Appendix B: Implementing a URL Polling Agent
A polling agent is a CGI script external to the Avere cluster that can monitor and report on the progress of the core filer’s backup job. The agent can be written in any language that supports CGI, including scripting languages such as Perl and Python, and compiled languages such as C, C++, and Java. (See RFC 3875 for information about the components of a CGI script.)
When URL polling is set, and all changed data has been written to the core filer, the Avere cluster checks the URL of the CGI script approximately every 10 seconds during the time period specified by the cluster schedule.
The CGI script must accept the arguments mass (a legacy term for a core filer) and targetTime, where mass is the name of the core filer and targetTime is a UNIX timestamp representing the next scheduled write-through period.
It must also return the string RELEASE Core Filer SYNC when the core filer’s backup process has completed.
CGI Script Example
A simple example CGI script written in Python follows.
Note that this script does not include any logic for monitoring the status of the core filer. For that information, refer to your core filer’s documentation, particularly any API documentation.
Caution
This simple example is not secure enough to be used in an internet-exposed system. To improve security, validate all inputs, and consider moving the state file out of /tmp and using a more secure filename pattern.
#! /usr/bin/env python
import os
import sys
import cgi
cgiArgs = cgi.FieldStorage(keep_blank_values=True)
corefiler = cgiArgs.getfirst('mass')
targetTime = cgiArgs.getfirst('targetTime')
response = "must supply corefiler and targetTime arguments"
if corefiler and targetTime:
statefile = "/tmp/STATE.%s_%s"%(corefiler,targetTime)
os.close(os.open(statefile, os.O_CREAT|os.O_RDWR))
f = open(statefile, "r+")
l = f.readline()
if not l:
response = "starting backup operation"
f.write("start\n")
elif l == "start\n":
response = "waiting for backup operation to complete (1)"
f.truncate(0)
f.seek(0)
f.write("wait1\n")
elif l == "wait1\n":
response = "waiting for backup operation to complete (2)"
f.truncate(0)
f.seek(0)
f.write("wait2\n")
elif l == "wait2\n":
response = "waiting for backup operation to complete (3)"
f.truncate(0)
f.seek(0)
f.write("wait3\n")
else:
response = "backup complete.\nIt's time to RELEASE Core Filer SYNC\n"
os.unlink(statefile)
f.close()
print "Content-Type: text/plain"
print "Pragma: no-cache"
print "Cache-Control: no-cache, must-revalidate"
print "Expires: Sat, 26 Jul 1997 05:00:00 GMT"
print
print response
sys.exit(0)