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So You Want to Develop on illumos

Written by Bryan Cantrill


Term Definition
FCS First Customer Ship
gate Source code repository
gatekeeper Release Engineer responsible for ensuring the gate is working
putback Committing code to the gate
jurassic Server in the Sun Microsystems engineering dept.


So you're new to open source, and/or you're new to illumos. Maybe you have lots of experience developing mission-critical software, maybe you have none. But if you haven't already figured it out, we take quality very seriously around here. Developing illumos is very hard, and it's very important. This is good news, not bad news -- solving easy problems is boring and solving irrelevant problems is, well, irrelevant. But you should be prepared for the fact that you will need to push yourself to deliver the highest quality software.

If you haven't already discovered it, illumos -- like any large software system -- has a complete range of software quality within its many subsystems.


Some illumos subsystems are beautiful works of engineering -- they are squeaky clean, well-designed and well-crafted. These subsystems are a joy to work in but (and here's the catch) by virtue of being well-designed and well-implemented, they generally don't need a whole lot of work. So you'll get to use them, appreciate them, and be inspired by them -- but you probably won't spend much time modifying them. (And because many of these subsystems have been implemented by engineers who are now part of the active illumos community, many of the changes will be done by the original implementer(s) anyway.)


Other illumos subsystems are cobbled-together piles of junk -- reeking garbage barges that have been around longer than anyone remembers, floating from one release to the next. These subsystems have few-to-no comments (or what comments they have are clearly wrong), are poorly designed, needlessly complex, badly implemented and virtually undebuggable. There are often parts that work by accident, and unused or little-used parts that simply never worked at all. They manage to survive for one or more of the following reasons:

  • They work just well enough to not justify the cost of rewriting them
  • The problem they solve isn't important enough to justify the cost of rewriting them
  • The problem they solve is so nasty that the cost of a rewrite is enormous -- or at least it dwarfs the cost of ongoing maintenance

If you find yourself having to do work in one of these subsystems, you must exercise extreme caution: you will need to write as many test cases as you can think of to beat the snot out of your modification, and you will need to perform extensive self-review. You can try asking for assistance, but you may quickly discover that no one is available who understands that particular subsystem. Your code reviewers may not be able to help much either -- maybe you'll find one or two people who have had the same misfortune that you find yourself experiencing, but it's more likely that you will have to explain most aspects of the subsystem to your reviewers. You may discover as you work in the subsystem that maintaining it is simply untenable -- it may be time to consider rewriting the subsystem from scratch. (After all, most of the subsystems that are in the first category replaced subsystems that were in the second.) One should not come to this decision too quickly: rewriting a subsystem from scratch is enormously difficult and time-consuming. Still, don't rule it out a priori.

Even if you decide not to rewrite such a subsystem, you should improve it while you're there in ways that don't introduce excessive risk. For example, if something took you a while to figure out, don't hesitate to add a block comment to explain your discoveries. And if it was a pain in the ass to debug, you should add the debugging support that you found lacking. This will make it slightly easier on the next engineer -- and it will make it easier on you when you need to debug your own modifications.


Most illumos subsystems, however, don't actually fall neatly into either of these categories -- they are somewhere in the middle. That is, they have parts that are well thought out, or design elements that are sound, but they are also littered with implicit intradependencies within the subsystem or implicit interdependencies with other subsystems. They may have debugging support, but perhaps it is incomplete or out of date. Perhaps the subsystem effectively met its original design goals, but it has been extended to solve a new problem in a way that has left it brittle or overly complex. Many of these subsystems have been fixed to the point that they work reliably -- but they are delicate and they must be modified with care.

Due Diligence

The majority of work that you will do on existing code will be to subsystems in this last category. You must be very cautious when making changes to these subsystems. Sometimes these subsystems have local experts, but many changes will go beyond their expertise. (After all, part of the problem with these subsystems is that they often weren't designed to accommodate the kind of change you might want to make.) You must extensively test your change to the subsystem. Run your change on your desktop, your laptop, your home machine and every kind of machine you can grab a tip line to. But you can't just be content with booting a machine with your change -- you must beat the hell out of it. Sometimes there is a stress test available that you may run, but this is not a substitute for writing your own tests. You should find any standards tests that might apply to the subsystem and run them. (If you don't know which standards tests might apply to your change, consult the gatekeepers or the C-team.) You should review your own changes extensively. Are you obeying all of the locking rules? What are the locking rules, anyway? Are you building new dependencies into the subsystem? (This can only be answered with extensive, laborious cscope'ing -- you cannot rely on code reviewers to pick up subtle new dependencies.) Review your changes again. Then, print your changes out, take them to a place where you can concentrate, and review them yet again. And when you review your own code, review it not as someone who believes that the code is right, but as someone who is certain that the code is wrong. As you perform your self-review, look for novel angles from which to test your code. Then test and test and test.

It can all be summed up by asking yourself one question: have you reviewed and tested your change every way that you know how? You should not even contemplate a putback until your answer to this is an unequivocal YES.. Remember: you are always empowered as an engineer to take more time to test your work. You can -- always take time to do the Right Thing. This is important to avoid the Quality Death Spiral. You must do your part by delivering FCS quality all the time.

Does this mean that you should contemplate ritual suicide if you introduce a serious bug? Of course not -- everyone who has made enough modifications to delicate, critical subsystems has introduced a change that has induced expensive downtime somewhere. We know that this will be so because writing system software is just so damned tricky and hard. Indeed, it is because of this truism that you must demand of yourself that you not integrate a change until you are out of ideas of how to test it. Because you will one day introduce a bug of such subtlety that it will seem that no one could have caught it.

And what do you do when that awful, black day arrives? Here's a quick coping manual from those of us who have been there:

  • Don't pretend it didn't happen -- you screwed up, but your mother still loves you (unless, of course, her home directory is on jurassic)
  • Don't minimize the problem, shrug it off or otherwise make light of it -- this is serious business, and we take it seriously
  • If someone spent time debugging your bug, thank them
  • If someone was inconvenienced by your bug, apologize to them
  • Take responsibility for your bug -- don't bother to blame other subsystems, the inherent complexity of illumos, your code reviewers, etc.

But most importantly, you must ask yourself: what could I have done differently? If you honestly don't know, ask a community member to help you. We've all been there, and we want to make sure that you are able to learn from it. Once you have an answer, take solace in it; no matter how bad you feel for having introduced a problem, you can know that the experience has improved you as an engineer -- and that's the most anyone can ask for.