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Mack_Hanson_107's avatar
Mack_Hanson_107
Icon for Nimbostratus rankNimbostratus
Oct 27, 2005

Error Handling, Test condition for variables

Is there some way to trap errors, so that the end-user doesn't experience pain? Whenever I refer to variable that isn't yet instantiated, BIGIP returns null, which in browser results in a JavaScript popup box that says "Document has no data." How do I check to see if a variable is valid without getting a BIGIP error? Is there a test condition?

My line '[session lookup ssl $sslsessionid]' is throwing errors when it can't find the session variable $sslsessionid.


when HTTP_REQUEST {
  set uri_path         [HTTP::uri]
  if {[regexp {^(/login)(.)*?$} $uri_path] == 1} {  
     set ssl session id, in the website login section
    set sslsessionid [SSL::sessionid]
  }  
}
when HTTP_RESPONSE {
  retrieve previously stored ssl session id, from any website section
  set sslid [session lookup ssl $sslsessionid]
}
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  • Joe_Pruitt's avatar
    Joe_Pruitt
    Oct 27, 2005
    There are several ways you can avoid this runtime error from occuring.

    1. Force set the variable to an empty string in the HTTP_REQUEST event and check if it's not an empty string in the HTTP_RESPONSE event.

    when HTTP_REQUEST {
  set uri_path         [HTTP::uri]
  set sslsessionid ""
  if {[regexp {^(/login)(.)*?$} $uri_path] == 1} {  
     set ssl session id, in the website login section
    set sslsessionid [SSL::sessionid]
  }  
}
when HTTP_RESPONSE {
  retrieve previously stored ssl session id, from any website section
  if { "" ne $sslsessionid } {
    set sslid [session lookup ssl $sslsessionid]
  }
}

    2. Use the "info exists" command to check if the variable is defined before trying to access it.

    when HTTP_REQUEST {
  set uri_path         [HTTP::uri]
  if {[regexp {^(/login)(.)*?$} $uri_path] == 1} {  
     set ssl session id, in the website login section
    set sslsessionid [SSL::sessionid]
  }  
}
when HTTP_RESPONSE {
  retrieve previously stored ssl session id, from any website section
  if { [info exists sslsessionid] and {"" ne $sslsessionid} } {
    set sslid [session lookup ssl $sslsessionid]
  }
}

    The later is probably more optimal as it doesn't require the variable to be initialized every connection.

    Also, I threw in an extra check for an empty string in there just to be safe.

    You can get the docs on the "info exists" command in the TCL Reference manual at SourceForge

    http://tmml.sourceforge.net/doc/tcl/

    Click here

    -Joe
  • Mack_Hanson_107's avatar
    Mack_Hanson_107
    Icon for Nimbostratus rankNimbostratus
    May 25, 2007
    This is more what I had in mind. It works great. I've been using it for a while now.

    
if { [catch {
   try some operation
} ] == 1 } {
   handle the error
}