Welcome!

Microsoft Cloud Authors: Yeshim Deniz, Janakiram MSV, Andreas Grabner, Stackify Blog, Liz McMillan

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
DX World EXPO, LLC., a Lighthouse Point, Florida-based startup trade show producer and the creator of "DXWorldEXPO® - Digital Transformation Conference & Expo" has announced its executive management team. The team is headed by Levent Selamoglu, who has been named CEO. "Now is the time for a truly global DX event, to bring together the leading minds from the technology world in a conversation about Digital Transformation," he said in making the announcement.
"We provide IoT solutions. We provide the most compatible solutions for many applications. Our solutions are industry agnostic and also protocol agnostic," explained Richard Han, Head of Sales and Marketing and Engineering at Systena America, in this SYS-CON.tv interview at @ThingsExpo, held June 6-8, 2017, at the Javits Center in New York City, NY.
"We are focused on SAP running in the clouds, to make this super easy because we believe in the tremendous value of those powerful worlds - SAP and the cloud," explained Frank Stienhans, CTO of Ocean9, Inc., in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
"DX encompasses the continuing technology revolution, and is addressing society's most important issues throughout the entire $78 trillion 21st-century global economy," said Roger Strukhoff, Conference Chair. "DX World Expo has organized these issues along 10 tracks with more than 150 of the world's top speakers coming to Istanbul to help change the world."
"We've been engaging with a lot of customers including Panasonic, we've been involved with Cisco and now we're working with the U.S. government - the Department of Homeland Security," explained Peter Jung, Chief Product Officer at Pulzze Systems, in this SYS-CON.tv interview at @ThingsExpo, held June 6-8, 2017, at the Javits Center in New York City, NY.
With tough new regulations coming to Europe on data privacy in May 2018, Calligo will explain why in reality the effect is global and transforms how you consider critical data. EU GDPR fundamentally rewrites the rules for cloud, Big Data and IoT. In his session at 21st Cloud Expo, Adam Ryan, Vice President and General Manager EMEA at Calligo, will examine the regulations and provide insight on how it affects technology, challenges the established rules and will usher in new levels of diligence...
The financial services market is one of the most data-driven industries in the world, yet it’s bogged down by legacy CPU technologies that simply can’t keep up with the task of querying and visualizing billions of records. In his session at 20th Cloud Expo, Karthik Lalithraj, a Principal Solutions Architect at Kinetica, discussed how the advent of advanced in-database analytics on the GPU makes it possible to run sophisticated data science workloads on the same database that is housing the rich...
SYS-CON Events announced today that Massive Networks will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Massive Networks mission is simple. To help your business operate seamlessly with fast, reliable, and secure internet and network solutions. Improve your customer's experience with outstanding connections to your cloud.
Internet of @ThingsExpo, taking place October 31 - November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 21st Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago. All major researchers estimate there will be tens of billions devic...
"The Striim platform is a full end-to-end streaming integration and analytics platform that is middleware that covers a lot of different use cases," explained Steve Wilkes, Founder and CTO at Striim, in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
SYS-CON Events announced today that Calligo, an innovative cloud service provider offering mid-sized companies the highest levels of data privacy and security, has been named "Bronze Sponsor" of SYS-CON's 21st International Cloud Expo ®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Calligo offers unparalleled application performance guarantees, commercial flexibility and a personalised support service from its globally located cloud plat...
Everything run by electricity will eventually be connected to the Internet. Get ahead of the Internet of Things revolution and join Akvelon expert and IoT industry leader, Sergey Grebnov, in his session at @ThingsExpo, for an educational dive into the world of managing your home, workplace and all the devices they contain with the power of machine-based AI and intelligent Bot services for a completely streamlined experience.
SYS-CON Events announced today that DXWorldExpo has been named “Global Sponsor” of SYS-CON's 21st International Cloud Expo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Digital Transformation is the key issue driving the global enterprise IT business. Digital Transformation is most prominent among Global 2000 enterprises and government institutions.
SYS-CON Events announced today that Datera, that offers a radically new data management architecture, has been named "Exhibitor" of SYS-CON's 21st International Cloud Expo ®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Datera is transforming the traditional datacenter model through modern cloud simplicity. The technology industry is at another major inflection point. The rise of mobile, the Internet of Things, data storage and Big...
While the focus and objectives of IoT initiatives are many and diverse, they all share a few common attributes, and one of those is the network. Commonly, that network includes the Internet, over which there isn't any real control for performance and availability. Or is there? The current state of the art for Big Data analytics, as applied to network telemetry, offers new opportunities for improving and assuring operational integrity. In his session at @ThingsExpo, Jim Frey, Vice President of S...
"MobiDev is a Ukraine-based software development company. We do mobile development, and we're specialists in that. But we do full stack software development for entrepreneurs, for emerging companies, and for enterprise ventures," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
SYS-CON Events announced today that DXWorldExpo has been named “Global Sponsor” of SYS-CON's 21st International Cloud Expo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Digital Transformation is the key issue driving the global enterprise IT business. Digital Transformation is most prominent among Global 2000 enterprises and government institutions.
In his opening keynote at 20th Cloud Expo, Michael Maximilien, Research Scientist, Architect, and Engineer at IBM, discussed the full potential of the cloud and social data requires artificial intelligence. By mixing Cloud Foundry and the rich set of Watson services, IBM's Bluemix is the best cloud operating system for enterprises today, providing rapid development and deployment of applications that can take advantage of the rich catalog of Watson services to help drive insights from the vast t...
SYS-CON Events announced today that EnterpriseTech has been named “Media Sponsor” of SYS-CON's 21st International Cloud Expo, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. EnterpriseTech is a professional resource for news and intelligence covering the migration of high-end technologies into the enterprise and business-IT industry, with a special focus on high-tech solutions in new product development, workload management, increased effic...
SYS-CON Events announced today that Massive Networks, that helps your business operate seamlessly with fast, reliable, and secure internet and network solutions, has been named "Exhibitor" of SYS-CON's 21st International Cloud Expo ®, which will take place on Oct 31 - Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. As a premier telecommunications provider, Massive Networks is headquartered out of Louisville, Colorado. With years of experience under their belt, their team of...