We Choose Coding Challenges and How We Made Ours Higher

We Choose Coding Challenges and How We Made Ours Higher

At Gravitational, we construct open supply techniques software program that allows multi-cloud entry and utility administration. This text is concerning the classes I realized whereas making an attempt to develop a hiring course of that
is clear for the candidates, whereas producing good outcomes for my staff.

Listed below are among the explanation why I want coding challenges:

  • Permits for a greater pattern work.
  • Hidden gems can shine.
  • Removes pointless stress.
  • Simulates precise working situations.

I began with a course of arrange at Mailgun, the earlier firm I labored at. Nevertheless, whereas reimplementing it, I spotted there was quite a bit to vary and enhance for Gravitational.

Under are some particulars on all of those factors and I additionally share considered one of our challenges, which may be fascinating for anybody enthusiastic about utilizing a coding problem as a hiring device.

Execs of Coding Challenges

Getting a pattern of labor

Now we have discovered one of the simplest ways to rent techniques engineers
is to get the very best attainable high quality (and the smallest attainable amount)
pattern of their work.

PeopleWare has a great analogy:

Circus Supervisor: How lengthy have you ever been juggling?

Candidate: Oh, about six years.

Supervisor: Are you able to deal with three balls, 4 balls, and 5 balls?

Candidate: Sure, sure, and sure.

Supervisor: Do you’re employed with flaming objects?

Candidate: Certain.

Supervisor: … knives, axes, open cigar bins, floppy hats?

Candidate: I can juggle something.

Supervisor: Do you’ve got a line of humorous patter that goes together with your juggling?

Candidate: It’s hilarious.

Supervisor: Properly, that sounds advantageous. I suppose you’re employed.

Candidate: Umm … Don’t you need to see me juggle?

Supervisor: Gee, I by no means considered that.

Discovering hidden gems

Relying solely on previous credentials (in depth expertise / prestigious levels) is an efficient approach to miss out on
“Hidden Gems” – candidates who’re thought of junior (or think about themselves junior),
with out related trade credentials however have the fitting abilities and talents
that qualify them as medium to senior degree engineers.

Nothing in Alex’s background provided a touch that this is able to occur.
He had Walter White’s resume, however Heisenberg’s aptitude. None of us noticed it coming.

by way of Thomas Ptacek’s “The Hiring Publish”.

Eradicating pointless stress

When graduating from highschool, we needed to move a time-based math customary take a look at. I vividly keep in mind myself sitting there for what felt like eternally, making an attempt to divide 1 by 2. I froze and it appeared like nothing in my prior information ready me for this – how would you divide one by two, in any case, if one is clearly much less?

Many occasions when I’ve been an interviewee myself, I discovered my stress degree to be so excessive
that one of the best I may do was to supply options I had memorized earlier than. I may actually not assume
about creatively fixing any issues and would typically get caught on the only of questions.

I feel many individuals can relate to my experiences. That’s the reason our coding challenges are
designed to alleviate pointless stress. We attempt to permit folks to focus in an atmosphere
that’s acquainted and cozy to them, at a tempo that enables them to be inventive and present their highest quality of labor attainable. Precisely the atmosphere any good supervisor want to create on their staff.

There may be all the time a certain quantity of unavoidable stress related to work:
product supply deadlines outlined in contracts, competitors profitable prospects over and different
challenges that maintain us on our toes.

Nevertheless, including superficial stress to the combo isn’t useful. Individuals will all the time
deal with any such stress as B.S. and act accordingly:

The corporate’s possibilities for one of the best second quarter in historical past have been in our arms, they mentioned.
They requested me to share that reality with the remainder of the staff, “to focus their efforts”.
I’ve by no means labored on a extra targeted staff in my life, however I dutifully handed the phrase
on the subsequent morning. The power went out of the staff like wind out of a sail.
The chief programmer summed all of it up: “Who offers a rat’s ass to their second quarter?”
Half an hour later, they’d all gone house.

PeopleWare Chapter 21

Simulating the work situations

Coding challenges permit us to simulate the work situations that exist on the job. At Gravitational, we create a Slack channel with the hiring staff of engineers, assessment candidates’ design docs and supply a regular code assessment for his or her pull requests.

This offers each us and the candidates the concept of what it’s wish to work at Gravitational.
Typically people find yourself chatting with our engineers about our work/life steadiness,
asking questions concerning the commute, places of work and perks.

Classes realized

Lacking out

There may be plenty of competitors for expertise and lots of senior and completed engineers
don’t have time for this problem. They’ve so many gives that writing code
for Four-Eight hours is simply not going to work for them. That is one thing that now we have come to phrases with and we hope the flexibility to search out “hidden gems” outweighs this limitation.

Overly aggressive deadlines

The town of Denver, Colorado, set out in 1988 to construct a brand new airport to interchange the prevailing one, Stapleton Airport. The brand new Denver Worldwide Airport (DIA) was scheduled to open on October 31,1993. That was the plan. On October 31, 1993, each different a part of the huge air-port complicated was able to go … sincere it was. Actually. Belief uson this. However the software program wasn’t prepared, so the airport couldn’t open!

This sort of dollars-to-dumpster simplification was a function of newspaper and journal protection of the DIA troubles from the primary signal of delay in early 1993 till the partial opening in 1995.

― Tom DeMarco, Waltzing with Bears: Managing Danger on Software program Initiatives

Essentially the most skilled engineers can beat our greatest timing estimates and ship excellent code. Nevertheless, our staff has discovered that revealing this to candidates units unreasonable expectations and people who spent greater than one of the best estimates felt that they have been doing one thing incorrect.

After I was in faculty, our professor requested us to jot down a easy shell interpreter in C with the intent
of educating us the fundamentals of course of lifecycle and interprocess communication.

Everybody in our class coded a easy shell in a few days. We had so many assignments,
that people neatly constructed a minimal viable product and moved on to different issues. Nevertheless, the entire thought fascinated me a lot that I disappeared from the lessons for the remainder of the week. I spent days and nights writing an BNF grammar spec in Bison, traversing summary syntax tree of the shell, and ended up with an actual interpreter.

I exploit among the tips from this interpreter even to this present day and I earned an important rating from my professor. Though, I had plenty of catching as much as do with the remainder of the lessons I skipped. I keep in mind this coding streak as one of many happiest occasions I had in faculty.

Would it not matter to me if an engineer wrote a posh piece of performance in in the future or every week? If there are little to no bugs and it runs for years, then the worth is a large multiplier, whatever the time spent.

That’s why I don’t pay plenty of consideration to cheap fluctuations in time, solely the ultimate high quality.

Protecting momentum

We’re nearly all the time hiring for numerous roles and so it’s not necessary to us
to get an important candidate employed by a sure time. So, we began off
with obscure deadlines or no deadlines in any respect – “Be at liberty to show it in every time”.

It turned out this was killing momentum and didn’t give people a way of significance. We had many instances when
folks by no means turned their code in, merely happening to different extra necessary issues of their lives.

As soon as we added a “comfortable deadline” of Four weeks, it considerably improved the completion charge. We made it clear although, that we don’t anticipate candidates to code all Four weeks.
The precise coding time is nearer to Four-Eight hours however with the flexibleness to unfold this time
over essentially the most handy time obtainable.

We let candidates know that in the event that they discovered themselves coding greater than Eight hours,
they need to get again to us to cut back the scope.

Scope explosion

Over time of operating our coding problem, now we have been consistently decreasing the scope.
We discovered that we had been asking candidates to jot down an excessive amount of code that didn’t actually matter. Issues like backend integration, complete take a look at protection (writing one or two take a look at samples is sufficient) or implementing
the configuration administration half for the device have been all faraway from the spec with out affecting the top end result.


Working a coding problem is a really costly approach to rent folks, because it takes our interview
staff of three folks a few full days per candidate. Typically we submit near 100 of pull request feedback and our CEO spends time with each single individual going via the interview course of.

This strategy may not be scalable for giant corporations, however works properly for small
to medium sized corporations, the place each single individual on the staff could make such a huge effect. To scale back the prices and filter out candidates who’ve very restricted programming expertise,
coding challenges could possibly be paired with exterior screening processes
by corporations like Triplebyte.


We discovered you will need to have the identical set of tips with the interview staff and the candidate, so everybody understands
what’s required for achievement. After we began sharing a scorecard with the interview staff and the candidate,
the opinions from completely different staff members turned extra constant over time.

Fictional mission

Large warning to readers:
If Gravitational asks you to finish an ‘engineering problem’ they’re utilizing you free of charge labour.

– Touch upon Hacker Information “Who’s Hiring” from somebody who had submitted our earlier era of coding problem.

We realized to choose a non-existent mission that has little to no relation
to what we do in manufacturing, to stop any considerations that we would
have concepts of utilizing their work. Even when there’s a slight resemblance
to an precise mission now we have been engaged on, it may create unease with some folks.

That’s why we ended up choosing a totally fictional mission that
is commodity software program, was carried out many occasions in open supply and has no use
by itself to us.

Design doc

At first, writing a quick design doc was non-compulsory. Nevertheless, afterward we made it
a requirement as a result of it allowed us to filter out the candidates who have been too inexperienced
in our space of labor.

Regardless of our steering, many of us have been going into “evaluation paralysis”, spending weeks writing docs ad infinitum.
Others have been submitting designs that will have by no means labored or confirmed lack of know-how of protocol necessities.

Design paperwork helped us to make clear plenty of misconceptions and switch
what would in any other case be a failed interview (due to misunderstanding of the mission
scope or intent) into successful.

Non-Adversarial hiring course of

“I’ve been hiring folks for 10 years, and I nonetheless swear by a easy rule: If somebody doesn’t ship a thank-you e-mail, don’t rent them.”

Jessica Liebman, Enterprise Insider

In most work environments, we don’t want or anticipate our co-workers to fail. In reality, we put forth plenty of effort for them to succeed – recommendation, code opinions, 1x1s, and many others.

We discover this strategy works properly for the best way we conduct our interviews, as properly. We make it clear that our coding challenges don’t include
any “hidden landmines”. There are not any particular components planted within the spec that the hiring staff expects the candidate to fail on or implement in a sure (incorrect) means. Each single expectation now we have from the candidate is clearly outlined in our interview information.

Reviewing the code, not the candidate

From    Linus Torvalds
Date    Solar, 23 Dec 2012 09:36:15 -0800
Topic    Re: [Regression w/ patch] Media commit causes 
consumer house to misbahave (was: Re: Linux Three.Eight-rc1)

> On Solar, Dec 23, 2012 at 6:08 AM, Mauro Carvalho Chehab
> <[email protected]> wrote:
> Are you saying that pulseaudio is getting into on some 
> bizarre loop if the returned worth will not be -EINVAL? 
> That appears a bug at pulseaudio.


It is a bug alright - within the kernel. How lengthy have you ever been a
maintainer? And also you *nonetheless* have not learnt the primary rule of kernel

Evidently, when reviewing code at work, we all the time assessment the answer and never the actual one that submitted it.
Any “character assassinations” or different private remarks will not be solely thought of poor style,
however prohibited in our each day routine.

It seems that is one of the simplest ways to conduct the interview, as properly. We all the time invite the
candidates to retry in Three-6 months in the event that they really feel they realized one thing new. We make it clear that they shouldn’t
deal with the submission as a verdict to their talent set from some authority, slightly merely our groups’ present suggestions.

Don’t share the resume with a hiring staff

Sharing a resume with a hiring staff supplies an excessive amount of context and expectation. I realized
that it’s best to not share the resume and prior expertise with the hiring staff, focusing them
on evaluating the code and speaking to the candidate.

Simply as Thomas Ptacek, I discovered the outcomes to be stunning. Some candidates self rated themselves as junior, nevertheless their submissions have been rated a lot stronger. In lots of instances the alternative was true.

Avoiding peer stress

“I assumed that the dialog didn’t stream.”

– A Hiring Supervisor

After the hiring supervisor mentioned this phrase on the hiring suggestions session,
the candidate bought a really poor rating from the staff.

Proper after discussing the candidates submissions, sharing professionals and cons, each hiring staff
member submits their rating anonymously. After I’m the hiring supervisor, I normally
ship my suggestions after everybody’s scores submitted.

This actually helped us to keep away from the peer stress to charge candidate in a sure means or
to set implicit expectations after listening to the hiring supervisor’s suggestions.

All the time cool to again out

I used to be promoting my automotive the opposite day and I discovered myself negotiating with the gross sales supervisor, though
via a gross sales consultant as a proxy.

I’m sorry, however my supervisor is agency on the ultimate worth. That may be “No” from him.

Why would they use this system? I feel one of many causes is that it’s a lot simpler to say “no” if in case you have by no means seen the individual or don’t must say “no” head to head.

We attempt to make it as snug as attainable for candidates to say “no” at any stage. It must be cool to easily
drop us an e-mail to say they don’t need to proceed or not begin the problem within the first place.

Presenting supply brackets upfront

It was a lot simpler for candidates making use of to us to decide to spending their time
on the problem as soon as we began presenting wage brackets earlier than the coding began.

We aren’t an authority

Regardless of all our efforts, we don’t all the time present one of the best expertise. We’re nonetheless studying
and making errors alongside the best way. We realized to make that clear in the course of the interview course of
to mitigate any emotions of failure if a candidate didn’t get sufficient votes from the staff.


The coding problem course of is delivering plenty of worth for our staff at Gravitational and the candidates. Though, it comes at an enormous operational worth and is a really demanding approach to conduct the interview course of. Now we have realized many classes alongside the best way and are consistently bettering the spec and the method.

A full problem spec is printed beneath. Aren’t we afraid that publishing the problem will put candidates who learn it ? Not a bit, if somebody can learn it and prepare to ace it, all the higher for them!

I want to thank Greg Kogan for reviewing the weblog publish and permitting us to make use of his fantastic illustrations.

Our coding problem spec

That is our V3 Methods engineer coding problem spec and the interview information beneath.


Implement a prototype job employee service that gives an API to run arbitrary Linux processes.


This train has two objectives:

  • It helps us to know what to anticipate from you as a developer. How do you write manufacturing code, the way you cause about API design and the way you talk when making an attempt to know an issue earlier than you resolve it.
  • It helps you get a really feel for what it’s wish to work at Gravitational, as this train goals to simulate our day-to-day and expose you to the kind of work we’re doing right here.

We consider this system will not be solely higher, but additionally is extra enjoyable in comparison with the whiteboard/quiz interviews so widespread within the trade.

We aren’t alone:


We recognize your time and we look ahead to hack on this little mission collectively.


The aim is to implement a small a part of a distributed job scheduler: a Linux job employee server that executes arbitrary linux processes based mostly on the direct API requests from shoppers.



Job employee ought to present an RPC API to start out/cease/question standing and get an output of a operating job course of. Any RPC mechanism that works for the duty and is acquainted to you is OK: GRPC, HTTPS/JSON API or anything that may assure safe and dependable client-server communication.
The API ought to present a easy however safe authentication and authorization mechanism.


Consumer command ought to be capable to hook up with employee service and schedule a number of jobs.
Consumer ought to be capable to question results of the job execution and fetch the logs.


Interview course of

The interview staff is assembled within the slack channel and consists of the engineers who shall be working with you.
You might be inspired to speak to them and ask questions concerning the engineering tradition, work and life steadiness, or anything that you simply want to study Gravitational.

We perceive that the interview is a two-sided course of and we’d be pleased to reply any questions!

Earlier than writing the precise code, we encourage you to create a small design doc in a Google Doc and share it with the staff. This doc ought to include key trade-offs and key design approaches. Please keep away from writing a very detailed design doc. Use this doc
to verify the staff can present design suggestions and reveal that you’ve investigated the issue house to supply an inexpensive design.

Cut up your code submission utilizing pull requests and provides the staff a chance to assessment the PRs. A very good “rule of thumb” to observe is that the ultimate PR submission is including a small function set – it signifies that the staff had a chance to contribute the suggestions throughout a number of properly outlined levels of your work.

Our staff will do their greatest to supply a top quality assessment of the submitted pull requests in an inexpensive timeframe. You might be spending your time on this. We’re going to contribute our time too.

After the ultimate submission, the interview staff will assemble and vote utilizing +1, -2 nameless voting system: +1 is submitted every time a staff member accepts the submission, -2 in any other case.

If there’s a optimistic end result, we’ll join you to our HR staff who will accumulate one/two references and can work out the opposite particulars. You can begin the reference assortment course of in parallel if you need to hurry up the method.

After reference assortment, our ops staff will ship you a proposal.

In case of a destructive rating end result, the hiring supervisor will contact you and ship an inventory of the important thing observations from the staff that affected the end result. Please don’t be discouraged. Our code assessment course of is targeted on the submission, not the candidate and we shall be excited so that you can take one other problem at a later time in case you really feel you’ve got addressed our feedback!

Code and mission possession

It is a take a look at problem and now we have no intent of utilizing the code you’ve got submitted in manufacturing. That is your work, and you’re free to do no matter you’d like with it.

Areas of focus

Gravitational focuses on networking, infrastructure and safety, so these are the areas we shall be evaluating within the submission:

  • Constant coding type. Gravitational follows https://github.com/golang/go/wiki/CodeReviewComments for the Go language. If you’ll use a special language, please decide coding type tips and tell us what they’re.
  • Please write one take a look at for authentication and one other for the networking part.
  • Reproducible builds. Choose any vendoring/packaging system that can permit us to get constant construct outcomes.
  • Constant error dealing with and error reporting. The system ought to report clear errors and never crash beneath non-critical situations.
  • Concurrency and networking errors. A lot of the points we’ve seen in manufacturing are associated to knowledge races, networking error dealing with or goroutine leaks. So we shall be on the lookout for these errors in your code.
  • Safety. Use sturdy authentication and easy, however strong authentication. Arrange the strongest transport encryption you’ll be able to. Take a look at it.
  • Error logging and dealing with. Constant errors are key.


It is very important write as little code as attainable. In any other case, this job may eat an excessive amount of time and the general code high quality will endure.

It’s OK and anticipated in case you minimize corners. For instance, configuration tends to take plenty of time and isn’t necessary for this job. So we encourage candidates to make use of onerous codes as a lot as attainable and easily add TODO gadgets exhibiting candidate’s considering.

For instance:

// TODO: Add configuration system. Think about using CLI library to help each
// atmosphere variables and cheap default values,
// for instance https://github.com/alecthomas/kingpin

Feedback like this are actually useful to us as a result of they prevent plenty of time and reveal to us that you simply’ve frolicked enthusiastic about this downside and supply a transparent path to an answer.

Contemplate making different cheap trade-offs and be sure to talk them to the interview staff. Listed below are another trade-offs that can enable you to spend much less time on the duty:

  • Don’t implement a system that scales or is extremely performing. As a substitute, talk the efficiency enhancements you add sooner or later.
  • Excessive availability. It’s OK if the system will not be extremely obtainable. Write down how would you make the system extremely obtainable and why your system will not be.
  • Don’t attempt to obtain full take a look at protection. This may take too lengthy. Take two key elements, e.g. authentication/authorization layer and networking and implement one or two take a look at instances that reveal your strategy to testing.

Pitfalls and gotchas

That can assist you out, we’ve composed an inventory of issues that resulted in a no-pass from the interview staff:

  • Scope creep. Candidates have tried to implement an excessive amount of and ran out of time and power. To keep away from this pitfall, use the only answer that can work. Keep away from writing an excessive amount of code. For instance, we’ve seen candidates’ code introduce caching and make errors within the caching layer validation logic. Not having caching would have solved this downside.
  • Information races. We are going to scan the code with a race detector and do our greatest to search out knowledge races within the code. Keep away from international state as a lot as attainable. If utilizing international state, write down a great description of why is it needed and defend it towards knowledge races.
  • Deadlocks. When utilizing mutexes, channels or another synchronization primitives, be sure the system gained’t impasse. We’ve seen candidates’ code holding mutex and making a community name with out timeouts in place. Be additional cautious with networking and sync primitives.
  • Unstructured code. We’ve seen candidates leaving commented chunks of code, having one massive file with all of the code or not having code construction in any respect.
  • Not speaking. Candidates who submitted all of the code to grasp department, which doesn’t give us the flexibility to supply suggestions on the varied implementation phases. As a result of we’re a distributed staff, structured communication is essential to us.
  • Implementing customized safety algorithms/authentication schemes is normally a foul thought until you’re a skilled safety researcher/engineer. It’s positively a foul thought for this job – attempt to stick with trade confirmed safety strategies as a lot as attainable.


We need to be as clear as attainable on how we shall be scoring your submission. The next desk supplies an outline of various areas you’ll be evaluated on and the way they are going to have an effect on your total rating.

Description Attainable Factors Awarded Attainable Factors Subtracted
The submitted code has a transparent and modular construction +1 -1
The candidate communicated their progress in the course of the interview +1 -1
This system builds are reproducible +1 -1
README supplies clear directions +1 -1
The candidate outlined the important thing design factors within the design doc +1 -1
The code has no apparent knowledge races and deadlocks +1 -1
The code supplies examples of checks masking key elements +1 -1
The code supplies clear error dealing with and error reporting +1 -1
This system is working based on the specification +1 -1
The candidate demonstrates capacity to deal with and apply suggestions +1 -1
The client-server communication is carried out in a safe means +1 -1

Asking questions

It’s OK (and inspired) to ask the interview staff questions. Some people steer clear of
asking inquiries to keep away from showing much less skilled, so we offered examples of questions
to ask and questions we anticipate candidates to determine on their very own.

It is a nice query to ask:

Is it OK to pre-generate secret knowledge and put the secrets and techniques within the repository for the needs of POC? I’ll add a be aware that we’ll auto-generate secrets and techniques sooner or later.

It demonstrates that you considered this downside area, acknowledge the commerce off and prevent and the staff time by not implementing it.

That is the kind of query we anticipate candidates to determine on their very own:

What model of Go ought to I exploit? What dependency supervisor ought to I exploit?

We anticipate candidates to have the ability to discover options to widespread non-project particular questions
like this one on their very own. Until specified within the necessities, decide the answer that works
greatest for you.

This job must be carried out in Go and may work on a 64-bit Linux machine with kernel higher than Three.19.zero.


When unsure, all the time err on the aspect of over speaking.
We promise that we’re not going to subtract any factors for seemingly “foolish” questions.

Lastly THANK YOU for taking your time to take the problem. We perceive that your time is effective and we actually recognize it!

We want you good luck!


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