Steve Vinoski’s Blog

Interested in Erlang? You might consider attending Erlang Factory SF Bay Area 2010. Below is a message that Francesco Cesarini, conference organizer and co-author of the most excellent book Erlang Programming, sent to the erlang-questions list yesterday providing more information about the conference, especially pointing out that the Very Early Bird registration deadline is tomorrow (Sunday January 31). Hope to see you there!

Hi All,

a note to say that we are almost done with the programme for the 2010 SF Bay Area Erlang Factory. This year, we are lucky to have keynote speakers such as Joe Armstrong, Bjarne Dacker, Kenneth Lundin and Steve Vinoski. They will be giving four of the 35 scheduled talks on the 25th and 26th of March in the San Francisco Bay Area. The almost complete programme is available here:

http://erlang-factory.com/conference/SFBay2010/programme

The conference will be preceded by three days of University courses taught by experts such as Simon Thompson, John Hughes, Thomas Arts, Henry Nystrom and Kevin Smith. Come and learn Erlang, OTP, QuickCheck or Web Development with Erlang. More information on the courses are here:

http://erlang-factory.com/conference/SFBay2010/university

This *Sunday* (January 31st) is the deadline for the very early bird deadline. Register by Sunday night and save $400 on the on-site registration price.

The conference hotel and venue is the SF Airport Hilton, a short BART / Caltrain ride from SF and the Valley. We have secured a very competitive price of US$109 per room and night at the conference hotel, this being one of the reasons for us choosing it. The other is the lower price of the venue, allowing us to pass on the savings to the delegates through a higher very early bird discount. We are planning an ErlLounge open to everyone who can’t make the two days on the 25th, and hope we will be able to surpass last year’s success. If you have thoughts or questions, you are welcome to drop me a line.

Hope to see you all there!

Francesco
–
http://www.erlang-solutions.com

The latest “Functional Web” column is now available. It’s entitled “Build Your Next Web Application with Erlang,” (PDF) and my co-author this time was Dave Bryson of BeepBeep fame. Over the past year or two there’s been a sharp increase in development efforts in the Erlang web framework space, as a number of folks, ourselves included, have found that Erlang works really well for server-side web development. In this column Dave and I provide an introduction to a number of Erlang web frameworks and servers. Future columns will dig into some of these in much more detail.

My thanks to Dave for his efforts on this column; he was a sheer pleasure to work with. If Dave ever invites you to co-author something, whether it’s an article or some code, I recommend you take him up on his offer.

P.S. Sorry for the quiet blog lately — it’s just that I have a whole lot of work on my plate these days.

Today Klacke released version 1.82 of Yaws. This is primarily a maintenance release, so nothing major has changed, except that yaws.conf and other files that used to go into etc now go into etc/yaws. For example, the default install prefix is /usr/local so for the installation on my laptop my yaws.conf file has moved from /usr/local/etc/yaws.conf to /usr/local/etc/yaws/yaws.conf. You’ll want to make sure you move any existing conf files you might have to the new directory.

Various other changes and bug fixes are described in the release notes on the main Yaws website. For the previous release Klacke moved the sources from sourceforge to github, so follow that link if you’re looking for source, and you can follow these instructions to build, install, and run. If you should run into any issues or problems, please send a message to the Yaws mailing list or create a new issue on the Yaws issue tracker.

As far as planning for the next release goes, aside from the usual little bug fixes and enhancements and adding tests, Klacke and I noticed Joe Williams’s recent Erlang Factory presentation that mentions Yaws CPU usage is sometimes higher than expected, so we’ll be looking into that and doing some general profiling and performance testing work.

QCon is always very, very good, but QCon London 2009 last week was the best one yet. Highlights:

• Ola Bini’s “Emerging Languages in the Enterprise” track on Wednesday had some great talks, especially Rich Hickey’s Clojure talk. Ola’s deep knowledge and love of programming languages made him the perfect host for this track, and he put together a brilliant lineup.
• Speaking of Rich, I consider myself very lucky to have gotten to meet and spend a fair amount of time with him. He’s very bright, talented, knowledgeable, and experienced, and both of his talks were outstanding.
• I introduced Rich to Joe Armstrong at the conference party Wednesday evening and they spent the next few hours talking at length about functional programming, their respective languages, VM implementation issues, concurrency issues, etc. Ola jumped in as well. They also continued the conversation the next day. I just sat back, listened, and learned.
• Getting to spend time again with Joe was excellent. He always has incredibly useful analyses and opinions to express, and in general is fun to be around and easy to learn from.
• I also finally got to meet Ulf Wiger, Erlang developer extraordinaire, in person. He’s a laid back guy, quite well-informed and a deep thinker who can cover a wide variety of topics in amazingly useful detail. His talk on multicore programming in Erlang covered cutting edge Erlang development and presented some very difficult concurrency issues.
• Ulf’s talk, as well as Rich’s second talk, which was on persistent data structures and managed references, were part of Francesco Cesarini’s “Functional and Concurrent Programming Languages Applied” track on Thursday. I met Francesco, who like Ulf is one of the world’s top Erlang developers, at QCon London last year. He assembled a great track for this conference, with Rich’s and Ulf’s back-to-back talks being way more than enough to sober up any developer who thinks that multicore is not an issue and that today’s methods for dealing with concurrency will continue to work just fine. Best of luck with that!
• Sir Tony Hoare’s talk about the null reference being his “billion dollar mistake” was great because of all the detail he recounted from some of the early days of computing. He was both informative and entertaining. I was also impressed with Ulf during this talk, whom Professor Sir Hoare invited to come up to the front and present what turned out to be a pretty convincing argument in favor of the null reference.
• Paul Downey’s talk on the downsides of standardization was by far the most humorous talk I heard, perfect to close out the track, but it also presented a number of hard-won useful lessons about the perils of standardization efforts.

As with all QCon conferences, there were a bunch of interesting tracks running in parallel, and unfortunately I still haven’t figured out how to be in multiple places at once. I had to miss Michael Nygard’s talk, for example, because my own talk got moved to the same time slot as his.

My talk (PDF) covered the history of RPC, why it got to be the way it was, and why the forces that created it really aren’t all that viable anymore.

The final conference panel was by far the most inventive panel I’ve ever been on. Modeled after the British game show “It’s a Bullseye!” and hosted by none other than Jim Webber, it had contestants from the audience throwing darts to become eligible for a prize. Once a contestant became eligible, Jim would ask the panel — Michael Nygard, Ian Robinson, Martin Fowler, and me — to answer a question submitted by conference attendees either earlier during the conference or live via Twitter. Based on our answers, audience members held up either a green card if they liked the answers or a red one if they didn’t, and if the majority was green, the contestant would win a book. The questions were hard! We had only two minutes each to answer, which for some questions seemed like an eternity but for most was way too short. Anyway, it was great fun, and given how many there were in the audience after three grueling conference days and how much they seemed to be enjoying themselves, it worked very, very well.

If you have any interest at all in leading edge software and computing topics being presented by the world’s most knowledgeable speakers in a fun atmosphere, go to QCon. I guarantee you won’t be disappointed.

Erlang’s heart feature provides a heartbeat-based monitoring capability for Erlang runtime systems, with the ability to restart a runtime system if it fails. It works reasonably well, but one issue with it is that if an error occurs such that it causes repeated immediate runtime crashes, heart will happily keep restarting the runtime over and over again, ad infinitum.

For yaws 1.80, released a few days ago on Feb. 12, I added a check to the heart setup in the yaws startup script to prevent endless restarts. I thought I’d share it here because it’s useful for Erlang systems in general and is in no way specific to yaws. It works by passing startup information from one incarnation to the next, checking that information to detect multiple restarts within a given time period. We track both the startup time and the restart count, and if we detect 5 restarts within a 60 second period, we stop completely. This is not to say that yaws is in dire need of this capability — it’s extremely stable in general and 1.80 in particular is a very good release — but I added it mainly because other Erlang apps sharing the same runtime instance as yaws may not enjoy that same high level of stability, especially while they’re still under development.

The command heart runs to start a new instance is set in the HEART_COMMAND environment variable. For yaws, it’s set like this (I’ve split this over multiple lines for clarity, but it’s just one line in the actual script):

HEART_COMMAND="${ENV_PGM} \
  HEART=true \
  YAWS_HEART_RESTARTS=$restarts \
  YAWS_HEART_START=$starttime \
  $program "${1+"$@"}

where

• ${ENV_PGM} is /usr/bin/env, which allows us to set environment variables for the execution of a given command.
• HEART is an environment variable that we use to indicate the command was launched by heart.
• YAWS_HEART_RESTARTS is an environment variable that we use to track the number of restarts already seen. The yaws script initially sets this to 1 and increments it for each heart restart.
• YAWS_HEART_START is an environment variable that we use to track the time of the current round of restarts. This is tracked as UNIX time, obtained by the script via the “date -u +%s” command.
• $program is the yaws script itself, i.e., $0.
• ${1+"$@"} is a specific shell construct that passes all the original arguments of the script unchanged along to $program.

The yaws script looks for HEART set to true, indicating that it was launched by heart. For that case, it then checks YAWS_HEART_RESTARTS and YAWS_HEART_START to see how many restarts we’ve seen since the start time. We get the current UNIX time and subtract the YAWS_HEART_START time; if it’s less than or equal to 60 seconds and the restart count is 5, we exit completely without restarting the Erlang runtime. Otherwise we restart, first adjusting these environment variables. If the restart count is less than 5 within the 60 second window, we increment the restart count and set the new value into YAWS_HEART_RESTARTS but keep the same YAWS_HEART_START time. But if the current time is more than 60 seconds past the start time, we reset YAWS_HEART_RESTARTS to 1 and set a new start time for YAWS_HEART_START. Look at the yaws script to see the details of this logic — scroll down to the part starting with if [ "$HEART" = true ].

Note that this approach is much like the way Erlang receive loops generally track state, by recursively passing state information to themselves.