… is this one. I’ve wasted many an hour searching for the cause of this. And it’s one you’re likely to run into pretty quickly when you try to write your first Hibernate configuration file.

The XML

<one-to-many type=”OtherClass”/>

delivers the error

Error parsing XML: Attribute “type” must be declared for element type “one-to-many”.

This looks like a perfectly self-explanatory error, however looking at the file, the element does have a “type” attribute. What should one do?

Thinking about it, I only just introduced the “type” attribute to the <one-to-many> element in my config. What happens if I change the attribute name to “fsdjkfdk”?

<one-to-many fsdjkfdk=”OtherClass”/>

The error is now:

Error parsing XML: Attribute “fsdjkfdk” must be declared for element type “one-to-many”.

What the error means is that the attribute must not be declared, as opposed to must.

It’s amusing to read even people on the Hibernate team get confused by this error, and can’t find a solution.

(Hibernate 3.3.1 - the most current version - although I encountered this error within the first hour of ever using Hibernate in Q1/2006.)

I saw a cool device today. One of my customers manages data centers and they have a new data center and need to connect it to their old ones, so they’ve bought/rented some “dark fiber” between their data centers and have fiber multiplexers at each end so that lots of fiber-optical devices at each end can all talk to one another over this single dark fiber. (Or something like that - I don’t know too much about networking equipment!)

So basically this multiplexer looks like any other switch thing, a long horizontal device with a bunch of identical ports on the back, each with a number etc, and one “master” port which connects to the dark fiber. (only the optical sockets look slightly different from e.g. RJ45 sockets..)

But then i noticed, this device only had the identical data ports/master port on the back. I couldn’t find a power socket, I looked at all of its sides, and was confused for a while. Until i worked out the reason why I couldn’t see the power socket - the device was purely optical and didn’t need power!

Imagine that, data center, a bunch of hard-core networking and computing equipment, the last thing one expects is devices that don’t need power!

Functions take parameters. What order should these parameters be in?

Perhaps a bit of a ridiculous question, given that it clearly doesn’t matter.

void writeUser(Connection c, User u) { ... }
void writeUser(User u, Connection c) { ... }

If one writes either of these functions, they will both work, and any performance differences between the two would be an extreme micro-optimization which wouldn’t be platform independent (e.g. in RISC OS the first 6 parameters to a C function were stored on the stack and the rest weren’t, unless one used a “pass by value” structure, so it might have an impact on performance… but I digress)

However, certain languages support the feature partial application, where passing only some of the parameters to a function produces a function taking only the remaining parameters. For example (in SML, syntax examples, BTW A sad sign of the times when Googling for SML yields “Did you mean: XML”)

fun add (a,b) = a + b;
add 1 2;                 (* prints 3 *) 

val successor = add 1;   (* generates function taking 1 parameter *)
successor 2;             (* prints 3 *)

So in those languages, it makes sense to place the most constant parameters as left-most as possible. (The syntax only allows left-most parameters to be partially applied.)

For example a function to write a user to a database connection might be defined as taking two parameters, a database connection and the user to be written, and partial application could be used as follows:

Clearly the first partial-application function is much more useful than the second.

Even in languages which don’t support partial application I’ve noticed a convention of functions being defined this way, probably because at least some (of the best?) code is written by people with experience of functional programming languages. I like this convention, and adhere to it myself.

The MySQL “lock tables” (and “unlock tables”) command has a nasty side-effect, it implicitly commits the current transaction.

This caused a bug in production code (a normally irrelevant temporary error, which should have normally caused a rollback, only rolled back to the last “unlock tables” command due to its implicit commit, and thus left the database in an inconsistent state, meaning that when the request was retried the software found the database in a state it wasn’t expecting and thus couldn’t process the request even though the irrelevant temporary error had now been fixed. So the request got retried indefinitely, and the result was that a lot of orphan invoices were created in the database, which wouldn’t have been a problem but invoice numbers were then consumed by these unused invoices, and Austrian law dictates that invoice numbers must be sequential, i.e. the numbering system shouldn’t have huge holes in it. Or, as in our case, a number of huge holes, as the system was up and running for other requests during this bug (a feature, that one faulty request shouldn’t take the whole system down), so other invoices did get correctly generated at the time, and their invoice numbers were then small islands in the sea of unused/invalid invoice numbers produced by the bug.)

I wish this had been more obviously documented (MySQL 5.1 lock tables documentation). It is there in the bullet points (and the comments!), but it’s hardly well emphasized, and I missed it.

The trouble is that the “lock tables” command comes from the stone-age of MySQL before it supported transactions at all.

This has been “fixed” in MySQL 6.0 (MySQL 6.0 changelog), via the introduction of a new options ”in share mode” / “in exclusive mode”.

In the mean-time, if you need to lock a table (which one needs to do in order to ensure table-wide sequential numbering, as a primary key isn’t designed for that) I would recommend creating a table with a name like “mutux” and creating a row for each type of table-lock you might like to acquire, and then “select for update” the row corresponding to your table. This creates an in-transaction row-level lock, doesn’t do an implicit commit, and the lock will be automatically released when the transaction ends.

Java’s slogan was “write once, run anywhere”. They received a certain amount of criticism but I have to say that compared to other programming languages it’s really true. You can use it for:

• Background jobs (without user-interface)
• Server-side web applications (many web servers & web frameworks available)
• In the web browser (applets)
• In the web browser (translation from Java to Javascript by GWT)
• On the desktop (using platform-independent Swing, or with native Apple UI using Cocoa)
• On some mobile phones (J2ME, Google Android) - although I haven’t tested how good that really works?

Writing the previous version of a certain application website in Perl, there was no easy way to give the customer a “tool” to test out new versions of the configuration file. These files would normally be installed on the server, were multiple megabytes in size, and the Perl would parse and use them. For testing, it was not ideal to have to upload potential new files to the test server, due to their size.

The new version is in Java and also takes a configuration file, but I have written a Swing (desktop) tool which simply allows the tester to select a new potential configuration file from their local hard disk, and the desktop tool reuses all the processing logic 1:1 that the web server in production would use.

That wouldn’t have been possible with the old version of the logic written in Perl. (I know there are windowing libraries for languages like Perl but they are hardly as easy to deploy - i.e. install on a Windows workstation - as a Java application - simply double-click the .jar file once Java is installed)

I am writing the web front-end for the new version in GWT so I can reuse certain (mainly user validation) code between the web browser (giving the user instant feedback in case of errors) and the web server (necessary for security in case someone bypasses the client and sends HTTP requests directly.) And simply pass Java objects between the web server and the web client, without having to worry about how that transfer works (JSON, XML, etc.)

Other mainstream candidates for languages which run on multiple places:

• Objective-C is not too bad, running on Apple desktops, Apple iPhones, and on the web server via WebObjects (does the current version of WO still use Objective-C or is it Java-only these days?) - but not in the browser
• Javascript runs on the web browser and server, and no doubt mobile browsers (but not desktops as far as I know)
• Perhaps C#? Certainly good desktop, web server integration, no doubt IE/Windows integration via ActiveX

Thanks the the internet archive I found one of the first (or one of the only?) websites I designed i.e. I did the graphics, did the “implementation” using a tool which produced HTML, etc. (Although the company logo etc already existed, I didn’t write the texts..)

This was for AAA+ which was the first company I worked for in my life, which was also the company I worked for just after coming to Vienna, so has a lot of associations of newness in my life.

And I think it doesn’t look too bad :) www.aaa-plus.com (Internet Archive)

Windows, Linux and OS X offer the ability to view the utilization of each CPU/core in the system. This is completely useless. On all these operating systems, tasks get switched from one core to another on a regular basis. (I don’t know why this happens, but I suppose there is no reason for it not to happen.)

Here is my CPU-bound single-threaded program running on a dual-core computer.

I suppose all one can really say is that if one has N cores and the average CPU% usage (over all cores) is approximately 100/N then probably one is running a program which can’t take advantage of multiple cores.

I would rather replace the current “CPU usage history, per core” multiple graphs with:

• One graph, showing a history of the average over all CPUs (visually the same as if one had a 1-core CPU).
• I would then add horizontal marker lines: If one had 4 cores, I would add 4 equally spaced marker lines. This would show that if the performance reached a marker line (e.g. 25% for the first line) then probably running the equivalent of 1 single-threaded program.

I mean it’s not a brilliant solution but I reckon it would be more meaningful than the way the information is currently displayed.

I see why people get a bit upset about the privacy aspects of Google Streetview! On the other hand, it is rather amazing. To sit here in my office in Vienna and be able to walk around where I used to live at Southampton University.

• First house (where I lived Summer 1996 - Summer 1997)
• Second house (where I lived Summer 1997 - Summer 1998)

In comparison to my first house, even the flat I’m living in now doesn’t look too bad :)

I’m spending the day at home after 3 nights of bad sleep (no idea why).

Just now I heard a banging in my spare bedroom (= “ghost room”) and I saw that the window was open. Why did it suddenly become open? It’s a “double-glazed” window in the old style, i.e. there are simply two windows, one on the inside and one on the outside, and in between there are blinds, which are drawn. They’ve been drawn for a few months. There is no reason to open the inside window, open the blinds, open the outside window, and then redraw the blinds, and close the inside window. And given that it’s a windy day, had anyone done this (anyone є { me, Christina }, and neither of us go into the room much) then it would have started banging much earlier, not just suddenly now, in mid-afternoon!

Down the coffee house this morning, reading the newspaper Österreich (which makes the UK’s Daily Mail seem like a quality newspaper), I learnt the following facts about the ongoing case against Fritzl:

• There are a lot of foreign media in Austria for the case but apparently a disproportionate number of them are from the UK. Why?
• It’s speculated that hiding his face in court is a way to increase the value of photos & interviews he will give later.
• In jail, Fritzl is getting “fan letters” from women. Can that really be true!?

Today and tomorrow I am at an Ask Tom Live Seminar in Prague. Tom Kyte works for Oracle and has written many books on Oracle. For me he’s a bit of a “star”, so when I heard he was talking in Europe, I had to come. (But is that wrong? Normal people are in to stars like Robbie Williams surely, not Oracle experts.)

Wow there is so much I don’t know about Oracle evidently. I mean the solutions that he is presenting concerning Oracle 11g, are solutions to problems in features I’ve never used, such as partitioning, etc.

Some fun things I learnt:

• In Oracle 6 “sql plus” used to be offered as a for-cost extra, as it was considered so good! (Apparently there was a previous tool, and it was worse.) Sql Plus still doesn’t support e.g. the “up” arrow key to get the previous command.
• In Oracle 3 “insert into x select from x” would be an infinite statement as it would constantly re-select then insert the data from x that it had just inserted. This was fixed in Oracle 4. 
• Oracle 11 can create “virtual columns”, i.e. “create table x(a number, b as (a+1))”. You can then use these virtual columns for indexes (like functional indexes), foreign key constraints, etc. A statement like “select .. from x where a+1=?” will be rewritten as “where b=?” which will then use the index. Audience member asked “can I write an update statement to update b?”
• You can do “select …. as of <date>” and if the data is still available in the rollback segment, it will be returned.
• Adding a column is now always instantaneous, i.e. “alter table x add y number default 4″ will not alter the data in the table, but when a row is read which was created before the alter table statement happened, the column will be added with its default value. (But presumably altering the column’s default value then causes the long write?)
• Apparently Oracle was the first RDBMS. Well that’s what Tom Kyte claims anyway.

P.S. I should have brought my Oracle One on One with me for him to autograph… OK that would have been ridiculous.