using functional programming within an industrial product group: perspectives and perceptions

USING FUNCTIONAL

PROGRAMMING WITHIN AN

INDUSTRIAL PRODUCT

GROUP: PERSPECTIVES AND

PERCEPTIONS

David Scott Citrix Systems

Richard Sharp Citrix Systems

ThomasGazagnaire INRIA

Anil Madhavapeddy University of

Cambridge

presenter

XenServer: an operating system virtualisation

platform

Why is XenServer interesting?

It is widely distributed

50k companies; largest customer >20k physical

machines

It is low-level mission-critical infrastructure

Hypervisor, kernel, management stack, Windows GUI

It is written in C,OCaml, Python, C#

Around 11 full-timeOCamldevs; 130kLOC

ofOCaml

About Us

Why did we chooseOCaml?

Several of us used it before and trusted it.

We believed it would make us more

productive.

As a startup, there was a culture of risk-

taking and innovation.

… and no-one was able to stop us

A mix of programming styles

Imperative Most I/O, Xen and OS syscalls. “A type-safe C”

Functional Bin packing algorithms for failure planning

Combinators for domain communicationwait_for (

any_of

[ „OK, value_to_appear “/path1”;

„Failed, value_to_become "/path" v

] )

Meta Programming Database accessors, RPC stubs, API specification

What (some) people said

“the tool-chain is not mature enough”

“we will not be able to hireOCamlpeople”

“no-one will buy the company”

“people in other teams will not be able to

contribute any code”

“the tool-chain is not mature

enough”

In 4.5 years of heavy use, we found one

(obscure) compiler bug

It was already fixed in CVS when we reported it

The binaries are subjected to 2,000 machine-

hours of testing every night, with regular week-

long soak stress tests

Last seg-fault was over 3 years ago

There was a missing “%s” in our custom C

bindings

… and an extra “%s” in some Python code

“we will not be able to

hireOCamlpeople”

FromOctober 2006 –April 2010, 12OCaml

engineers hired (25% of engineering hires, as

planned)

Only 2 had previousOCamlexperience

Training time was dominated by learning the

product, not the language

Advertising on caml-list, haskell-café got great

candidates

“no-one will buy the company”

Argument: in the due-diligence phase, a

suitor will detect the presence of a “non-

standard” “research” langage likeOCamland

conclude we‟re not serious

Reality: on 22nd October 2007, Citrix paid

$500M for XenSource after a comprehensive

code review.

“people in other teams will not be

able to contribute any code”

“… what about contribution

size”

Concerns

Foreign Function Interface

Requires much knowledge of the GC, and bugs

can be subtle. But we like how fast it is!

Profiling support

More of a generic UNIX issue, but interactions of

threads, process and hypercalls can be

“interesting”.

Conclusion

We‟re very happy withOCaml !

Great tools

Nice language

Excellent people

Our code is open-source (LGPL), check it out:

http://www.xen.org/products/cloudxen.html

Questions?