Published: Friday, June 02, 2000
Saving Application Variables Across Web Restarts
By Daniel Cooper
A while ago I had a problem with a site that I was working on. One of the requirements called for a featured
product to appear in the footer of the site, which would be changed monthly by the client through a back-end
interface.
Usually when one has to dynamic information for a site a database is the answer. In this case I thought "This
is stupid. I'm not going to do a database call to retrieve a single value and I'm not going to have an orphan
table in the database to only hold that value!"
The obvious solution is to use an application variable. After all, it’s global, can be set easily and have
little overhead. The problem is that the webserver gets rebooted every week, meaning that any settings in the
application variables would be lost. My problem was: "How do I make application wide values persistent?"
What I did was write two functions. One writes key/value pairs to a text file in a format that the second
function can read back. I set up the site so that when the site's operator set a value, it was written both
to the text file and to an application variable. Then when the application restarts, the Global.asa
file contains code to initialize those variables to the previous values set. Note that this approach will only
persist textual application variables since there is no way to represent a binary object through a text file.
So the code to record application variable values looks a little like this:
Application("featured_product_id") = someVariable
Call recordToFile(someFileName, "featured_product", someVariable)
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And in the Application_OnStart event in Global.asa we have a line like:
Application("featured_product_id") = readFromFile(fileName, "featured_product")
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So once you set a value in both an application variable and in the file you can access it throughout the site
with little overhead and the confidence of knowing that if your web application is restarted the value will
persist.
Here's how its done:
recordToFile() - writes a key/value pair to a text file you specify, I use data.txt
but it can be any filename. It creates the file if it doesn’t already exist and will create a key/value pair
or set the value if it exists.
readFromFile() - opens your data file and retrieves the value for a given key, or null if it
can’t find it.
Following is the code to recordToFile. In Part 2 we'll look
at the code for readFromFile and dissect both functions.
Part 2
function recordToFile(strFileName, strKey, strValue)
dim objFileSys, objFile, arrLine, objDict, thing
Set objFileSys = Server.CreateObject("Scripting.FileSystemObject")
Set objDict = Server.CreateObject("Scripting.Dictionary")
if objFileSys.FileExists(strFileName) then
'case where file already exists and presumably holds values
Set objFile = objFileSys.OpenTextFile(strFileName, 1)
do while not objFile.AtEndOfStream
arrLine = split(objFile.ReadLine, "|")
if isArray(arrLine) then
if Ubound(arrLine) = 1 then
objDict.Add arrLine(0), arrLine(1)
end if
end if
loop
objFile.close
objFileSys.DeleteFile(strFileName)
end if
'insert/update new item
objDict.Item(strKey) = strValue
Set objFile = objFileSys.OpenTextFile(strFileName, 2, true)
for each thing in objDict
objFile.writeLine thing & "|" & objDict.Item(thing)
next
objFile.Close
Set objDict = Nothing
Set objFile = Nothing
Set objFileSys = Nothing
End Function
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