Category Archives: Stack Overflow

Just how spiky is your traffic?

No, this isn’t the post about dynamic languages I promise. That will come soon. This is just a quick interlude. This afternoon, while answering a question on Stack Overflow1 about the difference between using an array and a Dictionary<string, string> (where each string was actually the string representation of an integer) I posted the usual spiel about preferring readable code to micro-optimisation. The response in a comment – about the performance aspect – was:

Well that’s not so easily said for a .com where performance on a site that receives about 1 million hits a month relies on every little ounce of efficiency gains you can give it.

A million hits a month, eh? That sounds quite impressive, until you actually break it down. Let’s take a month of 30 days – that has 30 * 24 * 60 * 60 = 2,592,000 seconds2. In other words, a million hits a month is less than one hit every two seconds. Not so impressive. At Google we tend to measure traffic in QPS (queries per second, even if they’re not really queries – the search terminology becomes pervasive) so this is around 0.39 QPS. Astonished that someone would make such a claim in favour of micro-optimisation at that traffic level, I tweeted about it. Several of the replies were along the lines of "yeah, but traffic’s not evenly distributed." That’s entirely true. Let’s see how high we can make the traffic without going absurd though.

Let’s suppose this is a site which is only relevant on weekdays – that cuts us down to 20 days in the month. Now let’s suppose it’s only relevant for one hour per day – it’s something people look at when they get to work, and most of the users are in one time zone. That’s a pretty massive way of spiking. We’ve gone down from 30 full days of traffic to 20 hours – or 20 * 60 * 60 = 72000 seconds, giving 14 QPS. Heck, let’s say the peak of the spike is double that – a whopping 28 QPS.

Three points about this:

  • 28 QPS is still not a huge amount of traffic.
  • If you’re really interested in handling peak traffic of ~28 QPS without latency becoming huge, it’s worth quoting that figure rather than "a million hits a month" because the latter is somewhat irrelevant, and causes us to make wild (and probably wildly inaccurate) guesses about your load distribution.
  • If you’re going to bring the phrase "a .com" into the picture, attempting to make it sound particularly important, you really shouldn’t be thinking about hosting your web site on one server – so the QPS gets diluted again.
  • Even at 28 QPS, the sort of difference that would be made here is tiny. A quick microbenchmark (with all the associated caveats) showed that on my laptop (hardly a server-class machine) I could build the dictionary and index into it 3 times 2.8 million times in about 5 seconds. If every request needed to do that 100 times, then the cost of doing it 28 requests per second on my laptop would still only be 0.5% of that second – not a really significant benefit, despite the hugely exaggerated estimates of how often we needed to do that.

There are various other ways in which it’s not a great piece of code, but the charge against premature optimization still stands. You don’t need to get every little ounce of efficiency out of your code. Chances are, if you start guessing at where you can get efficiency, you’re going to be wrong. Measure, measure, measure – profile, profile, profile. Once you’ve done all of that and proved that a change reducing clarity has a significant benefit, go for it – but until then, write the most readable code you can. Likewise work out your performance goals in a meaningful fashion before you worry too much – and hits per months isn’t a meaningful figure.

Performance is important – too important to be guessed about instead of measured.


1 I’m not linking to it because the Streisand effect would render this question more important than it really is. I’m sure you can find it if you really want to, but that’s not the point of the post.

2 Anyone who wants to nitpick and talk about months which are a bit longer or shorter than that due to daylight saving time changes (despite still being 30 days) can implement that logic for me in Noda Time.

Revisiting randomness

Almost every Stack Overflow question which includes the words "random" and "repeated" has the same basic answer. It’s one of the most common "gotchas" in .NET, Java, and no doubt other platforms: creating a new random number generator without specifying a seed will depend on the current instant of time. The current time as measured by the computer doesn’t change very often compared with how often you can create and use a random number generator – so code which repeatedly creates a new instance of Random and uses it once will end up showing a lot of repetition.

One common solution is to use a static field to store a single instance of Random and reuse it. That’s okay in Java (where Random is thread-safe) but it’s not so good in .NET – if you use the same instance repeatedly from .NET, you can corrupt the internal data structures.

A long time ago, I created a StaticRandom class in MiscUtil – essentially, it was just a bunch of static methods (to mirror the instance methods found in Random) wrapping a single instance of Random and locking appropriately. This allows you to just call StaticRandom.Next(1, 7) to roll a die, for example. However, it has a couple of problems:

  • It doesn’t scale well in a multi-threaded environment. When I originally wrote it, I benchmarked an alternative approach using [ThreadStatic] and at the time, locking won (at least on my computer, which may well have only had a single core).
  • It doesn’t provide any way of getting at an instance of Random, other than by using new Random(StaticRandom.Next()).

The latter point is mostly a problem because it encourages a style of coding where you just use StaticRandom.Next(…) any time you want a random number. This is undoubtedly convenient in some situations, but it goes against the idea of treating a source of randomness as a service or dependency. It makes it harder to get repeatability and to see what needs that dependency.

I could have just added a method generating a new instance into the existing class, but I decided to play with a different approach – going back to per-thread instances, but this time using the ThreadLocal<T> class introduced in .NET 4.0. Here’s the resulting code:

using System;
using System.Threading;

namespace RandomDemo
{
    /// <summary>
    /// Convenience class for dealing with randomness.
    /// </summary>
    public static class ThreadLocalRandom
    {
        /// <summary>
        /// Random number generator used to generate seeds,
        /// which are then used to create new random number
        /// generators on a per-thread basis.
        /// </summary>
        private static readonly Random globalRandom = new Random();
        private static readonly object globalLock = new object();

        /// <summary>
        /// Random number generator
        /// </summary>
        private static readonly ThreadLocal<Random> threadRandom = new ThreadLocal<Random>(NewRandom);

        /// <summary>
        /// Creates a new instance of Random. The seed is derived
        /// from a global (static) instance of Random, rather
        /// than time.
        /// </summary>
        public static Random NewRandom()
        {
            lock (globalLock)
            {
                return new Random(globalRandom.Next());
            }
        }

        /// <summary>
        /// Returns an instance of Random which can be used freely
        /// within the current thread.
        /// </summary>
        public static Random Instance { get { return threadRandom.Value; } }

        /// <summary>See <see cref="Random.Next()" /></summary>
        public static int Next()
        {
            return Instance.Next();
        }

        /// <summary>See <see cref="Random.Next(int)" /></summary>
        public static int Next(int maxValue)
        {
            return Instance.Next(maxValue);
        }

        /// <summary>See <see cref="Random.Next(int, int)" /></summary>
        public static int Next(int minValue, int maxValue)
        {
            return Instance.Next(minValue, maxValue);
        }

        /// <summary>See <see cref="Random.NextDouble()" /></summary>
        public static double NextDouble()
        {
            return Instance.NextDouble();
        }

        /// <summary>See <see cref="Random.NextBytes(byte[])" /></summary>
        public static void NextBytes(byte[] buffer)
        {
            Instance.NextBytes(buffer);
        }
    }
}

The user can still call the static Next(…) methods if they want, but they can also get at the thread-local instance of Random by calling ThreadLocalRandom.Instance – or easily create a new instance with ThreadLocalRandom.NewRandom(). (The fact that NewRandom uses the global instance rather than the thread-local one is an implementation detail really; it happens to be convenient from the point of view of the ThreadLocal<T> constructor. It wouldn’t be terribly hard to change this.)

Now it’s easy to write a method which needs randomness (e.g. to shuffle a deck of cards) and give it a Random parameter, then call it using the thread-local instance:

public void Shuffle(Random rng)
{
    …
}

deck.Shuffle(ThreadLocalRandom.Instance);

The Shuffle method is then easier to test and debug, and expresses its dependency explicitly.

Performance

I tested the "old" and "new" implementations in a very simple way – for varying numbers of threads, I called Next() a fixed number of times (from each thread) and timed how long it took for all the threads to finish. I’ve also tried a .NET-3.5-compatible version using ThreadStatic instead of ThreadLocal<T>, and running the original code and the ThreadStatic version on .NET 3.5 as well.

Threads StaticRandom (4.0b2) ThreadLocalRandom (4.0b2) ThreadStaticRandom (4.0b2) StaticRandom(3.5) ThreadStaticRandom (3.5)
1 11582 6016 10150 10373 16049
2 24667 7214 9043 25062 17257
3 38095 10295 14771 36827 25625
4 49402 13435 19116 47882 34415

A few things to take away from this:

  • The numbers were slightly erratic; somehow it was quicker to do twice the work with ThreadStaticRandom on .NET 4.0b2! This isn’t the perfect benchmarking machine; we’re interested in trends rather than absolute figures.
  • Locking hasn’t changed much in performance between framework versions
  • ThreadStatic has improved massively between .NET 3.5 and 4.0
  • Even on 3.5, ThreadStatic wins over a global lock as soon as there’s contention

The only downside to the ThreadLocal<T> version is that it requires .NET 4.0 – but the ThreadStatic version works pretty well too.

It’s worth bearing in mind that of course this is testing the highly unusual situation where there’s a lot of contention in the global lock version. The performance difference in the single-threaded version where the lock is always uncontended is still present, but very small.

Update

After reading the comments and thinking further, I would indeed get rid of the static methods elsewhere. Also, for the purposes of dependency injection, I agree that it’s a good idea to have a factory interface where that’s not overkill. The factory implementation could use either the ThreadLocal or ThreadStatic implementations, or effectively use the global lock version (by having its own instance of Random and a lock). In many cases I’d regard that as overkill, however.

One other interesting option would be to create a thread-safe instance of Random to start with, which delegated to thread-local "normal" implementations. That would be very useful from a DI standpoint.

OMG Ponies!!! (Aka Humanity: Epic Fail)

(Meta note: I tried to fix the layout for this, I really did. But my CSS skills are even worse than Tony’s. If anyone wants to send me a complete sample of how I should have laid this out, I’ll fix it up. Otherwise, this is as good as you’re going to get :)

Last week at Stack Overflow DevDays, London I presented a talk on how humanity had made life difficult for software developers. There’s now a video of it on Vimeo – the audio is fairly poor at the very start, but it improves pretty soon. At the very end my video recorder ran out of battery, so you’ve just got my slides (and audio) for that portion. Anyway, here’s my slide deck and what I meant to say. (A couple of times I forgot exactly which slide was coming next, unfortunately.)

Click on any thumbnail for a larger view.

Good afternoon. This talk will be a little different from the others we’ve heard today… Joel mentioned on the podcast a long time ago that I’d talk about something "fun and esoteric" – and while I personally find C# 4 fun, I’m not sure that anyone could really call it esoteric. So instead, I thought I’d rant for half an hour about how mankind has made our life so difficult.

By way of introduction, I’m Jon Skeet. You may know me from questions such as Jon Skeet Facts, Why does Jon Skeet never sleep? and a few C# questions here and there. This is Tony the Pony. He’s a developer, but I’m afraid he’s not a very good one.

(Tony whispers) Tony wants to make it clear that he’s not just a developer. He has another job, as a magician. Are you any better at magic than development then? (Tony whispers) Oh, I see. He’s not very good at magic either – his repertoire is extremely limited. Basically he’s a one trick pony.

Anyway, when it comes to software, Tony gets things done, but he’s not terribly smart. He comes unstuck with some of the most fundamental data types we have to work with. It’s really not his fault though – humanity has let him down by making things just way too complicated.

You see, the problem is that developers are already meant to be thinking about difficult things… coming up with a better widget to frobjugate the scarf handle, or whatever business problem they’re thinking about. They’ve really got enough to deal with – the simple things ought to be simple.

Unfortunately, time and time again we come up against problems with core elements of software engineering. Any resemblance between this slide and the coding horror logo is truly coincidental, by the way. Tasks which initially sound straightforward become insanely complicated. My aim in this talk is to distribute the blame amongst three groups of people.

First, let’s blame users – or mankind as a whole. Users always have an idea that what they want is easy, even if they can’t really articulate exactly what they do want. Even if they can give you requirements, chances are those will conflict – often in subtle ways – with requirements of others. A lot of the time, we wouldn’t even think of these problems as "requirements" – they’re just things that everyone expects to work in "the obvious way". The trouble is that humanity has come up with all kinds of entirely different "obvious ways" of doing things. Mankind’s model of the universe is a surprisingly complicated one.

Next, I want to blame architects. I’m using the word "architect" in a very woolly sense here. I’m trying to describe the people who come up with operating systems, protocols, libraries, standards: things we build our software on top of. These are the people who have carefully considered the complicated model used by real people, stroked their beards, and designed something almost exactly as complicated, but not quite compatible with the original.

Finally, I’m going to blame us – common or garden developers. We have four problems: first, we don’t understand the complex model designed by mankind. Second, we don’t understand the complex model designed by the architects. Third, we don’t understand the applications we’re trying to build. Fourth, even when we get the first three bits right individually, we still screw up when we try to put them together.

For the rest of this talk, I’m going to give three examples of how things go wrong. First, let’s talk about numbers.

You would think we would know how numbers work by now. We’ve all been doing maths since primary school. You’d also think that computers knew how to handle numbers by now – that’s basically what they’re built on. How is it that we can search billions of web pages in milliseconds, but we can’t get simple arithmetic right? How many times are we going to see Stack Overflow questions along the lines of "Is double multiplication broken in .NET?"

I blame evolution.

We have evolved with 8 fingers and 2 thumbs – a total of 10 digits. This was clearly a mistake. It has led to great suffering for developers. Life would have been a lot simpler if we’d only had eight digits.

Admittedly this gives us three bits, which isn’t quite ideal – but having 16 digits (fourteen fingers and two thumbs) or 4 digits (two fingers and two thumbs) could be tricky. At least with eight digits, we’d be able to fit in with binary reasonably naturally. Now just so you don’t think I’m being completely impractical, there’s another solution – we could have just counted up to eight and ignored our thumbs. Indeed, we could even have used thumbs as parity bits. But no, mankind decided to count to ten, and that’s where all the problems started.

Now, Tony – here’s a little puzzle for you. I want you to take a look at this piece of Java code (turn Tony to face screen). (Tony whispers) What do you mean you don’t know Java? All right, here’s the C# code instead…

Is that better? (Tony nods enthusiastically) So, Tony, I want you to tell me the value of d after this line has executed. (Tony whispers)

Tony thinks it’s 0.3 Poor Tony. Why on earth would you think that? Oh dear. Sorry, no it’s not.

No, you were certainly close, but the exact value is:

0.299999 – Well, I’m not going to read it all out, but that’s the exact value. And it is an exact value – the compiler has approximated the 0.3 in the source code to the nearest number which can be exactly represented by a double. It’s not the computer’s fault that we have this bizarre expectation that a number in our source code will be accurately represented internally.

Let’s take a look at two more numbers… 5 and a half in both cases. Now it doesn’t look like these are really different – but they are. Indeed, if I were representing these two numbers in a program, I’d quite possibly use different types for them. The first value is discrete – there’s a single jump from £5.50 to £5.51, and those are exact amounts of money… whereas when we measure the mass of something, we always really mean “to two decimal places” or something similar. Nothing weighs exactly five and a half kilograms. They’re fundamentally different concepts, they just happen to have the same value. What do you do with them? Well, continuous numbers are often best represented as float/double, whereas discrete decimal numbers are usually best represented using a decimal-based type.

Now I’ve ignored an awful lot of things about numbers which can also trip us up – signed and unsigned, overflow, not-a-number values, infinities, normalised and denormal numbers, parsing and formatting, all kinds of stuff. But we should move on. Next stop, text.

Okay, so numbers aren’t as simple as we’d like them to be. Text ought to be easy though, right? I mean, my five year old son can read and write – how hard can it be? One bit of trivia – when I originally copied this out by hand, I missed out "ipsum." Note to self: if you’re going to copy out "lorem ipsum" the two words you really, really need to get at least those words right. Fail.

Of course, I’m sure pretty much everyone here knows that text is actually a pain in the neck. Again, I will blame humanity. Here we have two sets of people using completely different characters, speaking different languages, and quite possibly reading in different directions. Apologies if the characters on the right accidentally spell a rude word, by the way – I just picked a few random Kanji characters from the Unicode charts. (As pointed out in the comments, these aren’t actually Kanji characters anyway. They’re Katakana characters. Doh!) Cultural diversity has screwed over computing, basically.

However, let’s take the fact that we’ve got lots of characters as a given. Unicode sorts all that out, right? Let’s see. Time for a coding exercise – Tony, I’d like you to write some code to reverse a string. (Tony whispers) No, I’m not going to start up Visual Studio for you. (Tony whispers) You’ve magically written it on the next slide? Okay, let’s take a look.

Well, this looks quite promising. We’re taking a string, converting it into a character array, reversing that array, and then building a new string. I’m impressed, Tony – you’ve avoided pointless string concatenation and everything. (Tony is happy.) Unfortunately…

… it’s broken. I’m just going to give one example of how it’s broken – there are lots of others along the same lines. Let’s reverse one of my favourite musicals…

Here’s one way of representing Les Miserables as a Unicode string. Instead of using one code point for the “e acute”, I’ve used a combining character to represent the accent, and then an unaccented ASCII e. Display this in a GUI, and it looks fine… but when we apply Tony’s reversing code…

… the combining character ends up after the e, so we get an “s acute” instead. Sorry Tony. The Unicode designers with their fancy schemes have failed you.

EDIT: In fact, not only have the Unicode designers made things difficult, but so have implementers. You see, I couldn’t remember whether combining characters came before or after base characters, so I wrote a little Windows Forms app to check. That app displayed "Les Misu0301erables" as "Les Misérables". Then, based on the comments below, I checked with the standard – and the Unicode combining marks FAQ indicates pretty clearly that the base character comes before the combining character. Further failure points to both me and someone in Microsoft, unless I’m missing something. Thanks to McDowell for pointing this out in the comments. If I ever give this presentation again, I’ll be sure to point it out. WPF gets it right, by the way. Update: this can be fixed in Windows Forms by setting the UseCompatibleTextRendering property to false (or setting the default to false). Apparently the default is set to false when you create a new WinForms project in VS2008. Shame I tend to write "quick check" programs in a plain text editor…

Of course the basic point about reversal still holds, but with the correct starting string you’d end up with an acute over the r, not the s.

It’s not like the problems are solely in the realm of non-ASCII characters though. I present to you…

A line break. Or rather, one of the representations of a line break. As if the natural cultural diversity of humanity hasn’t caused enough problems, software decided to get involved and have line break diversity. Heck, we’re not even just limited to CR, LF and CRLF – Unicode has its own special line terminator character as well, just for kicks.

To prove this isn’t just a problem for toy examples, here’s something that really bit me, back about 9 or 10 years ago. Here’s some code which tries to do a case-insensitive comparison for the text "MAIL" in Java. Can anyone spot the problem?

It fails in Turkey. This is reasonably well known now – there’s a page about the “Turkey test” encouraging you to try your applications in a Turkish locale – but at the time it was a mystery to me. If you’re not familiar with this, the problem is that if you upper-case an “i” in Turkish, you end up with an “I” with a dot on it. This code went into production, and we had a customer in Turkey whose server was behaving oddly. As you can imagine, if you’re not aware of that potential problem, it can take a heck of a long time to find that kind of bug.

Here’s some code from a newsgroup post. It’s somewhat inefficient code to collapse multiple spaces down to a single one. Leaving aside the inefficiency, it looks like it should work. This was before we had String.Contains, so it’s using IndexOf to check whether we’ve got a double space. While we can find two spaces in a row, we’ll replace any occurrence of two spaces with a single space. We’re assigning the result of string.Replace back to the same variable, so that’s avoided one common problem… so how could this fail?

This string will cause that code to go into a tight loop, due to this evil character here. It’s a "zero-width non-joiner" – basically a hint that the two characters either side of it shouldn’t be squashed up too closely together. IndexOf ignores it, but Replace doesn’t. Ouch.

Now I’m not showing these examples to claim I’m some sort of Unicode expert – I’m really, really not. These are just corner cases I happen to have run into. Just like with numbers, I’ve left out a whole bunch of problems like bidi, encodings, translation, culture-sensitive parsing and the like.

Given the vast array of writing systems the world has come up with – and variations within those systems – any attempt to model text is going to be complicated. The problems come from the inherent complexity, some additional complexity introduced by things like surrogate pairs, and developers simply not having the time to become experts on text processing.

So, we fail at both numbers and text. How about time?

I’m biased when it comes to time-related problems. For the last year or so I’ve been working on the Google’s implementation of ActiveSync, mostly focusing on the calendar side of things. That means I’ve been exposed to more time-based code than most developers… but it’s still a reasonably common area, as you can tell from the number of related questions on Stack Overflow.

To make things slightly simpler, let’s ignore relativity. Let’s pretend that time is linear – after all, most systems are meant to be modelling the human concept of time, which definitely doesn’t include relativity.

Likewise, let’s ignore leap seconds. This isn’t always a good idea, and there are some wrinkles around library support. For example, Java explicitly says that java.util.Date and Calendar may or may not account for leap seconds depending on the host support. So, it’s good to know how predictable that makes our software… I’ve tried reading various explanations of leap seconds, and always ended up with a headache. For the purposes of this talk, I’m going to assert that they don’t exist.

Okay, so let’s start with something simple. Tony, what’s the time on this slide? (Tony whispers) Tony doesn’t want to answer. Anyone? (Audience responds.) Yes, about 5 past 3 on October 28th. So what’s the difference between now and the time on this slide? (Audience response.) No, it’s actually nearly twelve hours… this clock is showing 5 past 3 in the morning. Tony’s answer was actually the right one, in many ways… this slide has a hopeless amount of ambiguity. It’s not as bad as it might be, admittedly. Imagine if it said October 11th… Jeff and Joel would be nearly a month out of sync with the rest of us. And then even if we get the date and the time right, it’s still ambiguous… because of time zones.

Ah, time zones. My favourite source of WTFs. I could rant for hours about them – but I’ll try not to. I’d just like to point out a few of the idiosyncrasies I’ve encountered. Let’s start off with the time zones on this slide. Notice anything strange? (Audience or whisper from Tony) Yes, CST is there three times. Once for Central Standard Time in the US – which is UTC-6. It’s also Central Standard Time in Australia – where it’s UTC+9.30. It’s also Central Summer Time in Australia, where it’s UTC+10.30. I think it takes a special kind of incompetence to use the same acronym in the same place for different offsets.

Then let’s consider time zones changing. One of the problems I face is having to encode or decode a time zone representation from a single pattern – something like "It’s UTC-3 or -2, and daylight savings are applied from the third Sunday in March to the first Sunday in November". That’s all very well until the system changes. Some countries give plenty of warning of this… but on October 7th this year, Argentina announced that it wasn’t going to use daylight saving time any more… 11 days before its next transition. The reason? Their dams are 90% full. I only heard about this due to one of my unit tests failing. For various complicated reasons, a unit test which expected to recognise the time zone for Godthab actually thought it was Buenos Aires. So due to rainfall thousands of miles away, my unit test had moved Greenland into Argentina. Fail.

If you want more time zone incidents, talk to me afterwards. It’s a whole world of pain. I suggest we move away from time zones entirely. In fact, I suggest we adopt a much simpler system of time. I’m proud to present my proposal for coffee time. This is a system which determines the current time based on the answer to the question: "Is it time for coffee?" This is what the clock looks like:

This clock is correct all over the world, is very cheap to produce, and is guaranteed to be accurate forever. Batteries not required.

So where are we?

The real world has failed us. It has concentrated on local simplicity, leading to global complexity. It’s easy to organise a meeting if everyone is in the same time zone – but once you get different continents involved, invariably people get confused. It’s easy to get writing to work uniformly left to right or uniformly right to left – but if you’ve got a mixture, it becomes really hard to keep track of. The diversity which makes humanity such an interesting species is the curse of computing.

When computer systems have tried to model this complexity, they’ve failed horribly. Exhibit A: java.util.Calendar, with its incomprehensible set of precedence rules. Exhibit B: .NET’s date and time API, which until relatively recently didn’t let you represent any time zone other than UTC or the one local to the system.

Developers have, collectively, failed to understand both the models and the real world. We only need one exhibit this time: the questions on Stack Overflow. Developers asking questions around double, or Unicode, or dates and times aren’t stupid. They’ve just been concentrating on other topics. They’ve made an assumption that the core building blocks of their trade would be simple, and it turns out they’re not.

This has all been pretty negative, for which I apologise. I’m not going to claim to have a complete solution to all of this – but I do want to give a small ray of hope. All this complexity can be managed to some extent, if you do three things.

First, try not to take on more complexity than you need. If you can absolutely guarantee that you won’t need to translate your app, it’ll make your life a lot easier. If you don’t need to deal with different time zones, you can rejoice. Of course, if you write a lot of code under a set of assumptions which then changes, you’re in trouble… but quite often you can take the "You ain’t gonna need it" approach.

Next, learn just enough about the problem space so that you know more than your application’s requirements. You don’t need to know everything about Unicode – but you need to be aware of which corner cases might affect your application. You don’t need to know everything about how denormal number representation, but you may well need to know how rounding should be applied in your reports. If your knowledge is just a bit bigger than the code you need to write, you should be able to be reasonably comfortable.

Pick the right platforms and libraries. Yes, there are some crummy frameworks around. There are also some good ones. What’s the canonical answer to almost any question about java.util.Calendar? Use Joda Time instead. There are similar libraries like ICU – written by genuine experts in these thorny areas. The difference a good library can make is absolutely enormous.

None of this will make you a good developer. Tony’s still likely to mis-spell his "main" method through force of habit. You’re still going to get off by one errors. You’re still going to forget to close database connections. But if you can at least get a handle on some of the complexity of software engineering, it’s a start.

Thanks for listening.

Iterating atomically

The IEnumerable<T> and IEnumerator<T> interfaces in .NET are interesting. They crop up an awful lot, but hardly anyone ever calls them directly – you almost always use a foreach loop to iterate over the collection. That hides all the calls to GetEnumerator(), MoveNext() and Current. Likewise iterator blocks hide the details when you want to implement the interfaces. However, sometimes details matter – such as for this recent Stack Overflow question. The question asks how to create a thread-safe iterator – one that can be called from multiple threads. This is not about iterating over a collection n times independently on n different threads – this is about iterating over a collection once without skipping or duplicating. Imagine it’s some set of jobs that we have to complete. We assume that the iterator itself is thread-safe to the extent that calls from different threads at different times, with intervening locks will be handled reasonably. This is reasonable – basically, so long as it isn’t going out of its way to be thread-hostile, we should be okay. We also assume that no-one is trying to write to the collection at the same time.

Sounds easy, right? Well, no… because the IEnumerator<T> interface has two members which we effectively want to call atomically. In particular, we don’t want the collection { “a”, “b” } to be iterated like this:

Thread 1 Thread 2
MoveNext()  
  MoveNext()
Current  
  Current

That way we’ll end up not processing the first item at all, and the second item twice.

There are two ways of approaching this problem. In both cases I’ve started with IEnumerable<T> for consistency, but in fact it’s IEnumerator<T> which is the interesting bit. In particular, we’re not going to be able to iterate over our result anyway, as each thread needs to have the same IEnumerator<T> – which it won’t do if each of them uses foreach (which calls GetEnumerator() to start with).

Fix the interface

First we’ll try to fix the interface to look how it should have looked to start with, at least from the point of view of atomicity. Here are the new interfaces:

public interface IAtomicEnumerable<T>
{
    IAtomicEnumerator<T> GetEnumerator();
}

public interface IAtomicEnumerator<T>
{
    bool TryMoveNext(out T nextValue);
}

One thing you may notice is that we’re not implementing IDisposable. That’s basically because it’s a pain to do so when you think about a multi-threaded environment. Indeed, it’s possibly one of the biggest arguments against something of this nature. At what point do you dispose? Just because one thread finished doesn’t mean that the rest of them have… don’t forget that “finish” might mean “an exception was thrown while processing the job, I’m bailing out”. You’d need some sort of co-ordinator to make sure that everyone is finished before you actually do any clean-up. Anyway, the nice thing about this being a blog post is we can ignore that little thorny issue :)

The important point is that we now have a single method in IAtomicEnumerator<T> – TryMoveNext, which works the way you’d expect it to. It atomically attempts to move to the next item, returns whether or not it succeeded, and sets an out parameter with the next value if it did succeed. Now there’s no chance of two threads using the method and stomping on each other’s values (unless they’re silly and use the same variable for the out parameter).

It’s reasonably easy to wrap the standard interfaces in order to implement this interface:

/// <summary>
/// Wraps a normal IEnumerable[T] up to implement IAtomicEnumerable[T].
/// </summary>
public sealed class AtomicEnumerable<T> : IAtomicEnumerable<T>
{
    private readonly IEnumerable<T> original;

    public AtomicEnumerable(IEnumerable<T> original)
    {
        this.original = original;
    }

    public IAtomicEnumerator<T> GetEnumerator()
    {
        return new AtomicEnumerator(original.GetEnumerator());
    }

    /// <summary>
    /// Implementation of IAtomicEnumerator[T] to wrap IEnumerator[T].
    /// </summary>
    private sealed class AtomicEnumerator : IAtomicEnumerator<T>
    {
        private readonly IEnumerator<T> original;
        private readonly object padlock = new object();

        internal AtomicEnumerator(IEnumerator<T> original)
        {
            this.original = original;
        }

        public bool TryMoveNext(out T value)
        {
            lock (padlock)
            {
                bool hadNext = original.MoveNext();
                value = hadNext ? original.Current : default(T);
                return hadNext;
            }
        }
    }
}

Just ignore the fact that I never dispose of the original IEnumerator<T> :)

We use a simple lock to make sure that MoveNext() and Current always happen together – that nothing else is going to call MoveNext() between our TryMoveNext() calling it, and it fetching the current value.

Obviously you’d need to write your own code to actually use this sort of iterator, but it would be quite simple:

T value;
while (iterator.TryMoveNext(out value))
{
    // Use value
}

However, you may already have code which wants to use an IEnumerator<T>. Let’s see what else we can do.

Using thread local variables to fake it

.NET 4.0 has a very useful type called ThreadLocal<T>. It does basically what you’d expect it to, with nice features such as being able to supply a delegate to be executed on each thread to provide the initial value. We can use a thread local to make sure that so long as we call both MoveNext() and Current atomically when we’re asked to move to the next element, we can get back the right value for Current later on. It has to be thread local because we’re sharing a single IEnumerator<T> across multiple threads – each needs its own separate storage.

This is also the approach we’d use if we wanted to wrap an IAtomicEnumerator<T> in an IEnumerator<T>, by the way. Here’s the code to do it:

public class ThreadSafeEnumerable<T> : IEnumerable<T>
{
    private readonly IEnumerable<T> original;

    public ThreadSafeEnumerable(IEnumerable<T> original)
    {
        this.original = original;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return new ThreadSafeEnumerator(original.GetEnumerator());
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    private sealed class ThreadSafeEnumerator : IEnumerator<T>
    {
        private readonly IEnumerator<T> original;
        private readonly object padlock = new object();
        private readonly ThreadLocal<T> current = new ThreadLocal<T>();

        internal ThreadSafeEnumerator(IEnumerator<T> original)
        {
            this.original = original;
        }

        public bool MoveNext()
        {
            lock (padlock)
            {
                bool ret = original.MoveNext();
                if (ret)
                {
                    current.Value = original.Current;
                }
                return ret;
            }
        }

        public T Current
        {
            get { return current.Value; }
        }

        public void Dispose()
        {
            original.Dispose();
            current.Dispose();
        }

        object IEnumerator.Current
        {
            get { return Current; }
        }

        public void Reset()
        {
            throw new NotSupportedException();
        }
    }
}

I’m going to say it one last time – we’re broken when it comes to disposal. There’s no way of safely disposing of the original iterator at “just the right time” when everyone’s finished with it. Oh well.

Other than that, it’s quite simple. This code has the serendipitous property of actually implementing IEnumerator<T> slightly better than C#-compiler-generated implementations from iterator blocks – if you call the Current property without having called MoveNext(), this will throw an InvalidOperationException, just as the documentation says it should. (It doesn’t do the same at the end, admittedly, but that’s fixable if we really wanted to be pedantic.

Conclusion

I found this an intriguing little problem. I think there are better ways of solving the bigger picture – a co-ordinator which takes care of disposing exactly once, and which possibly mediates the original iterator etc is probably the way forward… but I enjoyed thinking about the nitty gritty.

Generally speaking, I prefer the first of these approaches. Thread local variables always feel like a bit of a grotty hack to me – they can be useful, but it’s better to avoid them if you can. It’s interesting to see how an interface can be inherently thread-friendly or not.

One last word of warning – this code is completely untested. It builds, and I can’t immediately see why it wouldn’t work, but I’m making no guarantees…

API design: choosing between non-ideal options

So, UnconstrainedMelody is coming on quite nicely. It now has quite a few useful options for flags enums, "normal enums" and delegates. However, there are two conflicting limitations which leave a couple of options. (Other related answers on Stack Overflow have suggested alternative approaches, basically.)

Currently, most of the enums code is in two classes: Flags and Enums. Both are non-generic: the methods within them are generic methods, so they have type parameters (and constraints). The main benefit of this is that generic type inference only applies to generic methods, and I definitely want that for extension methods and anywhere else it makes sense.

The drawback is that properties can’t be generic. That means my API is entirely expressed in terms of methods, which can be a pain. The option to work around this is to have a generic type which properties in. This adds confusion and guesswork – what call is where?

To recap, the options are:

// Option 1 (current): all methods in a nongeneric class:
// Some calls which are logically properties end up
// as methods…
IList<Foo> foos = Enums.GetValues<Foo>();
// Type infererence for extenion methods
// Note that we couldn’t have a Description property
// as we don’t have extension properties
string firstDescription = foos[0].GetDescription();
        
// Option 2: Use just a generic type:
// Now we can use a property…
IList<Foo> foos = Enums<Foo>.Values;
// But we can’t use type inference
string firstDescription = Enums<Foo>.GetDescription(foos[0]);
        
// Option 3: Use a mixture (Enums and Enums<T>):
IList<Foo> foos = Enums<Foo>.Values;
// All looks good…
string firstDescription = foos[0].GetDescription();
// … but the user has to know when to use which class

All of these are somewhat annoying. If we only put extension methods into the nongeneric class, then I guess users would never need to really think about that – they’d pretty much always be calling the methods via the extension method syntactic sugar anyway. It still feels like a pretty arbitrary split though.

Any thoughts? Which is more important – conceptual complexity, or the idiomatic client code you end up with once that complexity has been mastered? Is it reasonable to make design decisions like this around what is essentially a single piece of syntactic sugar (extension methods)?

(By the way, if anyone ever wanted justification for extension properties, I think this is a good example… Description feels like it really should be a property.)

Generic constraints for enums and delegates

As most readers probably know, C# prohibits generic type constraints from referring to System.Object, System.Enum, System.Array, System.Delegate and System.ValueType. In other words, this method declaration is illegal:

public static T[] GetValues<T>() where T : struct, System.Enum
{
    return (T[]) Enum.GetValues(typeof(T));
}

This is a pity, as such a method could be useful. (In fact there are better things we can do… such as returning a read-only collection. That way we don’t have to create a new array each time the method is called.) As far as I can tell, there is no reason why this should be prohibited. Eric Lippert has stated that he believes the CLR doesn’t support this – but I think he’s wrong. I can’t remember the last time I had cause to believe Eric to be wrong about something, and I’m somewhat nervous of even mentioning it, but section 10.1.7 of the CLI spec (ECMA-335) partition II (p40) specifically gives examples of type parameter constraints involving System.Delegate and System.Enum. It introduces the table with "The following table shows the valid combinations of type and special constraints for a representative set of types." It was only due to reading this table that I realized that the value type constraint on the above is required (or a constructor constraint would do equally well) – otherwise System.Enum itself satisfies the constraint, which would be a Bad Thing.

It’s possible (but unlikely) that the CLI doesn’t fully implement this part of the CLR spec. I’m hoping that Eric’s just wrong on this occasion, and that actually there’s nothing to stop the C# language from allowing such constraints in the future. (It would be nice to get keyword support, such that a constraint of "T : enum" would be equivalent to the above, but hey…)

The good news is that ilasm/ildasm have no problem with this. The better news is that if you add a reference to a library which uses those constraints, the C# compiler applies them sensibly, as far as I can tell…

Introducing UnconstrainedMelody

(Okay, the name will almost surely have to change. But I like the idea of it removing the constraints of C# around which constraints are valid… and yet still being in the key of C#. Better suggestions welcome.)

I have a plan – I want to write a utility library which does useful things for enums and delegates (and arrays if I can think of anything sensible to do with them). It will be written in C#, with methods like this:

public static T[] GetValues<T>() where T : struct, IEnumConstraint
{
    return (T[]) Enum.GetValues(typeof(T));
}

(IEnumConstraint has to be an interface of course, as otherwise the constraint would be invalid.)

As a post-build step, I will:

  • Run ildasm on the resulting binary
  • Replace every constraint using EnumConstraint with System.Enum
  • Run ilasm to build the binary again

If anyone has a simple binary rewriter (I’ve looked at PostSharp and CCI; both look way more complicated than the above) which would do this, that would be great. Otherwise ildasm/ilasm will be fine. It’s not like consumers will need to perform this step.

As soon as the name is finalized I’ll add a project on Google Code. Once the infrastructure is in place, adding utility methods should be very straightforward. Suggestions for utility methods would be useful, or just join the project when it’s up and running.

Am I being silly? Have I overlooked something?

A couple of hours later…

Okay, I decided not to wait for a better name. The first cut – which does basically nothing but validate the idea, and the fact that I can still unit test it – is in. The UnconstrainedMelody Google Code project is live!

Recent activities

It’s been a little while since I’ve blogged, and quite a lot has been going on. In fact, there are a few things I’d have blogged about already if it weren’t for “things” getting in the way.

Rather than writing a whole series of very short blog posts, I thought I’d wrap them all up here…

C# in Depth: next MEAP drop available soon – Code Contracts

Thanks to everyone who gave feedback on my writing dilemma. For the moment, the plan is to have a whole chapter about Code Contracts, but not include a chapter about Parallel Extensions. My argument for making this decision is that Code Contracts really change the feel of the code, making it almost like a language feature – and its applicability is almost ubiquitous, unlike PFX.

I may write a PFX chapter as a separate download, but I’m sensitive to those who (like me) appreciate slim books. I don’t want to “bulk out” the book with extra topics.

The Code Contracts chapter is in the final stages before becoming available to MEAP subscribers. (It’s been “nearly ready” for a couple of weeks, but I’ve been on holiday, amongst other things.) After that, I’m going back to the existing chapters and revising them.

Talking in Dublin – C# 4 and Parallel Extensions

Last week I gave two talks in Dublin at Epicenter. One was on C# 4, and the other on Code Contracts and Parallel Extensions. Both are now available in a slightly odd form on the Talks page of the C# in Depth web site. I no longer write “formal” PowerPoint slides, so the downloads are for simple bullet points of text, along with silly hand-drawn slides. No code yet – I want to tidy it up a bit before including it.

Podcasting with The Connected Show

I recently recorded a podcast episode with The Connected Show. I’m “on” for the second 2/3 of the show – about an hour of me blathering on about the new features of C# 4. If you can understand generic variance just by listening to me talking about it, you’re a smart cookie ;)

(Oh, and if you like it, please express your amusement on Digg / DZone / Shout / Kicks.)

Finishing up with Functional Programming for the Real World

Well, this hasn’t been taking much of my time recently (I bowed out of all the indexing etc!) but Functional Programming for the Real World is nearly ready to go. Hard copy should be available in the next couple of months… it’ll be really nice to see how it fares. Much kudos to Tomas for all his hard work – I’ve really just been helping out a little.

Starting on Groovy in Action, 2nd edition

No sooner does one book finish than another one starts. The second edition of Groovy in Action is in the works, which should prove interesting. To be honest, I haven’t played with Groovy much since the first edition of the book was finished, so it’ll be interesting to see what’s happened to the language in the meantime. I’ll be applying the same sort of spit and polish that I did in the first edition, and asking appropriately ignorant questions of the other authors.

Tech Reviewing C# 4.0 in a Nutshell

I liked C# 3.0 in a Nutshell, and I feel honoured that Joe asked me to be a tech reviewer for the next edition, which promises to be even better. There’s not a lot more I can say about it at the moment, other than it’ll be out in 2010 – and I still feel that C# in Depth is a good companion book.

MoreLINQ now at 1.0 beta

A while ago I started the MoreLINQ project, and it gained some developers with more time than I’ve got available :) Basically the idea is to add some more useful LINQ extension methods to LINQ to Object. Thanks to Atif Aziz, the first beta version has been released. This doesn’t mean we’re “done” though – just that we think we’ve got something useful. Any suggestions for other operators would be welcome.

Manning Pop Quiz and discounts

While I’m plugging books etc, it’s worth mentioning the Manning Pop Quiz – multiple choice questions on a wide variety of topics. Fabulous prizes available, as well as one-day discounts:

  • Monday, Sept 7th: 50% of all print books (code: pop0907)
  • Monday, Sept 14: 50% off all ebooks  (code: pop0914)
  • Thursday, Sept 17: $25 for C# in Depth, 2nd Edition MEAP print version (code: pop0917) + C# Pop Quiz question
  • Monday, Sept 21: 50% off all books  (code: pop0921)
  • Thursday, Sept 24: $12 for C# in Depth, 2nd Edition MEAP ebook (code: pop0924) + another C# Pop Quiz question

Future speaking engagements

On September 16th I’m going to be speaking to Edge UG (formerly Vista Squad) in London about Code Contracts and Parallel Extensions. I’m already very much looking forward to the Stack Overflow DevDays London conference on October 28th, at which I’ll be talking about how humanity has screwed up computing.

Future potential blog posts

Some day I may get round to writing about:

  • Revisiting StaticRandom with ThreadLocal<T>
  • Volatile doesn’t mean what I thought it did

There’s a lot more writing than coding in that list… I’d like to spend some more time on MiniBench at some point, but you know what deadlines are like.

Anyway, that’s what I’ve been up to and what I’ll be doing for a little while…