How asynchronous is SmtpClient.SendAsync?

Posted by Annie on Jan 30, 2012 in .NET, Coding | 3 comments

In a previous role, I was tasked with writing a newsletter email sender. How hard can this be, I thought to myself, and set off to complete my mission.

Initial Thoughts

We were probably going to be sending tens of thousands of emails at a time so although I figured I’d need to use some threading, I thought I’d start by using the asynchronous version of the SmtpClient.Send method, SmtpClient.SendAsync instead of the blocking SmtpClient.Send. I figured that way I’d be able to send batches of emails asynchronously and that way we’d be able to get through all the emails super fast.

The Test

I wrote some basic prototype code using SmtpClient.SendAsync and ran a test to send a couple hundred emails in it. Although my test ran without errors, I pretty quickly discovered that I wasn’t receiving all the emails that I was apparently sending by calling SmtpClient.SendAsync! At first I wondered if I’d somehow overflowed my inbox or something but that didn’t seem to be the problem. Then I started doing a bit more research and discovered something which explained the behaviour I was seeing…

The Findings

From the ‘remarks’ section of the MSDN documentation on SmtpClient.SendAsync:

After calling SendAsync, you must wait for the e-mail transmission to complete before attempting to send another e-mail message using Send or SendAsync.

And…

To receive notification when the e-mail has been sent or the operation has been canceled, add an event handler to the SendCompleted event.

OH! So in order to successfully send multiple emails asynchronously, I must add an event handler to the SmtpClient.SendCompleted event and wait until the first SmtpClient.SendAsync has completed before triggering the next one. Hmm… this does not seem all that asynchronous to me! I realize that there must be good reasons why it was implemented this way but in theory, using SmtpClient.SendAsync to send multiple emails really isn’t all that much more asynchronous than lining up a bunch of synchronous calls to SmtpClient.Send.

Conclusion

I guess it all comes down to your interpretation of what ‘asynchronous’ means – in this case, SmtpClient.SendAsync is indeed asynchronous in that it allows the program to carry on executing without blocking. This is great in most cases, unless what you want to do next is send another email.

So to sum up, it seems the only way to send multiple emails at the same time using .NET’s SmtpClient is to use threading after all. Spawn up a few worker threads with a separate instance of the SmtpClient in each and just send the emails using SmtpClient.Send.

ASP.NET Response.Redirect

Posted by Annie on Jan 23, 2012 in Coding | 1 comment

In ASP.NET web applications, the Response.Redirect(string url) method is often used to control the flow of an application (both user browsing and logic) by redirecting the client to a different URL. For example, on a page that requires an authentication user, it is quite standard to first check if the user is logged in and if they aren’t, call Response.Redirect(“login.aspx”). This is all great, except that many developers probably don’t know what’s actually going on under the covers.

Under the covers

Internally, Response.Redirect (and Server.Transfer for that matter) calls Response.End to end the page’s execution (rather abruptly) and then shifts the execution to the Application_EndRequest event in the application’s Global.asax. We rely on this behaviour to bail out of wherever we were when we decided to call Response.Redirect. What most people probably don’t realize is that Response.End raises a ThreadAbortException exception. Exceptions are expensive, and if you’re calling Response.Redirect often, you might find this particular behaviour detrimental to the performance of your site.

So, how do I avoid these exceptions?

Well, Response.Redirect actually has an overload that takes two parameters – Response.Redirect(string url, bool endResponse). If you use this overload instead and pass false to the endResponse parameter, the internal call to Response.End will be suppressed and as such, no ThreadAbortException exceptions will be raised.

Great! But wait, there’s more…

There is a side effect! Using this overload and passing false so that Response.End doesn’t get called means that any code after the call to Response.Redirect will now be executed where it wouldn’t have previously. This makes perfect sense – if Response.End isn’t called then the execution isn’t shifted to the Application_EndRequest event and if the execution isn’t shifted, then it will carry on executing wherever it currently is. That’s right – although you no longer get ThreadAbortException exceptions being thrown, Response.Redirect no longer controls the flow of your logic.

This is fine if you are calling Response.Redirect at the end of a method at the bottom of your call stack because there was probably no more code to be executed anyway. On the other hand, if you are calling this from inside a method that isn’t at the bottom of your call stack, you have to be careful with the remaining stack calls. Even if you call return straight after you call Response.Redirect, where are you ‘returning’ to? Another method that executes more code? Eeek!

In conclusion

None of this is news – it’s all explained in this Microsoft Support article. However, I really don’t feel that it emphasizes enough the negative effects that switching over to the Response.Redirect overload may have on your application. You simply can’t go through your application and replace every Response.Redirect with it’s overload without thinking carefully about what this might do to the logic of your application. The ‘flow’ may still look like it’s working as it should because the user will still be redirected to another page, but under the covers you might find that you’re now executing code that you weren’t before and that may have some pretty nasty unwanted effects.

Localization and skinning in ASP.NET MVC 3 web applications

Posted by Annie on Jan 11, 2012 in ASP.NET MVC, Coding | 0 comments

Working in Europe has introduced me to the world of localization and skinning web applications. Not only do we have to support several different locales for the same web application, but in some cases, depending on the host url, the site should be skinned completely differently. A common reason for needing to do this is if your e-commerce website also sells products under the guise of a different company in certain locales. Obviously, skinning sites involves a lot of css styling, but you may also need to provide different values for titles, labels, urls, etc, for each skin. You really don’t want to have to build and deploy several copies of your website to do this, and luckily, you don’t have to!

Localization in .NET using resource files

In .NET, localization is generally done via resource files. For each language / locale you wish to support, you should have a SiteA.resx file, e.g.

• SiteA.resx (This is the fallback resource file, in case no other more specific resource files match the current culture)
• SiteA.es.resx (Spanish)
• SiteA.de.resx (German)
• SiteA.nl.resx (Dutch)
• SiteA.nl-BE.resx (Belgian Dutch)

If this is all you need, then .NET makes localization pretty straight forward. When you build your solution, .NET automatically creates a strongly-typed resource class with the same name as your resource files (without the extensions). In my example above, .NET would have created a class called SiteA in the Resources namespace, with public readonly string properties for each key in my resources files.

When you access one of the properties in this resource class, as long as you have set your current culture correctly, .NET will take care of picking exactly which resource file to read the values from. How it picks which resource file to use is done via a fallback mechanism which is handled by the convention-based naming. Scott Hanselman provides a pretty good explanation of how this works here.

Localizing skinned sites using resource files

If you also want to use resource files to provide different localized values for each of your skinned sites, you’ll need to create a set of resource files for each skin, e.g. SiteB.resx and SiteC.resx:

• SiteA.resx
• SiteA.es.resx
• SiteA.de.resx
• SiteB.resx
• SiteB.es.resx
• SiteB.de.resx
• SiteC.resx

From these resource files, .NET creates three separate strongly-typed resource classes for you to use:

However, you cannot edit these classes manually, they do not inherit from a common base class nor do they implement a common interface, so how do you elegantly switch between skins without having some massive switch statement everywhere you need to read a value from the resource files?

As with any programming problem, I’m sure there are many ways to do. I’m going to explain how I solved this particular problem in an ASP.NET MVC 3 web application.

ActionFilterAttributes and a LocalizedResourceManager

First of all, I needed something to intercept every action in my application and figure out which culture to use. In my application, I allow the user to change the language of the site by setting a ‘lang’ parameter in the querystring. If that parameter isn’t set, then I look in the user’s browser languages for the main supported language. If you wanted to, you could also read and write a user’s chosen culture from a cookie, but I didn’t need to implement this. I also needed to figure out which skin to display based on the host url that the user requested.

I was able to do this by sub-classing the ActionFilterAttribute class and overriding it’s OnActionExecuting method:

ActionFilterAttribute.cs

You’ll see that on line 18 of the LocalizationAttribute, I resolve the site that the user has requested. I do this by reading the host url. Once I know which site the user has requested, I initialize my LocalizedResourceManager by setting it’s CurrentSiteName property.

LocalizedResourceManager.cs

The LocalizedResourceManager is very simple – it only exposes one method, GetResource(string), to which you pass the name of the resource key that you wish you read out of your resource files. Internally, this method calls HttpContext.GetGlobalResourceObject(string, string) to actually read the property out of the resource files. This method here is really the crux of my solution.

HttpContext.GetGlobalResourceObject(string, string) takes two string parameters: className and resourceKey. In essence, this method allows you to pick which resource class to use and then uses the normal fallback mechanism to figure out which localized resource file to actually read the value from based on which culture is currently set.

All that’s left to do now is to register the LocalizationAttribute as a global filter in the Application_Start() method of Global.asax.cs. I also provide a new route which takes a lang parameter so that the user can change which language they wish to see the site in.

Global.asax.cs

And finally, to make use of this setup, all I have to do is call the static LocalizedResourceManager.GetResource(string) method from within my controllers or views, passing it the resource key name for which I would like to get a localized (both locale and skin-wise) value. By this time, the ActionFilterAttribute will have done it’s magic by setting a current culture and the LocalizedResourceManager.CurrentSiteName property, thereby telling the LocalizedResourceManager which resource class to use and letting .NET use it’s fallback mechanism to pick the right localized resource file as per the current culture.

Index.cshtml

Software deployment processes

Posted by Annie on Oct 28, 2011 in Coding, Technology | 7 comments

During my career as a Software Engineer, I have seen a variety of different approaches to ‘deployments’. This is one aspect of the software development life cycle that wasn’t touched on at all during my 4 year Computer Science degree and seems to be a point of contention for a lot of software companies.

In my experience, a lot companies seem to almost fear deployments, leading to unmanageable 6-week deployment cycles, middle-of-the-night deployments that take hours and inevitably result in unplanned outages, a lot of error-prone manual configuration, unnecessary friction between developers and sys admins, no rollbackability, no version history, etc etc.

Image sourced from http://sgale.blogspot.com

Deployments shouldn’t be that hard

Really, they shouldn’t. If you have a well-planned, stable and automated deployment process, you should be able to deploy early and often and hopefully without any glitches. Your customers will be happier and you won’t end up looking like the poor cat in the picture above.

Below is a list of what I consider to be the most important qualities of a good deployment strategy (not necessarily in order of priority):

• Manageability
  Deployments really shouldn’t be as complicated and scary as a lot of companies make them out to be. It should be a one-button action and it shouldn’t require taking the whole system offline for hours in the middle of the night by the whole sys admin team. The number of steps involved in deploying may vary depending on your system and how major the deployment is, but they should be easy enough to follow.
• Configurability
  It should be fairly easy to deploy any of your projects – from static files to binaries, web applications that need IIS to be recycled and even windows services that need to be uninstalled, installed then restarted.
• Visibility
  Developers, sys admins and anyone else in the company who wants to know should be to quickly and easily be able to see a history of all deployments, what the last version deployed was, what the changes were, who worked on it and when exactly it went live.
• Rollbackability
  For those odd occasions when a production release doesn’t go quite as expected, it should be relatively easy to rollback to a previous version. I say relatively easy because it really depends on how major the deployment was and how complicated your software is.

My deployment system

In one of my previous roles, I was asked to come up with a process for deploying the various components that made up the software we were building. The components that needed to be deployed (sometimes together, sometimes independently) were .NET web services, ASP.NET MVC web apps, windows services and static files. What I ended up building was a deployment process that involved Teamcity, MSBuild scripts, custom MSBuild tasks, a .NET ‘deployment web service’ and last but not least, a page on our intranet to show version history and comments.

Since building this process, I’ve been asked to describe it several times so I thought to myself, why not blog about it?!

Requirements:
I built this system for one company, with their requirements in mind. It may not suit everyone’s deployment needs. For example, my deployment system did not deploy SQL scripts as this was not a requirement. Any scripts / tables that needed to be created or altered / migration scripts would be run manually before deploying the code. This became a part of the greater deployment process, but it was not handled automatically. And finally, I only built the code aspect of this system and wired it all up through Teamcity – our sys admins set up all required VPN tunnels, FTP servers, etc etc…

Process:
For every code solution, we had an MSBuild script that Teamcity would use to build and publish the binaries / static files on dedicated build agents. Apart from giving Teamcity instructions on how to build the solutions, these MSBuild scripts would also zip up the binaries and static files using the version number as the filename and then copy the zip files to a build server. The sys admins set up an FTP server pointing to all these zip files so that the servers we deployed to had access to them.

The next step was to set up new Teamcity build tasks that used different MSBuild scripts to actually deploy the zipped up binaries and static files. We had one MSBuild task per solution and environment. These MSBuild scripts used custom MSBuild tasks to call the deployment web service on a particular server / environment. In cases where a particular solution was load balanced, we would pass the host names of each server hosting the solution to the custom MSBuild task which would then spin up a bunch of threads and call the deployment web service on each of those servers.

The Deployment Web Service – the brains of the system:
The ‘deployment web service’ was the brains of the system – the rest of the stuff I just described simply wired everything up. The first step in setting up a new server was always to deploy and configure the deployment web service. Once it was installed and configured properly, this is more or less what the deployment web service would do when it was called:

• Download the appropriate zip file containing the built solution from the FTP server onto the server you are deploying to.
• Unzip the zip file into a temporary folder.
• Modify the config files – depending on the environment being deployed to, the deployment web service would pick the right config file, delete all the others and rename the remaining one to Web.config (or App.config…).
• If the solution being deployed was a windows service, the deployment service would now stop and uninstall it if it were already installed and running.
• Copy all the files from the temporary directory over the top of the actual binaries using robocopy.
• If the solution being deployed was a website or a web service, the deployment service would now recycle the app pool that the site was running under.
• If the solution being deployed was a windows service, the deployment service would now install it.
• Delete all temporary files.

Simple, right? Seriously though, that was the brains of the system and it really didn’t do anything out-of-this-world. Deploying binaries and/or files really isn’t that hard!

But what about environment-specific configuration?

This is a much debated point and I’m not claiming that the way we dealt with environment-specific configuration is the only way to go, but it was certainly simple and enabled us to have automated stress-free deployments.

For every application we built, we would maintain separate config files for each environment. That’s right – we had a Web.config (for localhost), Web.Development.config, Web.Test.config, Web.Staging.config and Web.Production.config. I know some of you reading this will be rolling your eyes right about now and thinking… “So every time I need to add a new key to my config file, I have to add it to 5 different files?!? What a nightmare!!”. Trust me, it really isn’t that hard to keep all these files in sync and seriously, it’s much easier than trying to manually merge config files at the last minute before deploying. Think about it this way – once a system is up and running, you don’t add keys anywhere near as often as you have to deploy (x number of environments you’re deploying to).

So how did the right config file get deployed to each environment? Well, all these environment-specific config files would be included as part of the project to be deployed. Each instance of the deployment web service knew which environment it was sitting in and deploying to (this was configured via it’s own web.config) so as I described above, the deployment web service would pick the correct config file based on it’s filename, delete all the others and rename the correct one to Web.config (or App.config). Easy peasy.

Benefits

Having used several different techniques for deploying code through environments and out to production over the years, this system certainly demonstrated various benefits and in particular, satisfied all of the qualities held by a good deployment process which I listed above.

• Manageability
  Since this system was triggered from Teamcity, it really couldn’t get any easier to manage deployments. Deployments became a matter of pushing one button and waiting for a couple of minutes to see the result. There was no manual merging of config files, no remoting onto the server to restart an app pool. Just one button click.
• Configurability
  In terms of set up, although this system is made up of several components, it was fairly easy to configure a new project to be deployed. Sure, there were a few steps involved but they were all simple and more or less a copy-and-paste of an existing project’s deployment scripts. It was also pretty straight-forward to set up a new server to deploy to. This involved setting up the deployment web service on the new server and updating any MSBuild scripts with the hostname of the new server for projects that you want to deploy to it.
  Because it was highly configurable, the system was able to deploy several components at the same time, to various different servers.
• Visibility
  Because we used Teamcity to trigger the deployments, we had all the whole build log that Teamcity produces. I put some logging into the custom MSBuild tasks so that we could see what was going on from within Teamcity’s build logs. Later on, I added some code to update a version databases so that deployments were visible from within our intranet.
• Rollbackability
  Rolling back to a previous version was a matter of looking up which version number you wish to roll back to and setting this version number in the parameters passed to the deployment MSBuild script by Teamcity. The custom MSBuild tasks and deployment web service would then pick up that particular version and re-deploy it.

Final thoughts

In over a year of daily deployments of several different web sites, web services, windows services and static files, my deployment system never failed. Having this system in place enabled us to focus on writing cool software and getting it out often and early instead of planning stressful deployments. Everyone benefited from this system – developers and testers could easily and quickly deploy to dev, test and staging as often as they wanted, without waiting for sys admins to be free, and production deployments were no harder. Sys admins didn’t need to know anything about how to deploy, managers were able to see when a new version had been deployed to a particular environment and all the automation meant that there was very little room for human error. A definite win for everyone!

If your team doesn’t have an automated deployment system already in place, I implore you to either build your own (like we did) or use an off-the-shelf one. It may take a bit of effort to set it all up at the beginning, but it’s definitely worth it in the long run.

Deploying ASP.NET MVC 3 to IIS 6

Posted by Annie on Oct 19, 2011 in ASP.NET MVC, Coding, Technology | 4 comments

I recently built a new ASP.NET MVC 3 (.NET 4) web application which unfortunately has be hosted on Windows Server 2003 running IIS 6. Guess I’d figured getting it running on IIS 6 would be as easy as installing .NET 4 and ASP.NET MVC 3 on the server… but it wasn’t. This isn’t a step by step tutorial on how to get this working, but more of a list of things to look out for and some links to helpful material which all helped getting my site up and running.

Installations

• Install the .NET 4 Framework.
• Install ASP.NET MVC 3. Apparently you can avoid installing ASP.NET MVC 3 on the server and just deploy all the appropriate MVC assemblies in the bin folder of your app instead, but I couldn’t get this to work.

IIS Configuration

• Make sure ASP.NET v4.0.30319 is ‘Allowed’ under Web Service Extensions in IIS – it is ‘Prohibited’ by default.
• Create a new app pool for your web site to use – IIS 6 app pools cannot run sites under different frameworks so make sure this app pool is only used for your new site.

IIS Website Configuration

• Under the ‘ASP.NET’ tab, make sure that the ASP.NET version is set to ‘4.0.30319’.
• Under the ‘Home Directory’ tab, make sure that ‘Execute permissions’ is set to ‘Scripts only’ (at least).
• Click on the ‘Configuration’ button in the ‘Home Directory’ tab and make sure that all the Application extensions are mapped to .NET 4 versions of the dlls.
• Add a ‘Wildcard application mapping’ – click on ‘Insert’ and enter ‘C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll’ as the ‘Executable’. Leave ‘Verify that file exists’ unchecked – checking this made my routes stop working.

Permissions

If you’re running under an account with limited permissions, you may need to grant it ‘List Folder Contents’ and ‘Read’ permissions on the Windows Temp directory. I needed to do this because I’m calling out to web services and .NET creates temporary classes in the Windows Temp directory with which to serialize objects from XML. Not sure if this is needed if you’re not doing any serializations, but worth mentioning.

Conclusion

I kept getting 403 and 404 errors when I first deployed my new ASP.NET MVC 3 (.NET 4) web application to IIS 6, but after doing / checking all of the above, it all seems to be working smoothly.

And last but not least: if you’re allowed to, try recycling your application pool and resetting IIS if some changes don’t seem to be making much difference.

Useful links

http://haacked.com/archive/2010/12/22/asp-net-mvc-3-extensionless-urls-on-iis-6.aspx
http://johan.driessen.se/posts/getting-an-asp.net-4-application-to-work-on-iis6
http://blog.stevensanderson.com/2008/07/04/options-for-deploying-aspnet-mvc-to-iis-6/