My First Presentation

Last Thursday[+] saw me popping my “presentation” cherry as I stood up in front of a small crowd of around 35 people and waffle on for 60 minutes about unit testing database code (SQL). The size of the audience and the number of questions at the end and in the pub afterwards makes me feel that it can probably be chalked up as a success. I certainly got a buzz out of it and more importantly I’m glad to get one under my belt before the ACCU Conference in April where I’ll be presenting “Using xUnit As a Swiss Army Testing Toolkit”.

It was pretty obvious from my twitter feed that I was more than a little bit nervous and even though I had a few calming replies from my fellow ACCU members I was still somewhat pensive. The ACCU is full of very smart people and the topic on which I was speaking was not what I would consider one of my core skills and so I was well aware that the room would contain people far more knowledgeable than me on the subject. The nightmare scenario was one of those people asking the kind of ‘obvious’ question that would invalidate the entire talk. The basis for this is some of the ‘heckles’ that I’ve seen at past conferences and talks. Fortunately they were unfounded… this time.

So what did I learn from the experience? Well, it’s kind of nice being the centre of attention for an hour or so :-) The biggest thing, and I’ve said this before about blogging, is that it makes you think even harder about your topic so you feel doubly sure you don’t say anything really stupid. When first asked by Allan Kelly to do a talk for ACCU London I reluctantly said ‘yes’ as I find originality a big stumbling block – I’m always assuming that I need to say something novel. Because of the ACCU’s heritage[*] I didn’t want to say anything C++ related, I don’t know anything about Java, the iPhone, Python etc and anyway those angles are already well covered by other prominent members so that pretty much left something C#/SQL based. I also didn’t want to do a shorter version of my upcoming conference talk so I looked back over my blog for inspiration and realised I had a few posts relating to database development that I could probably stitch together into a coherent piece And so the following blog posts were fused together to the form the meat of the session:-

xUnit Style Database Unit Testing
Simple Database Build & Deployment With SQLCMD

…with the following posts providing some additional minor content:-

Debug & Release Database Schemas
Implementing Constants & Enumerations in a Database

Naturally I had extravagant plans when planning it 3 months earlier. I was going to develop my own test runner and follow Paul Grenyer’s fine example by running actual tests with a live demo. I was also going to have a few vibrant slides and follow Martin Fowler’s lead by eschewing bullet point lists and unnecessary images. And then Xmas came and went and like any real student the deadline was looming and still I only had a rough outline.

My wife, who fortunately knows a thing or two about presenting[#], and more importantly knows a thing or two about her lazy husband, provided some sage advice to get me bootstrapped. It didn’t take long to realise I was never going to have time for the live demos (either beforehand or during the talk for that matter) and I also dropped the ‘clever’ slides in favour of creating what were essentially a giant bunch of queue cards. Once I had gone through the entire talk for real I decided I probably shouldn’t try and run before I can walk…

This may seem really obvious to everyone else but there is a world of difference between talking in your head and saying it out loud. I spent a number of train journeys on my commute planning the talk and then going through it in my head, i.e. actually talking in my head to practice what I was going to say – I didn’t think my fellow passengers would be that interested in the content and they might also find it a little repetitive :-). So when I came to practice it out loud I got to hear all the pauses and umm’s and errr’s and it was truly disheartening. I practiced it again over the next few evenings and although the talk become more fluent, I was still annoyed with myself for forgetting to say certain things. My wife stepped in once more with another ‘obvious’ piece of advice “The audience doesn’t have a script, so they won’t know what you did and didn’t mean to say”. She also said that if a point was truly important I wouldn’t forget it. Boy it annoys me how that women is always right! I still ditched one slide right at the last minute as I realised the content was effectively irrelevant, it also was hard to convey fluently and would help bring me back to nearer the 60 minute mark.

On the night it seemed to go pretty much according to plan, except for the technical problems getting my laptop and the projector to acknowledge each other’s presence. I only remember losing my train of thought once and I believe I got all the key points in just like my wife said I would :-). I felt the urge on a number of occasions to ad-lib as I thought of something new or different to say but I held fast and stuck to the content I’d been practicing. If I ever get to do the talk again I guess I can always ‘refactor’[^] it. It was also nice to be able to point out two members of the audience (Phil Nash & Steve Love) whilst acknowledging their ‘indirect’ contributions.

After I had finished rabbiting there were a number of questions ranging from the classic “how do you get your developers to write tests?” to one from fellow member Ed Sykes about whether it’s possible to get code coverage for SQL based tests? No, I don’t know. In the pub afterwards we got to chew the fat further and a few more ideas popped into my head. The question now is whether the alcohol killed off too many brain cells or whether I can still recall what they were for future blogging purposes…

All-in-all a wonderful experience and hopefully an excellent bit of practice in preparation for the main event - the ACCU conference in April.

FWIW Here is a link to the slides (PowerPoint).

[+] The photo was taken by fellow ACCU member Schalk Cronje.

[*] There are members who sit on the C++ panel! They know more than than just their onions…

[#] She’s a producer of live events and one of her many talents is to train and rehearse speakers.

[^] I guess that means “the message” stays the same but I say it in a different way.

Intriguing SCHTASKS Bug

To get our system up and running quickly we chose to use the built-in Windows task scheduler to invoke our nightly batch. During initial development we added a couple of tasks by hand using the Scheduled Tasks UI, but as we got closer to our release date we created a batch file to script creation of the scheduled tasks so that we had a formal (and version controlled) deployment process, e.g.

C:\> SCHTASKS /CREATE /SC WEEKLY /D … /TN “RunBatch”

The script was written and tested on a Windows XP desktop machine, but when we came to test it on our Windows server the script failed with the following error:-

ERROR: The creation of the scheduled task failed.
Reason: The Task Name may not contain the characters: <, >, :, /, \,|.

This was most curious as the server was running Windows Server 2003 (which is the same family as XP) and the task names were all pretty simple using just a number, dash and short name, e.g. “01-EstablishValueDate” – there were no out-of-the-ordinary characters at all. Naturally I took the dash[*] out first – no luck. Then the numbers – nope. Finally I resorted to a binary search to try and pin down exactly which character(s) it was…

It turned out to be the letter ‘D’ at the start of the word “Date”! The first three tasks I tried all had the word “Date” in them somewhere and they all worked after just removing the letter ‘D’. To be more specific it was a capital letter D as a lower case ‘d’ worked! This became our temporarily workaround whilst I did a little Googling in the background as I just couldn’t entertain the idea that this was the real problem; I’ll always assume I’m doing something wrong until Occam's Razor tells me it’s not me :-). Fortunately I quickly discovered the following forum post “Error In Schtasks Command Line” where there author was having no joy in creating a scheduled task called “Defrag”. Sadly it doesn’t say they resorted to just calling their task “defrag” as a workaround. Anyway a short time later I managed to discover an MSDN article that appeared to confirm the behaviour. In the KB article it says the following:-

The Schtasks command verifies the task name by comparing the task name with an invalid character set. If the task name contains one or more characters from the invalid character set, the Schtasks command denies the task creation request.

However, in race conditions, the invalid character set may contain a valid character. Therefore, the Schtasks command cannot create some tasks even if the task names contain only valid characters.

My mind boggles at what the SCHTASKS code is doing to create such a race condition? And why just the letter D?

[*] A common source of error is pasting command lines out of Word documents or web pages into console windows; this often results in curious error messages from the application. The problem is the “clever” document editor converting a dash (en dash, minus sign, etc) into a proper hyphen (em dash) or equivalent Unicode character at a different code point from the ASCII one. It may look like a dash to you and me but the “computer says no”.

Exception Safe By Design

I’ve recently gone back to my simple database query tool to add a few essential usability features and like any legacy codebase there were a couple of surprises in store. Now, I’m not making excuses for myself, but most of the code is the best part of 10 years old and like any journeyman I’ve learnt a lot since that was written. Also the recent episodes of Matthew Wilson’s Quality Matters column in Overload (one of the ACCU journals) focused on exception handing and so these two examples became even more prominent.

Resource Management

One of the new features I added was to properly support non-query type statements, i.e. INSERT/DELETE (it was originally written to be a query tool but it’s been useful to run ad-hoc SQL as well). This involved me digging into the ODBC facade and eventually I came across this piece of code:-

CSQLCursor* CODBCSource::ExecQuery(...)
{ 
  CODBCCursor* pCursor = new CODBCCursor(*this);

  try 
  { 
    ExecQuery(pszQuery, *pCursor); 
  } 
  catch (const CODBCException&) 
  { 
    delete pCursor; 
    throw; 
  } 

  return pCursor;
}

Now clearly you’ll have to put aside your disgust of Hungarian Notation for a moment and focus on the real issue which is the shear verbosity of the code. It’s also wrong. On the one hand I’ve been a good boy and only caught what I can knowingly recover from (a specific CODBCException type exception) but I’ve also missed the point that any other exception type will cause a memory leak. Clearly I should have used catch(…) because the exception type is irrelevant in this case.

Only I shouldn’t, because the try/catch block is entirely unnecessary. Although now I’m firmly in the camp that believes RAII is probably the single most important idiom within C++, I obviously wasn’t then. I try really hard to write code that ensures there are no memory leaks and I use tools like the Debug CRT Heap in Visual C++ and BoundsChecker to point out my mistakes. And yet this piece of code highlights how hard it is to do that, especially in the face of exceptions; if you don’t have 100% code coverage in your testing it’s just too easy to miss something. What is all the more ridiculous though is that the correct solution is less work anyway:-

CSQLCursor* CODBCSource::ExecQuery(...)
{ 
  Core::UniquePtr<CODBCCursor> pCursor(new CODBCCur…);

  ExecQuery(pszQuery, *pCursor);

  return pCursor.detach();
}

Assuming I can make no changes to the method signature this ticks the box of being exception safe by design. The Core::UniquePtr smart pointer class is analogous to boost::scoped_ptr which ensures single ownership until it’s given away at the end. But there’s still a bad code smell which is that the method returns a bald pointer so the callee could still leak it. What we should return instead is the smart pointer object so that we explicitly transfer ownership to the callee:-

Core::SharedPtr<CSQLCursor> CODBCSource::ExecQuery(..)
{ 
  Core::SharedPtr<CODBCCursor> pCursor(new CODBCCur…);

  ExecQuery(pszQuery, *pCursor);

  return pCursor;
}

One unpleasant taste in the mouth is that because of the change in scope we cannot just return a Core::UniquePtr because that would require temporarily copying the object[*] so instead we use a smart pointer that allows shared ownership – Core::SharedPtr. Funnily enough this transfer of ownership from caller to callee is actually right up std::auto_ptr’s street and for the knowledgeable this would be preferable from a performance perspective. But sadly you end up worrying about the caller and whether they know the pitfalls of auto_ptr and so perhaps you go back to the safer option.

Side Effects

Another feature I wanted to added was to avoid requesting credentials when the connection doesn’t need them and this put me right into the client method responsible for opening a new database connection:-

void CAppCmds::Connect(. . .)
{ 
  try 
  { 
    OnDBDisconnect(); 
    . . . 
    App.m_oConnection.Open(strConnection); 
    . . . 
  } 
  catch(CSQLException& e) [#] 
  { 
    App.AlertMsg(TXT("%s"), e.m_strError); 
    OnDBDisconnect(); 
  }
}

This piece of code I found puzzling because I couldn’t obviously see why it called OnDBDisconnect() both at the start of the method and also in the exception handler as the connection will either open successfully or throw, in which case we had already cleaned up earlier. So I looked at what OnDBDisconnect() did to see why it might be necessary:-

void CAppCmds::OnDBDisconnect()
{ 
  if (App.m_oConnection.IsOpen()) 
    App.m_oConnection.Close(); 

  App.m_pCurrConn = NULL; 
  . . .
}

Nothing earth shattering there – just run-of-the-mill cleanup code. And so I went back to the Connect() method and looked more closely. And then I spotted it:-

  OnDBDisconnect(); 
  . . . 
  App.m_pCurrConn = App.m_apConConfigs[nConnection]; 
  . . . 
  App.m_oConnection.Open(strConnection);

There are side-effects that occur between the point when the previous connection is closed and when we attempt to open a new one. A quick scoot back through the version control repository[+] to see what changes have been made to this method and I see that it was checked in as a fix for another problem after failing to open a connection. Clearly my mindset was “fix the broken exception handling” instead of “fix the poorly written method”. The first rule of writing exception safe code is to perform all side-effects off to one side and then use exception safe mechanisms like std::swap()[^] to “commit” the changes.

So, for my example code above we need to introduce some temporaries to hold the unstable state, perform the Open and then persist the new state:-

  OnDBDisconnect(); 
  . . . 
  Core::SharedPtr<…> pConfig(App.m_apConConfigs[…]); 
  Core::SharedPtr<…> pConnection(new CODBCConnection); 
  . . . 
  pConnection.Open(strConnection); 
  . . . 
  App.m_oConnection.swap(pConnection); 
  App.m_pCurrConn.swap(pConfig);

Taking a cue from the preceding section about resource management I have used my Core::SharedPtr[$] smart pointer type as the connection will live on after this method returns. The catch handler remains but only to report the error message to the user, no cleanup now needs to be performed.

But we can do even better. If we take a closer look at what happens as a result of using swap() we see that the call to OnDBDisconnect() may actually be superfluous. In fact it may be better not to do it all. The call to swap() effectively means that the new connection will live on as App.m_oConnection and the old one will get destroyed at the end of the scope (the try/catch block). Now if we look at what OnDBDisconnect() does:-

void CAppCmds::OnDBDisconnect()
{ 
  App.m_oConnection.Close(); 
  App.m_pCurrConn = NULL;

  UpdateScriptsMenu(); 
  UpdateUI();
}

It closes the existing connection and resets m_pCurrConn (these both now happen automatically near the end of Connect()) and it resets the UI. If we look at the tail end of the Connect() method we see this:-

  UpdateScriptsMenu(); 
  UpdateUI();

What this all means is that if an exception is thrown then the user loses their current connection as well. If we perform all the work off to the side and don’t modify any state (i.e. don’t call OnDBDisconnect()) up front then the user at least gets to keep their current connection as a consolation prize. If the new connection does work then we just save the new state and let the old one disappear at the end of the try/catch block. Is that just being clever or an attempt to write the simplest code possible? A topic for another day…

[*] I need to go back and read up on the new std::unique_ptr type in C++0X as I’m sure this was designed for this kind of scenario – and also the std::vector<std::auto_ptr<T>> gotcha.

[#] Using an exception to signal failure when opening a database connection falls right into the “Using Exceptions For Flow Control” category in Matthew Wilsons Quality Matters column and would almost certainly fall into the real of “Not Exceptional” for Kernighan & Pike too. In this case I’m inclined to agree and will be adding a TryOpen() method at the next opportunity.

[+] Lone developers need Source Configuration Management (SCM/VCS) too. At least, if you want to perform any software archaeology to avoid regressions, that is.

[^] It would be great if std::swap() had a ‘nothrow’ guarantee as that would make it excellent for this idiom. But I’m not sure it is guaranteed by the standard - is it just a convention like not throwing exceptions from destructors? My Google skills found lots of chatter but no quote citing chapter & verse.

[$] Naturally you wouldn’t use the full type name everywhere but use a typedef instead. Given how prevalent the use of the SharedPtr type is (especially with types stored in containers) I tend to add a typedef right after a  new entity class declaration for the smart pointer wrapper, e.g. class Xxx { . . . }; typedef Core::SharedPtr<Xxx> XxxPtr;

Reacquainting Myself With Sed & Awk

I’ve always felt somewhat uncomfortable using Visual Studio as my main text editor because there seems to be legions of programmers out there that swear by vi and emacs’ text processing prowess. For me (a C++/C# Windows server-side programmer) I find that day-to-day all I need is syntax highlighting, cursor key movement and basic search & replace; plus the basic code navigation aid “Go to definition”. But that’s pretty much it. So I posed a question on accu-general to find out if I’m some kind of programming Neanderthal or whether all these supposed fancy text processing aids are actually used day-to-day. My straw poll suggests not and so I don’t feel quite so pathetic now.

Still, I had one of those moments the other day when I needed to perform a really simple text processing task – I needed to add 6 tab characters to the end of every line in a large (10’s of MB) text file. This kind of thing (modifying[*] a file that large) is somewhat unusual for me. Instinctively I opened the file in my (already running) Visual Studio and tried to work out how I was going to do it. The two options I would normally use are Find & Replace and a macro (which I then run repeatedly by holding the ‘run macro’ key down :-). Given the size of the file I didn’t have all day for the macro to run so it had to be a replace using a regex. The one I came up with replaced the \n character with \t\t\t\t\t\t\n and then I waited for VS to do its thing…

Whilst waiting I thought to myself how much easier that probably would have been with Sed and/or Awk. I could even hear the sniggers of those Unix programmers around me. But I haven’t used either tool in 20 years and I don’t have a copy installed by default because I’m running Windows of course. I remember trying ports of the major Unix tools way back when I started programming professionally and it was painful because of the fundamental difference between who is responsible for handling wildcard expansion (the shell in Unix vs the program in DOS). Plus the memory constraints of DOS meant they weren’t altogether reliable[#]. Time has obviously marched on but I’m still jaded by the experience of the very distant past and I’ve also discovered various alternatives that have made up the shortfall in many respects.

And then I ran into another classic issue – trying to work out which SQL scripts had been saved in ASCII and which in Unicode (SQL Server Management Studio annoyingly uses Unicode by default). This was causing my colleague’s (Cygwin[^] based) grep not to match various files. Once again I remembered the old Unix ‘file’ tool that attempts to detect what format a file is and whist searching for a Windows port of that I came across the UnxUtils project on SourceForge. This looked to be a native port of the most commonly found Unix tools such as awk, sed, grep, egrep etc. It didn’t contain a version of ‘file’ but I got one of those from somewhere else. Anyway, I unzipped the UnxUtils package, added it to my PATH, and then waited for a moment to use them.

That moment came sooner than I expected. I needed to analyse our on disk trade repository and provide some idea of its size and daily growth. My instinct this time was somehow to use Excel as that is pretty much what the financial world uses for all its analysis and reporting. But we were running an old version that still has the 65K row limit and our repository was well over 100K files per day. So I Googled for an Awk based solution which (adapted for Windows) initially became:-

c:\> dir /s *.trade | gawk “{ sum += $3 } END { print sum }”

This was pretty close, but I needed to strip the chod out of the dir listing for the daily comparison. Also, recursively searching the repository whilst I was getting this right was going to be way too slow so I captured the output from dir once and applied a sprinkling of Sed to remove the rubbish:-

c:\> dir /s *.trade > filelist.txt
c:\> sed –n /^[0-9][0-9]/p < filelist.txt | gawk “{ sum += $3 } END { print sum }”

And that was it for the current repository size. The day-on-day growth just involved extra calls to Sort and Diff to trim out the common items - it was a textbook example of pipeline processing!

Of course I got stuck for ages not knowing that Sed has a limited regex capability or that the ‘END’ in the gawk script had to be in uppercase. But it inspired me to put the effort in to learn the tools properly and so I popped onto Amazon and found a second hand copy of Sed & Awk (2nd Edition) by Dougherty & Robbins for under £10 and also picked up the O’Reilly Sed & Awk Pocket Reference for a couple of quid to leave in my drawer at work.

In the last couple of months I’ve read the majority of the book so that I feel more equipped to notice when this pair of tools would be a better choice. One of the most useful features I discovered about Sed (that I don’t believe VS can do easily) is to apply a transformation to repeating sections of a file (which are specified by an ‘address range’, i.e. a pair of regex’s that denote the start and end of the region). I’ve also found it easier to use Awk to generate, say, SQL statements where I have the output from a query that then needs to be turned back into a bunch of stored procedure calls with the previous result set as the parameters. This is doable in VS with regex based search & replacing but it’s probably not the most efficient use of my time.

The only real problem I have is that the Sed regex syntax is a subset of the normal POSIX one, so some common features are not available. It’s also another regex syntax to learn (along with Visual Studio’s own warped variation).

By this point I suspect that I’m preaching to the converted and there is much nodding of heads. There has also probably been the quiet whispering of “the right tool for the job” and all that. So yes, I’m incredibly late to the party, but I’m most pleased with my re-acquaintance. I don’t expect to ever be writing multi-line scripts but even if it makes the one-liners easier it will have been ten quid well spent.

 

[*] I search and analyze much larger log files every day with BareTail, BareGrep and LogParser. It’s just that I don’t need to modify them or generate other files from them.

[#] When it came to searching my entire (400 MB) hard disk it was actually quicker to close Windows 3.1, reboot into Linux (0.91pl15 IIRC) and use find + grep, save the results and then reboot back into 16-bit Windows than it was to do the search directly in Windows (or via a DOS prompt) in the first place.

[^] I want my tools to behave naturally on Windows - I have no desire to simulate a Unix environment.

My ACCU 2011 Session Proposal - Using xUnit As a Swiss Army Testing Toolkit

I’ve decided to be brave and submit a session proposal for next year’s ACCU Conference. I know if I get accepted that means I’ll have to curtail the relentless late nights in the bar – at least until I’ve given my presentation. Anyway, the catalyst for my proposal was my blog back in March titled Integration Testing with NUnit. Not long after posting that I watched Paul Grenyer give a presentation to the London branch of the ACCU that touched on similar ground. My session is intended to take the idea further still and cover use of the xUnit style in other areas where perhaps custom test scripts and tools might have been written such as API exploration and verifying external systems.

Session Summary

Title: Using xUnit As a Swiss Army Testing Toolkit
Type: Tutorial/Case Study
Duration: 90 minutes
Speaker: Chris Oldwood

Speaker Biography

Chris started out as a bedroom coder in the 80s writing assembler on 8-bit micros; these days it’s C++ and C# on Windows in big plush corporate offices. His career has covered both shrink wrapped applications and in-house systems with the past 5 years focusing on grid-based distributed systems in the Finance industry. When not attached to a keyboard and screen he has a wife and four children to entertain, dips his toe in the local swimming pool and provides the commentary for the annual Godmanchester Gala Day Duck Race.

Session Description

Modern Unit Testing practices act as a conduit for improved software designs that are more amenable to change and can be easily backed by automation for fast feedback on quality assurance. The necessity of reducing external dependencies forces us to design our modules with minimum coupling which can then be leveraged both at the module, component and subsystem levels in our testing. As we start to integrate our units into larger blocks and interface our resulting components with external systems we find ourselves switching nomenclature as we progress from Unit to Integration testing. But is a change in mindset and tooling really required?

The xUnit testing framework is commonly perceived as an aid to Unit Testing but the constraints that it imposes on the architecture mean that it is an excellent mechanism for invoking arbitrary code in a restricted context. Tests can be partitioned by categorisation at the test and fixture level and through physical packaging leading to a flexible test code structure. Throw in its huge popularity and you have a simplified learning curve for expressing more that just unit tests.

Using scenarios from his current system Chris aims to show how you can use a similar format and tooling for unit, component and integration level tests; albeit with a few liberties taken to work around the inherent differences with each methodology.

Implementing Constants & Enumerations in a Database

How often have you come across this kind of code in a stored procedure:-

select abc
from   xyz
where  Status = 3

…or if you’re lucky (or perhaps unlucky if the comment is out-of-date):-

select abc
from   xyz
where  Status = 3 --Failed

One of the downsides of SQL (at least with T-SQL) is a lack of support for constants, and by extension enumerations.

No Magic Numbers

It’s long been a best practice that you avoid the use of so called Magic Numbers like the ‘3’ in the examples above in favour if some more symbolic name. This aids both readability and maintainability because you don’t have to go and lookup what the number represents (in the case of an enumeration) and if the enumeration value needs to change you only have to change the relationship with the symbol and not the code where the symbol is referenced.

Local Constants

If you only need a constant for use within a single stored procedure then you can just use a normal variable:-

declare @Failed int
set     @Failed  = 3
. . .
select abc
from   xyz
where  Status = @Failed

This will make the latter SQL more readable and hopefully more maintainable - it should be far easier to grep for “Failed” than “3” if the numeric value does need to change[*].

Global Constants

Sadly the need for one-off constants is pretty rare. What is more likely is the need for a globally defined constant so that we don’t violate the DRY/SPOT principle by defining local constants in every procedure where it’s used. The best solution I could come up with[#] was to use a User Defined Function:-

create function Failed() 
  returns int
as 
  return 3
go

This means that you can use it like so:-

select abc
from   xyz
where  Status = Failed()

However it’s not always possible to use a UDF in the same place that you can a constant or variable (I’ve yet to read up on why) and so sometimes in a stored procedure you still need to use a local variable as well:-

declare @Failed int
set     @Failed = Failed()
. . .
exec SomeOtherProc @Status = @Failed

Lack of Namespaces

Although some databases have support for schemas, they are often quite simplistic and not designed for nesting like you would in C++ and C#. This means that to avoid conflicts we need to resort to the old style of using some form of prefix or suffix on the name:-

select abc
from xyz
where Status = TaskStatus_Failed()

This becomes a little less distracting with enumerations as in C# you are forced to prefix the enumeration value with it’s type name anyway (unlike C++, although may people nest C++ enumerations within a separate struct to get the same effect) so it’s not too unnatural to read. Of course you could still create a separate namespace, called say ‘constants’, in which you put all these UDFs if you felt that it helped matters.

An Alternative - Lookup Tables

The main thing I like about the UDF solution is that the function name means the symbol appears as code rather than data. One alternative I’ve seen is to use a small lookup table that has the integer value and a string for the symbol. This makes the numbers less ‘magic’ but it means you have invoke a query to get the enumeration value:-

declare @Failed int
select  @Failed = EnumValue from TaskStatusEnum
                  where EnumSymbol = ‘Failed’

I’ll be honest and admit that I’ve done no performance comparisons between this and the UDF idea. It also still feels to me like you’re expressing the enumeration with data rather than code though but that’s probably nonsense.

Client-Side Use

The other benefit of using a UDF is that you can also use it client side should you happen to need to embed queries within your code so your magic numbers don’t leak further afield. We use proper enumerations in our C# code that are kept in sync manually but tested automatically as part of a build. Ideally we should generate the UDF’s and C# enum’s from a single source, but for the moment the automated tests acts as a nice safety net.

 

[*] Of course in the real world you would never rely on the consistent use constants like this and would actually search for all references to the table and/or column name instead, but hey this is theory :-)

[#] Remembering that I’m not a SQL expert by any stretch of the imagination.

What’s Exceptional Depends on Context

The latest edition of Overload (one of the journals from the ACCU) has another episode of Matthew Wilson’s excellent Quality Matters column. The latest topic is exception handling in which he makes a reference to The Practice of Programming by Kernighan & Pike – a book which I’ve only ever skimmed so far. Hence I thought I should to read the relevant chapter and somewhat surprisingly I found the following statement:-

“Exceptions are often overused. Because they distort the flow of control, they can lead to convoluted constructions that are prone to bugs. It is hardly exceptional to fail to open a file; generating an exception in this case strikes us as over-engineering. Exceptions are best reserved for truly unexpected events, such as file systems filling up or floating-point errors.”

My gut reaction to that was “No it’s not – it depends on context”…

External Resources Always Fail

Messer’s Kernighan & Pike are obviously far more knowledgeable and experienced than me so why did they make such a bold statement? Generalising somewhat, I presume that they were suggesting that any interaction with an external resource should be handled with due care as you should expect it to fail – after all we always encounter problems with files, sockets, databases etc. And as a general rule this makes perfect sense.

Full Environmental Control

But… In the world of in-house server-side software you are often completely in control of the entire environment in which your code runs. Even for in-house desktop software the environment may be a little more hostile but you can still ensure certain invariants. Taking the “opening a file” example again and given the following conditions, would failing to open a file still be considered unexceptional?

1. The file exists (say we just created it and control its lifetime)
2. The permissions on the folder are correct (our server installation script set them)
3. The server has plenty of internal resources (memory, handles etc)
4. The process has not suffered any unrecoverable errors

In this situation I would be on the side of it being exceptional. In my experience failures under these kinds of scenarios usually point to a system configuration error [1 or 2], which means it would never have worked, or the server/process itself has become unstable [3 or 4] due to some earlier (and hence unexpected) issue.

Recoverability

Of course the issue is perhaps moot as all errors fall into two categories – recoverable and unrecoverable. In the scenario described above the recovery is effectively manual[*] – fix the system configuration or restart the process/server. In the real world I can’t see how you could justify the cost of designing a system to automatically cope with some/all of those potential edge cases[+]. Even the last one (process instability) can be hard to handle fully via process recycling because too much badly written code masks nasty errors like Out Of Memory or Access Violations by sinking everything so you don’t get to react to it.

In contrast the times where I can see you might be able to recover from failing to open a file are when the choice of file is controlled by the user, such as in a desktop application, or when the contents of the file can be considered optional, such as user settings.

Applying the issue wider afield to databases and remote services we end up at clustering/replication as the obvious recovery option. Even so, given the general reliability of in-house networks and (non-blade) hardware these days, I would still consider it pretty exceptional for a failure to occur. By-and-large misconfiguration is the most common reason I see for failures involving resources.

Do Or Do Not (Or There May Be a Try)

The chapter of The Practice of Programming in question was about designing interfaces so do we assume that this message is aimed more at the writers of libraries rather than applications? Given that you don’t know the context in which your code is being called do you have to choose between error codes or exceptions? Would your interfaces be better or worse if you somehow supported both so that the caller could decide? I’m sure there are many developers out there right now wincing as they stare at their legacy COM heavy codebases that have a mixture of wrappers generated by Visual C++’s #import feature where some methods throw and others return HRESULTs, some have a Chk prefix and others don’t. What I’m suggesting has more moderation to it…

In the .Net world there is a common pattern where methods by default succeed or throw an exception and a parallel set of methods named TryXxx that return a bool and use an output parameter instead. The latter ‘overloads’ allow you to avoid the cost[#] of handling an exception such as when parsing numbers and DateTimes that you can expect to be iffy (format wise) but they will still throw other exceptions which are orthogonal to the task such as Out of Memory.

Was It Just a Poke At Java?

Or maybe I’m just being too literal? The book was written over 10 years ago (back when exceptions where still not exactly mainstream) and it follows an example in Java so perhaps they were just having a pop at the choice Java had made. After all, at that time Java was still popular as a client-side technology – even outside the browser.

The fact that people like Matthew Wilson are still regurgitating this argument a decade later means there is still much to learn. I think the TryXxx pattern is a nice compromise as you get the flexibility of handling a domain failure without resorting to an unnecessary try/catch block and you also don’t accidentally mask orthogonal failures by ignoring the return code or catching an exception of too generic a type. I wonder if this also satisfies Kernighan & Pike or do they find it muddies the waters even further and still stand by their original statement?

 

[*] I say manual because what normally seems to happen is that the system monitoring software gets bombarded with errors and someone has to make sense of them. Once that’s done it usually involves bouncing something…

[+] In the embedded world, where lives may be at risk, there are forces way outside the normal risk/reward trade offs of an in-house corporate system.

[#] I’m referring to both the performance and maintenance costs of using exceptions.