From: Alexander Lamb <Alexander.J.Lamb@...>
Date: 2005-10-21T22:09:35+09:00
Subject: Re: Threads, timing and HTTP


On Oct 21, 2005, at 3:01 PM, Robert Klemme wrote:

> 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...
>
>
I could go single thread (since indeed I am doing a ping per 30  
seconds more or less). However, if the first one I try hangs (until a  
timeout for example), I am pushing back the time at which I will ping  
the second url. Logically I would need to do something like "ping  
each url one after another unless one of them seems to take longer  
and then detach a thread to wait for the answer".
For the time being I will test forking a process.

Alex