Sustaining Software program Correctness

This text is a write-up of a chat I gave at MinneBar 2022. As a substitute of studying this, you can additionally watch the recording or view the slides.

The title of this discuss is “sustaining software program correctness.” However what precisely do I imply by “correctness”? Let me set the scene with an instance.

Years in the past, when Trello Android adopted RxJava, we additionally adopted a reminiscence leak drawback. Earlier than RxJava, we would have, say, a button and a click on listener; when that button would go away so would its click on listener. However with RxJava, we now have a button click on stream and a subscription, and that subscription may leak reminiscence.

We may keep away from the leak by unsubscribing from every subscription, however manually managing all these subscriptions was a ache, so I wrote RxLifecycle to deal with that for me. I’ve since disavowed RxLifecycle because of its quite a few shortcomings, considered one of which was that you just needed to bear in mind to use it appropriately to each subscription:

  .bindToLifecycle() // Overlook this and leak reminiscence!

When you put bindToLifecycle() earlier than subscribeOn() and observeOn() it would fail. Furthermore, in case you outright neglect so as to add bindToLifecycle() it doesn’t work, both!

There have been lots of (maybe 1000’s) of subscriptions in our codebase. Did everybody bear in mind so as to add that line of code each time, and in the proper place? No, in fact not! Individuals forgot consistently, and whereas code overview caught it typically, it didn’t at all times, resulting in reminiscence leaks.

It’s simple in charge folks for messing this up, however truly the design of RxLifecycle itself was at fault. Relying on folks to “simply do it proper” will ultimately fail.

Let’s generalize this story.

Suppose you’ve simply created a brand new structure, library, or course of. Over time you discover some points that stem from folks incorrectly utilizing your creation. If folks would simply use every little thing appropriately there wouldn’t be any issues, however to your horror everybody continues to make errors and trigger your software program to fail.

That is what I name the correctness dilemma: it’s simple to create however arduous to keep up. Getting folks to align on a code model, correctly contribute to an OSS venture, or persistently releasing good builds – all of those processes are simple to give you, however errors ultimately creep in when folks do not use them appropriately.

The core mistake is designing with out conserving human fallibility in thoughts. Anticipating folks to be excellent is just not a tenable answer.

When you pay no consideration to this side of software program design (like I did for a lot of my profession), you’re setting your self up for long run failure. Nevertheless, as soon as I began specializing in this drawback, I found many good (and customarily simple) options. All it’s a must to do is strive, just a bit bit, and typically you’ll arrange a product that lasts eternally.

How will we design for correctness?

Human error is an issue in any trade, however I believe that within the software program trade we’ve a singular superpower that lets us sidestep this drawback: we are able to readily flip human processes into software program processes. We will take unreliable chores finished by folks and switch them into reliable code, and sooner than anybody else as a result of we’ve bought all of the software program builders.

What will we do with this energy to keep away from human fallibility? We constrain. The important thing thought is that the much less freedom you give, the extra doubtless you’ll preserve correctness. In case you have the liberty to do something, then you could have the liberty to make each mistake. When you’re constrained to solely do the proper factor, then you don’t have any alternative however to do the proper factor!

There are all kinds of methods we are able to make use of for correctness, laying on a spectrum between flexibility and rigidity:

Let’s take a look at every technique in flip.

Institutional Data

In any other case generally known as “stuff in your head.”

That is much less of a method and extra of a place to begin. Every part has to begin someplace, and normally that’s within the collective consciousness of you and your teammates.

Ideas are nice! Pondering comes naturally to most individuals and have many benefits:

Ideas are extraordinarily low-cost; the going charge has been unaffected by inflation, so it’s nonetheless only a penny for a thought. Brainstorming relies on how low-cost ideas are; “The place ought to this button go?” you may ask, and also you’ll have fifteen totally different attainable areas within the span of some minutes.

Ideas are extraordinarily versatile. You’ll be able to pitch a brand new course of to your crew to check out for every week, see the way it goes, then abandon it if it fails. “Let’s strive posting a fast standing message every morning”, you may recommend, and when everybody inevitably hates it then you may rapidly give it up every week later.

Institutional data can clarify and summarize code. Would you quite learn via each line of code, or have somebody focus on its construction and objectives? Trello Android may function offline, which suggests writing modifications to the consumer’s database then syncing these modifications with the server – I’ve simply now described tens of 1000’s of traces of code in a single sentence.

Institutional data can clarify the “why” of issues. By itself, code can solely describe the way it will get issues finished, however not why. Any hack you write to resolve an answer in a roundabout manner ought to embrace a touch upon why the hack was crucial, lest future generations surprise why you wrote such wacky code. There might need been a sequence of experiments that decided that is the very best answer, despite the fact that that’s not apparent.

Institutional data can describe human issues. There’s solely a lot you are able to do with code. Your trip coverage can’t be absolutely encoded as a result of workers get to decide on once they take trip, not computer systems!

There’s rather a lot to love about considering, however in the case of correctness, institutional data is the worst. Low-cost and versatile doesn’t make for a robust correctness basis:

Institutional data might be misremembered, forgotten, or depart the corporate. I are likely to neglect most issues I did after just some months. Coworkers with skilled data can give up anytime they need.

Institutional data is laborious to share. Each new teammate needs to be taught each little bit of institutional data by another person throughout onboarding. Everytime you give you a brand new thought, it’s a must to talk it to each current teammate, too. Scale is inconceivable.

Institutional data might be tough to speak. The sport “phone” is based on simply how arduous it’s to cross alongside easy messages. Now think about enjoying phone with some tough technical idea.

Institutional data doesn’t remind folks to do one thing. Do you want somebody to press a button each week to deploy the most recent construct to manufacturing? What if the one who does it… simply forgets? What in the event that they’re on trip and nobody else remembers that somebody has to push the button?

Like I stated, institutional data is nice and essential – it’s the place to begin, and an inexpensive, versatile option to experiment. However any institutional data that’s recurrently used ought to be codified ultimately. Which leads us to…


I’m certain that somebody was screaming at their monitor whereas studying the final part being like “Documentation! Duh! That’s the reply!”

Documentation is institutional data that’s written down. That makes it tougher to neglect and simpler to transmit.

Documentation has a lot of the benefits of institutional data – although not fairly as low-cost or versatile, additionally it is in a position to summarize code and describe human issues. It is usually a lot simpler to broadcast documentation; you don’t have to sit down down and have a dialog with each one that must study.

There’s additionally a pair bonuses to visible data. Documentation can use photos or video. A very good circulation chart or structure abstract is price 1000 phrases – I may spend a bunch of time speaking about how Trello Android’s offline structure works, or you can take a look at the circulation charts on this article. I personally discover that video can click on with me simpler than simply speaking; I believe this is the reason the fashionable video essay exists (over written articles).

Documentation also can create checklists for complicated processes. We automated a lot of it, however the technique of releasing a brand new model of Trello Android nonetheless concerned many unavoidably handbook steps (e.g. writing launch notes or checking crash reviews for brand new points). A very good guidelines may also help lower down on human error.

Regardless of documentation’s advantages, there’s a motive this discuss was initially titled “documentation is just not sufficient.”

Right here’s a typical scenario we’d run into at work: we’d give you a brand new crew course of or structure, and other people would say “that is nice, however we’ve bought to jot down it down so folks received’t make errors sooner or later.” We’d take the time to jot down some nice documentation… solely to find that errors stored occurring. What offers?

Properly, it seems there are numerous issues that may come up with documentation:

Documentation might be badly written or misunderstood. A doc can clarify an idea poorly or inaccurately, or the reader may merely misapprehend its that means. There’s additionally no option to double-check that the knowledge was transmitted successfully; speaking to a different particular person permits for clarifying questions, however studying documentation is a one-way transmission.

Documentation might be poorly maintained and go outdated. Maybe your doc was correct when first written, however years later, it’s a web page of lies. Retaining documentation up-to-date is dear and laborious, in case you even bear in mind to return and replace it.

Documentation might be arduous to search out or just ignored. Even when the doc is ideal, you want to have the ability to discover it! Perhaps you understand it’s someplace on Confluence however who is aware of the place. Even worse, folks may not even know they should learn some documentation! “I’m sorry I took down the server, I didn’t know that you just could not lower releases at 11PM as a result of I by no means noticed the discharge course of doc.”

Documentation can’t function a reminder. Very similar to with institutional data, there’s no manner for documentation to inform you to do one thing at a sure time. Checklists get you barely nearer, however there’s no assure that an individual will bear in mind to verify the guidelines! Trello Android had a launch guidelines, however oftentimes the discharge would roll round and we’d uncover that somebody forgot to verify it, and now we are able to’t translate the discharge notes in time.

Documentation is critical. Some ideas can solely be documented, not codified (like high-level structure explanations). And in the end, software program growth is about working with people. People are messy, and solely written language can deal with that messiness. Nevertheless, it’s just one step above institutional data by way of correctness.


Let’s take a detour into the dictionary.

An affordance is “the standard or property of an object that defines its attainable makes use of or makes clear the way it can or ought to be used.”

I used to be first launched to this idea by “The Design of On a regular basis Issues” by Don Norman, which matches into element learning seemingly banal design selections which have enormous impacts on utilization.

The quilt of the e-book is traditional, exhibiting how absurd a tea kettle with the spout and deal with on the identical facet could be

A traditional instance of excellent and unhealthy affordances are doorways. Good doorways have an apparent option to open them. Crash bar doorways are a very good instance of that; there’s no universe during which you’d suppose to pull these doorways open.

The alternative is what is called a Norman door (named after the aforementioned Don Norman). Norman doorways that invite you to do the flawed factor, for instance by having a deal with that begs to be pulled however, in reality, ought to be pushed.[email protected]/40530223463

Right here’s why I discover all this fascinating: We will use affordances in software program to invisibly information folks in direction of correctness in software program. When you make “doing the proper factor” pure, folks will simply do it with out even realizing they’re being guided.

Right here’s an instance of an affordant API: in Android, there’s nobody stopping you from opening a connection to a database everytime you need. A dozen builders every doing their very own customized DB transactions could be a nightmare, so as an alternative, on Trello Android we added a “modification” API that might replace the DB on request. The modification API was simple – you’d simply say “create a card” and it’d go do it. That’s rather a lot less complicated than opening your individual connection, establishing a SQL question, and committing it – thus we by no means needed to fear about anybody doing it manually. Why would you, when utilizing the modification API was there?

What about enhancing non-software conditions? One instance that involves thoughts is submitting bug reviews. The tougher it’s to file a bug report, the much less doubtless you’re to get one (which, hey, perhaps that’s a function for you, however not for me). The teams that put the onus on the filer to determine precisely the place and the way to file a bug tended to not hear essential suggestions, whereas the groups that stated “we settle for all bugs, we’ll filter out what’s not essential” bought numerous suggestions on a regular basis.

If, for some motive, you may’t make the “proper” manner of doing issues any extra affordant, you may as an alternative do the other and make the flawed manner un-affordant (aka arduous and obtuse). Is there an escape hatch API that most individuals shouldn’t use? Disguise it in order that solely those that want it may well even discover it. Getting too many developer job functions? Add a easy algorithm filter to the beginning of your interview pipeline.

I consider this idea like how governments can form financial coverage via subsidies and taxes: make what you need folks to do low-cost; make what you do not need folks to do costly.

Although not precisely an affordance, I additionally think about peer strain a associated option to invisibly nudge folks in the proper path. I don’t suppose I’m alone after I say that the very first thing I do in a codebase is go searching and attempt to copy the native model and logic. If somebody asks me so as to add a button that makes a community request, I’m going to search out one other button that does it first, copy and paste, then edit. If there are 50 other ways to jot down that code, nicely, I hope I discovered the proper one to repeat; if there’s only one, then I’m going to repeat the write methodology. Consistency creates a flywheel for itself.

I like affordances as a result of they information folks with out them being consciously conscious of it. A whole lot of the correctness methods I’ll focus on later are extra heavy handed and obtrusive; affordances are mild and invisible.

Their essential draw back is that affordances and peer strain can solely information, not prohibit. Typically these methods are helpful while you can’t cease somebody from doing the flawed factor as a result of the coding language/framework is simply too permissive, you might want to present exceptions for uncommon circumstances, otherwise you’re coping with human processes (and something can go off the rails there).

Software program Checks

Software program checks are when code can verify itself for correctness.

When you’re something like me, you’ve simply began skimming this part since you suppose I’m gonna be speaking about unit assessments. Properly… okay, sure, I’m, however software program checks are a lot extra than unit assessments. Unit assessments are only one type of a software program verify, however there are numerous others, such because the compiler checking grammar.

What pursuits me right here is the timing of every software program verify. These checks can occur as early as while you’re writing code to as late while you’re working the app.

The sooner you may get suggestions, the higher. Quick suggestions creates a good loop – you neglect a semicolon, the IDE warns you, you repair it earlier than even compiling. In contrast, sluggish suggestions is painful – you’ve simply launched the most recent model of your app and oops, it’s crashing for 25% of customers, it’ll be at the least a day earlier than you may roll out a repair, and also you’ll should undo some structure selections alongside the way in which.

Let’s take a look at the timing of software program checks, from slowest to quickest:

The slowest software program verify is a runtime verify, whereby you verify for correctness as this system is working. Gathering analytics/crash knowledge out of your software program because it runs is nice for locating issues. For instance, in OkHttp, every Name can solely be used as soon as; attempt to reuse it and also you get an exception. This verify is inconceivable to make earlier than working the software program.

There are massive drawbacks to runtime checks: your customers find yourself being your testers (which received’t make them completely happy) and there’s an extended turnaround from discovering an issue to deploying a repair (which additionally received’t make your customers completely happy). It’s additionally an inconsistent option to take a look at your code – there may be a bug on a code path that’s solely accessed as soon as a month, making the suggestions loop even slower. Runtime checks are price embracing as a final resort, however counting on them alone is poor apply.

The subsequent slowest software program verify is a handbook take a look at, the place you manually execute code that runs a verify. These might be unit assessments, integration assessments, regression assessments, and many others. There might be a variety of worth in writing these assessments, however it’s a must to foster a tradition for testing (because it takes time & effort to jot down and confirm the correctness of assessments). I believe it’s price investing in these kinds of assessments; in the long term, good assessments not solely prevent effort but in addition drive you to architect your code in (what I think about) a typically superior manner.

One step up from handbook assessments are automated assessments, that are simply handbook assessments that run routinely. The core drawback with handbook assessments is that it requires somebody to recollect to run them. Why not make a pc bear in mind to do it as an alternative? Bonus factors if failed checks stop one thing unhealthy from occurring (e.g. blocking code merges that break the construct).

Subsequent up are compile time checks, whereby the compilation step checks for errors. Usually that is in regards to the compiler implementing its personal guidelines, akin to static kind security, however you may combine a lot extra into this step. You’ll be able to have checks for code model, linting, protection, and even run some automated assessments throughout compilation.

Lastly, the quickest suggestions is given at design time, the place your editor itself tells you that you just made a mistake while you’re writing code. As a substitute of discovering out you mis-named a variable throughout compilation, the editor can immediately inform you that there’s a typo. Or while you’re writing an article, the spellchecker can discover errors earlier than you publish the article on-line. Very similar to compile time checks, whereas these are usually about grammatical errors, you may typically insert your individual design time model/lint/and many others. checks.

Whereas quick suggestions is healthier, the sooner timings are likely to constrain what you may take a look at. Design-time checks can solely particular bits of logic, whereas runtime checks can cowl principally something your software program can do. In my expertise, whereas it’s simpler to implement runtime checks, it’s typically price placing in a bit of additional effort to make these checks go sooner (and be run extra persistently).


Constraints make it in order that the one path is the proper one, such that it’s inconceivable to do the flawed factor. Let’s take a look at a couple of circumstances:

Enums vs. strings. When you can constrain to just some choices (as an alternative of any string) it makes your life simpler. For instance, individuals are typically tempted to make use of stringly-typing when deciphering knowledge from server APIs (e.g. “card”, “board”, “record”). However strings might be something, together with knowledge that your software program is just not in a position to deal with. By utilizing an enum as an alternative (CARD, BOARD, LIST) you may constrain the remainder of your utility to only the legitimate choices.

Stateless features vs. stateful lessons. Something with state runs the chance of ending up in a nasty state, the place two variables are in stark disagreement with one another. When you can execute the identical logic in a self-contained, stateless perform, there’s no threat that some long-lived variables can find yourself out of alignment with one another.

Pull requests vs. merging to essential. When you let anybody merge code to essential, then you definitely’ll find yourself with failing assessments and damaged builds. By requiring folks to undergo a pull request – thus permitting steady integration to run – you may drive higher habits in your codebase.

Not solely can constraints assure correctness, in addition they restrict the logical headspace you might want to wrap your thoughts round a subject. As a substitute of needing to contemplate each string, you may think about a restricted variety of enums. In the identical vein, it additionally limits the variety of assessments you might want to cowl your logic.


Whenever you automate, a pc does every little thing for you. This is sort of a constraint however higher as a result of folks don’t even should do something. You solely have to jot down the automation as soon as, then the computer systems will take over doing all your busywork.

One efficient use of this technique is code technology. A traditional instance are Java POJOs, which don’t include an equals(), hashCode(), or toString() implementations. Within the previous days, you used to should generate these by hand; these implementations would rapidly go stale as you modified the POJO’s fields. Now, we’ve libraries like AutoValue (which generate implementations based mostly on annotations) or languages like Kotlin (which generate implementations as a language function).

Steady integration is one other nice automation technique. Having hassle remembering to run all of your checks earlier than merging new code? Simply get CI to drive you to do it by not permitting a merge till you cross all of the assessments. You’ll be able to even have CI do automated deployments, such that you just barely should do something after merging code earlier than releasing it.

There are two essential drawbacks of automation. The primary is that it’s costly to jot down and preserve, so it’s a must to verify that the payoff is price the price. The second drawback is that automation can do the flawed factor over and over, so it’s a must to watch out to verify that you just applied the automation appropriately within the first place.

Now that we’ve reviewed the methods, enable me to reveal how we use them in the true world.

Earlier than fixing any given drawback, it’s best to take a step again and work out which of those methods to use (if any) earlier than committing to an answer. You’ll in all probability find yourself with a mixture of methods, not only one. For instance, it’s not often the case you could simply implement constraints or automation with out additionally documenting what you probably did.

There are a couple of meta-considerations to have in mind as nicely:

First, whereas inflexible options (like constraints or automation) are higher for correctness, they’re worse for flexibility. They’re costly to vary after implementation and unforgiving of exceptions. Thus, you might want to stability correctness and adaptability for every scenario. Typically, I development in direction of early flexibility, then shifting in direction of correctness as crucial.

Second, you may implement correctness badly. You’ll be able to have flakey software program checks, overbearing code contribution processes, tough automation upkeep, or no escape hatches for brand new options or exceptions. Correctness is an funding, and you might want to be sure to can afford to take a position and preserve.

Final, you want buy-in out of your teammates. I are likely to make the error of considering that as a result of I like an answer that everybody else can even prefer it, however that’s positively not at all times the case. When you get settlement from others, correctness is less complicated to implement (particularly for crew processes); folks will go together with your plans, and even pitch in concepts to enhance it.

Disagreements, alternatively, can result in toxicity, akin to folks ignoring or purposefully undermining your creation. At my first job they tried to implement a code model checker that prevented merges, however did not have a plan for the way to repair previous information. There was no automated formatter (as a result of it was a customized markup language), so nobody ever needed to repair the large information; as an alternative everybody simply stored utilizing a workaround to keep away from the code model checker! Whoops!

Taking a while to assemble proof then presenting the case to your coworkers could make a world of distinction.

Now, let’s take a look at a couple of examples and analyze them…

Code Fashion

For instance, how do you get everybody to persistently use areas over tabs?

❌ Institutional data – Dangerous; this doesn’t stop folks from going off the code model in any respect.

❌ Documentation  – Simply as unhealthy as institutional data, however written down.

✅ Affordances – Semi-effective. You’ll be able to configure your editor to at all times use areas as an alternative of tabs. Even higher, some IDEs allow you to verify a code model definition into supply management so everyone seems to be on the identical web page style-wise. Nevertheless, by way of correctness, it guides however doesn’t prohibit.

✅ Software program checks – Utilizing lint or code model checkers to confirm code model is a superb use of CPU cycles. Individuals can’t merge code that goes off model with this in place.

❌ Constraints – Not likely attainable from what I can inform. I’m undecided the way you’d implement this – ship everybody keyboards with out the tab key?

❌ Automation – You possibly can have some hook routinely rewrite tabs to areas, however actually this offers me the heebie jeebies a bit!

Ultimately, I like implementing your model with software program checks, however making it simpler to keep away from failures with affordances.

Code Contribution to an OSS Challenge

How do folks contribute code to an open supply codebase? When you’ve bought a selected course of (like code opinions, working assessments, deploying) how do you guarantee these occur when a random particular person donates code?

❌ Institutional data – Inconceivable for strangers.

✅ Documentation – When you write strong directions, you may create a extra welcoming surroundings for anybody to contribute code. Nevertheless, documentation alone is not going to end in a dependable course of, as a result of not everybody reads the handbook.

✅ Affordances – There’s loads you are able to do right here, like templates for explaining your code contribution, or giving folks clear buttons for various contributor actions (like signing the contributor license settlement).

✅ Software program checks – Having loads of software program checks in place makes it a lot simpler for folks to contribute code that doesn’t break the prevailing venture.

✅ Constraints – Repository hosts allow you to put all kinds of good constraints on code contribution: stop merging on to essential, require code opinions, require contributor licenses, require CI to cross earlier than merging.

✅ Automation – CI is critical as a result of it feeds info into the constraints you’ve arrange.

For this, I take advantage of a mixture of all totally different methods to attempt to get folks to do the proper factor.

Cleansing Streams

Let’s revisit the story from the start of this text – the way to clear up sources in reactive streams of knowledge (particularly with RxJava).

❌ Institutional data – You’ll be able to train folks to wash up streams, however they may neglect.

❌ Documentation – No extra appropriate than institutional data, simply simpler to unfold the knowledge.

✅ Affordances – We used an RxJava instrument referred to as CompositeDisposable to wash up a bunch of streams directly. AutoDispose provides simpler methods to wash up streams routinely as nicely. Nevertheless, all these options nonetheless require remembering to make use of them.

✅ Software program checks – We added RxLint to confirm that we truly deal with the returned stream subscription. Nevertheless, this doesn’t assure you keep away from a leak, simply that you just made an try to keep away from it. When you’re utilizing AutoDispose, it gives a lint verify to ensure it’s getting used.

✅ Constraints – I’m fairly excited by Kotlin coroutines’ scopes right here. As a substitute of placing the onus on the developer to recollect to wash up, a coroutine scope requires that you just outline the lifespan of the coroutine.

❌ Automation – Understanding when a stream of knowledge is now not wanted is one thing solely people can decide.

What technique you employ right here is determined by the library. The perfect answer IMO are constraints, the place the library itself forces you to keep away from leaks. When you’re utilizing a library that may’t implement it (like RxJava), then affordances and software program checks are the way in which to go.

Clearly, not each possibility is accessible to each drawback – you may’t automate your manner out of all software program growth! Nevertheless, at its core, the much less folks should make selections, the higher for correctness. Free folks’s minds up for what actually issues – growing software program, quite than wrestling with avoidable errors.

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