A Philosophy of Software Design | John Ousterhout | Talks at Google
[MUSIC PLAYING] SPEAKER 1: OK. So, welcome. Good morning, everyone. It’s my pleasure to welcome
Professor Ousterhout for this Talk at
Google presentation. My name is [INAUDIBLE]. I work in the Technical
Infrastructure Cloud. A very quick intro to John to
get things started here– John is a Professor of Computer
Science at Stanford University. He spent, before that,
14 years in industry where he founded two companies,
Scriptics and Electric Cloud and, before that, 14 years as
a professor at UC Berkeley. Anybody here who has taken
a class with John before– so a bunch of his
students as well.
So he’s pretty
distinguished, so I’m not going to go through a
list of his awards here– a member of the National Academy
of Engineering, ACM Software System Award, UC Berkeley
Distinguished Teaching Award, to name a few here. But more relevant
to this talk, John has built several influential
systems, the Sprite operating system, Tcl, Tk, log-structured
file systems, Raft, and, more recently, RAMCloud. And I think his bios
says he has graded, like, thousands
of programming student assignments. And today, we are going to
hear the synthesis of all of John’s first-hand knowledge
both from his research and teaching on a topic that’s
very close to us, how to design and how not to design software. There is a book here. I don’t have it to show. Let me show it to
the camera here. There is a book or “A
Philosophy of Software Design.” I was pretty lucky to read
the pre-print of this.
It’s a really good book. And hopefully, he’ll
tell us a little bit more about that as well. And last note, if
you have questions, since this is being recorded,
just raise your hand, and you’ll get the mic to you. And then you can ask questions. So, welcome, John. JOHN OUTSERHOUT: Great. Thank you. [APPLAUSE] First of all, I do not
have an hour’s worth of material prepared,
because there will be lots of questions. I’m happy to take
them in line as we go. At some point, probably,
there’ll be too many and we’re running late,
then I’ll cut them off. But feel free to ask questions,
you know, express disbelief, argue, insult my mother,
whatever you wish, during the talk.
But do make sure that the
insults are on the mic. [LAUGHTER] OK. So people have been
programming computers for more than 80 years now. And yet software design is
still basically black art. There’s essentially no agreement
on how to do software design or even what a good piece
of software looks like. You know, we talk a lot
about software engineering, software tools,
testing, and processes. But we have almost
no conversation about the fundamental act
of designing software. And this has bugged
me for a long time. So today, I’d like to tell
you about some things I’ve been doing to try and change
that, to start the conversation, and to see if we can somehow
create a more, greater, sense of design awareness
in the software community.
And that consists
of a new course I developed at
Stanford and then, based on that, a book
on software design. OK. So let’s start with the basics. If you had to pick one
the idea, one concept that’s the most important thing in
all of computer science, one thing that threads through
every aspect of computer science from AI to systems
to theory, first of all, what would you pick? AUDIENCE: Abstraction.
JOHN OUTSERHOUT: Abstraction. AUDIENCE: Testing. JOHN OUTSERHOUT: Sorry? AUDIENCE: Testing. JOHN OUTSERHOUT: Testing. Other ideas? AUDIENCE: Composition. JOHN OUTSERHOUT: Sorry? AUDIENCE: Composition. JOHN OUTSERHOUT: Composition. AUDIENCE: Complexity. JOHN OUTSERHOUT: Sorry? AUDIENCE: Complexity. JOHN OUTSERHOUT: Complexity. So I asked Don
Knuth this question. He said, layers of
abstraction, which I would say is almost right. [LAUGHTER] Although, some would say,
by definition, it’s right. What I would say is
problem decomposition. How do you take a
complicated problem or system and chop it up into
pieces that you can build relatively independently? And yet, as far as I know,
aside from what you’re going to hear today, there
is no course anywhere where that concept is one
of the most important ideas in the course. We simply don’t teach it. Second, we all know that
some programmers are way, way more productive than others. Google has coined the
term the 10x programmer. And yet, as far
as I know, no one attempts to teach
these skills either. And these ideas have been
sitting in the back of my head and bugging me for a long time. How can we have these things
that are so important, and yet we don’t
make any attempt to teach people this stuff? You know, if you’re
a great programmer, somehow you just figure
it out on your own.
So that leads to the
question, can we do this? And there are three
questions there. First, is it even possible? You know, is this just
some sort of innate skill that you’re born
with within your genes, or is it something
that can be taught? So first let’s do
a quick survey. How many of you think this is
something that it’s, you know, either you have it or you don’t? You can’t teach it. It’s just an innate skill. A few. How many think
that this could be taught in some way or another? Good. A few summers ago, I
read a great book by Geoff Colvin titled
“Talent Is Overrated.” If you want a fun read, read it. It’s a relatively quick read
in which he goes through study after study that
shows in these fields where we think of some people
as being tremendously talented, the only thing
that differentiates the top performers from
the average performer is how much they’ve practiced.
That’s the one consistent
correlating factor they could find. It’s all about practice. So that got me
thinking, I mean, I believe it’s got to be
possible to teach this. But the second thing is,
who’s going to do it? Well, unfortunately, not
most of today’s faculty. The problem is the
typical faculty member writes a little bit of
code as a grad student, and never really gets
any instruction. You throw out some crappy
code for your dissertation, and then you become a professor. And then you decide coding
is what graduate students do, so you stop coding. And you never
write any more code. And so, you know, the only way
to become a great developer and learn design skills
today is with tons of personal experience,
sort of trial and error. Faculty don’t have that. So they can’t teach
their students. And so the whole process
just continues over and over and over again. But, fortunately, I’m not
one of those faculty members. So maybe I have a better
chance of doing this. So, I
love programming.
I mean, programming, it’s
one of the top two or three things I live for– you know,
my family, programming– [LAUGHTER] –really bad golf game. There’s a sort of few
things [AUDIO OUT] the things that drive my life. But if I don’t write 5,000
lines of code a year at least, I feel like it hasn’t
been a great year for me. So my career,
I’ve probably written, I think now, 250,000 to
300,000 lines of code. So I’ve had a chance
to observe a lot. And, again, as
I’ve been doing it, I’ve been thinking
about design stuff.
So I thought maybe, you know if
anybody can do it in academia, I probably got about as
good a shot as anybody. Then the third question
is, well, how do you do it? So I combined all these
three things. I finally decided only
one way to find out. I’m just going to
have to try it. You know, maybe I’ll crash and
burn, but I’ll give it a try.
So I created a new course
at Stanford, CS 190 Software Design Studio. It’s not taught like a
typical [INAUDIBLE] course. It’s taught more
like the way you learned English
writing in high school with an iterative approach. So, you know, the way about
writing is you write something. Your teacher marks it up. You get it back again. You rewrite.
They marking up again. And so the whole process
iterates several times. And it’s that process of
getting criticism, incorporating the criticism and
learning and seeing how that makes things better. That’s how you
become a good writer. So I thought we’ll try the
same idea in the class. And so that’s the way it works. Now, we only have 10
weeks in a quarter system. So we can only do three
iterations of this cycle. But the basic way it works
is in the first couple of weeks of the course,
students in teams of two build the largest
system they can build in three weeks. That’s typically about 2,000
to 3,000 lines of code, so, you know, still
not a huge system. Then after week three, we
go into the code review phase. Students read each
code and write reviews. And I read every
single line of code written by all of the students.
This is sort of one of the
limitations of the class right now. So I end up reading 20,000
to 30,000 lines of code in that week. And then we do code
reviews in class where students will present
pieces of their project and other students
will critique it. And then I do
longer code reviews. I read everybody’s projects. I typically make 50 to 100
comments on every team. And then I meet with the
teams individually for an hour and go over it with them.
Then the second
phase is they revise based on the code reviews. They add a little bit
of additional functionality as well in the second phase. We do another round
of code reviews. And then the third phase,
they get a new project. They start from scratch again. And we finally do a third
round of code reviews. And when they do this, the
students get no ideas from me about how to design. I just tell them what
the system has to do. They have to start from
scratch and figure everything out themselves. So they make lots of
mistakes in the first phase, but that’s part of the
learning of the class. Yeah, another question. Mic’s coming here. AUDIENCE: I watched
your YouTube video on the same topic you gave. And in that one, phase three,
was past the project to the person on the left and
inherit someone else’s code. Why did you change it? JOHN OUTSERHOUT: Yeah.
The first two times
I taught the class, I scrambled the
projects in the third phase and made people add over
somebody else’s project. That was a good
learning experience, but the problem is people
end up spending a lot of time decoding other people’s projects
and being frustrated with it. That has some value,
but they don’t spend as much time
doing real design stuff. And so I finally
decided if what I want is to teach
people design, I think they’ll learn more if
they get to start from scratch and do a new design project.
So I changed it. I may change back again. I’ll try this for a while
and see if it works. OK. So that’s the basic
idea of the course. Now, you may be wondering,
what are the magic secrets? How do you do software design? And by the way,
what makes me think I know what the secrets are? So first of all, I don’t
claim to know all the answers. I have, you know, some ideas
that have kind of coalesced in my mind over the
years, which I’m using in the course and the book. I don’t claim those are the end. I’m hoping
to use those to start a discussion of
all of them over time. But in the class, there are about
sort of 10 high-level ideas.
They’re very abstract,
almost philosophical. I can’t give you a recipe. You know, follow these
10 steps, and you will produce a great design. I’m not sure that
there is such a recipe. So instead, there
are these vague ideas that I talk about while
students are doing their first round of design. I’m not sure they
sink in very well, because they’re so abstract.
It’s probably hard to figure
out how to apply them. The way they work
best, though, was when we do the code reviews. Then I can show the
students how they violated some of these
principles and what the consequence
was in their code and how they should change it. And then they can go back
and apply the principles as they revise. And then I think the
ideas start sinking in. So it’s sort of a– question in back, yeah. AUDIENCE: Yeah. I’m curious what you think about
how the sort of end-to-end principle in sort of
software design factors into this in terms of trying
to push all the complexity to the edges of the system. JOHN OUTSERHOUT: End-to-end principle? AUDIENCE: So like
the [INAUDIBLE] example of TCP, the checksum,
you know, is done end to end.
It’s not in the
intermediate router stages. And that was a design
principle that they had. JOHN OUTSERHOUT: I’ll
have to think about that. I’m not sure that relates in a
direct way to these principles, but I’ll have to
think about that. So there are these
vague principles. Then the other thing I try and
do is talk about red flags. So red flags are
very specific things that, if you see this sort
of behavior or pattern, you’re probably in trouble. And actually, I
think for beginners in particular, red flags
are really useful to people. Because even if you don’t know
how to design the right system if you can see
you’re going wrong, then just try something
else until eventually, the red flags go away. And you’ll probably end up
in a pretty decent place. So red flags are
very useful. So I’d like
to do is I’m going to talk about the
four things that are in red just to give you
a sense of some of the ideas.
Two of them are
design principles. Classes should be deep. Define areas out of existence. And two of them are
more about mindset. I’ll talk about
those afterward. So the first one is this notion
that classes should be deep. This is just
another way of thinking about the idea of information
hiding David Parnas first put out in a paper in
the early 1970s. By the way, to me,
this paper is one of the two most important
classic papers and all software design and engineering. Has anybody here read
this paper in classes? Great. It’s still topical
and relevant today. The last third maybe
not be so much so, but the first part’s still–
and it’s got a great example and is easy to read. So the way I think
about it, think about a class as a rectangle. And the area of the rectangle
is the functionality that the class provides.
So you can think of that’s the
benefit that the class provides to the rest of the system. Then think about the
top edge as that’s the interface to the class. And by the interface, I
mean everything someone has to have in their mind
to use that class. It’s not just the signatures
for the functions, but things like side effects
and dependencies and things like that. That’s the cost. You think that that’s
the complexity cost that this class imposes
on the rest of the system. So we’d like that to be
as small as possible. So ideally, what you’d
like is the greatest cost– sorry, greatest
benefit, least cost. So you’d like the smallest
interface and then the largest area. So the opposite of what
I call a shallow class. So that’s something
that has either not a whole lot of
functionality or a complicated interface or both. Those classes, just
don’t give us much leverage against complexity.
In fact, in the worst
case, the additional overhead
of the interface adds more complexity
than what you’ve hidden underneath the interface,
and it’s a net negative. What we’d like to have, of
course, on the other side is a deep class– very simple interface
with a very large amount of functionality underneath it. I mean, that’s another way of
thinking about abstraction, right? The idea of abstraction
is we’re trying to provide a simple
way of thinking about something
quite complicated underneath. So deep classes are
good abstractions. And this notion you can
apply to classes. You can apply it to
methods within classes. You can apply it to interfaces
in general or modules in a system or
subsystems, or anything like that that has an
interface or an implementation. Yeah, question. AUDIENCE: Is this
one of the concepts that you give your
students before they start the first project or after? Because it seems important
to give at least a few hints at the beginning.
JOHN OUTSERHOUT: This is
what I give them before– well, it’s concurrent
while they’re doing the first project. We don’t have enough time for
me to go through all the ideas before they start designing. So while they’re
designing, we’re talking about this
stuff in class. Here’s a classical example
of a shallow method, which I have to say I
see distressingly often. There is essentially
no information hiding in this method. To use
it, you pretty much need to understand the
complete implementation. And, by the way, there’s
almost no implementation there. This is so bad that
it takes more keystrokes to invoke the method
than if you had just done the body of the
function yourself.
So it’s
a complete loss, just adding complexity and
getting nothing back for it. Another example, actually,
of a relatively shallow class I see all the time is a
class for list manipulation. This is very shallow. I mean, lists are really
easy to manipulate. It’s two-pointers. We know the pointers are
hiding underneath there someplace anyhow. And so that’s a
very shallow class. Now, you can’t always
eliminate shallow classes. You know, sometimes
you end up the best you can do is a shallow class. And so I wouldn’t say you
should never, ever have them. But a shallow class
doesn’t buy you much from a design standpoint. It doesn’t help you in the
fight against complexity.
AUDIENCE: [INAUDIBLE] AUDIENCE: So I think one thing
that is perhaps a bit subtle is that the size of the class
is not the amount of code that is in it right now. It might be the amount of code
that you eventually have to add or if you have to have multiple
implementations of it, right? So maybe this doesn’t
make sense in isolation, but perhaps there
is something else, there is another
implementation of it maybe right now or in the future
which is going and storing stuff in a database or
something like that. And so I kind of
wonder if you disagree with that
characterization that you need to keep track of the
evolution of the class in mind when deciding on its depth. JOHN OUTSERHOUT: Well,
I agree with that. The whole idea
behind software design is we’re doing things
for the future. It’s we’re doing things today
to make it easier for us to develop in the future. And you have to think
a little bit ahead. Although, of course, you
know the classic problem with software is we can’t
visualize the future very well.
So it’s dangerous to try
and think too far ahead. But I agree with your
concept in general. In this particular case,
though, the problem is that the interface is
already so specialized that there’s just
probably not a lot of room for wiggle room under there. Almost certainly anything you
change to the implementation is probably also going
to change the interface. And so, again, it’s probably
not going to help you. But, you know, if you
have reason to believe it will, well, yeah, sure. So the problem that leads
to this– and by the way, I think this is one of the
biggest mistakes people make, too many, too small,
too shallow classes. And the reason people do
that is that they’ve been taught classes and
methods should be small. How many of you at
some time in some course was taught your
method should be small? And how many of you have been
given a number, like any method larger than n lines,
you should chop it up? How many of you have heard
something like that said? OK.
Tell me a small value
of n you’ve heard. AUDIENCE: 20. JOHN OUTSERHOUT: 20? [INTERPOSING VOICES] JOHN OUTSERHOUT: Has
anybody heard 10? [LAUGHTER] I’ve heard 10 sometimes. So if you take this
to the extreme, this results in what
I call it classitis. Classitis is when somebody
says classes are good and somebody else
thought what they heard was more classes are better. [LAUGHTER] And so under
classitis, your goal is to have as many small
classes as possible where each class
adds the tiniest possible amount of
new functionality to the previous classes.
If you want more
functionality than that, make another class for it. Probably the best example
of this in the world today is the Java Class Library. There’s nothing in the Java
language that requires this. But somehow this has
just become the culture in the Java world, these tons of
little tiny shallow interfaces. Like, for example, the thing
that just bugged me about Java when I started programming,
if I want to open a file and read serialized
objects from it, I have to create three
objects for that. First, I have to create
this file stream. But for some reason,
I don’t get buffering. If I want to buffer– it’s like, do you
want it to eat today? [LAUGHTER] If I want to buffer, I have
to create another object. And then if I want read
serialized objects, you add another
object on top of that.
And each of these
can throw exceptions that I have to catch. And then exceptions can
come halfway through, and I have to do clean up– an enormous amount of
complexity for something simple. So I think they lost sight of
the idea that, first of all, in managing complexity, the
common case matters a lot. You want to make the common
case simple. I shouldn’t have
to do all of this. OK. Maybe I want to be able
to not have to buffer, but that should be an
option that– you know if I don’t want to buffer, that
should be where things get more complicated, not to have to
remember to ask especially for buffering.
So to me, it’s not about length. Length is not
the fundamental issue. I don’t have
problems with methods that are hundreds of lines long
if they’re relatively clean and if they have a
deep interface to them. It’s an abstraction. That’s the most important thing. So rather than
striving for length, you should first try and
get these deep abstractions. And then if you end
up with something big and long,
see if you can chop it up. But depth is more important. So what’s an example
of a deep interface? My favorite in the
world, probably one of the most beautiful
interfaces ever invented is that Unix file
I/O interface– five functions, falling
over easy interfaces to every function.
The only complexity is the flags
and the permissions for opening are a little bit funky,
but everything else is very, very simple. And then think
about what’s hidden behind this simple interface. There are typically hundreds
of thousands of lines of code behind that that do everything
from managing disk space to file caching to, you
know, device drivers– an enormous amount of code and, of
course, many other lower-level interfaces behind this, but
just this amazingly beautiful five functions. Now, today, people look at
this and think, well, duh, of course, that’s the
way you design things. But it was not like this. Has anybody in this
room programmed it at around the time
Unix came out? Only a few of us are old
enough to have done that.
Things were pretty
horrible before then. Like, for example, in
pre-Unix file systems, there would be a
different set of kernel calls if you wanted to open a file for
random access versus sequential access– two different sets of kernel
calls, different kinds of files. You’d have to declare your file
random access or sequential. And Unix just had this
amazing simple interface. So anyhow– a beautiful example. OK. Let me go to my second
design principle, which is to define errors out of existence. So we all know that exceptions
are a huge source of complexity in systems. For example, in the
RAMCloud storage system, we thought we were building
a low-latency storage system and figured out we’d be spending
all of our time making things fast and low-latency. But 90% of our time
went into doing crash recovery.
So they’re a huge
source of complexity. And, again, the common wisdom
kind of leads people astray. You’re taught to program
defensively, which is good. But people then
think, ah, I should be throwing lots of exceptions. The more exceptions
I’m throwing, the better defense I’m doing. And as long as I, building my
module, catch all the errors and heave them up in
the air as exceptions, I don’t have to worry
about where they land or who’s going to
deal with them.
I’ve done my job. The more of them I
throw, the better programmer I am. But then that just
results in this explosion of exceptions that have
to be caught and handled. And then the exceptions
themselves create more bugs. And you have secondary
exceptions and so on. So I would argue,
in general, overall, we should try to minimize
the number of places where you have to
handle exceptions. You can’t eliminate
it but try to minimize that. And the best case of all– which I found you can
do surprisingly often– is simply to redefine
the semantics so that there is no error. There is no exception at all. That’s the best. It’s just gone completely. So let me give you
three examples of that. First one– when I was designing
the Tcl scripting language many years ago, I introduced
an unset command, which deletes one and more variables. When I was doing
this, I thought– by the way, this is a
classic mistake, people– I thought why in
their right mind would anybody ever delete a
variable that doesn’t exist? That makes no sense.
So I’m going to throw
an exception for that. Well, turns out people
do that all the time. For example, you’re halfway
through a computation. You decide you
just want to bail. So you want to clean up all
the mess you might have made. So you go through and
delete every variable that you might have
created, but you don’t know which
ones you got around to creating yet. And so some of them don’t exist. So in practice, people complain
about this all the time. And, if you
look at the Tcl code, I bet virtually every
invocation of the unset command is wrapped in a catch command
that will catch the exception and throw it away
when it happens. So what I should have done– in fact, I don’t know
why I didn’t fix it once I found the problem. That was my second mistake. What I should have done
was redefine the semantics, so that unset makes
a variable not exist. And if you think
of it that way, ah, well, if the variable doesn’t
exist already, we’re done, clean, no harm, no foul.
So that’s what I
should have done, should not have
thrown that exception. Another example
is file deletion. In Windows, at least
in the early days, if you tried to
delete a file when some process had the file
open, that was not allowed. Has anybody experienced this? Does anybody that has used
Windows experience this? It’s this horrible thing. And then now you figure,
OK, who’s got it open? You start going around killing
programs to try and make it so you delete the file.
And you can’t find which
program has it open. So finally, you just
give up, and you reboot. And then it turns out– [LAUGHTER] And then it turns out
that it’s a system daemon that has the file open. So when you reboot,
it’s still in use. You can’t delete it. So Unix has a beautiful
really lovely solution that makes this error go away. In Unix, if you delete
a file while it’s open, what happens is it deletes
the file from the directory and the namespace. It no longer appears
anywhere in the file system. But the actual
contents of the file still hang around, so that any
process that’s using the file can continue to access the file. And then when the last
open instance of the file is closed, finally it
cleans up and throws away everything else. It’s a lovely solution. Because you could have
made the other mistake, which is that I’m going
to delete the file. And now, anybody doing I/O
on the file gets an error.
That would probably
be even worse because now every
program would have to be written to
handle the case where the file goes away in the
middle of your accessing it. So Unix had this beautiful
solution where there were no errors on either side. It just does the right thing. So Windows, eventually
realized this was a problem. And I don’t know where they are
in the evolution of fixing it. But I know that the
first stage they did is they had some
special flag you could set that would say, delete
the file even if it’s open. Or you can maybe when you
open the file– so then you can allow it to be
deleted while it’s open. But they didn’t get rid
of the directory entry. They kept the
directory entry around. And so you couldn’t recreate
the file while it was open. So if you were, for
example, trying to rebuild and, you know, the
binary was in use, you couldn’t create a new
version of the binary file, because the directory
entry was locked down.
So they still
didn’t get it right. So, again– examples of why you
just want to make these errors go away. A third example,
which people often consider more controversial when
I say this one, is substrings. So in Java, various methods will extract a
substring out of a string. And these methods are
very exception happy. If either of the indices is outside
the range of the substring and throws an exception– and I
believe if they’re kind of out of order, it throws
an exception– I find this a huge pain. I end up, typically,
having to write my code to effectively clip my indices
to the range of the string before I invoke the
Java substring command, so I won’t get these exceptions. I consider this just a bad idea. Instead, it’d be so
much simpler just to find the substring,
so that it returns the overlap between the
indices you’ve specified and the available
contents of the string.
So if both of the indices
are outside the range of the string, it
returns an empty string. You know, if they
are in reverse order, clearly there can’t be
anything in the right order. It doesn’t return and just does
the clipping automatically. This would eliminate so much
extra code people have to write and also, I suspect, a lot of
runtime exceptions that occur, because people forgot
to clip their indices to the range of the string
before they invoke the command. So, again, the overall idea
here is to try and reduce the number of
places where people have to deal with exceptions.
The book talks about a few
other ways of doing this. But the best case of all is just
to define the exceptions away. Just make the normal behavior
and always do the right thing. And nobody’s objecting
to the Java substring. Usually, somebody raises their
hand and objects to that. Yeah. AUDIENCE: Yeah. So not objecting, but defining
[INAUDIBLE] The semantics, perhaps, is a more
complete definition. I mean, you could
simply say, OK, whatever set of
parameters you pass, you have random behavior that
is difficult to explain.
But the examples you gave
were very, very intuitive, like when you defined
overlap between the indexes and the available space
or the Tcl, et cetera. JOHN OUTSERHOUT: Does somebody want
to argue that Java substrings should throw exceptions? AUDIENCE: Yeah. [INAUDIBLE] JOHN OUTSERHOUT: Let’s bring
the microphone over here. AUDIENCE: [INAUDIBLE] JOHN OUTSERHOUT: I know some
of you thinking it, so yeah. AUDIENCE: I think it should
throw a runtime exception, because, like, you can– sometimes it could be
just the wrong assumptions on the client side. Like, they’ve just
done something wrong. And they’re assuming that’s
the correct range, but it’s not. I think it should be
a runtime exception. JOHN OUTSERHOUT: This is
part of the philosophy about we’re going to
try and keep people from making mistakes, which
it’s kind of a noble thought. But the problem is that it’s
really hard to keep people from making mistakes.
And so typically what
happens when we try and do this is we introduce lots of
complexity, which makes it hard to even do the right thing. And so I’d argue, in general,
we want to make it easy to do the common
case and the right thing. You still have to do
testing to catch mistakes. And so this isn’t
going to replace the need for unit tests. But, in general, I think
when you try and build systems that can keep
people from making mistakes, you usually create a whole lot
of complexity for everybody. AUDIENCE: So how
do you distinguish between Java substring versus
java.string.character@index? Should that throw an exception
if the index is out of range? JOHN OUTSERHOUT: Good question. That one probably should,
because it can’t return a character. You know, what do you– you know, you could redefine
it kind of like IEEE Floating-Point did,
where there were special values that
represented arithmetic errors and things like that.
So if you have some
sort of a null character that you could return,
that might make sense. But we have that
in our character sets. So in that case,
you really can’t. I’m not sure what you
would return if you were going to return something. So, yeah, in that
case, you probably do have to throw an exception. AUDIENCE: [INAUDIBLE] AUDIENCE: -optional character. AUDIENCE: So one
question– so when is it a good idea to
throw exceptions? So the intuitive case is you
want to eliminate exceptions as much as possible, then
how do you decide, no, there’s nothing that I can do? My changing semantics
doesn’t work. And I have to
throw an exception now.
JOHN OUTSERHOUT: Well, if you
fundamentally can’t carry out your contract with
your caller and if you can’t implement your
interface, then sort of have to throw an exception. You know, like,
if you’re reading [INAUDIBLE] you’re
doing a read operation and the reader can’t succeed,
because you got an I/O error, you have to. That’s information that you
do have to reflect the caller. So you can’t eliminate
all exceptions. You know, some of the
students in the class thought this was
a great idea. And so they
wrote no exception handlers in their code– [LAUGHTER] –and said, I’m just defining
them out of existence. I said, no, no, some of
them matter. You have to– and a lot of
software design, I think, is figuring out what matters
and what doesn’t matter. And, ideally, you’d like to make
as little matter as possible. You know, you’d like to be as– to not depend on
very many things.
But you have to recognize
the things that are important and do matter. And those you do have to
reflect in the system. Yeah. AUDIENCE: Do you have
thoughts on exceptions versus extra status codes
or extra error code returns? JOHN OUTSERHOUT: Boy, that’s
a classic argument, exceptions versus return values. Now, there are times
when each makes sense. I believe exceptions
provide the most value when you throw them farthest. If they’re going a
long way up the stack, that’s where they provide
the biggest benefit. Because you’ve
made it unnecessary for a whole bunch of
intermediate layers to deal with the situation. You can take it back
to the source and deal with it at the source. If you are catching exceptions
in methods you call, you know, there’s not as
much value for that compared to just getting a return value. It’s not a whole lot different. One of the problems
with exceptions is, also, they all
have clunky syntax. And
checking a return value is simpler
syntactically than declaring an
exception handler. But, you know, a lot of times,
some of these exceptions you really will want them
to propagate a long way.
And so then even if the
caller is going to look at it, you probably want to manifest
it as an exception rather than a return value. AUDIENCE: Any
thought on crashing as a way of, like,
avoiding [INAUDIBLE], like, passing [INAUDIBLE]? JOHN OUTSERHOUT: Any thoughts
on crashing as a way? AUDIENCE: Yes. JOHN OUTSERHOUT: Yeah, yeah. Crashing
is a fine thing to do in certain situations. Like, for example,
in most programs, I would argue you should not
be trying to [AUDIO OUT] out of memory exceptions.
It’s just hopelessly complicated to try and do that because it depends
so much on memory. For most applications,
if you run out of memory, just crash. I mean, print a
message and crash. It rarely happens. Machines today have
tons of memory anyhow. You know, again,
use with caution. But I think there are
situations where just crashing is a fine thing, a
fine way to do it. The alternative would
create so much complexity that you probably wouldn’t get
it right in the first place and it’d end up crashing anyway. SPEAKER 1: Hey, John, there’s
17 questions on the Dory. So I’ll just ask
the top-ranked one so that we don’t
run out of time.
The question is
from [INAUDIBLE]… And in real life, development is
often under these constraints– for example, tight
schedule, need to experiment and be
flexible about design, lack of mature tools,
library, et cetera. Does your approach to software
design philosophy change base on real-world constraints? JOHN OUTSERHOUT: Yes. And actually, that’s
a good lead-in. Let me move on to the
next part of the talk, which I’m going to
talk about that. And I think I want
to get moving, too, because we’re starting to
run a little tight on time. [AUDIO OUT] about a couple
of specific techniques or philosophies you can use. But I think one of the biggest
obstacles to good design is a mindset. If you don’t have
the right mindset, you will just never
produce a good design. And I think there are two ways
of approaching the programming or design process, which I
call tactical and strategic. Unfortunately, most people
take the wrong approach, which is the tactical approach. In the tactical
approach, your goal is to get something working– you know, your next feature,
fixing a bug, whatever.
But that’s your goal, to
get something working. So you think, how
can that be wrong? Isn’t that the whole goal of,
you know, building systems? We want them to be working. Well, the problem
with this approach is that when you do
it, then you usually have a mindset that, well, I’ll
try and make it mostly clean. But if I have a little– if I take a couple of, you
know, shortcuts or a little bit kluges, as long as I
don’t do too many of them, if I get it working,
that’s what counts. The problem with that is that
those kluges build up fast. Because not only
are you doing it, but everybody else on
your team is doing it. And then after a while you
start saying, well, I probably shouldn’t have made that
decision back there, but we’ve got our next
deadline we’ve got to hit. You know, I’ll try and find
time after that deadline.
But then by the time
the deadline is done, you’ve introduced
so many kluges, you realize it’s going to
take weeks to fix all these. And you don’t have
time for that. And so it just never happens. So the result of this
inevitably is the system turns to spaghetti
fast. And the problem
is that complexity isn’t one mistake you make. It’s not a single big thing that
makes the system complicated. It’s hundreds or
thousands of mistakes made by many people
over some time. And so that means,
first of all, you don’t notice it as it’s
happening, because it’s just a bit at a time.
But then, even worse,
once it’s happened, it’s almost impossible to fix. Because there’s no one
thing you can go back and fix to fix the problem. It’s thousands of things
in hundreds of places. And so it’s just
so overwhelming, you never get to it. So, by the way,
there’s an extreme of the tactical programmer,
sort of personality type, I call the tactical tornado. This is a person who turns
out enormous amounts of pretty shoddy code that kind of
80% work at a huge rate and leaves a wake of
destruction behind them.
[LAUGHTER] And in many organizations, these
people are considered heroes. This is who management
goes to when we need the new feature for tomorrow. And they’ll have
a feature tomorrow that works for most of
the day tomorrow, anyhow. When I gave
this talk, somebody said, oh, is that what you mean
by the 10x programmers? [LAUGHTER] No. No. How many of you feel
like you’ve been in the presence of one or
more tactical tornadoes over your career? Yeah. Every organization has them. I’m sure they’re here at Google. And, unfortunately, one
of the things I hope to do is to provide a
name for this, so we can start calling them out
and maybe educate management about them. How many of you feel like
your management rewards technical tornadoes? Yeah, that’s even
worse, unfortunately.
So, you know, the
tactical approach is easy to slide
into, very hard not to do that. But so the first thing
you have to realize, if you want to do great
design, you have to realize working code is not enough. That can’t be the only goal. It’s sort of table stakes. Of course, things have to work. But that shouldn’t
be the real goal. Instead, you should take what I
call a strategic approach where the goal is a great design. That’s the most important thing. Yes, it’s got to work today, but
we have to have a great design. Why do we want a great design? It’s so that we can
develop fast in the future. So it’s all
about investing. We’ll invest in
good design today because most of
the code we develop is going to be in the future. And so if we mess
things up today, we’re slowing ourselves
down for the future.
So you have to think
about complexity, and try and find ways to drive
complexity out of the system. And, fundamentally–
this is the hard part– you have to sweat
the small stuff. And you sort of have to have
a zero-tolerance approach. Don’t let those little
bits of lint creep in. Because if they do, then
you’re going to slide back into a tactical mode again. You know, so if you
program strategically, you’re going to go slower first.
But, presumably, in the end,
you’ll end up going faster. I mean, even with a
strategic approach, you’re development will
still slow down over time, because complexity
is inevitable. We can’t prevent it. We can only kind of slow the
growth as much as we can. But, eventually, you’ll end
up [INAUDIBLE] being faster. So the issue is
you have to invest.
If you’re not willing to take
a little bit of extra time today, at least a little bit,
you can’t do good design. But in my opinion,
it all pays back. You always get your
investment back. Now, the challenge is what are
the parameters of these curves? Like, how much slower do
I have to go initially? You know, when do
I get to the point where the slopes are the same,
so I’m at least now running as fast as I was tactical? When do I catch up
so I have a net gain? And then how much
faster do I go at the end? Unfortunately, I don’t know
of any quantitative answers to any of those questions.
You know, I have opinions. Like, my opinion is that
the crossover period is somewhere in the 6 to
12-month range, roughly how long it takes you
to forget about why you wrote that last chunk of code. But I don’t have any data to
back that up, unfortunately. So that’s the challenge. Because, you know, people
can see the cost today, but they can’t
estimate when the benefit’s going to come
or how long in the future it’s going to be.
And so it’s really easy just
to optimize for this part down here. AUDIENCE: Yeah,
just a quick question. Have you thought of analyzing
open-source development and seeing whether or
not you can observe this? I mean, it would require
some judgment calls, I guess. But– JOHN OUTSERHOUT:
I’m not sure how you do– you can look
at the rate of commits from the new lines added. But the problem is you
don’t know how much effort went into those. AUDIENCE: Yeah. JOHN OUTSERHOUT: So– AUDIENCE: I was thinking like
when a new storage device comes out and people are writing
new file systems or something. You know, you could look at
this across different OSs and see how things happen. JOHN OUTSERHOUT: Yeah,
that’s a thought. It’s hard. So the question is,
how much do you invest? Let me charge
for a couple more slides because we’re starting to
run a little tight on time. So how much? What’s the right
amount to invest? And, you know, if you
look at most start-ups, honestly, they’re mostly
completely, 100%, tactical. They feel tremendous pressure
to get the products out. You know, our financing is
going to run out in six months.
Whatever evil we
put in, you know, when we become famous and
wealthy and do our IPO, we’ll be able to
hire more engineers to clean up this mess. And so, you know, they end up
with these horrible spaghetti code bases. And, unfortunately, of
course, once you get that, it’s pretty much
impossible to clean it up. I’ve never heard
of anybody– maybe if you could go back
and completely rewrite, you could do it. Maybe there have been
examples of that, but it rarely happens. So to me, Facebook is the
poster child for this. They even built it into
their company motto– move quickly and break things. And they did both of those. [LAUGHTER] So now, in a sense,
developers kind of like this.
So fresh college
hires could come in. They’re super empowered. They push their first
commits, and they’re changing the website in one week. The first week on the job
feels like, you know, really empowering for them. But their code base was
just a notorious mess. One summer, a couple
of my graduate students went there for internships. They came back, and
they just couldn’t believe what they’d seen. And this was interesting
because my students had been whining for the
whole previous year about why we had to write unit
tests and write documentation. Linux doesn’t have
any unit tests. And Linux doesn’t
have any comments. Why do we have to do this? I never heard that again after
they came back from Facebook.
[LAUGHTER] So Facebook has
eventually realized this. And so they changed their
motto to “move quickly with a solid infrastructure.” It’s like, you know,
no, no, I meant to say wouldn’t, not would. [LAUGHTER] So have they been
able to fix it? I don’t know. Now, people hear this,
they say, well, yeah, but Mark Zuckerberg is
the fourth wealthiest person on the planet. How can this be a bad approach? And, of course, the
answer is, you know, you can be successful
with crappy code. You can be tactical and
build companies that succeed. Facebook has done it. But I have two thoughts on this. One is you can also succeed
with the other approach. And Although I don’t
have personal experience with either of these
companies, I sense that both Google and VMware
took a much more, sort of, stronger design culture
approach at least particularly in the early
years, you know, 2000 to 2010.
Those were the two companies
known in Silicon Valley as this is where the
great software developers go. They care about design. And they do really good coding. And the reason this matter is
that if you have that culture, I think you’re in a
much better position to recruit the best programmers. And we know about
this 10x phenomenon. You know, the best way
to get great products out fast is to get the
best programmers. And so I think the
strongest argument in favor of a good
design culture is that it allows you
to hire top people. And that will give
you an advantage. So you can do things the
crappy way if you want, but there are enough
success stories to show that you
can make this work. You can do the
investment approach. OK. But then again, OK,
but how much to invest? I would say how
much can you afford? You know, just ask
yourself what’re the most we can afford at
this stage in our lifetime to invest.
I think in terms of maybe 10%
or 20%, or something like that. I bet, almost everybody, you can
afford to go 10% slower today realizing you’re going
to get it all back. It’s not a sunk cost. It’s all coming back to you. In somewhere, you know,
six months to a few years, you’re going to get
it all back again. You can afford to do that. And I think in terms of
small steps, not heroics. You know, you can’t
spend six months and completely design
the whole system, you know, from first principles
before you write any code. That doesn’t work. We know one of the
problems of software is we can’t visualize
the way our systems are going to turn out. You have to use a somewhat
iterative approach. So I’d say, think of it
in terms of small steps.
So when you’re
building new modules, take a little bit of time
to design the interface. Try to come up
with deep classes. Write the documentation
as you’re going, unit tests, of course. And then whenever
you make changes, you have to realize you can’t
get it right the first time. It’s one of the
rules of software. We just can’t. So assume every time
you’re in a system, you’re going to have
to be improving things. Always look for something
you can make better. One reason for this is you’re
probably making something worse when you go in also. So even if you just
want to break even, you’ve got to find
something to improve. So I just think that I’m just
trying to at least break even. So this is against
the normal philosophy. I think, oftentimes, when people
are changing existing code, they go for what’s the smallest
number of lines of code I can change to do this.
I think sometimes it’s
because they’re afraid. I don’t understand this. I’m going to break
something, so I’ll make the fewest lines of change. So I’ll just kind of directly
access a global variable, another file, rather
than trying to find a clean interface for it. So don’t do that. Try and find a clean
way of doing things. And ideally, in the
best case, you’d like to end up where
you would have ended up if you had built the whole
system from scratch knowing what you know today.
That’s what you’re like. Is this the way we would have
done it if we had built it from scratch? That’s the ideal. Now, you know, you
can’t always do that. Sometimes that would
be a refactoring that’s way beyond the
scale of what you can do. So, again, I would just say
ask yourself at any given point in time, am I doing the
most I possibly can? Don’t just give in
and the boss says we’ve got to get a
release out tomorrow, so I’m just going to
hack this thing in to make the boss happy.
Ask, is this the best
I can do that. Yeah. AUDIENCE: John, will
you touch on layering, which is often considered very
good in software engineering, but also has a
challenge to the performance of the code when it executes? Often, the functionality is
replicated in multiple layers. And it’s not clear
how to distill that without mucking up the code. JOHN OUTSERHOUT: So is your
question, are the layers good? AUDIENCE: Yeah. So how to do [AUDIO OUT]
deal with layers, the demand for
abstraction layers, and the demand for
performance both contradict each other here? JOHN OUTSERHOUT: We need
layers for managing complexity. So in that sense, they’re good. They do have problems
with performance. I think people do
too many layers. And so I think that’s a
common mistake is just to throw in lots
of skinny layers, again, rather than having
a smaller number of meatier layers.
That’s one problem. The other thing is that
for cases where performance matters if you think
a little bit about what your key performance
metrics are, I believe you can often define
your layers in a way that allows you to achieve very high
performance even with layering. But you have to be thinking a
little bit about sort of this, the overall, kind of gross
level performance metrics while you’re doing
your system design. OK. So anyhow, I think this
investment thing is the biggest challenge, particularly because
a lot of managers, I think, don’t get it. And they’re sort of pushing
for the next feature or the next release. I don’t know how to
combat that except to have the people that
everybody recognizes as the great engineers in
the company go to management and try and get
management buy-in. And I think it’d be
great if companies can have sort of explicit–
this is in our engineering philosophy. We will invest at
10% [INAUDIBLE]… Roughly, on average,
10% of our resources are going towards making designs
better, not just cranking out the next features. Yeah, John. AUDIENCE: Hi, John. I haven’t looked at
this for 20 years now, but the quantitative analyses
that you’re describing were the domain of
the social engineering community for a while.
I’m wondering if you sort
of poked into that space to see if they have
experimentation, numbers, and analyses, that might be useful. Or have they just
dropped off the map? JOHN OUTSERHOUT: I haven’t
looked very carefully, partly because I’m sort
of skeptical that it is possible to measure
it in a meaningful way. So I haven’t– but I would
[AUDIO OUT] See numbers if there were [AUDIO OUT]
were believable. But I haven’t looked
enough to know. OK. So I just have a
couple more slides, then I’ll open it
up for questions. So is the course working? You know, it’s hard
to tell for sure.
I mean, you can’t become a
great programmer in one quarter. It’ll take, still, lots of
experience for these people. We’ll have to come back
and ask them once they’ve been out in the industry for a while. This is just sort of the first
step in the process I think. But the class has a really
fun energy to it, really fun. The students start in the
first round of code reviews.
And they’re very
tentative, kind of afraid to criticize each
other, and not sure what they should be saying. So the first round’s
kind of tentative. Then they meet with me,
and they get unloaded. And then they start– oh,
OK, I can do that, too. And so about halfway through the
second code review, typically the tone of discussions changes
and students are now more critical of each other. We have these interesting
arguments in class about whether one approach is
more complicated than another. And it’s really clear by
the end of the quarter that the students are
thinking about programming in a very different way than
they thought about it before. So I think that’s
encouraging. I will have to say, though, even
if the students don’t become better designers, actually,
I’m becoming a better designer from teaching the class. It’s really interesting for me. You know, you might
think it’s tedious reading 20,000 lines
of code in a week three times over a quarter. And it does take some time. But I
learn a lot from that because the students are
all doing the same thing in the same language.
And so I can see
different approaches. And I can see the kind of common
mistakes that they make. And, what’s
ended up happening is I’ve changed my view about a
couple of the design issues as a result of
teaching the course. And in particular, the one
at the bottom of this slide I realized that
making classes just slightly general purpose,
even though you’re only going to use them in
one place, actually makes the amazingly
simpler and deeper. I hadn’t realized
that until I saw some student projects,
actually, the second time I was teaching the course. So it’s been really
interesting for me to see this. And I feel like
I’m learning more about the process
of software design and getting better ideas
for how to teach it as I go.
Then as I’ve talked to
people about the course, people kept saying, well, you
should write a book on this. And I was on
sabbatical last year, so I decided, OK,
I would do that. So I have written this book. It’s a relatively small
book, 170 pages, again, still fairly philosophical more than prescriptive. So the hope is that I can reach
more people because I can only teach 20 people a
year in the class. I can’t do more than that. And ideally, I’m hoping the
book will kind of be a lightning rod that maybe we can
kick start a discussion. I hope people write
to me and disagree with things in the book or
suggest other design ideas or provide better examples. I’m also trying to define
terms that people can use. One of the things I hope
maybe the book can do is people can use
that in design reviews to have ideas and
ways of talking about design and design ideas.
A have a
question for you. How many people here
do design reviews? I assume almost everybody. And how many of
you in your design review are it common
to discuss design issues as opposed
to coding style and bugs? How many of you commonly
talk about design things? Good. Great. I’d like to see more of that. So now the book’s been out
for a couple of months. If you read it and have
thoughts or comments, I’d love to get
your input on it. My view is this is
just a first stab. If this ends
up being a useful book, it’ll probably be– it’s kind of like software. At about version
three, you know, where it hits
its stride, I’ll need to get feedback and revise
and fix wrong things and add new ideas. So just to conclude,
I believe we can teach software design. I believe we can. My current approach,
unfortunately, is woefully non-scalable. I haven’t figured that out.
I need somebody
with a lot of experience I think reading
the student’s code. I feel like some
principles are emerging. You know, if you read the book
and agree or disagree with me, I’d be curious to hear
your take on that. What I hope to do in
the future is just to somehow leverage this
out into the community. I’d love to see just a greater
sense of design awareness in the software community as a
whole can we have a discussion? We make an important
part of our community it’s just sad that
we don’t even have discussions today. So I created a mailing
list for discussion topics about the book. Hasn’t been much
traffic on it so far, but I hope there will
be more in the future. And, again, I hope to
take ideas from people and make the book better and
better and the class better over time.
And the long-term
the goal is it’d be great if, 5 or 10 years
from now, we have a relatively
robust set of principles that most developers agree
are good for software design. So thanks for your
attention, and I’d be happy to take more questions. [APPLAUSE] SPEAKER 1: So let me get a
few questions from the Dory out here. Do testing considerations,
like unit testing or marks, influence interfaces, and abstractions? JOHN OUTSERHOUT:
Testing considerations– I’m a huge fan of,
particularly, unit tests– crucially important.
I have debated whether to try
and teach them in class. I did a little bit
at one time in the class, but it was just too much of
a distraction for the class. We only have 10 weeks. And so I finally
decided, for the class, I just got to focus on design. So I don’t do unit
testing in class. But I’m a complete
advocate of unit tests. I can’t imagine doing
software without unit tests. AUDIENCE: So I was
intrigued by the first part of the discussion when you said
in the iterative process you dropped– you initially had students
build on other students’ code. To me, that’s a very,
very real-world problem. And I’m just
surprised that you– I mean, I wish you
would teach that. JOHN OUTSERHOUT: You
know, I agree with you. In a perfect world, that would
do one of the things we do. The problem is
we’ve got 10 weeks.
How do I use that 10 weeks? And I decided I want to try and
do the most I can on design. And so I have to
sacrifice other things. It’s the same reason why
unit testing got thrown out I felt like
it was better for them to be thinking about more
design issues than unit testing issues. But I agree with your
thought completely. Yeah. AUDIENCE: Do you have
TAs for the course? JOHN OUTSERHOUT: Do we have TAs? Yeah, me. [LAUGHTER] Not right now. Because I just
worry that the TAs would not be able to give
high enough level of feedback.
I’ve started wondering if
maybe students who have taken the course previously could TA. So far, the demand has
not been more than my capacity. And so I’ve been able
to keep it small. If the demand grows, I
may try an experiment to see if previous course
participants can TA. But, again, I worry. Also, for the first
few offerings, I wanted to read all
the code myself, too, because I wanted to
learn from it as well.
AUDIENCE: Have you thought
about using software engineers as TAs? We read and write
code every day. [LAUGHTER] JOHN OUTSERHOUT: I’m not
opposed to it in principle. But I’d want to
make sure students don’t get mixed messages. And so particularly
because the ideas I’m teaching in
the course are not ideas that are not necessarily
widely spread in the community. The worst
thing that would happen is if a software engineer
comes in and says, no, this method is
longer than 20 lines. You’ve got to break it up. So if I could find people
that are sort of bought into the philosophy–
and maybe, you know, the book gets out there
and more people buy in, maybe we can do more of that.
SPEAKER 1: OK. One question from the Dory,
the second highest rate one– are threads still a bad idea? [LAUGHTER] JOHN OUTSERHOUT: You know, one
of the interesting things when the only form in which
you publish something is a set of slides rather
than a paper– which that was. It was just an invited talk
at a [INAUDIBLE] conference a long time ago. It’s interesting. People can come
back and interpret that in whole bunches of
different ways over the years. So that was done in a very
narrow context of the time when people were starting to
introduce threads for things that didn’t even need threads. They were doing threading on
machines that didn’t even have multiple processors on them. And so that talk was
a reaction to that. Threads are unavoidable. I mean, for certain things,
you just have to use them. But they’re still sort
of the next-to-worst alternative where the
worst alternative may be, in some cases,
not using threads– incredibly hard to program.
So I’m still not
a fan of threads, but they’re kind
of a fact of life. Yeah. AUDIENCE: I have a
question on Tcl language. I think the language is sort
of very simple in itself, but eventually not gained,
like, the larger scope of adoption. I want to ask because you
are the language inventor, is your high-level
issues in the language of design you saying you got
it wrong, which makes it– JOHN OUTSERHOUT: Whatever
happened to Tcl, I think, would be your question. I don’t want to diverge
too much from that. I would say two things. First, one of the
things Tcl was best at was doing kind of simple
interactive GUIs with Tk.
And when the web came
along, the web took that over and Tcl did not
make the transition to the web. So a lot of things people would
have used Tcl Tk for, they end up doing on the web. The second thing is, is it
really with the right language? I’m not sure, in retrospect,
that it’s the right language. I built it for
a very different purpose. I built it as more of a
textual command language that you’d be typing
commands to programs, almost like a shell language. And then it ended up getting
used as a scripting language. It has a bunch of
interesting properties. But I don’t know, if
I were doing it again, I’m not sure I would
design the same language.
But I think that the
biggest thing that happened is kind of
the web came along and Tcl did not effectively
make the transition to the web. SPEAKER 1: OK. So there are a lot of really
interesting questions both on the Dory
and in the room. In the interest
of time, I’m going to do two more Dory questions
and two more in the room.
And then John is going to
stay around and hopefully answer more questions as well. OK. So any thoughts on
the hiring process based on what you’ve observed? And the question
has, how would you change the hiring process
which is so algorithm heavy? Can you share your experience
when you ran your company? JOHN OUTSERHOUT: Hiring
process I’m not sure it relates
to software design. But I have a very strong
opinion about the hiring process, which is you should hire
for slope, not y-intercept. I think people, when they hire,
do a pattern match of what this person has
done, try and find the person who has exactly
done the job they want to do.
I think that’s the wrong
way to hire in general. I think you want to
hire the person who’s the fastest learner, who
has the fastest slope. I don’t particularly
care whether they’ve done the job I want to do. Because particularly
at a startup, when things change so rapidly,
you know, things change. So I look for people
who are really smart fast learners rather. Also, by the way, the person
who has done this job five times before, you sort of
wonder, well, you know, why haven’t they moved
on to the next job? Oftentimes, those people
kind of hit their plateau. Whereas, the person with the
rapid slope has the potential. AUDIENCE: OK. JOHN OUTSERHOUT:
That’s my [INAUDIBLE]… SPEAKER 1: Last
question from the Dory– what do you think
about Hyrum’s Law? JOHN OUTSERHOUT: Sorry,
remind me what Hyrum’s Law is. SPEAKER 1: It’s a– I googled it and it says with
a sufficient number of uses of an API, it does
not matter what you promise in the contract. All observable
behaviors of your system will be depended on by somebody.
[LAUGHTER] JOHN OUTSERHOUT: I
think there’s truth to that, that, yeah, over time
what happens is the sort of, application, they
find every crevice and sink their roots under that. They will discover. So unless you’re very
careful with your APIs, I think that’s probably true– sad, but true. Yeah. AUDIENCE: Hi. Have you observed any
change in the simplicity or the complexity
of a software design based on the choice of
languages and support ecosystem? JOHN OUTSERHOUT:
Oh, a good question. Some languages somehow
lend themselves to better design than other languages. You know, there probably are– I think that’s mostly a
secondary, sort of second-order thing. But, for example, today, I’d
rather a program in C++ than C, because I liked some of the
object-oriented features in C++. Now, C++ is sort of a big beast
of a language with a horrible learning curve, but I
appreciate its power of it. But, honestly, I think
the design principles are fairly universal. You can use them
in any language. You know, that simple file
system interface for Unix was done in C with no
object-oriented stuff.
And then people would
say, oh, no, actually, you notice it’s
object-oriented because there’s a
file descriptor that gets passed around. And the object-oriented
was crucial to that. I say, no, they just found
a really sweet interface. So I haven’t found one language
that somehow was dramatically better than others for design. But if people have
opinions, I’d be curious to hear other
opinions about that. SPEAKER 1: So back
to hiring, you mentioned we should hire on
a slope, not on a y-intercept. And I radically agree. How do I estimate the slope? JOHN OUTSERHOUT:
How do measure that? SPEAKER 1: Right? Especially, like– JOHN OUTSERHOUT: I have no idea. SPEAKER 1: –you need at
least two points, right? In an interview, I get only one. And I am terrified of hiring
more tactical tornadoes. [LAUGHTER] JOHN OUTSERHOUT: I don’t know. You know, so I’ll tell you
what correlated best for me, but it’s sort of crazy that
I found in my experienced people who worked out best were
the ones whom I just really enjoyed my conversation
with them during the interview.
And that’s sort of weird. I feel kind of
funny saying that because that means somehow
somebody sucked up to me. And I liked them and hired them. I don’t think that’s it because
I have a pretty sensitive meter for suck-upness. And that pisses me off. And people have also pointed out,
if you take that edge, that’s kind of a risk that
you’re just hiring more people like yourself. And so you’re not going
to do diversity very well. So I have no idea
what to make of that or whether it’s
even a good idea. But that’s the one
thing that seemed to correlate in my experience. SPEAKER 1: OK. Yeah. So let’s thank John
once again now. [APPLAUSE].
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