ASP.NET: Session state has created a session id, but cannot save it because the response was already flushed by the application - asp.net

Question

in an old ASP.NET Web Forms application, developed in Visual Studio 2010,
suddenly does not run anymore, and in the log file appears this message:
Session state has created a session id,
but cannot save it because the response was already flushed by the application.
No new deployment has been made, and no code modifications take place.
Until now I didn't find any solution to this.
What I have to check?
I state that the source code is no longer available, and therefore it would be very difficult to change the code and proceed with a new deployment.
Thanks in advance.
Luis

Answer 1

This would suggest that someone might be hitting the site and jumping directly to some URL (and thus code) that say does some response redirect to another page or some such.
Remember, when code behind runs, and say re-directs to another page, in most cases the running code for the current page is terminated, and that is normal behaviors.
However, the idea that you going to debug code and debug a web site when you don't have the code to debug? Gee, I don't see how that's going to work at all. As noted, if this just started, then it sounds like incoming requests are to pages that don't expect to be hit "first", but some pages that expect to be ONLY called from other pages in the site when some session() and imporant values are setup BEFORE such pages are to be hit.
It also not clear if the site is using sql based sessions, or just in-memory sessions. In memory can (and is) faster, but it also not particually relaible. Now, if you deployed to a new web server or new hosting, then often session errrors can now start to appear, and this is due to the MASSIVE HUGE LARGE DIFFERENT of using cloud based hosting vs that of older hosting soluions that run on a single server.
Clould computing is real utility computing, and thus when you host a web site on such systems, then in-memory session() cannot be used anymore, since multiple servers can and will be used to "dish out" web pages. Since more then one server might be used, then obvisouly in-memory sesson() can't work, since a few web pages might be served out by one server, and then a few more pages might be served out by another server. And using shared memory for a session is limited to ONE server, since multiplel servers don't and can't transfer their memory to other servers.
So, this suggests that you want to be sure that sql server based sessions are being used here - and for any kind of server farm, or any kind of system that does load balances between more then one server, then of course you HAVE to use sql server based sessions, since in memory can't work in that kind of environment.
The error could also be due to excessive server loads - often the session database is "locked" for a short period of time, and can thus often be a bottleneck. So, for say years you might not see a issue, but then as load and use of the web site increases, then this can become noticed where as in the past it was not. I suppose the database used for storing sessions could be checked, or looked at, since as you note, you don't have the ability to test + debug the code. I would REALLY but REALLY work towards solving and fixing this lack of source code for the web site, since without that, you have really no means to manage, maintain, and fix issues for that web site.
But, abrupt terminations of web pages? As noted, this could be a error triggered in code, and thus the page never finished what it was supposed to do. And as noted, perhaps a page that expects some session() values but does not have them as explained above could be the problem. It not clear if your errors also shows what URL this was occurring for.
While nothing seems to have changed - something obviously did.
Ultimate, you need to get that source code, or deal with the people + vendor that supplies the code for that site. If you don't have a vendor, and you don't have source code, you quite much attempting to work on a car that you cant even open the hood to check what's going on under that hood.
so, one suggestion here? Someone is hitting a page that expected some value(s) in session to exist. Often the simple solution is to shove ANY simple or dummy value into session so session REALLY does get created, and then when the page attempts to save the session(), there is one to save!!!
In other words, this error often occurs when session is attempted to be saved, but no sesison exists. For such pages, as noted, a simple tiny small code change of doing this session("zoozoo") = "my useless text" will fix this error. So, it sounds like session is being lost.
As noted, a error on a web page can also trigger a app-pool re-start. If app-pool re-starts, then session is lost (in memory session). Now, with session being lost, then any page that decides to terminate early AND ALSO having used session() will trigger this error.
So, this sounds like app-pool is being re-started and session is being lost. (you can google why app-pool restarts and for the many reasons). However, critical to this issue would be are you using sql based sessions, or in-memory (server) sessions? So, this sounds like some code is triggering a error, and with a error triggered, app-pool re-starts. And with app-pool being restarted, then in-memory session is blown away. And now, without ANY session at all, then attempts to save the session trigger the exact error message you see. (and this is why shoving a dummy value into the session allows and can fix some pages - since you can't save a "nothing" session, and if you do, then you get that exact error message.
but, as noted, you can't make these simple changes to code anyway, right?
But, first on this issue - are you using memory based sessions or not? And that feature can be setup and configured in IIS, and without changes to the code base. So, one quick fix might be to turn on sql server based sessions. It will cost web site performance (10%), but the increased reliability is more then worth the performance hit.
Another area to look at? Are AJAX calls being made to a page, and again without any previous session having been created? So, once again, we down to a change in end user behaviors, and possible those hitting a page first before having logged in, or done other things - and again one would see this error crop up.