, but not necessarily only GUI-related. If your application is database-driven, you have to store this data, which usually means you also have to store it in a class or struct in your programming language. For example, you might not just need to store an employee name, you might also need to store employee name richtext data, employee name display flags and filter category information, and so on. Trying to strictly separate model data from view-related data basically just gets you twice as many classes and tables for a little bit of gain in "purity." That is true even if you follow a strict MVC pattern (which I usually do), and in the end you store all of the data in the same database file anyway.

I'm currently using Go, but the same problems also came up in Racket for in the past. I've never read any software design article that provided concrete solutions to these problems. I wish those articles would stop discussing toy examples about cars and employees and instead explain how real-world problems are solved.

mdpopescu4• 7 years ago

1. Decorators. See https://blog.ploeh.dk/2010/04/07/DependencyInjectionisLooseC...

2. Yes, you need a lot of model types (groups, layers). You need the view models, the database models, the business logic models, and possibly others. Sure, that's annoying at first because they're pretty much identical. That's fine though, because in that case you can just use AutoMapper or something similar.

I have never seen a project where the various models didn't change independently in a short timeframe (weeks, months at the most).

dirkg2• 7 years ago

Does SOLID align with functional programming?

I've come to think that IOC/DI should almost be an anti pattern as it depends on interfaces and I've never seen anything use it that wasn't abstraction hell.

Functional programming seems like a breath of fresh air, which is ironic since it predates all this modern buzzword centric world and was around with LSIP.

Solid Principles: A Software Developer’s Framework to Robust, Maintainable Code

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