From: Robert Klemme <bob.news@...>
Date: 2005-10-21T22:01:59+09:00
Subject: Re: Threads, timing and HTTP

Alexander Lamb wrote:
> Hello list,
>
> I am implementing a very simple script to ping web servers or
> services (to monitor how our environment is functionning).
>
> Some production url's run on more than one host. Therefore, I start a
> thread for each separate url.
>
> The function run in my threads is:
>
>      def doPing(uri_string, probe)
>        s = uri_string
>        while true
>          begin
>            timeout(@seconds_before_timeout) do |timeout_length|
>              start = Time.new
>              begin
>                open(s) do |result|
>                  if result.status[0] != "200"
>                    probe.addToLogFile([s,'ERR',0,result.status[1]])
>                  else
>                    probe.addToLogFile([s,'OK',Time.new - start,''])
>                  end
>                end
>              rescue Exception
>                probe.addToLogFile([s,'ERR',0,$!])
>              end
>            end
>          rescue Timeout::Error
>            probe.addToLogFile([s,'ERR',0,'timeout'])
>          end
>          sleep(@seconds_between_ping)
>        end
>      end
>
> However, this is a problem. Indeed, I want also to measure the time
> (round-trip) it takes for the ping (these are only simple pings for
> the time being). As you can see I get the local time before and after
> the call. But this doesn't work with threads. Indeed, since the
> process is shared among threads, the time will be dependent on the
> number of threads I am running and not a correct view of the actual
> time it takes to ping.
>
> I can't define the piece of code between the two times as critical
> and only for one thread because if the open-uri blocks, it will
> prevent another thread to ping another url in the mean time.
>
> Any idea? Maybe use processes instead of threads?

Maybe you can exploit one of the result headers.  Chances are that there
is a timestamp somewhere.  Then you *only* need to synchronize clocks on
your machine and on servers...

Or you switch to a single thread solution.  I don't know your ping
interval but if you don't need to ping too often and don't have too many
servers that should be ok.  You can create a simple scheduling that always
picks the URL with the closest ping point...

Regards

    robert