Tuesday, May 13, 2008
#
DevTeach: Home Grown Production System Monitoring and Reports
That was an interesting talk, Owen Rogers talked about how to setup an Operation Database in order to get more visibility on your production system. This is the first time that I sat in a talk that is a "Release It!" influenced talk, and it was very interesting.
The type of Operation Database is focused on adding more information for the developers, rather than exposing more information to the operation team. What was especially interesting is that the amount of data being capture is very small. The standard log data (time, message, exception, etc) and a command log, which I think about as a message handling log. This log capture some statistics about messages / commands that the application has handled. Message size (in & out), time, processing time, etc.
From that, you can get pretty interesting data about your application (just showing avg. message processing time over a period of time is extremely valuable).
The nice part about this is that the entry cost is basically zero.
Zero friction IoC: Auto registration is mandatory
This is the entire Binsor config file for a real application:
import Castle.MonoRail.Framework
import Castle.MonoRail.WindsorExtension
import Rhino.Commons.Facilities from Rhino.Commons.ActiveRecord
facility MonoRailFacility
facility RhinoTransactionFacility
facility ActiveRecordUnitOfWorkFacility:
assembly = "HibernatingRhinos"
for type in AllTypesBased of IController("HibernatingRhinos"):
component type.Name, type
for type in AllTypes("HibernatingRhinos").WhereNamespaceEq("HibernatingRhinos.Services"):
component type.GetServiceInterface(), type
And I am pretty confident that I am not going to have to do much in the future with those.
And yes, you can do it with the fluent registration API as well.
Binsor & Auto Registration - Making it even simpler
Here is the syntax that I am getting at...
for type in AllTypesBased of IView("Rhino.Commons.Test"):
component type
for type in AllTypesWithAttribute of ControllerAttribute("Rhino.Commons.Test"):
component type
for type in AllTypes("Rhino.Commons.Test") \
.WhereNamespaceEq("Rhino.Commons.Test.Binsor"):
component type
for type in AllTypes("Rhino.Commons.NHibernate") \
.Where({ t as System.Type | t.Name.Contains("NHRepository") }):
component "nh.repos", type.GetSeriveInterface(), type
And this seems to cover just about any scenario that I can think of. Combine that with Binsor's extend facility, and we are more or less done.
Setting up MonoRail Windsor Integration with Binsor
I think I'll let it stand on its own:
facility Castle.MonoRail.WindsorExtension.MonoRailFacility
for type in AllTypesBased of Controller("HibernatingRhinos"):
component type.Name, type
Done.
I didn't believe it, to be fair.
Saturday, May 10, 2008
#
Open seats for the DSL course
There are still open seats for the DSL course in Austin, next week (19th - 20th May).
You can register here: http://ayende.eventbrite.com/
WPF & Prism: Before & After
Glenn posted the before/after pictures of the Prism Reference Implementation application.
I was very impressed when I saw how they got from the traditional developer styled UI:
To this UI:
Something else that Glenn said couldn't be stressed enough, this metamorphosis has happened in two weeks by a single developer.
I have worked on skinnable (win forms) applications, and that kind of thing just doesn't take just two man weeks, if it is possible at all.
Wow!
This make the WPF story much more compelling.
Talks Abstracts
This is an update of an old post of mine, listing all the talks that I am thinking of / had prepared. I am using this mostly as a way to centralize all my talks. Comments are welcome.
Level 200: Producing Production Quality Software
Working software is no longer the only thing that we need to produce. We need to create a software system that has a chance of surviving in the cruel world of production system, outside the clean room and sterile environment of development and QA. Understanding bottlenecks in the system, preventing cascading failures and recovery strategies have ceased being the problems of the very high end players. With the cost of system downtime being measures in $$$/second, this is an area we have to consider all the way.
In this talk we will cover how we can map common weaknesses in the system design, preemptively protect ourselves from them, and produce software systems that can withstand the real world hostile environment.
Level 4/300: Object Relational Mapping += 2: More then just data <-> object
Object relational mapping are becoming only more popular, as people developing complex systems find that they need more than the tabular model to work with in their applications. A sophisticated ORM can do a lot more than merely get the data out of the database in object form, it can be a valuable assest in simplifying development and making things possible. In this session, you will see how you can utilize an ORM in untraditional ways to get an additional, better, approach to solving complex issues.
Some of those ways include business rules, localization, state transitions, inversion of control, etc. All done via the ORM layer, and all can be used to drasticly simplify the complexity of the given scenarios.
Level 100: Using Active Record to write less code
What would you say if I told you that you can stop writing data access code in .Net? Aren't you tired of writing the same thing over and over again, opening connection, querying the database, figuring out what to return, getting back untype data that you need to start putting on the form? Do you really see some value in writing yet another UPDATE statement?
The Active Record framework allows you to fully utilize the power of the database, but without the back breaking work that it used to take. Active Record uses .Net objects to relieve you from the repeating task of persistance. Those objects are schema aware and can persist and load themselves without you needing to write a single line of SQL. Building business application using Active Record is a pleasure, the database stuff just happens, and you are free to implement the business functionality.
Presentation for this can be found here: Using Castle Active Record
Level 200: Rapid (maintainable) web development with MonoRail
If you're a fan of Ruby on Rails and want to see similar capabilities in .NET, or you're an ASP.NET developer looking for an easier way to do things, MonoRail will be irresistible once you find out what it can do for you. Strong support for Ajax makes writing buzzward compliant web applications a breeze. Utilization of the Model-View-Controller architecture and convention over configuration makes web development with MonoRail a pleasure. Free yourself from page-life cycle issues and viewstate worries, start working with MonoRail, where the framework works for you.
This talk will introduce the general concepts of the framework, and how you can use them
Level 100: Interaction based testing With Rhino Mocks
Beyond the simplest scenarios, all objects had collaborators that they work with. This flies in the face of testing objects in isolation. This is the problem that mock objects were created to solve. In this talk you will learn what mock objects are, how to utilize them and best practices on when / how to utilize them. Rhino Mocks is a mock objects framework for .Net whose core goals are to let the developer rely on the compiler work well with refactoring tools.
Level 200: Inversion of Control and Dependency Injection: Breaking out from the dependecy hell
Responding to change is the holy grail of software development. Inversion of Control (IoC) and Dependency Injection (DI) are two related patterns that allows to make significant changes to an application without having to touch every part of the application. IoC and DI encourage breaking the application into discerete, highly cohesive parts, so a change, when it eventually comes, is very local. A nice benefit is that applications that uses IoC are also very testable applications.
This talk will introduce the concepts of IoC and how to use them in your application.
Presentation can be found here: Inversion Of Control and Dependency Injection
Level 4/300: Advance usages of Inversion of Control containers
You already understand the concepts of Inversion of Control and Dependency Injection, now is the time to see how far we can make the IoC container works for us. This talk will focus on using an IoC container in complex scenarios. We will talk about generic decorator chains and generic specialization, contextful containers and IoC DSLs. These powerful concepts can greatly enhance your ability to respond to change in your application.
Level 300: Writing Domain Specific Languages in Boo
Domain Specific Langauge is not just the DSL SDK from Microsoft. A DSL can make working with the domain much easier, since you are capable of leveraging the domain concepts directly. The other alternative to a DSL is an XML file, and we all know how well declarative model can work when you need imperative concepts, just consider NAnt for a minute and you will see the issue. Usually, writing a DSL in .Net would be a complex issue, requiring writing a parser, interpreter, etc. Boo already handles all of that, and its open architecture means that it is very easy to extend it to express the concepts of the domain. This talk will show you how to build DSLs in Boo and how to utilize this power in your applications.
Presentation can be found here: Writing Domain Specific Language in Boo
SvnBridge - Success
My personal success criteria for SvnBridge is when I no longer care what I am working against. And I think we got to that point. There are still issues, to be fair, but they won't hit you anywhere near mainline development.
I am listening to Rob Conery MVC Storefront 9 in which he makes the mistake of referring to CodePlex as Subversion. With that, I think that I can say, we are there.
Friday, May 09, 2008
#
Writing unreliable software
This had surprised me, to say the least. I run into a bug during stress testing for SvnBridge, after a while, it would simply get stuck.
I am not the best in figuring out exactly what got an application stuck, but I generally manage to put some effort before I call the big guns. This time, I had managed to get a working theory, and prove that that the symptoms that I am seeing are consistent with my theory. I decided to dig into it a bit, and came up with interesting results.
The jury is still out with regards to whatever that was the real reason for the issue that SvnBridge has, but I went to some trouble to try research my theory. Along the way, I came up with some interesting conclusions. Just a reminder, I recently read Release It! and I consider this a very influential book. One of the things that kept coming up in the book how a chain reaction can take down an application. A single server stop responding, and the other just crumble. The examples that were brought up were of two kinds: a flawed implementation of a connection pool and a mismatch in the various capacities of the systems.
In SvBridge, there is a location where I am doing something similar to this code:
revision = webService.GetLatestVersion()
items = webService.GetItems(revision, downloadPath)
items.ForEach { item | item.BeginDownload(webService) }
comment = webService.GetLog(revision)items.WaitForAllToFinishDownloading()
SendToClient( Revision: revision, Comment: comment)
for item in items:
item.WaitUntilLoaded()
SendToClient( item.Data )
This isn't the exact code, but is is a good, and simple, representation of what is going on. This code can lead, under a set of special circumstances, to a hung server.
The important thing to understand here is that this code is using BeginDownload to perform an async invocation over HTTP. Another important data point is that we tend to hit the same physical server for a lot of our work.
Take a look at what is actually happening...
It is very important to observe that GetLog is actually never executed. The .NET framework is allowing (by default) only 2 connections to a server. GetLog will only be executed when there is a free slot, and the async requests are ahead of it in the queue. (Actually, I haven't verified the exact sequence in which this would be executed).
Until the async requests complete, we are stuck. This is important because it is not limited to the current request. This means that one request can block another, and since a single checkout request can cause a few thousands sub requests to the backend server, a single user can take down the system for a significant amount of time. (What I am actually seeing is a bit different, it looks like the async request never returns from the server under high load, but never mind that).
There are solutions for all of that, connection groups and overriding the number of allowed connections per server are just few of them.
I remember reading the Release It book and being thankful that so much of that seems to be focused on Java (thankfully, I have never had to write my own connection pool, which seems to be a pastime in the Java land).
What caused this post was actually my defensive approach failing with spectacular results. I started by defensively specifying Timeout on my requests. It would kill the request, but it would also keep the server alive. Unfortunately, Timeout has no affect on async requests. I found this very surprising, even though it is documented, I consider this a bug. Even worse, the actual example in BeginGetRequest is for fixing this issue, to support Timeout in async requests.
Leaving aside my own annoyance for hitting this tripwire, it brought to mind sharply that we should be very aware of the things that we do, and how they affect the longevity and scalability of our solutions. I spent most of the beginning of this week and the end of last week doing just that, throwing huge amounts of code and requests at SvnBridge. This was the most interesting problem that it had, and it was very interesting to see how we will solve that.
Tuesday, May 06, 2008
#
Choose the appropriate medium
I recently started using Tweet, which also means that I mostly watch conversations go past. It has made me feel even more strongly about selecting the appropriate medium for discussion.
You cannot have a meaningful discussion in Twitter or IM, the conventions and limitations of the platform. Email is a better medium to expression complex concepts, but voice or video are far better methods of communication. Obviously, nothing can supercede the quality of discussion face to face, especially in open space format.
The other side of the coin is the cost of this interaction.
- IM / Twitter have very little cost, and almost zero expectation. I can send you an "r u thr?" msg without being disturbed by that.
- Doing the same in email, however, would be unacceptable, because I have different expectations from email.
- Escalating to voice or video is far more costly. Now you have tied people in time, which tends to be very hard. I just had a discussion with someone that is 8 hours away from me. Finding the right time to schedule the conversation was... interesting.
- Face to face ties use in both space and time
- Etc...
So you have to choose the appropriate medium for the message you are trying to send.
Sending the message using inappropriate medium tends to cause difficulties.
Pluggable Domain Model
Rhino Security is a good example of a pluggable domain model, in which we can plug some functionality into different and varied domains.
Here is an interesting demo to show how you can use it.
public static void DemoUsingCustomerCareModule<TCustomer>(string schema)
where TCustomer : ICustomer, new()
{
Configuration cfg = new Configuration()
.Configure("hibernate.cfg.xml")
.AddAssembly(typeof(Lead).Assembly)
.AddAssembly(typeof(TCustomer).Assembly);
cfg.MapManyToOne<ICustomer, TCustomer>();
cfg.SetSchema(schema);
new SchemaExport(cfg).Execute(true, true, false, true);
ISessionFactory factory = cfg.BuildSessionFactory();
object accountCustomerId;
using (var session = factory.OpenSession())
using (var tx = session.BeginTransaction())
{
var customer = new TCustomer { Name = "ayende"};
session.Save(customer);
var customerCareService = new CustomerCareService(session);
customerCareService.GenerateLeadFor(customer, "phone call");
customerCareService.GenerateLeadFor(customer, "email ");
tx.Commit();
accountCustomerId = session.GetIdentifier(customer);
}
using (var session = factory.OpenSession())
using (var tx = session.BeginTransaction())
{
var customer = session.Get<TCustomer>(accountCustomerId);
var customerCareService = new CustomerCareService(session);
Console.WriteLine("Leads for: " + customer.Name);
foreach (var lead in customerCareService.GetLeadsFor(customer))
{
Console.WriteLine("\t" + lead.Note);
}
tx.Commit();
}
}
This can be used with:
DemoUsingCustomerCareModule<AccountingCustomer>("Accounting");
DemoUsingCustomerCareModule<HelpDeskCustomer>("HelpDesk");
Course: Building Domain Specific Languages in Boo
You can register here for a two days course in building DSL with Boo.
It is going to take place two weeks from today, in Austin. (19 - 20 May)
I know that this is short notice, but it wasn't something that was planned well in advance. It came out of the ALT.Net conference.
Topics:
- Creating Domain Specific Languages
- The Boo Language
- Flexible compiler and malleable language
- Creating applications with embedded DSL
- Management, tracing and debugging
- Tooling support
- Testing and maintainability concerns
There are ten seats open for that.
I hope we would have fun.
I would also like to thank Jeffrey Palermo and Headspring for hosting the course.
Monday, May 05, 2008
#
How do you track that?
I have an interesting problem with SvnBridge.
After around 5000 full revision request (a set of requests that can occur), the application get hung making a web service call to TFS. This comes after making quite a few calls to TFS, and is generally fairly easily reproducible. The actual call being made is not an expensive one (nor is it the same call). TFS is responsive during that time, so it is not its fault.
It looks very much like I am hitting the 2 concurrent HTTP requests, except that all requests are serialized, and there is no multi threaded work involved.
I have been unable to reproduce this under a profiler or debugger...
Thoughts?
Actively enforce your conventions
Glenn posted about a test I wrote PrismShouldNotReferenceUnity, in which I codified an assumption that the team made.
This is something that I try to do whenever I decide that I will have some sort of convention. If at all possible, I will try to make sure that the compiler will break if you don't follow the convention, but that is often just not possible, therefor, tests and active enforcement of the system convention fill that place.
Wait a minute, I haven't even defined what a convention is! By convention, I means things like:
- All services should have a Dispose() method - add a Dispose abstract method to AbstractService
- All messages should be serializable - create a test that scan the message assemblies and check for non serializable messages.
- You may only call the database twice per web request - create an http module that will throw an exception if you call it more than that
- A request should take less than 100 milliseconds - add an interceptor that would fail if this is not the case.
- The interface assembly may not contain logic - add a test that would fail if it find a class with a method on the interface assembly.
All of those are ways to increase the feedback speed. This is especially true if there is some extra step that you need to perform, or you want to draw a line in the sand, from which you will not deviate.
Actively enforced conventions keep you honest.
Sunday, May 04, 2008
#
Matching SVK revision numbers to the SVN revision number
I am using SVK as a way to learn Distributed SCM, and it has been fine so far, except for one big issue. I am trying to apply SVN patches against SVK repositories. Actually, I setup SVK so I work against a local mirror, so the workflow is exactly the same, but the revision numbers are different. Trying to apply a patch just doesn't work because of that.
You can go into the patch and edit the root revision, so it is not hard to make this work, except...
SVK doesn't give me any mapping between the local revision and the source revisions (maybe it does, and someone will tell me). This means that in order to find the right version I have to do some annoying digging.
Here is a small program to solve the problem. Usage: svkrev [repository path] [source revision to look for]
Example:
C:\Users\Administrator>svkrev //mirror/nhcontrib 195
exec: svk log --xml //mirror/nhcontrib
exec: svk propget svm:headrev //mirror/nhcontrib --revprop -r 10298
exec: svk propget svm:headrev //mirror/nhcontrib --revprop -r 10260
:=349
exec: svk propget svm:headrev //mirror/nhcontrib --revprop -r 10092
:=195
Found: 10092
By the way, can you find the huge perf boost here?
class Program
{
static void Main(string[] args)
{
string path = args[0]; // "//mirror/nhcontrib"
var revisionToSearchFor = int.Parse(args[1]); // 195
string logAsXml = ExecuteProcessAndReturnOutput("log --xml "+ path);
var doc = new XmlDocument();
doc.LoadXml(logAsXml);
var list = new ArrayList();
foreach (var node in doc.SelectNodes("/log/logentry/@revision"))
{
list.Add(node);
}
for (int i = 0; i < list.Count;i++ )
{
XmlNode node = (XmlNode)list[i];
int revision = int.Parse(node.Value);
string output = ExecuteProcessAndReturnOutput("propget svm:headrev " + path + " --revprop -r " + revision);
if (string.IsNullOrEmpty(output.Trim()) == false)
{
var currentRevision = int.Parse(output.Split(':')[1].Trim());
Console.WriteLine(":=" + currentRevision);
if (currentRevision == revisionToSearchFor)
{
Console.WriteLine("Found: " + revision);
return;
}
i += (currentRevision - revisionToSearchFor -1);
}
}
}
public static string ExecuteProcessAndReturnOutput(string args)
{
var output = new StringBuilder();
string err = null;
var psi = new ProcessStartInfo
{
FileName = @"c:\Program Files\svk\svk.bat",
Arguments = args,
RedirectStandardOutput = true,
RedirectStandardError = true,
CreateNoWindow = true,
UseShellExecute = false
};
Console.WriteLine("exec: svk "+ args);
Process svk = Process.Start(psi);
ThreadPool.QueueUserWorkItem(delegate
{
err = svk.StandardError.ReadToEnd();
});
ThreadPool.QueueUserWorkItem(delegate
{
string line;
while ((line = svk.StandardOutput.ReadLine()) != null)
{
Debug.WriteLine(line);
output.AppendLine(line);
}
});
svk.WaitForExit();
if (string.IsNullOrEmpty(err) == false)
throw new InvalidOperationException(err);
svk.WaitForExit();
Thread.Sleep(500);
return output.ToString();
}
}
Rhino Mocks new feature: Persistent Mock Repository
Here is something that most people just don't think about, but it hits some people at the high end. Rhino Mocks is extremely efficient mocking framework, for just about any scenario. However, when it comes the time to mock large amount of interfaces (where large is in the hundreds to thousands), or if you have large interfaces (with dozens or hundreds of methods), it is costly to create the mocks. Rhino Mocks does some caching internally, but it is noticeable issue to some people.
We are talking about an extra second or two for even most pathological cases here, so in general it wouldn't warrant anything special from the library, except that there is a twist to that. Under the debugger, the amount of time spent actually generating the mock can go up by several orders of magnitudes. To add insult to injury, Visual Studio 2005 has several bugs in the debugger that will cause it to crush & burn under several such scenarios.
Karl Lew has been kind enough to not only provide a patch for Rhino Mocks to help this problem, but also document it.
Thanks, and sorry for taking so long to get to it.
Solving the impendence mismatch between Hierarchical Data and XML
I was very impressed when I saw how Subversion handles the complexity of having data of hierarchical nature that needs to be serialized to XML. Check this out.
<?xml version="1.0" encoding="utf-8"?>
<S:editor-report xmlns:S="svn:">
<S:target-revision rev="11"/>
<S:open-root rev="-1"/>
<S:open-directory name="tags" rev="-1"/>
<S:add-directory name="tags/asd"/>
<S:close-directory/>
<S:close-directory/>
<S:close-directory/>
</S:editor-report>
And here is another one:
<?xml version="1.0" encoding="utf-8"?>
<S:editor-report xmlns:S="svn:">
<S:target-revision rev="15"/>
<S:open-root rev="-1"/>
<S:open-directory name="trunk" rev="-1"/>
<S:open-file name="trunk/a.txt" rev="-1"/>
<S:apply-textdelta checksum="eabc96676e7defda414a1eed33bdfb09">
U1ZOAAAQEwETk2FzZDENCjINCjMNCjQNCjUNCjY=
</S:apply-textdelta>
<S:close-file checksum="c6301e5dad1330a7b9bd5491702c801b"/>
<S:close-directory/>
<S:close-directory/>
</S:editor-report>
I was, as they say incredibly happy with this Work Time Fun.
Saturday, May 03, 2008
#
Go with High End Solutions
About a year and a half ago, I start an exciting new project (there is a demo of the actual project here). The actual application is fairly complex, and has some it gave me the chance to explore some very interesting ideas. Rhino Security is a generalization of the security scheme used in this project, and it is pretty much the driving force for Rhino Igloo. But that is not what I want to talk about.
What I do want to talk about is the infrastructure that we used for the project. We used IoC, OR/M, AoP, MVC and many other buzz worthy TLD. It was the first time that I had the chance in implementing real high end complexity reduction techniques. I left the team 10 months ago. In the meantime, the application was under active development, got a few new team members and had two major releases.
I am really proud of that system.
A few weeks ago I got a phone call from the current team lead, asking me about the windsor.boo file that is sitting there. The last time anyone touched it was shortly after I left, after which, it just... existed. I had the chance to do a code review on the new stuff that the team developed, about three months ago. I couldn't find any real difference between the code develop before and after I left.
Anyway, I had to spend 15 minutes on the phone, explaining the process that was going on there. Before I left (and during the time I was the team lead), I made sure that I passed on all the knowledge that I had about the system, the design decisions and the overall picture. However, there was a period of nearly three months in which I forgot that we even had this infrastructure, because we hadn't have to deal with it for that time period. After I left...
- 9 months.
- 2 major releases.
- Zero issues with the infrastructure.
I asked the team lead what she thinks about that. Since it is her project now, and if she thinks that it was the right decision to make. She love the infrastructure, and wouldn't hear about using a lower end solution. Most of what we did was actually going over the file and explaining historical decisions, for that matter.
As an additional data point, I was able to look at a piece of code I have last seen over a year ago and figure out not only what it does, but the how and why of it with no ramp up time.
I consider this a success.
How to test this?
I have an interesting problem. I am currently working on adding sync support to SvnBridge. This mostly involves tracking down what SVN does and duplicating it myself. There isn't that many new code (I had to add a class and a method). I have nothing that I can _unit_ test. Oh, I could probably craft something, but I am reluctant to mock the world when what I want is to test the actual integration between the SVN client and the TFS server.
The problem is that I have no real way of testing this. Why do I mean by that? syncing is an operation that works over the entire repository, from start to finish. I can't perform an integration test, because that would take too long in most scenarios. I would need to setup a whole new TFS server per test. Not really a good solution, no matter how you turn the dice. And the other problem is that I need a rich set of situations to actually test this.
Right now I am driving that by going against the production CodePlex servers, and trying to see if I can get all the information from there. This is incredibly valuable, because it gives me access to a lot of source control practices that are out there, and expose a lot of false assumptions. But I can't write tests for those, because they are too big a scenario.
I can probably set an integration test that would execute against the production servers (they are publicly exposed, after all, so no issue there), but I am... less than thrill about having a single test that can potentially run for hours or days.
Right now I think that I am leaning toward partial fake of both the client and the server. That is, create a set of input values which, while not being the real world values, would still test that logic.
Considering that the actual interaction we are talking about take place over many requests, this is tricky, but possible.
Suggestions?
Friday, May 02, 2008
#
Avoid retrofitting unit tests
I was asked this a few days ago, should we spend time creating proper unit tests for our code?
The team in question already have a set of integration tests, but very few tests that qualify as unit tests.
My response was rambling and long, but it can be put down to the following statement: TDD is a design technique, not a testing technique. TDD, and especially test first, have the nice side affect of leaving tests as part of the process, which can be incredibly helpful when you are working with the code. But retrofitting tests? That tends to be a waste of time.
Writing a unit test before touching the code is absolutely the way to go, but going and adding unit tests, as a task of its own? I don't see the value in that.
If you have integration tests there, that tends to be good enough, and you will write unit tests when you change the code, so eventually you'll have enough unit tests ( eventually you will have enough unit tests on the hot spots ).
Thursday, May 01, 2008
#
Playing with numbers
For some reason, it looks like it is a good day to be impressed by numbers, here it the most interesting one...
And here is the reason you don't want to use the file system as your database:
Dynamic Mapping with NHibernate
A while ago I posted how to handle dynamic mapping with Active Record, it was incredibly easy to do, because Active Record has a lot of smarts internally, and output the XML, on top of which NHibernate adds quite a bit of convention over configuration as well. Doing the same using NHibernate directly is possible, but a bit long winded. Here is the sample code, which link all the Employee properties to the correct entity:
Configuration cfg = new Configuration()
.AddAssembly(typeof (Employee).Assembly)
.AddAssembly(typeof(ScheduledTask).Assembly);
Mappings mappings = cfg.CreateMappings();
foreach (PersistentClass persistentClass in mappings.Classes)
{
if (persistentClass.MappedClass.GetProperty("Employee") == null)
continue;
Property prop = new Property();
PersistentClass employeeClass = cfg.GetClassMapping(typeof (Employee));
Table table = employeeClass.Table;
ManyToOne value = new ManyToOne(table);
value.ReferencedEntityName = typeof (Employee).FullName;
Column column = new Column("Employee");
value.AddColumn(column);
prop.Value = value;
prop.Name = "Employee";
prop.PersistentClass = employeeClass;
persistentClass.AddProperty(prop);
persistentClass.Table.AddColumn(column);
persistentClass.Table.CreateForeignKey("FK_EmployeeTo" + persistentClass.MappedClass.Name,
new Column[] {column,}, typeof (Employee).FullName);
}
cfg.BuildSessionFactory();
new SchemaExport(cfg).Execute(true, true, false, true);
As you can see, there is a lot that needs to be done, we have to tell NHibernate a lot of things it would generally be able to figure out on its own. We can shove this to an extension method and get really nice syntax:
public static void MapManyToOne<TEntityInterface, TEntity>(this Configuration cfg)
{
Mappings mappings = cfg.CreateMappings();
foreach (PersistentClass persistentClass in mappings.Classes)
{
var propertyNames = new List<string>();
foreach (PropertyInfo property in persistentClass.MappedClass.GetProperties())
{
if (property.PropertyType == typeof (TEntityInterface))
{
propertyNames.Add(property.Name);
}
}
if (propertyNames.Count == 0)
continue;
var prop = new Property();
PersistentClass targetClass = cfg.GetClassMapping(typeof (TEntity));
foreach (string propertyName in propertyNames)
{
Table table = targetClass.Table;
var value = new ManyToOne(table);
value.ReferencedEntityName = typeof (TEntity).FullName;
var column = new Column(propertyName);
value.AddColumn(column);
prop.Value = value;
prop.Name = propertyName;
prop.PersistentClass = targetClass;
persistentClass.AddProperty(prop);
persistentClass.Table.AddColumn(column);
string fkName = string.Format("FK_{0}To{1}", propertyName, persistentClass.MappedClass.Name);
persistentClass.Table.CreateForeignKey(fkName,
new[] {column,}, typeof (TEntity).FullName);
}
}
}
Now we can use this with the following syntax:
cfg.MapManyToOne<IEmployee, Employee>();
Which is much nicer.
The single line bug fix
It is amazing how much time you can hunt for the exact cause of a bug. In this case, it took me almost two days (intersperse with other work, however) to track down and find the issue.
Remember, there is no reason to use ASCII, ever. I actually run a blame on the code (a new feature for SvnBridge!) to find out who wrote it. And then I sent a nasty email about it to /dev/null, just to clear my mind.
Wednesday, April 30, 2008
#
Extreme Patterns Video
I was asked a few times about recording a course, so I think that a few people would be happy to know that Glenn Block has posted a discussions that we had about a month ago.
You can find it here
I don't like distributed source control
Let us see how much of a furor that will cause :-)
The reasons that I don't like distributed source control systems have nothing to do with technical reasons, and everything to do with how they are used and promoted.
The main reason that I don't like them is that they encourage isolated work. I don't want anyone in my project to just go off and work on their own for a few weeks, then come back with a huge changeset that now needs to be merged. The way DVCS are promoted, this is a core scenarios, "no one have to see your mistakes".
I completely disagree. I want to see everyone's mistake, and I want them to see mine. There is little to learn from successes, and much to learn from failures. Far worse, I want to be able to peak into other people work, so I can give my input on it, even if it is just "wow, nice", or "yuck, sucks!"
The main advantage of DVCS is speed, trying to browse the repository when you have the entire thing on your HD is lightning fast, compared to doing the same with a remote repository. I tend to use SVK now, but I use it strictly as a local cache, and nothing more.
And that is why I don't like DVCS, I don't want people to work in isolation.