Hi,
In my GWT application, I am using JSON async callback mechanism
(JSONP) to retrieve data from server side. Earlier I used cgi-bin perl
scripts on server side to retrieve data and return http object back to
the client (browser) via async callback function call. I figured that
resource usage is much higher for cgi-bin scripts. Hence I moved away
from cgi-bin scripts and instead resorted to passing data directly
from a data file in web root's path through a HTTP get request. That
is I am populating a data file located at WEB-ROOT/json/query_data
periodically. This is being updated a separate polling process on the
same server. This is read by client periodically to fetch JSON data.
Instead of calling a cgi-bin script such as WEB-ROOT/cgi-bin/query.pl
which does nothing but read query_data and package it into a HTTP
header and passes it back through output stream back to client, I am
attempting to directly read the JSON data file in my new
implementation where the data file is pre-formatted to look like a
callback function call.
eg: INITIALLY
------------------
WEB-ROOT/json/query_data:
{..},...{..}
which is just an array of JSON objects.
WEB-ROOT/cgi-bin/query.pl :
#!/usr/bin/perl
use CGI;
use Fcntl qw(:DEFAULT :flock);
$q = CGI->new();
print $q->header();
# Read data from /tmp/json/query_data on an exclusive file lock
open($file, "< /tmp/json/query_data")
or die "{\"type\":\"JSONError\"}\n";
flock($file, LOCK_SH)
or die "{\"type\":\"FileLockError\"}\n";
@raw_data=<$file>;
close($file)
or die "can't close file: $!";
# Print JSON objects
print "callback123([";
foreach $line(@raw_data) {
print $line;
}
print "]);";
When this script is called by client side code, JSON data from
query_data is packaged into "callback123();" so that data is
transferred over as a JSON object through a asynchronous callback
mechanism. In my current implementation, I chose to bypass the above
cgi-bin script and instead, directly access the /tmp/json/query_data
file by creating a symbolic link in WEB-ROOT so that client can access
this file by just specifying URL "http://<IPADDR>/json/query_data".
CURRENTLY:
-------------------
WEB-ROOT/json/query_data:
callback123([{..},...{..}]);
client calls - http://<IPADDR>/json/query_data and expects JSON object
to be returned. This seems to work fine for the first request. However
subsequent requests still fetch cached data from previous
http://<IPADDR>/json/query_data access. I verified this by opening the
above URL on browser. On repeated navigation to this URL, instead of
getting updated data, I keep getting stale data from previous trial
and only a page "refresh" seems to actually go and fetch updated info.
How do I go around this?
Thanks,
Rajiv
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