Wednesday, October 19, 2011

Long Polling with Suneido

It started with our summer student deciding to build an "instant messenger" in Suneido. I'm not sure what prompted this, but it seemed like an interesting challenge so I let him run with it.

The trick is the "instant" part. His initial approach was to store the messages in a database table, and poll the table for changes. To make it "instant" he wanted to poll very frequently, at least once a second, and preferably more. We have about 50 users, if they are all polling every second, that's an extra 50 queries per second. I wasn't too keen on this.

I suggested we set up a separate HttpServer for the messenger, and keep the "outstanding" messages in memory, so polling didn't have to hit the database. (New messages were still written to a history table in the database, but that's not as frequent as the polling.) This took the load off the main database server, and made the requests faster.

I explained that a better way to make it "instant" was to use Comet style long polling where the server simply doesn't respond to the request until something is available. This means a lot more connections to the server since every client always has an outstanding request, which can be an issue depending on the server. But we were using jSuneido for the messenger HTTP server, and 50 connections wasn't a problem.

But the client side is also an issue. If we're going to be waiting a potentially long time for a response, the client can't be waiting "blocked", we need to make the request asynchronously. Suneido doesn't have a lot of support for aynchronous programming, but it does have a Thread function that allows you to execute code in a separate "fiber" - not a real thread, more like a coroutine. (On jSuneido "Thread" uses a real thread.)

We've never used Thread much because it has always seemed a little flaky. But it is basically sound because it's what the Suneido database server uses to handle multiple users - and that is solid. And it's also how SocketServer is implemented, and that also works well (e.g. in HttpServer)

One issue is that you want to avoid updating the GUI from a thread. That is easy to do by using Delayed with a 0 delay to process the response. Delayed works via the Windows message queue, which is processed by the main fiber. The code running inside the thread looks like:

     if "" isnt response = Http.Get(uri)
          Delayed(0, ResponseProcessor(response))

(plus catching exceptions etc.)

However, we haven't eliminated polling, we've just moved it to the messenger HTTP server, since it must poll for new messages. (Since Suneido doesn't have any way to "wait" for something.) The code looks something like:

     if Suneido.MessagesAvailable
          return ...
return ""

400 times 100ms is 40 seconds, less than the default timeout of 60 seconds.  Checking once every 100 ms (1/10 of a second) is a light load on the server, most of the time it's sleeping. (Note: this wouldn't work well on a cSuneido HTTP server because Sleep would block all the other fibers.)

This has ended up working remarkably well - the messenger is "instant" with little load on the server or client.

Tuesday, October 18, 2011

JUnit Max

I've been using JUnit Max for the last few days. It's an Eclipse extension that automatically runs your tests every time you save a change.

Normally I'd stick to open source options (which this is not), but I figure Kent Beck deserves some support.

It's taking me a little getting used to - makes me nervous not running the tests manually. It's designed to be unobtrusive, but part of me wants more blatant feedback that my tests passed.

One of my motivations for using it is to make sure I'm running all the tests, not just the one I'm working on. I've been caught a few times where I didn't discover I broke something elsewhere till some time afterwards.

Even with a fast machine, it still takes a while for the tests to run (although I'm the only one to blame for the speed of the tests), but at least they run in the background and I can continue working.

The web site is pretty minimal and there is no documentation as far as I can tell, but then again, it's pretty self explanatory.

One of the things that prompted me to try it was listening to a podcast with Kent Beck on Software Engineering Radio (recommended).

JUnit Max has had a bit of a rocky history, pulled in 2009, relaunched in 2010. I think it's a worthwhile product and I hope it's successful.

Sunday, October 16, 2011

Zite: Personalized Magazine for iPad

I've been playing with this a little. I like how it analyzes your on-line presence and suggests material. And then lets you "vote" material up or down. It found quite a few interesting computer articles I hadn't seen. Of course, the last thing I need is more stuff to read!

Zite: Personalized Magazine for iPad (free, ad supported)

Friday, October 07, 2011

The Grass Always Seems Greener

After tracking down the third bug to the same area of the code, I decided I had to stop adding more special cases and come up with a better solution.

The code was in the B-tree (technically B+-tree) implementation in my new append-only storage engine for jSuneido.

I'll try to explain the problem. I combine keys with a child node pointer. That leaves the question of whether a key points to the values less than it, or greater than it. I've always gone with greater than. But the problem is that when you do a lower bound binary search, the resulting position is one higher than the one you want. For example, if the tree node contains the keys 2, 4, 6, 8, the leaf containing 5 will be pointed at by the "4". But a lower bound binary search will give the position where you would insert 5, i.e. the position of the "6" key.

You can adjust for this, but it gets ugly. After the multiple bugs, I started to think that making the keys point to values less than the key would be cleaner because you wouldn't have the off by one problem.

So yesterday morning I set out to change the code. It's not a large amount of code and the changes were straightforward, but the code is fairly complex, and the "greater than" assumption was hidden in more places than I would have liked.

Eight hours later I had all my tests passing again. And it did simplify some parts of the code. But it also turned out (not surprisingly) that this approach had its own drawbacks. For example, a standard optimization is to detect when keys are being added in order (i.e. to the "end" of the B-tree) and to put only the new key in the new node, keeping the old full node intact. The way my code works, this is much easier with the "greater than" approach.

I'm sure I could have got it all worked out, but switching approaches was not the clear winner that I'd hoped.

Actual written code, with all it's messiness, always seems inferior to an imagined alternative, which is always perfectly clean. (And therefore, the grass is always greener title.) But when you implement the alternative, it often ends up just as messy as what you had before. (This relates to never rewrite from scratch)

I came up for air, ate some supper, realized I hadn't stepped foot outside the house all day, and considered my options. A glass of wine in hand, I returned to the computer. I wanted to "finish" working on it while all the details were fresh in my mind. I knew I wouldn't get a chance to work on it again for a week or so, and then I'd have to get back up to speed.

I threw out all the changes I'd made. (Except for a few additional tests I'd written.) Annoying, but at least it was only a days work.

By carefully adjusting the code I was able to keep the old approach, but remove most of the special cases that had been causing me trouble. The code is quite a bit cleaner now. I still have the off by one adjustment, but it's in a single place now.

Meanwhile, I've been reading Tokutek's blog about how COLA (cache oblivious lookahead array) style fractal trees are so much better than B-trees. That grass sure looks greener over there :-)

Thursday, October 06, 2011

Farewell to Steve

I sat down with my iPad last night to catch up on my blog reading and the first thing I saw was about Steve having died. We knew it was probable, but it was still a shock. When someone is so much larger than life, you don't want to believe they are bound by the same physical laws as the rest of us.

Apple started in 1977, at a time when I was becoming increasingly absorbed by computers. My first job (only job really, other than Axon) was at a local computer store. In the service department I repaired countless Apple II's. I remember when we first saw the Lisa, and then the Mac. It's hard to communicate what a revolution it was. Nowadays the world is filled with similar amazing gadgets, but not back then.

What a long way Apple has come, and a bumpy road at times. As an entrepreneur myself at that point, I remember my shock at Steve getting kicked out of his own company. Apple struggled and Microsoft grew to dominate. Back then we could not have imagined Apple ending up bigger than Microsoft. (Any more than we imagined Microsoft overtaking IBM.) And the idea that Apple would become the most valuable company in the world would have seemed like total fantasy. Steve returning to Apple and leading it to world domination still seems like a fairy tale.

Steve's abilities as a showman and salesman were alien to a geek like me. To me, what set him apart was his focus on design and user experience. Obviously, he didn't personally do all the design work. But I have to think he played a huge part in gathering and guiding his team, and shaping their culture and priorities. I can't help but wonder how things will change at Apple with Steve gone. In the short term, they will, no doubt, continue on much the same path. But in the long run, I wonder. (Just as I wonder how Axon will change when I'm not there. Not necessarily for the worse, but inevitably different.)

I don't agree with everything that Steve and Apple have done. Their closed, controlled, curated (and taxed) world is not my ideal, but it's hard to argue with what they have achieved. Apple's financial accomplishments are impressive. Even more impressive, is that they have one of the highest customer satisfaction ratings. What other company has grown so large, and yet still pleased so many people?

All in all, his life was an insanely great story.

We'll miss you Steve.