Display PDF from database without caching
in [ASP]
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From: rolfejr on 29 Sep 2006 11:28 I am trying to display a PDF in the users browser that is pulled from a binary field in our database, and keep that PDF from caching on the client computer. I can successfully pull the PDF and display it using the following code: Response.ContentType = "application/pdf" Response.BinaryWrite objRS("Attachment") where objRS("Attachment") is a reference to the binary field retrieved from the database. However, I have tried adding virtually every header known to have anything to do with caching, and I cannot seem to prevent the PDF from caching in the client's browser. So then I tried to use the adodb.stream object, as follows: set objStream=server.createObject("ADODB.Stream") objStream.Open objStream.Type=1 'adTypeBinary objStream.write objRS.fields("Attachment").value Response.ContentType = "application/pdf" Response.BinaryWrite objStream.Read() This follows, more or less, several examples I've found on the web, although most examples are reading a file into the stream, not a binary field returned from a database. This code gives me the following error: Response object, ASP 0106 (0x80020005) An unhandled data type was encountered. I'm looking for a way to make the stream work, or any other suggestions on how to display the pdf to the client without it caching in their browser.
From: Anthony Jones on 29 Sep 2006 12:10 <rolfejr(a)gmail.com> wrote in message news:1159543683.609567.76990(a)i42g2000cwa.googlegroups.com... > I am trying to display a PDF in the users browser that is pulled from a > binary field in our database, and keep that PDF from caching on the > client computer. I can successfully pull the PDF and display it using > the following code: > > Response.ContentType = "application/pdf" > Response.BinaryWrite objRS("Attachment") > > where objRS("Attachment") is a reference to the binary field retrieved > from the database. However, I have tried adding virtually every header > known to have anything to do with caching, and I cannot seem to prevent > the PDF from caching in the client's browser. So then I tried to use > the adodb.stream object, as follows: > > set objStream=server.createObject("ADODB.Stream") > objStream.Open > objStream.Type=1 'adTypeBinary > objStream.write objRS.fields("Attachment").value > > Response.ContentType = "application/pdf" > Response.BinaryWrite objStream.Read() > > This follows, more or less, several examples I've found on the web, > although most examples are reading a file into the stream, not a binary > field returned from a database. This code gives me the following > error: > > Response object, ASP 0106 (0x80020005) > An unhandled data type was encountered. > > I'm looking for a way to make the stream work, or any other suggestions > on how to display the pdf to the client without it caching in their > browser. > You can't prevent the caching of the PDF on the client by modifying how the PDF is streamed. At the end of the day the client sees the exact same sequence of bytes. What did you try in the headers. The following should prevent a cache from re-using the content:- Response.Expires = 0 Response.CacheControl = "private; max-age=0; no-cache" You could also go with:- Response.CacheControl = "private; max-age=0; no-store" Also you could use a negative number for expires to make sure that a slow clock on the client doesn't result in the content being cached. Browsers using HTTP 1.1 will favor Cache-Control over Expiry date anyway. How are you determining that a cache version is being re-used. The back button on a browser for example may not be affected by any of these HTTP headers.
From: rolfejr on 29 Sep 2006 12:54 I have tried the following headers: response.addheader "Expires","Mon, 26 Jul 1997 05:00:00 GMT" response.addheader "Cache-Control","no-store, no-cache, must-revalidate" response.addheader "Cache-Control","post-check=0, pre-check=0', FALSE" Response.AddHeader "Pragma", "no-cache" Response.CacheControl="no-cache" Response.expires=-1 I've tried various combinations of these as well. The way I am determining whether or not it is cached is by clearing my cache, loading the page, and looking at the cache - the PDF is there in the cache still there. I'm not so concerned about the browser showing a cached version instead of the latest version, I'm more concerned with privacy. These PDF's contain sensitive information. I am worried about someone viewing the PDF in their browser, then someone else walking up to their computer and getting the PDF from their cache. That's why I was wondering if by streaming the PDF if I could keep it from saving an actual PDF file in their cache folder. The interesting thing is that there are two pages involved - the first is gerenated HTML that shows the list of available PDF's from the database. I have successfully been able to prevent this page from being cached with the following meta tags: <meta http-equiv="Pragma" content="no-cache"> <meta http-equiv="Cache-Control" content="no-cache"> <meta http-equiv="Expires" content="-1" I have also added a cache-control:no-cache header using IIS on this specific page (actually, all pages in this directory. The user clicks one of the PDF links, and in a new window it opens the ASP page that is application.pdf content type in a new browser window. Obviously I can't put meta tags on this page, because it is not HTML - it's the binary PDF, so I am stuck with HTTP headers. I will keep experimenting, using your specific examples below and see what happens. Anthony Jones wrote: > You can't prevent the caching of the PDF on the client by modifying how the > PDF is streamed. At the end of the day the client sees the exact same > sequence of bytes. > > What did you try in the headers. The following should prevent a cache from > re-using the content:- > > Response.Expires = 0 > Response.CacheControl = "private; max-age=0; no-cache" > > You could also go with:- > > Response.CacheControl = "private; max-age=0; no-store" > > Also you could use a negative number for expires to make sure that a slow > clock on the client doesn't result in the content being cached. Browsers > using HTTP 1.1 will favor Cache-Control over Expiry date anyway. > > How are you determining that a cache version is being re-used. The back > button on a browser for example may not be affected by any of these HTTP > headers.
From: rolfejr on 29 Sep 2006 13:02 One other thing that is strange - if I look at my cache using internet explorer (tools -> options -> (general tab, settings button under Temporary Internet Files section) -> View Files, then the PDF is not there. The same thing is you navigate to C:\Documents and Settings\username\Local Settings\Temporary Internet Files. However, this is a special folder, and the actual files are stored in various subdirectories in a content.ie5 subdirectory to the folder Temporary Internet Files - windows just doesn't show it to you. So instead if you navigate to \\computername\c$\documents and settings\username\local settings\temporary internet files\content.ie5\ (windows no longer treats it as a special folder) and search all files in this directory and subdirectory, then you will find the PDF. Maybe I am just dealing with the fact that this is just how internet explorer works, and there is no way to prevent the actual file from existing on the client computer?
From: rolfejr on 29 Sep 2006 13:10
By the way, I have the same problem with firefox - the cache is a little more cryptic as there is no file extension, but nevertheless, the file is still there, and is easily renamed and accessed. |