Introducing Sentries

I recently needed a friendly to use circuit breaker in a Scala program. What I found was okay, but not nearly good enough for high volume, highly concurrent applications. So I set out to make it better.

After some iterations of changing I realized that the circuit breaker could be combined with other stuff like rate limiting, monitoring and such. Now, three months later, Sentries is ready for the world.

Here is an example usage: Please visit Sentries on Githib and let me know what you think.

Update 2012-09-04: Version 0.2 will be available in Maven central around 2012-09-04 18:00 GMT.

Update 2012-10-08: Version 0.3 has just landed in Maven central. Its only feature is the new 'clean()' method on the sentry registry which is useful during testing.

Update 2013-02-06: Version 0.5 is out. This is the first to support Scala 2.10. Breaking change is that all durations are now of type akka.util.Duration or scala.concurrent.duration.Duration (depending on the Scala version) instead of Long.

Getting size of a file in shell script

This is really too silly. There seems to be no consistent way to get the size of a file (in bytes) on multiple platforms. Here is a solution that works in bash on Linux (tested under Debian and Ubuntu) and BSD (tested under OSX):

# Echo's size of a file (first argument). # Tested under Linux (Debian) and BSD (OSX). function filesize() { echo $(stat --format=%s "$1" 2>/dev/null || stat -f '%z' "$1" 2>/dev/null) } # Example usage: echo $(filesize readme.txt)

X forwarding as root

Often is useful to run jconsole on a remote (production) machine. One basically has 2 options to do so. First, you can pierce your firewall and let your application listen to the appropriate JMX and RMI ports. This however always tricky (in particular the RMI). In addition, creating the connection string that jconsole accepts is not nice at all.

The other option, is to do X forwarding. I'll describe here how to do this on MacOS X, with jconsole on any Unix server with, ssh-server, no root password and sudo installed.

1. Start X11 (you'll need to install it from the installation CDs).
2. Start iterm2.
3. Login to the target system with ssh -X remote_user@system
4. Switch to root with sudo -s.
5. Get X authorities with xauth merge ~remote_user/.Xauthority
6. Start jconsole

Breaking a Java HashSet

Can it be?

Set set1 = new HashSet(5); Set set2 = new HashSet(5); // add of bunch of strings to both sets assert set1.equals(set2) == false;

Yes it can!

We actually had this problem in an integration test. The cause was that the strings to one set were added concurrently. Interestingly, the sets seemed to be the same, when printed they contained the same strings, just in a different order (which is also interesting). However, the internals were apparently so damaged that an equals invocation return false.

We replaced the HashSet with a ConcurrentSkipSet, and all was fine again.

Conclusion: Okay, so this is a boring conclusion, but just be careful with using normal collections in concurrent situations.

DNS for Version 99 is off-line

To alleviate some pain with using the commons-logging framework, I created version 99. It was hosted at the hostname no-commons-logging.zapto.org courtesy of no-ip.com. Unfortunately, due to lack of traffic I had to affirm usage of the the hostname once per month. Though this was slightly annoying, a complete pain is that I missed the last deadline, and that recreation is out as dashes in hostnames are no longer allowed.

As of 2011-08-15 16:11:00 GMT no-ip did credit to its name; no ip for no-commons-logging.zapto.org.

As 1) the current situation is disruptive for any version-99 user anyway, 2) there is a better workaround using the provided scope, I decided to not put back version 99 online under another hostname.

If you really need to use version 99 for some more time, just add an entry to your /etc/hosts file:

83.163.41.27 no-commons-logging.zapto.org

As promised, I'll keep version 99 for 5 years, that is until October 2012. Please check back here for IP address changes.

Update 2011-12-19: I just found an alternative with almost the same name: version99.qos.ch. It is a static maven repository with a limited number of version 99 jars.

Lock-less singleton pattern

Although the singleton pattern is now know as an anti-pattern, I think it still is a valid choice when you only need one instance in a particular context. Anyway, in Java there are several ways to implement a singleton.

For example this one uses a synchronized block:

private Singleton singleton = null; protected Singleton createSingleton() { synchronized (this) { // locking on 'this' for simplicity if (singleton == null) { singleton = new Singleton(); } return singleton; } }

To prevent locking all the time you can use the double-check idiom on a modern JVM. But I just thought about another implementation that switches locking for preventing code re-ordering, making it a lock-less implementation:

private AtomicReference singletonRef = new AtomicReference(null); protected Singleton createSingleton() { Singleton singleton = singletonRef.get(); if (singleton == null) { singleton = new Singleton(); if (!singletonRef.weakCompareAndSet(null, singleton)) { singleton = singletonRef.get(); } } return singleton; }

There is only one catch to this implementation: although createSingleton() will always return the same instance, it must be allowed to call the constructor of Singleton twice without side-effects.

Update 2011-06-18: changed compareAndSet to weakCompareAndSet as suggested by Adriano.

Apache Wicket Cookbook — book review

Some time ago I reviewed the drafts of the new book from Wicket rockstar programmer Igor Vaynberg: Apache Wicket Cookbook. If you are serious about using Wicket, this book is for you. It is fast, to the point, has very clear code samples and teaches you all the relevant (both clean and dirty) stuff you need and which Wicket in Action could not cover.

Conclusion: this is the book to read after 'Wicket in Action'.