alphanumeric - Generating unique, random alpha numeric strings

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I am developing an
application which allows users to share a link to a simple survey. For this, I want to
generate unique URLs for each survey, so having a URL
like:

http://myapp.com/aBcDe1F

I
want the alpha numeric identifier part of the URL to be pseudo random and somewhat short
(6-8 characters). Now, generating that is easy, but how do I ensure that they are unique
but also pseudo random? Do I have to generate it, then check with a query to the
database to ensure it's not been generated before, and if not, regenerate another string
and try the same process again?

I am aware that
obfuscating the URL this way does not really ensure security by any means, but password
based authentication is ruled out for this application, so I am trying to use a pseudo
random character string.

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class="normal">Answer

Yes - I
think you have to do it as you describe. But to be completely pedantic (ummm, I mean
"safe") do not do
this:

do
{

            generate a value
 check the database
}
while (the value
            did not exist)

insert a new row into the
            db

There is
a (very) small chance that you could generate the same value for two different users
simultaneously.

Rather, use the value as a
primary key within the database and do
this

do
{

            generate a value
 insert a new row into the
            db
}

while (there was a PK
            violation)