Marco Cecconi – I don’t love the single responsibility principle:

The principle is arbitrary in itself. What makes one and only Reason To Change™ always, unequivocally better than two Reasons To Change™? The number one sounds great, but I’m a strong advocate of simple things and sometimes a class with more reasons to change is the simplest thing. I agree that we mustn’t do mega-classes that try to do too many different things, but why should a class have one single Reason To Change™? I flatly disagree with the statement. It’s not supported by any fact. I could say “three” reasons and be just as arbitrary. Interesting critique of the single responsibility pattern. The quote above is perhaps one of the weaker arguments against the single responsibility pattern but it struck a nerve with me because all too often we assume that because a rule sounds good it has some validity. In reality, picking one responsibility for a class is quite arbitrary and, as the article argues, deciding what a responsibility is is even more arbitrary.

Previously in the Testing@LMAX series I’ve mentioned the way we’ve provided isolation between tests, allowing us to run them in parallel. That isolation extends all the way up to supporting a multi-tenancy module called venues which allows us to essentially run multiple, functionally separate exchanges on a single deployment of the LMAX Exchange.

We use the isolation of venues to reduce the amount of hardware we need to run our three separate liquidity pools (LMAX Professional, LMAX Institutional and LMAX Interbank), but that’s not all. We actually use the isolation venues provide to extend our testing all the way into production.

One of the most common reasons people avoid writing end-to-end acceptance tests is how difficult it is to make them run fast. Primary amongst this is the time required to start up the entire service and shut it down again. At LMAX with the full exchange consisting of a large number of different services, multiple databases and other components start up is far too slow to be done for each test so our acceptance tests are designed to run against the same server instance and not interfere with each other.

Testing time related functions is always a challenge – generally it involves adding some form of abstraction over the system clock which can then be stubbed, mocked or otherwise controlled by unit tests in order to test the functionality. At LMAX we like the confidence that end-to-end acceptance tests give us but, like most financial systems, a significant amount of our functionality is highly time dependent so we need the same kind of control over time but in a way that works even when the system is running as a whole (which means it’s running in multiple different JVMs or possibly even on different servers).

LMAX has invested quite a lot of time into building a suite of automated tests to verify the behaviour of our exchange. While the majority of those tests are unit or integration tests that run extremely fast, in order for us to have confidence that the whole package fits together in the right way we have a lot of end-to-end acceptance tests as well.

These tests deliver a huge amount of confidence and are thus highly valuable to us, but they come at a significant cost because end-to-end tests are relatively time consuming to run. To minimise the feedback cycle we want to run these tests in parallel as much as possible.

One of the things we tend to take for granted a bit at LMAX is that we store the results of our acceptance test runs in a database to make them easy to analyse later.  We not only store whether each test passed or not for each revision, but the failure message if it failed, when it ran, how long it took, what server it ran on and a bunch of other information.

The key to making continuous integration work well is to ensure that the build stays green – ensuring that the team always knows that if something doesn’t work it was their change that broke it. However, in any environment with a build pipeline beyond a simple commit build, for example integration, acceptance or deployment tests, sometimes things will break.

When that happens, there is always a natural tendency to commit an additional change that will fix it. That’s almost always a mistake.

Here’s a fun one, we have some code generated using javassist that looks a bit like:

public void doStuff(byte actionId) {<br />  switch (actionId) {<br />    case 0: doOneThing(); break;<br />    case 1: doSomethingElse(); break;<br />    default: <br />      throw new IllegalArgumentException("Unknown action: " + actionId);<br />  }<br />}

This works perfectly fine on Java 6 and Java 7 but fails on Java 8.  It turns out the problematic part is "Unknown action: " + actionId. When run on Java 8 that throws “java.io.IOException: invalid constant type: 15” in javassist.bytecode.ConstPool.readOne.

The truth is out: money is just an IOU, and the banks are rolling in it:

What the Bank of England admitted this week is that none of this is really true. To quote from its own initial summary: “Rather than banks receiving deposits when households save and then lending them out, bank lending creates deposits” … “In normal times, the central bank does not fix the amount of money in circulation, nor is central bank money ‘multiplied up’ into more loans and deposits.” In other words, everything we know is not just wrong – it’s backwards. When banks make loans, they create money. The complexity and subtlety in our financial system is delightful and frightening all at the same time. Almost everything has multiple contributing factors that are impossible to isolate and subtle shifts in world view like this can have huge implications on decision making.

Esoteric Curio – 折伏 – as it relates to coding

Our job as a team is to reinforce each other and make each other more productive. While I attempt to conquer some of the engineering tasks as well as legal tasks, and human resource tasks I face, I need concentration and it world certainly help to not be interrupted. I used to think I needed those times of required solitute pretty much all the time. It turns out that I have added far more productivity by enabling my teams than I have personally lost due to interruptions, even when it was inconvenient and frustrating. So much so that Ive learned to cherish those interruptions in the hope that, on serendipitous occasion, they turn into a swift, sprititual kick to the head. After all, sometimes all you need for a fresh persective is to turn out the light in the next room; and, of course, to not have avoided the circumstance that brought you to do it. Too often we confuse personal productivity, or even just the impression of it, for team productivity. So often though having individuals slow down is the key to making the team as a whole speed up.  See also Go Faster By Not Working and The Myth of Measurable Productivity.

Geekdom is not a club; it’s a destination, open to anyone who wants to put in the time and effort to travel there…

…dive headfirst into the things that interest you. Soak up every experience. Lose yourself in the pursuit of knowledge. When you finally come up for air, you’ll find that the long road to geekdom no longer stretches out before you. No one can deny you entry. You’re already home.

As a follow up to the earlier link regarding the performance of animations in CSS vs JavaScript, Christian Heilmann – Myth busting mythbusted:

Jack is doing a great job arguing his point that CSS animations are not always better than JavaScript animations. The issue is that all this does is debunking a blanket statement that was flawed from the very beginning and distilled down to a sound bite. An argument like “CSS animations are better than JavaScript animations for performance” is not a technical argument. It is damage control. You need to know a lot to make a JavaScript animation perform well, and you can do a lot of damage. If you use a CSS animation the only source of error is the browser. Thus, you prevent a lot of people writing even more badly optimised code for the web. Some good points that provide balance and perspective to the way web standards evolve and how to approach web development.