Category: Programming

Mountain West Ruby Conference
Ruby Queues (RQ) by Ara Howard
16 March 2007

http://www.linuxjournal.com/article/7922
Ara works for NOAA — primarily with satellite data sets. 50KLOC, all paid for by tax payer dollars. Builds medium sized 10-20 node distributed systems.

RQ helps build instant distributed linux clusters. When presenting RQ to scientists, he rarely mentions Ruby. Today, he will talk about the technical side of RQ. RQ isn’t one of the most interesting pieces of software he’s written, but he learned more than average while writing it. One of the reasons he teaches and presents is because he learns while doing it.

RQ has been used to help generate power outage maps after hurricanes hit. Why did he develop it? The lab purchased a bunch of linux machines instead of a Cray, because it was cheaper. His job was to make them work together. He tends to believe that the first link on Goggle will yield the information he needs, so he went looking for a simple distributed computing framework. The solutions he found were the wrong fit, or overly heavyweight. In their environment, the programs that act on the data follow the data, because it’s more expensive to move data than to move programs. He decided to write RQ.

Tried using MySQL for the server queue controller. However, it adds complication with setting up usernames and passwords, and getting approval for the security thereof. He decided to leverage what was already approved – traditional UNIX file permissions and NFS for shared access to data. He also couldn’t run a process as root, or have it listen on a TCP port.

Needed NFS-safe lock files.

gem install lockfile # he wrote this package

NFS lockd wasn’t very good at throughput or fairness. One node would get the lock 500 times in a row, then the next node 500 times, etc. He wrote lock-polling code with a back-off algorithm. It took a while to get it right.

Ended up using SQLite for the shared data store. “Beats the pants off pstore, fsdb, Madeline, etc.” Most of these un-ideal solutions didn’t work well with NFS-heavily-cached data. They would run for 2 weeks, then get corruption. In contrast, SQLite is very robust over NFS — it detects and recovers from corruption.

gem install slave

How does a normal user install daemon processes? RQ cron.

nrtq query – input and output in YAML. He didn’t tell scientists that it was YAML. They didn’t need to know. Using YAML meant he didn’t have to write his own parser, and it’s human readable.
RQ is being used on a single host to queue jobs. There’s a Rails plugin.

Lessons learned:

• NFS is quirky, but it’s the defacto standard. We get to live with it and work around the quirks.
• LVM kills performance.
• Roll your own NFS locking. The standard one is insufficient.
• Use NFS hard mounts. Puts nodes to sleep until NFS server comes back online.
• RQ does not move data around. They use vsftpd to allow data to be moved.
• Constraints are good. Turns out many people and organizations operate under the same constraints.

I’ll be at the Mountain West Ruby Conference March 16-17, 2007 9:00 AM – 5:00 PM each day. Registration is only $50 USD. http://mtnwestruby.org

I’ve long had an interest in version control systems (VCS), also known
as source code management (SCM) systems — beginning with RCS, SCCS and
CVS. CVS
was already showing it’s age when I started using it in 1998. When the
company I worked for, Axent, was acquired by Symantec in 2000, we switched to
using Perforce. At first, I
thought Perforce was a step backwards from CVS. After using it heavily
for a few months, it was clear that CVS and WinCVS
didn’t come close to the ease-of-use and features of Perforce and
p4win. CVS was dreadfully slow compared to Perforce, which was
lightning fast (and still is).

Perforce encourages third-party developers to develop add-ons for use
with their software, which is almost as good as what you get with an
active open-source project. Alough Perforce is proprietary, it’s about as open
as I’ve ever seen a commercial project. It runs on many platforms, has
conversion scripts to migrate CVS repositories to Perforce, etc. It’s not
cheap, unless you’re working on an open-source project, in which case, you can
get free licenses to use it.

At some point, I heard about the Subversion
project, which aimed to correct many of the deficiencies of CVS. Those
were the pre-1.0 days, and it was interesting to watch the development
of Subversion.

About the same time, Bitkeeper was in the news. It was different than CVS, Subversion and Perforce because it was a distributed
version control system. The idea appealed to me because of the
idea that a developer could have version control for his/her private
changes without having to check-in to the main repository until they
were ready. At that time, there weren’t any mature open-source
distributed version control systems to investigate.

I switched jobs late in 2004, and my new company was using Subversion.
Overall, I have been very pleased with Subversion in day-to-day use.
It’s much better than CVS. We had some reliability problems with the
Subversion server. It was running on Windows with the BDB database
storage back-end. When it was switched to a Linux server with the FSFS
back-end, it became much more reliable. My team uses TortiseSVN — an
excellent user interface that integrates with Windows Explorer.

I’ve periodically kept tabs on version control systems. Many open-source variants have sprung up over the last few years: Mercurial, Bazaar-NG, Git/Cogito, Darcs, SVK, Arch and Monotone.
Lately though, I haven’t seen any great reviews on which ones are the
most mature, or what the pros and cons are of each. So, I’ve done some
google research to figure it out, focusing primarily on the distributed
variants.

The conclusion I’ve come to is that the developers of each version
control system are learning from the developers of the other version
control systems, and each project is improving. The Subversion developers are
learning from the distributed version control developers. Recently, there was
an SVN developer summit and they tried out Mercurial, which tells me that there’s merit to the distributed approach.

If you’re already using a modern version control system, the cost to
switch may outweight the benefit. Organizations seem to be able to
cope with legacy tools like Visual SourceSafe and CVS, although better tools
can make developer’s lives easier.

Here’s my own highly subjective comparison table. I’ve marked, in red, some of the things
I think are noteworthy. I focused my efforts on the compeitors that
seem to have garnered the most community adoption. I’ve included one
commercial system, Perforce. Each item is rated on a scale of 1 to 10, 10 being the best. (Update: There’s a better table than mine at http://bazaar-vcs.org/RcsComparisons and various comparisons at Wikipedia)

Comparison of Source Code Management systems

January 31, 2007	Subversion	SVK	Git/Cogito	Mercurial	Bazaar-NG	Darcs	Perforce	Notes
Command-line name	svn	svk	git / cg	hg	bzr	darcs	p4
Cross-Platform	10	9	6	10	10	9	10	Windows, Linux, Mac, Solaris, etc.
Maturity	9	6	8	7	5	8	10	Maturity based on lifetime, and project flux in code
Maturity: GUI	9	0	5	4	3	1	10
Disconnected/offline operation	2	10	10	10	10	10	0	Disconnected 1. editing of files, 2. branching, 3. merging, 4. history, etc. Especially handy when there’s no network connectivity, such as when on an airplane.
Community Adoption	10	2	8	7	5	2	1
Documentation Quality	10	7	7	8	6	8	10
Storage Format: Robustness	5	5	10	8	7	5	5	Storage format least susceptible to corruption.
Storage Format: Not in flux	1	1	10	8	1	1	?
(re)Merging support	0	9	9	9	9	10	4	Remembers prior merges, cherry-picking, etc.
Repository Size	1	9	10	9	?	?	?
Speed	2	7	10	8	6		10
Scalability	9	9	10	9	5	5	9
Commercial Backing	10	5	10	10	10	5	10
Subversion Integration	10	8	6	5	4	4	?	Tailor can be used to migrate changes between all systems
Totals:	88	87	119	112	81	68	79

If I were to pick a VCS system today, it would probably be Git, followed by Mercurial. What follows are my unpolished notes and ideas.

Git/Cogito

Git is very scalable, and is
the fastest
open-source version control system available. Git has a wide community
of professional engineers supporting it, and it has a bright
future. There are graphical user interfaces available for Git such as
gitk and qgit, although none of them are as mature as the user interfaces available for Subversion. Cogito is the easy-to-use command-line wrapper around git. See also the Cogito Wiki.
According to Keith Packard of xorg fame, Git has the most
robust/reliable repository storage format. Advantages of git and all distributed VCSes include 1. offline repository access, 2.
private branches, 3. distributed backups including change history.

For those wishing to use Git/Cogito on Windows, use Cygwin and select the git and/or cogito packages and read the information at http://git.or.cz/gitwiki/WindowsInstall. For those organizations wishing for excellent Windows-Explorer integration, use git-cvsserver in combination with TortiseCVS.

To install git and cogito on Fedora, run the following as root:
  yum install git cogito qgit

I’ve reluctantly decided that Git isn’t as mature as Subversion, which
shouldn’t be surprising because Subversion has been around for longer. Git
isn’t the right fit for all projects. Git was designed for monolithic code
bases, not for modular code bases, although work is in progress to allow it to
support sub projects
(similar to svn:externals).
“Such flexibility is an implicit feature of centralized SCMs, but is much more
difficult to implement in a distributed system like git. As a result, git
currently lacks built-in subproject support, although gitweb does have a notion
of subprojects.”

There’s a document that describes Common Mistakes made when using Git. Unfortunately, most of it isn’t written yet — there’s only a loose outline.

Tutorials:

• http://wellington.pm.org/archive/200510/git/
• Crash course for CVS users: http://git.or.cz/course/cvs.html
• Crash course for Subversion users: http://git.or.cz/course/svn.html
• Howto use git to hack on sources coming from a Subversion repository.
  http://tw.apinc.org/weblog/2007/01/03#subverting-git

Tools — See http://git.or.cz/gitwiki/InterfacesFrontendsAndTools

Mercurial

The OpenSolaris project decided between Bazaar-NG, Git and Mercurial.
Mercurial was chosen primarily because 1. it was fast (although Git is
faster), 2. the Mercurial developers were very responsive to the
OpenSolaris developers and 3. OpenSolaris developers felt like they
could hack Python code, and 4. the repository format works well with ZFS &
NetApp filesystem snapshotting. Their evaluation of Git is here,
and it looks like the listed downsides are now out-of-date or superficial. The Mozilla project had a “version control shootout“, and although they haven’t yet made a decision, Mercurial and Bazaar-NG sounded the best to them.

The following has diagrams to illustrate distributed merging:
http://www.selenic.com/mercurial/wiki/index.cgi/UnderstandingMercurial

Mercurial is more mature than Bazaar-NG, and Mercurial is faster:
http://sayspy.blogspot.com/2006/11/bazaar-vs-mercurial-unscientific.html

“Technologically, centralized systems are a single point of failure–
any problems with the central server are problems for all people using
it.” — http://bazaar-vcs.org/WhyUseBzr

Mercurial supports access control, email notify, line-ending conversion,
etc.:
http://www.selenic.com/mercurial/wiki/index.cgi/UsingExtensions

SVK

SVK is built on top of Subversion, so it should, in theory, integrate
well with an existing Subversion repository, allowing developers to use
a distributed tool even if the master server remains a Subversion
server. Community adoption is high enough to have some confidence
in the future of the project, although adoption isn’t nearly as high as with Git, Mercurial or Bazaar-NG.

It used to be difficult to install, but you can now get a prebuilt
installer for Windows and probably for Linux as well. Working copies
(sandboxes) have no extra meta data (no .svn directory which interfere
with find, etc.) The repository format is significantly smaller than
with Subversion. I’ve found that SVK is much faster than Subversion,
although I haven’t used it much. There is not yet a graphical
user interface — a must for many organizations/communities.

The good, the bad and the ugly about SVK (Sept 2006):
http://kitenet.net/~joey/blog/entry/svk.html

Darcs

Users of darcs, including myself, appreciate its simplicity and
ease-of-use (note: Cogito, Mercurial and Bazaar-NG are also easy to
use). Downsides of darcs are that 1. Darcs is implemented in Haskel,
which limits the contributing developer community (perhaps it will inspire
people to learn Haskel), 2. depends on having Haskel libraries installed and
3. there’s no graphical user interface, unless you consider darcsweb. Still, I like darcs, and I use it on my
home linux box. Like Perforce and SVK, darcs doesn’t clutter up directories
with .darcs metadata. It used to be that Darcs wasn’t very scalable, but
I hear that it’s become much more scalable as of mid-2006. I’ve read that
Mercurial and Darcs feel somewhat similiar in their command-line user
interface.

Mirroring Subversion with Darcs and Tailor (Sept 2006):
http://fiatdev.com/articles/2006/09/10/mirroring-subversion-with-darcs-and-tailor

Subversion

Subversion has a bright future, I think, and we may yet see some of the
advantages of distributed systems appear. For those who need merge
history tracking, which makes future merges from the same branch
easier, there’s svnmerge.py. In a future release, Subversion will have this feature built-in.

The Subversion 1.4 release brought impressive speedups for working copy operations.

Control/Power

Changing information flow by switching from a centralized system to a
distributed system will empower or disempower different sets of people.
I wouldn’t be surprised if one encounters resistance in switching.

In the centralized model, developers are empowered to make any change
they want, which may affect everyone, without consulting others. Of course,
if they abuse that power, they may lose commit access. With a distributed system, an integrator pulls in people’s changes based on what and whom they trust. If
you’re aiming for quality code that doesn’t destabilize a system, it
sounds like a good approach, and it works well for Linux kernel development. Most distributed systems can be used similiar to a
centralized system, so that no integrator is required — individuals can push their changes to the master repository.

No-hassle online backup software for Windows XP: http://mozy.com and http://carbonite.com. Five dollars per month. Not bad.

I heard about these from listening to this podcast on usability of software

Why Software Sucks by David Platt
http://cdn.itconversations.com/ITC.TM-DavidPlatt-2007.01.02.mp3

What is the most important thing to the average computer user? They want their machine to “just work”. Why does Google know how to correctly translate a United Parcel Service tracking number, while the actual UPS website requires multiple entries just to get to the point where the tracking number can be entered? Programmer David Platt is the author of “Why Software Sucks…and What You Can Do About It”.

While average users are expected to use the computer as an everyday tool, programmers too often produce software that has poor functionality, especially compared to other devices used to perform other routine tasks.

One of the other major problems is that software is too often marketed to enterprises rather than individuals, and that constant updates are meant to convince companies to regularly upgrade, with little or no thought given to the end user.

The discussion is both enlightening and entertaining. While Platt believes the problem can be solved, he thinks it won’t happen unless software designers change their point of view to better consider the needs of the end user.

Tim Bray explains what is to like about the Ruby programming language:

I’ll jump to the conclusion first. For people like me, who are proficient in Perl and Java, Ruby is remarkably, perhaps irresistibly, attractive. Over the last week I’ve got an unreasonable amount of work done in a ridiculously short period of time, with lots of interruptions, in a language I previously didn’t know. It’s intuitive enough that I’ve often found myself guessing at a syntax or a method or a usage and getting it right first time.

Maybe the single biggest advantage is readability. Once you’ve got over the hump of the block/yield idiom, I find that a chunk of Ruby code shouts its meaning out louder and clearer than any other language. Anything that increases maintainability is a pearl beyond price.

I’ve been programming in C and Java for a quarter-century and I find Ruby easier to read, only a week in. Of course, a language’s culture is often more important than all that technical crap. I’ve found the ruby-talk mailing list to be a fount of wisdom and friendly to ignorant newbies too.

http://www.tbray.org/ongoing/When/200x/2006/07/24/Ruby

Follow the link to find out “What’s Lame” about Ruby.

Apple explains how to controll Symbol Visibility in GCC 4.x. The benefits and drawbacks of hiding symbols are outlined. Has example code and compiler switches.