Here comes Kotlin

Here comes Kotlin and Guice! Thank you Alex Tkachman.

Type Theory for the Uninitiated

Kalani Thielen presents The Algebra of Data, and the Calculus of Mutation, or Type Theory for the Uninitiated. Fine calculus a bit soft, algebra a little dull, programming a walk in the park? Type theory is for you.

1-1 beats N-M

SpiderMonkey, the JavaScript engine in Firefox, is moving towards a 1-1 threading model, away from a N-M model: JSRuntime is now officially single-threaded. Let me explain by looking back.

In Solaris the threading model is M-N, that is, for each program with N user threads there are M kernel threads servicing the program. This was thought to be the most flexible design and kernel threads were a limited resource.

Linux eventually settled on a 1-1 model, that is, for each program with N user threads there are N kernel threads each mapped uniquely to a user thread. This was found to be the simplest to code and the most performant in practice.

Back to JavaScript:

A single SpiderMonkey runtime (that is, instance of JSRuntime) — and all the objects, strings and contexts associated with it — may only be accessed by a single thread at any given time. However, a SpiderMonkey embedding may create multiple runtimes in the same process (each of which may be accessed by a different thread).

I read this to say: 1-1 mapping from SpiderMonkey to user thread. The same simplifications and performance gains Linux saw from the 1-1 model will be gained for SpiderMonkey.

Old lessons learned again; better than the alternative.

Spawn of Java

Brian McCallister writes Java Daemonization with posix_spawn(2), or as I like to think of it, Spawn of Java. The problem description:

The traditional way of daemonizing a process involves forking the process and daemonizing it from the current running state. This doesn’t work so well in Java because the JVM relies on several additional worker threads, and fork only keeps the thread calling fork. So, basically, you need to start a new JVM from scratch to create a child process.

The traditional way of launching a new program is to fork and exec the program you wish to start. Sadly, this also fails on Java because the calls to fork and exec are seperate, non-atomic (in the platonic sense, not the JMM sense) operations. There is no guarantee that the exec will be reached, or that the memory state of the JVM will even be sound when the exec is reached, because you could be in the middle of a garbage collection and pointers could be all over. In practice, this would happen exceptionally rarely, at least. Charles Nutter has talked about this problem in JRuby as well.

Visit Brian's post for the solution.

(You may also enjoy his related POSIX from Java post.)

Major code irritants

I could not agree more with this Java anti-pattern: Storing money in floating point variables.

In my interviews of Java candidates (or C++, for that matter) I always include questions about creating a hypothetical Money class, and pay close attention to their ideas for representation. International candidates often fare better on not assuming US dollars, but most all candidates fail completely on avoiding floating point.

Defaulting message fields in protobuf

Not well documented is the technique for defaulting message fields in protobuf. Say you have a field of message type, named "start" here:

message Complex {
    required int64 real = 1;
    optional int64 imaginary = 2 [default = 0];
}

message Vector {
    required Complex start = 1;
    required int64 length = 2;
}

How to you provide a default value, say at the origin? Like this:

import "google/protobuf/descriptor.proto";

extend google.protobuf.FieldOptions {
    // Pick the field number that is right for you!
    optional Complex complex = 50000;
}

message Vector {
    optional Complex start = 1
    [(complex) = { real: 0 imaginary: 0 }];
    required int64 length = 2;
}

See the Custom Options section for details.

Write your own kernel

James Malloy will show you how!

Better than null

A rich, delightful post from Joel Neely on an alternative to returning null to signal "not found" or "No": have the caller supply a callback with appropriate methods for condition signaling. In Java:

interface Found {
    void present(final String name);

    void absent(final String name);
}

class Phonebook {
    private final Set<String> names = new HashSet<>;

    void add(final String name) {
        names.add(name);
    }

    void remove(final String name) {
        names.remove(name);
    }

    void find(final String name, final Found found) {
        if (names.contains(name))
            found.present(name);
        else
            found.absent(name);
    }
}

class LookupPhone extends Phone implements Found {
    private final Phonebook phonebook;

    LookupPhone(final Phonebook phonebook) {
        this.phonebook = phonebook;
    }

    @Override
    public void present(final String name) {
        ringUp(name);
    }

    @Override
    public void absent(final String name) {
        complainIsUnknown(name);
    }

    void placeCall(final String name) {
        phonebook.find(name, this);
    }
}

(Keyed directly into the editor: apologies for typos.)

Bill Bejeck writes on Guava EventBus. This is very similar to the "MagicBus" code I wrote for a client while at ThoughtWorks in the mid-naughts. Essentially it is an in-process event bus respecting Java method signatures rather than a string-based topic space. You can write decoupled code in a pub-sub style within your own application.

In some weird sense it is an implementation of the COMEFROM instruction. When a "caller" publishes the right parameter list, subscribing methods are invoked as if called directly. Adding new subscribers changes the meaning of the call site. The difference is that control returns to the publisher after all subscribers run; COMEFROM is a complete transfer of control and does not multiplex.

Long live INTERCAL!

Loading the dice

A thoughtful algorithm/data structure post from Keith Schwartz on dice rolling, applicable to a broad range of simulations, using the Alias Method.

Linking profile to Google+

I've linked my Blogger profile to my Google+ account. Please comment if you have any thoughts on this, or if you see anything odd or missing as a result. Thanks!

Java server hiccups

Gil Tene at Azul has great insight into Java server performance gaffs. He calls them hiccups and provides tooling to find them.

Monad in Java

The shortest, clearest description of monads in Java I have read:

public class OptionalMonad {
    public static <T> Optional<T> unit(T value) {
        return Optional.of(value);
    }

    public static <T, U> Optional<U> bind(
            Optional<T> value,
            Function<T, Optional<U>> function) {
        if (value.isPresent()) return function.apply(value.get());
        else return Optional.absent();
    }
}

Thanks to François Sarradin in a very nice post.

Netflix circuit breaker

Ben Schmaus of the Netflix team posts a lovely description of the degraded service features of their API. The real life use case is compelling. The dashboard is just brilliant.

The death of HyperCard, a whodunit

Stanislav Datskovskiy posts his theory on the murder of HyperCard. It wasn't the butler.

Baker's Treadmill and Disruptor

Flying Frog Consultancy in Cambridge, England posts a nice comparison between Disruptor and low-latency GC algorithms:

The core of [Disruptor's] idea is to accomplish all of this message passing using a single shared data structure, the disruptor, rather than using several separate concurrent queues.

Baker's Treadmill is a low-latency garbage collection algorithm. Allocated blocks in the heap are linked together to form a cyclic doubly-linked list that is divided into four segments using four iterators that chase each other around the ring as heap blocks are allocated, traced and freed.

Cleverly observed with a good diagram. As they say, read the whole thing.

Microbenchmarking and JVM magic

Martin Thompson posts on what I call microbenchmarking and JVM magic, Biased Locking, OSR, and Benchmarking Fun. It is a good praticum to Cliff Click's What the heck is OSR and why is it Bad (or Good)?. OSR is the kind of JVM magic that both surprises and frightens, but makes perfect sense. I walked a colleague through OSR and he agreed with a shrug: the thing you are glad someone else wrote, excess hair.

Colebourne on Scala: EJB2 redux?

Stephen Colebourne posts a lengthy critique of Scala, Scala feels like EJB2. Scala attracts my inner physicist but brings concern to my practical programmer. Colebourne makes these implicit thoughts explicit.

Defeating SSL certificate validation in Java

This post is a hack. You have been warned.

My employer swaps out SSL certificates traversing the corporate firewall. It is a financial firm with regulatory responsibilities that require monitoring all traffic leaving the company, including encrypted traffic.

One side effect is trouble with well-meaning programs which validate SSL certificates before trusting an encrypted connection. These validations fail using the self-signed replacement certificate.

Maven is such a tool:

[WARNING] Could not transfer metadata com.devspan.vendor.envjs:envjs-rhino/maven-metadata.xml from/to sonatype-oss (https://oss.sonatype.org/content/groups/public): Error transferring file: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

The remark about "PKIX path building failed" is characteristic of the probelm.

There is lots of good advice on fixing this correctly, say, here, here or here. But none of them work just now; I cannot edit the locked-down trust store for Java, it does not already contain the aforementioned replacement certificate, and I do not want to build a custom Maven to work around the problem.

Enter the hack.

$ mvn -Djavax.net.ssl.trustStore=/dev/null package

Martin Thompson on lock cost

Anything Martin Thompson says about performance is usually worth a good study. No exception is Locks & Condition Variables - Latency Impact.

His takeaway points:

1. The one-way latency to signal a change is pretty much the same as what is considered current state of the art for network hops between nodes via a switch.
2. The impact is clear when letting the OS choose what CPUs the threads get scheduled on rather than pinning them manually.

Azul's approach for low-latency Java servers

I no longer work in the low-latency space. Still Azul's completion of a non-VM "pause-less" JVM for Linux is exciting. (Also running a JVM on a VM was too recursive.) May it be a benefit to those who need low-latency java servers.

Whither JDK8?

Stephen Colebourne posts on the future of JDK8 based on current JEPs. Among the possiblities, I'm looking forward to:

• Getting the PID of the current process
• Multi-style collections
• Reflection for parameter names
• Significant help for annotation writers (the compiler model stinks for usability)

Punk rock languages

I love Chris Adamson's description of Punk Rock Languages. His introduction should get your blood flowing:

That C has won the end-user practicality battle is obvious to everyone except developers.

The year is 1978, and the first wave of punk rock is reaching its nihilistic peak with infamous U.K. band the Sex Pistols touring the United States and promptly breaking up by the time they reach the West Coast. Elsewhere, Brian Kernighan and Dennis Ritchie are putting the finishing touches on their book The C Programming Language, which will become the de facto standardization of the language for years. While totally unrelated, these two events share a common bond: the ethos of both punk rock and C have lasted for decades, longer than anyone in 1978 could possibly have imagined.

And in many important ways, C is the programmer’s punk rock: it’s fast, messy, dangerous, and perfectly willing to kick your ass, but it’s also an ideal antidote to the pretensions and vanities that plague so many new programming languages. In an era of virtual machines and managed environments, C is the original Punk Rock Language.

Peregrine: Help for programming concurrency

Good news from Columbia University. Peregrine is automated software to aid in programming concurrency. More in the ACM: Major Breakthrough Improves Software Reliability and Security:

"Our main finding in developing Peregrine is that we can make threads deterministic in an efficient and stable way: Peregrine can compute a plan for allowing when and where a thread can 'change lanes' and can then place barriers between the lanes, allowing threads to change lanes only at fixed locations, following a fixed order," says Columbia professor Junfeng Yang. "Once Peregrine computes a good plan without collisions for one group of threads, it can reuse the plan on subsequent groups to avoid the cost of computing a new plan for each new group."