Why does Scriptroute use Ruby?

Neil Spring

Abstract: Scriptroute is a network measurement system composed of a privileged daemon and a safe-mode Ruby interpreter. This note explains my choice of Ruby and presents a starting point for discussions on whether to settle on Ruby or seek another language with different features.

1  Introduction

My choice of Ruby as a scripting language for Scriptroute measurements has proven controversial. These discussions surprise me because choosing a language wasn't without precedent: the choice of Tcl for the network simulator, ns, has made the research community learn Tcl. And Ruby is far easier to learn than Tcl. Ruby was the right choice for Scriptroute, even though it is not as widely known as Perl, as widely supported with external libraries as Python, as trusted in safe systems as Modula-3, or as flexible as Java.

The choice of interpreted language doesn't really matter, so feel free to argue with me or propose an alternative. There is a narrow interface between the interpreter and the daemon, and this interface can be accessed from other languages. When first working on Scriptroute, I convinced my advisors, David Wetherall and Tom Anderson, that we could choose the wrong language and still have a valuable system: all of the interesting research was in the philosophy of untrusted code and in the daemon that enforced policy. I built into the architecture the possibility that a better language might come along: Scriptroute clients ask for srrubycgi as the CGI script to execute to allow sr[something-else]cgi to exist. If the Scriptroute daemon becomes the preferred mechanism for allowing users to measure their own networks, replacing set-uid programs as I hope it will, I expect others will build interfaces to other languages.

As Scriptroute grows and becomes popular, more people are affected by the choice of Ruby. This sketch of a document is an argument that the decision was not taken lightly and that your effort in learning Ruby is not wasted.

2  Caveats

I hope someday to propose Scriptroute-enhanced Ruby as a lingua franca for active network measurement. However, this note does not make the case either that a single language should be chosen or that Ruby is the only language for the task. This note is only a starting point for that discussion.

There are several good language comparisons on-line. This is not one of them. This note does not assert that your favorite language lacks some feature: most languages will allow the programmer to kludge a feature even if it isn't part of the original design of the language. That's what function pointers are for.

My philosophy is that learning new languages is fun, and that junior-level undergraduates should have no trouble becoming proficient in Ruby.

3  Why Ruby

3.1  Crucial features

Ruby has a flexible safe mode. A Scriptroute script must be able to write results back to the client, perhaps interact with the runtime to inject packets and lookup a name or two, but not open files or fork threads.

Ruby can be embedded in C. For safety, the Scriptroute environment can't be just a native language module linked by the interpreter. Resource limits and the connection to the local daemon must be set up before the safe mode is entered and the script is run. To do this, Scriptroute embeds Ruby---the Ruby interpreter's functionality is implemented in a library that can be linked by the Scriptroute interpreter.

3.2  Niceties

Ruby is designed as an object oriented language. At first I thought this just meant that creating a hash table of arrays is not a kludge. I realized later that this means that it is easy for one script to extend and modify another through inheritance and encapsulation. I've used these features when merging traceroute code with bandwidth probing code and name-decoding code.

Ruby has a rich standard library. Like Java or Perl, you don't have to implement lists or hashes. Unlike Python, regular expressions are literals in the language.

Ruby supports anonymous functions. I've grown to dislike for loops when map will work just as well. Sometimes the best parameter is code, and iterating through a data structure makes more sense to me with lambdas.

Ruby's speed is comparable to that of other scripting languages. See The Great Computer Language Shootout. That the speed is comparable is nice, but not required for Scriptroute.

I had also seen a comparison of Python and Ruby which suggested that Ruby was cleaner and easier to learn for those who knew neither.

Ruby is infectious. Once you stop writing semicolons, you will not want to go back. Once you stop writing @{$foo{$asn}->{"$prefix/$prefixlen"}} and other unclean constructions, you cannot go back.

4  Why Not Ruby

In the interest of fairness, there are weaknesses in Ruby, some of which are acknowledged by the Ruby community and being actively worked on.

Ruby does not allow named arguments to be reordered (unlike Python).

Ruby is dynamically typed, like most (all?) scripting languages. I like static validation and use splint when I can in my C code.

Ruby has relatively few external libraries. While good libraries exist for parsing XML and many other tasks, the depth of extension interfaces for Python and Perl dwarfs that of Ruby. This lack of external libraries is a problem for tasks such as statistical processing. I hope that SWIG will make it easy to port extension libraries to several languages at once.

Ruby tries to do the right thing with ambiguous code, and sometimes it makes the wrong decision. Ratul found that it is sometimes counterintuitive: if(0) evaluates to true because 0 is an object of class integer, not nil and not false.

Some object that there are too many ways to do the same thing in Ruby. I never understood this complaint.

5  Success

Ruby has taken over the UW networking research group. After writing some involved Scriptroute scripts, Ratul Mahajan, a capable Perl hacker, now uses Ruby for Scriptroute-free work with Maya Rodrig, who is still borrowing my pick-axe book. Tom Anderson, faculty, rewrote our undergraduate course programming assignment software in Ruby, and has described the process as ``joyful.'' He gives undergraduates a week to learn Ruby. Graduate students Gang Zhao and Stavan Parikh use Ruby too.

Barath Raghavan, a graduate student at UCSD, made a point at a workshop to thank me for introducing him to Ruby via Scriptroute.

If you really don't like Ruby, or have a lot of code in another language to integrate with, the interface offered by the Scriptroute daemon is intentionally very simple and easy to replicate. Just search for calls to fprintf in ruby_classes.c. And of course, you don't need to learn Ruby to use Scriptroute, just to develop for it.

This document was translated from LATEX by HEVEA.