I’ve been looking at several MQTT brokers recently, and whilst I shan’t go into all the details of that, I shall post my hard-earned learnings on how to get VerneMQ built and running on macOS 10.12. This is all cobbled together from Googling various issues I ran into and bludgeoning my way to success, but hopefully this end to end description will be useful to somebody. To be honest, I have been massively put off VerneMQ because of this poor out-of-box developer experience and the extremely limited documentation.


  • Ensure XCode and its command line tools are installed, because we’ll need the command line compiler tools.
  • VerneMQ is built on Erlang, so we need to install that. Unfortunately the official “Erlang Installer” for mac didn’t work, giving an error: “erl could not be removed”. Thankfully brew works nicely. Don’t know why I didn’t try that straight off.
    • If you haven’t already got brew, install it from https://brew.sh. It’s really very quick and simple and a must-have tool anyway. Then install Erlang:
      > brew install erlang
  • Get the VerneMQ code:
    > git clone git://github.com/erlio/vernemq.git vernemq_git
    > cd vernemq_git
  • The build would fail with “vmq_passwd.c:32:10: fatal error: ‘openssl/evp.h’ file not found”, on macOS 10.11+, so we need to specify openssl location in CFLAGS. Ensure openssl is installed first if necessary, with brew:
    > brew install openssl
  • Get past an erlang rebar bug, caused by files being readonly, by making them readable (bit of a hacky workaround):
    > chmod -R u+w /usr/local/Cellar/erlang
  • Actually build it, now we have everything we need, adapting the path here as necessary to match the openssl version you actually have.
    > CFLAGS="-I /usr/local/Cellar/openssl/1.0.2k/include -L/usr/local/Cellar/openssl/1.0.2k/lib" make rel
  • If that succeeded (after a fair while) you should have the binaries in _build/default/rel/vernemq/bin/


It’s trivial to start the binary with default config:

_build/default/rel/vernemq/bin/vernemq start

Note that this starts it up then quits, but leaves the server running. You can do the same but with ‘stop’ to shut it down. See the docs on further things you can do with the vernemq binary.


If you’ve just built it and are running from the _build location, the config file it is using is at _build/default/rel/vernemq/etc/vernemq.conf

I turned on anonymous users and added a websockets listener by following the instructions in that file. I restarted the server (vernemq restart) and then was able to successfully publish and subscribe with the handy HiveMQ online WebSocket MQTT client.

This site replaces my original Typepad blog which was at alwaysthecritic.typepad.com. I will probably disable the Typepad blog entirely in the long run (possibly even the short run) but for now I figured out a way to have the Typepad pages redirect to the relevant articles on this new site.

Simply add an HTML widget to your Typepad sidebar, containing a little bit of JavaScript to modify the URL:

<script type="text/javascript">
document.location.href = document.location.href
    .replace('alwaysthecritic.typepad.com', 'justthesam.com')
    .replace('/atc/', '/')
    .replace('.html', '/');

This not only modifies the domain, but also loses the /atc prefix and the .html from the end, as they’re not needed on this new site. Magic!

I have blogged on my employer’s blog about the simple, but relatively featureful, Knockout.js page router that I whipped up recently. The core router.js file is only 61 lines but is packaged up into a nice demo app, src on GitHub, that you can just clone, then double click index.html to see it working (no server required).

Inspired by a random tweet about their added Scala support, I tried out the Codility sample test. I rather liked my solution and I think it’s a perfect example of some of the niceties of Scala, in a small way, so here it is:

def solution(a: Array[Int]): Int = {
  // Partial sums are Longs to avoid Int overflow.
  val sums = a.scanLeft(0L)(_ + _).tail
  def equilibrium(index: Int) = leftSum(index) == rightSum(index)
  def leftSum(index: Int) = if (index == 0) 0 else sums(index - 1)
  def rightSum(index: Int) = sums.last - sums(index)
  (0 until a.length) find (equilibrium(_)) getOrElse(-1)

The use of scanLeft to build up all the partial sums is particularly handy. Having done that it’s very easy to run through all the indexes until we find one that satisfies us. Note that the find method returns an Option[A] so we use getOrElse to return -1 if no solution was found (as per the requirements).

Allow me to wax philosphical for a moment with an observation about where computers and their operating systems are heading.

In the world of software development CQRS = Command Query Responsibility Segregation, which in its simplest sense recognises that it's sometimes better to use a different mechanism for reading data than it is for writing it. See Martin Fowler's exposition of the concept if you want to know more, but this post isn't actually about software development at all!

I reckon that we're at a critical juncture in the evolution of personal computing devices and that the CQRS principle is necessarily coming to the fore to save the human race.

Tablet computers are taking the world by storm, in case you hadn't noticed. Apple could barely make enough iPad Minis for me to be able to get my wife one for Xmas, though I did manage it at the very last minute, and shortly thereafter bagged one for myself too. Frankly it's bloody brilliant, but I use it predominantly for consuming rather than creating and I'm far from alone. This is partly because the human populace is inexorably dumbing down towards being fat blobs with brains wired directly into the 'net, consuming inane banter, amusing picture of cats and the latest celebrity news, 140 characters at a time. But that aside, it's just not very pleasant to write large quantities of text, manipulate images or perform other expansive creative works by prodding a tiny screen. Or even a big screen.

To write software, construct lengthy blog posts (ahem), edit movies, sequence the human genome or design great buildings requires a proper computer! On that basis I posit that there will always be a place for desktops and laptops, or indeed whatever replaces them but which necessarily has a non-trivial input mechanism. I genuinely worry that the market for serious computers will be increasingly neglected by the manufacturers, refocussing as they are on the mass consumer market, inevitably leading to the downfall of humankind. Perhaps I exaggerate – at least I hope so.

Now I've never used Windows 8, indeed I shudder at having to use Windows 7 on a daily basis at work, but I understand it represents something of a chimera. It is best known for its shiny, touchy, slidey 'Metro' UI, beckoning your greasy fingers to caress its tiles. However it also allows you to fall back into the more staid world of traditional Windows where presumably you can get some proper work done, as long as you have a keyboard and a pointing device other than your finger. I understand critics are conflicted about this hybrid approach, but it's CQRS writ large and may therefore be the way forwards. One way or another, at least some people will need to create great works. I do hope to be one of them, and to have the equipment to be able to do it.

I see irritating problems. Every tangible thing I use, every service I consume, I can't help but notice all the little things that could have been a bit better. And it really frustrates me when these are details that seem so very simple to fix.

For instance, when I buy a train ticket from FCC's machines at St Pancras I can't actually see the text on the card payment screen unless I squat uncomfortably, because it's low down and inset out of sight. I know it's low down so disabled/short people can use it, but at least provide a direct line of sight for the average sized person standing in front of the machine – i.e. 95% of your customers!

I recently bought a D-Link powerline networking kit on Amazon, which is a wonderful product by the way, that worked straight out of the box with zero configuration. But the Amazon page did not state that there were two ethernet cables in the box – though I suspected there might be and was explicitly looking for this information. Worse than that, it provided a handy link to buy the product along with two separately supplied ethernet cables. So I went with that (better safe than sorry) but lo and behold: two cables in the box and now I have two extras I paid for and don't need. I feel I have been poorly served by Amazon.

Often when travelling by train, announcements are made that are inaudible due to the low volume (compared to background noise) or muffled by unhelpful acoustics. But these are sometimes really very important announcements and there's precious little point making them if they can't be heard.

Whenever I visit the hairdresser they ask me what I want doing, and I struggle to articulate it, especially when it comes to the intricacies of the back of my head. Why don't they take photos and notes (in special hairdresser shorthand) so that they can sort me out consistently every time, even when it's somebody new cutting my hair. As far as I'm concerned this is a no-brainer.

There are many little disappointments just like this that I spot on a daily basis, with physical and virtual products and services and I'm increasingly wondering whether there's a way to turn this into a job: detail consultant. Available for hire by companies small and large, I would tell them what could be improved to give their customers a delightful rather than frustrating experience. It's not rocket science but apparently there's a gap for this sort of common sense supplied from an external viewpoint. Please form an orderly queue!

In fact I'm surprised that larger organisations don't have full time employee roles with exactly this mandate: roving throughout the company and in the field spotting and fixing these subtle issues. In a competitive world where bad publicity is only a Tweet away, this seems like a very good use of resources to my naive mind. If you have any pride at all in what you're doing that is.

I've been doing some trivial benchmarking of Play 2 with ab (Apache Bench) just to get an idea of its raw capabilities for serving simple requests – and because it's what I always do when picking up a new framework so I know what I'm dealing with. In doing so I ran into a bit of a puzzler that had me thinking Play 2 was bugged – but my spidey sense soon kicked in and told me it was more likely to be an OS or ab issue. I had done approximately the following, using Play 2.0.1 on OS X 10.7.3, and I'm pretty certain you'll see the same results if you do this on a Mac:

> play new hello  [select option 1 - basic Scala app]
> cd hello
> play start
> ab -c 50 -n 16000 http://localhost:9000/ [Runs fine - about 3700rps]
> ab -c 50 -n 16000 http://localhost:9000/ [Gives up with timeout]
> ab -c 50 -n 16000 http://localhost:9000/ [Runs fine - about 3700rps]
> ab -c 50 -n 16000 http://localhost:9000/ [Gives up with timeout]

It took me a bit of experimentation to establish that it's about 16000 requests that work fine, followed by timeouts, in a reliable pattern. That's a suspicious number, being near enough a power of 2, which is what clued me into it being an OS limit that I was running into. I ran the same ab test (with the same result) against the built in Apache https serving a static file, confirming that Play 2 probably wasn't to blame.

Sure enough, a quick Google turns up the goods. My OS was running out of the approximately 16000 ephemeral ports available and having to wait for them to be released before it could reuse them. So not Play 2 or ab's fault at all. Actually in some senses it is Play 2's fault for being so fast that I've run into this limit.

I'm not going to go into the details of what ephemeral ports really are, as others have done that perfectly well, and there is a good StackOverflow answer with some key ways to work around the problem by modifying parameters of the OS' network stack – but be careful and make sure you understand what you're doing.

However, one very simple way to workaround the issue is to simply pass the -k option to ab, to use HTTP keepalive (assuming the server you're testing supports it). Note that this changes the nature of your test though, as you're no longer really simulating large numbers of separate connections – but for basic sanity check testing it may help. For the record `ab -c 50 -n 100000 -k http://localhost:9000/` benchmarked Play 2 at about 7000 requests per second on my 2.4GHz Core Duo MacBook.

The hype around the Scala programming language just got too much recently and I decided to give it a go. I'm two thirds of the way through Programming in Scala, a massive but very good tome and I've been experimenting with the language and tools, though only a little bit so far. I have also just received the newly published Scala for the Impatient (a deliberately much more compact book) and will be working through that as well.

To put it mildly, Scala is not for the faint hearted, or anyone who just wants to get some stuff done ASAP. There is a big, steep learning curve and the relative immaturity of the language and the small community means you'd better be used to the pains of the bleeding edge. I thought Ruby was hard work, but Scala is frankly more so in my experience so far.

However the language is somewhat addictive – or perhaps I just like the challenge of learning stuff that takes a bit of grokking. It's a big language, with many very clever facilities and features. It's mind-boggling to start with but I think it's starting to sink in. Of course there's the functional paradigm to understand and master, but I did plenty of that at university so it doesn't frighten me.

Enough people have written about the frustrations and wonders of writing code with Scala, so I think I will reflect on a few miscellaneous findings, mostly of a practical bent.

Compiler speed

The Scala compiler is surprisingly slow and it seems to be one of the main bitching points amongst people trying to get up to speed with the language. But seriously, when you're used to instantaneous results with Grails, or even plain old Java (especially with JRebel) then waiting several seconds even for a trivial app is most upsetting. But I'm trying to be Zen about it.

The mailing lists tend to be full of griping about the compiler's speed, usually met with promises that it's getting faster, but the Scala compiler is so much more complicated than the Java compiler that it seems unlikely to do its work in the blink of an eye anytime soon. See Martin Odersky's explanation of why it's slow on Stack Overflow.

Eclipse support

Eclipse supposedly has good Scala support via the Scala-IDE plugin, but I found the out of box experience so terrible that I have given up. Having created a "new Scala project" I still had to manually add the Scala libraries to the project and manually create run configurations, which would very often still not appear in the lists of ways to run the project. I just want to hit "Go" and for my app to run – it should be trivially easy to make this happen. In my experience the best open source projects are the ones that deliver a delightful out-of-box experience and this has disappointed me on that front.

IntelliJ IDEA support

Luckily the Community (i.e. free) edition of IDEA supports Scala. Even though I hadn't used IntelliJ previously, I figured out how to download their Scala plugin and create a Scala app within just a couple of minutes and it was a very smooth experience compared to installing the Scala support into Eclipse (don't get me started).

However compilation of a Hello World app took 7 seconds every time I modified the single file! A bit of Googling led me to discover FSC – the Fast Scala Compiler – which is part of the standard Scala toolset. Support for FSC is built-in to IDEA but that support is turned off by default. Once I turned it on for my project things hotted up and compilation took just 2 seconds. That's still lamentable compared to most other languages, but just about tolerable for now. I have no idea how things go for a big project, though I get the impression from mailing lists that waiting tens of seconds or even minutes for a compile is fairly commonplace. We shall see.

Play 2.0

One of the reasons I decided to try Scala in the first place was the fuss over the Play 2.0 framework, which has just recently been released in its first 'complete' form. I have only messed with a couple of tutorials so far and frankly it's bewildering and strange coming from frameworks like Spring MVC, Grails, and Ramaze (a small Ruby framework).

There is much wailing and gnashing of teeth on the Play mailing list from people upset about its radical new direction, and its emphasis on Scala compared to Play 1. Maybe it's just the pain of change, but there's no doubt Play 2 is hard work to get to grips with.

I get the impression that a lot of people are missing the point though – it's a case of horses for courses. Play 2 is rather exotically architected using cunning non-blocking approaches that require it to abandon the classic Java Servlet container entirely. It also requires you to write obtuse and sometimes verbose code (compared to Grails say – though it's usually less verbose than Java) and partly because of that non-blocking architecture. Reading and understanding the Play 2 docs on Action Composition may very well require a PhD, but Action Composition is a technique that must be used to achieve relatively common ends. It's all a bit overwhelming for newbies.

This clever shenanigans enables it to handle 40,000 requests per second (albeit very very simple requests) using less than 20MB RAM. I saw Guillame Bort demonstrate this at QCon and it's certainly impressive. The point though is that it's all architected for the sort of new-breed web app that's dealing with connected rich-clients rapidly pushing and pulling data. If you want to create a few CRUD pages for a small admin team to look after a database then I very much doubt that Play 2 is for you.

Updated 17/3/2011 with a solution to Spring Security redirecting.

I had a bit of a fight against Grails to get security exceptions handled the way I wanted, but having figured it out I thought I'd write it up. It's really very simple – I had just had the wrong end of the stick.

I want to be able to throw an org.springframework.security.access.AccessDeniedException from anywhere in my code and have that appear to the user as a custom error page of my design, but with a 403 response code (that's 403 Forbidden, rather than 500 Internal Server Error) and maintaining the originally requested URL rather than redirecting to an error URL as Spring Security sometimes prefers to do (because then you can't refresh to retry).

My main confusion arose from the way Grails' UrlMappings.groovy handles custom exception mapping. It turns out that you need to do it like this:

// Handling specific exceptions requires a 500 code on the left for the
// mapping to pick them up, but we can send back another code in the
// controller that sends back the response.
"500"(controller:"error", action:"error403", exception:AccessDeniedException)

The only surprising thing is that you need a "500" code at the left rather than "403". The reasoning seems to be that this is the incoming error code to the mapping function, and all exceptions are deemed to represent a 500 error code at this point, i.e. they represent an internal server error, which on reflection isn't entirely unreasonable. To then send the error back to the user I have an ErrorController and an associated view in /views/error/error403.gsp.

class ErrorController {
def error403 = {
// Ensure that the correct response code gets sent. If we don't do
// this, it may send a 500 (internal server error) response because
// of the way we had to configure the UrlMappings for handling specific
// exception types with 500.
return response.sendError(javax.servlet.http.HttpServletResponse.SC_FORBIDDEN)

I also figured out how to stop Spring redirecting to a new URL when it decided to deny access. I'd rather the originally requested URL remain in the address bar. The trick is to put the following in Config.groovy.

grails.plugins.springsecurity.adh.errorPage = null

The major remaining annoyance is that these exceptions get reported in the logs whereas I'd rather they weren't (they represent the system working correctly) but I imagine I have to reconfigure logging to resolve that.

I just upgraded to latest Spock and Geb releases (0.5 and 0.5.1 respectively) but have had to undergo a strange sort of hell of odd errors before getting back on track. Turned out none of these were due to any actual code incompatibilities and eventually (after a couple of hours of fruitless head scratching) I got past them by running "grails clean" (which got me to some new errors) and finally by deleting ~/ivy2/cache. The latter step sadly seems to be required quite often when adding or upgrading plugins.

Personally I'm bitterly disappointed that Ivy (or Maven, if that's your poison), the so-called solutions to dependency hell, seems to introduce its own dependency nightmare more often than not. Of course every time you delete that Ivy cache folder you have to wait fifteen minutes the next time you run the app, whilst it downloads hundreds of jars. If you're on the train and not connected to t'Internet, you're really stuck.

I'm sure one day I'll grok what's really going on with this stuff, but for now I feel it ought to just work, but it really doesn't, and many people must waste a lot of time (or give up entirely) as a result.