Welcome!

Microsoft Cloud Authors: Elizabeth White, Mihai Corbuleac, Pat Romanski, David Bermingham, Steven Mandel

Related Topics: Microsoft Cloud

Microsoft Cloud: Article

How ASP.NET PostBacks and Redirects Work

A question from some of our clients

Last week I got the following two questions from one of our clients

  • “We use ASP.NET PostBacks but can’t find the PurePath for the request triggering the PostBack handler – any hints?”
  • “We see many ThreadAbortExceptions in our ASP.NET Application and we are not sure why they happen – are they expected?”

Time for a little blog that gives some internals on PostBacks as well as Redirects (which are commonly used in PostBack handlers).

What is a PostBack and how does it work?
It’s not my intention to write the 50th blog post about PostBacks – I’d rather recommend checking out the following links for more details on ASP.NET PostBacks and the ASP.NET Page LifeCycle. I am also not necessarily advocating PostBacks here either as there are other ways of implementing web applications (e.g.: ASP.NET MVC) – but – as the question came up I am sure many out there use this feature and may wonder how it works.

The important thing to understand is that a PostBack – as the name implies – POSTs data back to the current url. This allows ASP.NET Developers to implement PostBack code similar to event handlers in rich UI’s where you even get access to before/after values of controls.

In the sample application I use I have a MainForm.aspx page that displays a login control with a username and password field and a login button. When the user clicks the login button the data of the form elements is POSTed back to the server which ultimately calls the Login Button Click event handler. When a user therefore opens a browser and browses to my MainForm.aspx page and then clicks the login button we end up having 2 HTTP Requests to the same MainForm.aspx page. The second request responds with a different page, e.g.: Yes you are logged in or Authentication failed even though it goes to the same URL.

The following illustration shows my login button handler that takes the values POSTed to the MainForm.aspx page and checks whether the username/password combination is correct.

PostBack triggers Login Button Event Handler which does user  authentication and redirects depending on logon success

PostBack triggers Login Button Event Handler which does user authentication and redirects depending on logon success

Also – depending on whether it is the initial page request or a PostBack you may want to execute different code. This can be done by checking the IsPostBack property on the Page objects. Remember our earlier example? The user basically requested the same url – MainForm.aspx – but obviously expects a different result for the 2nd request. The following illustration shows how to implement conditional code depending on PostBack or initial request:

Conditional Code depending on whether this is a PostBack or initial  page request

Conditional Code depending on whether this is a PostBack or initial page request

The answer to the first question I received last week therefore is: Identify the 2 requests on the same initial URL. The following illustration shows how dynaTrace displays the individual URL’s that have been traced so far and how to actually drill to the PurePath for the PostBack request. We see the 2 requests to MainForm.aspx and the two corresponding PurePaths where the second is the one that reflects the PostBack request which ultimately calls our click handler:

Identifying the PostPack PurePath and spotting the click event  handler

Identifying the PostPack PurePath and spotting the click event handler

What’s the difference between Response.Redirect and Server.Transfer?
You probably wonder what really happens after a PostBack. Are we now always staying on the same MainForm.aspx page? Or is there a way to redirect the user to a different url? ASP.NET offers two options (at least I am only aware of these two). There is the option of redirecting the user to a different URL or let the server transfer over to process a different URL.

Let’s go back to my example from above. My user logs in. In case the login is successful I want the user to get to his personalized site. In case the login was not successful I want the user to get back to the login page with an error displayed that the login was not successful. One option to redirect the user to a different url is to use Response.Redirect telling ASP.NET to redirect to a different URL. Technically – Response.Redirect actually sends an HTTP Redirect Response back to the browser causing the browser to request the new URL. This works great – but – it causes an extra roundtrip between browser and server. The following illustration (using the FREE dynaTrace AJAX Edition) shows the Network Requests between browser and server when browsing through the pages of my application. We can spot the redirects that the PostBacks cause by using Response.Redirect:

PostBack's causing an additional roundtrip between browser and  server as it uses HTTP Redirects

PostBack's causing an additional roundtrip between browser and server as it uses HTTP Redirects

My sample application redirects to the same URL but using a different url parameter to indicate which feature of the web site my user should see, e.g.: overview. It is easy to spot what is going on here. First we have the initial request to MainForm.aspx which displays the login dialog (because my user is not yet logged in). The PostBack on MainForm.aspx comes back telling the browser to redirect to MainForm.aspx?action?overview which is then requested by the browser. This behavior goes on for every interaction my user does – basically causing an extra roundtrip that should be avoided. It also causes many exceptions on the server – and that brings me to the second question I received: In order to do the redirect ASP.NET aborts the currently executing thread with a ThreadAbortException. So – whenever you use Response.Redirect – expect to see ThreadAbortExceptions (in case you use something like dynaTrace that actually gives you insight into these internal exceptions).

Fortunately ASP.NET offers a second option to transfer to a different page. Server.Transfer shortcuts the redirect by executing another page request with the new URL in your current ASP.NET thread. When changing my implementation from Response.Redirect to Server.Transfer I get rid of all HTTP Redirects. The following illustration shows what a PurePath that uses Server.Transfer looks like:

Server.Transfer executes transfered URL in same execution thread

Server.Transfer executes transferred URL in same execution thread

Conclusion
Make yourself familiar with internals of ASP.NET. If you use other frameworks such as ASP.NET MVC make sure you do enough research and reading on the options the framework gives you and how things work internally. It is important to save on unnecessary HTTP roundtrips as they can become really expensive for end-user browsing experience. As always, feedback and your thoughts on this topic are welcome.

Related reading:

  1. SharePoint: Performance Problem with LookupField Control The Microsoft.SharePoint.WebControls.LookupField control is used as the default control when editing...
  2. SharePoint: List Performance – How list column indices really work under the hood Have you ever wondered what is really going on under...

More Stories By Andreas Grabner

Andreas Grabner has been helping companies improve their application performance for 15+ years. He is a regular contributor within Web Performance and DevOps communities and a prolific speaker at user groups and conferences around the world. Reach him at @grabnerandi

@ThingsExpo Stories
The IoT has the potential to create a renaissance of manufacturing in the US and elsewhere. In his session at 18th Cloud Expo, Florent Solt, CTO and chief architect of Netvibes, will discuss how the expected exponential increase in the amount of data that will be processed, transported, stored, and accessed means there will be a huge demand for smart technologies to deliver it. Florent Solt is the CTO and chief architect of Netvibes. Prior to joining Netvibes in 2007, he co-founded Rift Technol...
Join IBM June 8 at 18th Cloud Expo at the Javits Center in New York City, NY, and learn how to innovate like a startup and scale for the enterprise. You need to deliver quality applications faster and cheaper, attract and retain customers with an engaging experience across devices, and seamlessly integrate your enterprise systems. And you can't take 12 months to do it.
This is not a small hotel event. It is also not a big vendor party where politicians and entertainers are more important than real content. This is Cloud Expo, the world's longest-running conference and exhibition focused on Cloud Computing and all that it entails. If you want serious presentations and valuable insight about Cloud Computing for three straight days, then register now for Cloud Expo.
IoT device adoption is growing at staggering rates, and with it comes opportunity for developers to meet consumer demand for an ever more connected world. Wireless communication is the key part of the encompassing components of any IoT device. Wireless connectivity enhances the device utility at the expense of ease of use and deployment challenges. Since connectivity is fundamental for IoT device development, engineers must understand how to overcome the hurdles inherent in incorporating multipl...
Machine Learning helps make complex systems more efficient. By applying advanced Machine Learning techniques such as Cognitive Fingerprinting, wind project operators can utilize these tools to learn from collected data, detect regular patterns, and optimize their own operations. In his session at 18th Cloud Expo, Stuart Gillen, Director of Business Development at SparkCognition, will discuss how research has demonstrated the value of Machine Learning in delivering next generation analytics to im...
Manufacturers are embracing the Industrial Internet the same way consumers are leveraging Fitbits – to improve overall health and wellness. Both can provide consistent measurement, visibility, and suggest performance improvements customized to help reach goals. Fitbit users can view real-time data and make adjustments to increase their activity. In his session at @ThingsExpo, Mark Bernardo Professional Services Leader, Americas, at GE Digital, will discuss how leveraging the Industrial Interne...
The paradigm has shifted. A Gartner survey shows that 43% of organizations are using or plan to implement the Internet of Things in 2016. However, not just a handful of companies are still using the old-style ad-hoc trial-and-error ways, unaware of the critical barriers, paint points, traps, and hidden roadblocks. How can you become a winner? In his session at @ThingsExpo, Tony Shan will present a methodical approach to guide the holistic adoption and enablement of IoT implementations. This ov...
SYS-CON Events announced today that Stratoscale, the software company developing the next generation data center operating system, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. Stratoscale is revolutionizing the data center with a zero-to-cloud-in-minutes solution. With Stratoscale’s hardware-agnostic, Software Defined Data Center (SDDC) solution to store everything, run anything and scale everywhere...
Angular 2 is a complete re-write of the popular framework AngularJS. Programming in Angular 2 is greatly simplified – now it's a component-based well-performing framework. This immersive one-day workshop at 18th Cloud Expo, led by Yakov Fain, a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay, will provide you with everything you wanted to know about Angular 2.
Digital payments using wearable devices such as smart watches, fitness trackers, and payment wristbands are an increasing area of focus for industry participants, and consumer acceptance from early trials and deployments has encouraged some of the biggest names in technology and banking to continue their push to drive growth in this nascent market. Wearable payment systems may utilize near field communication (NFC), radio frequency identification (RFID), or quick response (QR) codes and barcodes...
SYS-CON Events announced today that Men & Mice, the leading global provider of DNS, DHCP and IP address management overlay solutions, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. The Men & Mice Suite overlay solution is already known for its powerful application in heterogeneous operating environments, enabling enterprises to scale without fuss. Building on a solid range of diverse platform support,...
You deployed your app with the Bluemix PaaS and it's gaining some serious traction, so it's time to make some tweaks. Did you design your application in a way that it can scale in the cloud? Were you even thinking about the cloud when you built the app? If not, chances are your app is going to break. Check out this webcast to learn various techniques for designing applications that will scale successfully in Bluemix, for the confidence you need to take your apps to the next level and beyond.
The increasing popularity of the Internet of Things necessitates that our physical and cognitive relationship with wearable technology will change rapidly in the near future. This advent means logging has become a thing of the past. Before, it was on us to track our own data, but now that data is automatically available. What does this mean for mHealth and the "connected" body? In her session at @ThingsExpo, Lisa Calkins, CEO and co-founder of Amadeus Consulting, will discuss the impact of wea...
Whether your IoT service is connecting cars, homes, appliances, wearable, cameras or other devices, one question hangs in the balance – how do you actually make money from this service? The ability to turn your IoT service into profit requires the ability to create a monetization strategy that is flexible, scalable and working for you in real-time. It must be a transparent, smoothly implemented strategy that all stakeholders – from customers to the board – will be able to understand and comprehe...
So, you bought into the current machine learning craze and went on to collect millions/billions of records from this promising new data source. Now, what do you do with them? Too often, the abundance of data quickly turns into an abundance of problems. How do you extract that "magic essence" from your data without falling into the common pitfalls? In her session at @ThingsExpo, Natalia Ponomareva, Software Engineer at Google, will provide tips on how to be successful in large scale machine lear...
SYS-CON Events announced today that Ericsson has been named “Gold Sponsor” of SYS-CON's @ThingsExpo, which will take place on June 7-9, 2016, at the Javits Center in New York, New York. Ericsson is a world leader in the rapidly changing environment of communications technology – providing equipment, software and services to enable transformation through mobility. Some 40 percent of global mobile traffic runs through networks we have supplied. More than 1 billion subscribers around the world re...
SYS-CON Events announced today that Fusion, a leading provider of cloud services, will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. Fusion, a leading provider of integrated cloud solutions to small, medium and large businesses, is the industry's single source for the cloud. Fusion's advanced, proprietary cloud service platform enables the integration of leading edge solutions in the cloud, including cloud...
There is an ever-growing explosion of new devices that are connected to the Internet using “cloud” solutions. This rapid growth is creating a massive new demand for efficient access to data. And it’s not just about connecting to that data anymore. This new demand is bringing new issues and challenges and it is important for companies to scale for the coming growth. And with that scaling comes the need for greater security, gathering and data analysis, storage, connectivity and, of course, the...
The IETF draft standard for M2M certificates is a security solution specifically designed for the demanding needs of IoT/M2M applications. In his session at @ThingsExpo, Brian Romansky, VP of Strategic Technology at TrustPoint Innovation, will explain how M2M certificates can efficiently enable confidentiality, integrity, and authenticity on highly constrained devices.
We’ve worked with dozens of early adopters across numerous industries and will debunk common misperceptions, which starts with understanding that many of the connected products we’ll use over the next 5 years are already products, they’re just not yet connected. With an IoT product, time-in-market provides much more essential feedback than ever before. Innovation comes from what you do with the data that the connected product provides in order to enhance the customer experience and optimize busi...