in reply to Script run as a Cron Job

Apart from the issue of creating user accounts on your server via a web form which I leave to your discretion now Abigail-II has warned you.

The three ideas I tried to describe in the CB are:

  1. Write your own server process (another perlscript) that listens on a socket or a pipe and pass the data to the server process from your cgi script. No cron, actioned as soon as it is passed.

  2. Instead of using a single large file to pass the data between the cgi script and the cron job, use small ones. One per account creation. That way the cgi creates them and the cron job process and deletes them. No locking or race conditions. Use the datetime to name the files to avoid conflict. Use a seperate directory to hold the files.

  3. Use two data files and a sentinel file.

    When the cron job starts, it looks for the sentinel file(an empty file whos job is to act as a signal) if it exists it deletes the file and waits a minute or two to make sure that the cgi script has finished writing to datafile1.

    Then it opens, processes and empties datafile1 and dies.

    If the sentinel file doesn't exist, it creates and waits a minute or two to make sure the cgi script has finished writing to datafile2. It then opens, processes and empties datafile2 and the dies.

    When the cgi script is ready to add data to a datafile, it first checks for the existance of the sentinel file. If the sentinel exists, it writes to datafile1. If it doesn't, it writes to datafile2.

    That way, the two processes can never be attempting to read/write to the same file at the same time.

    The length of the delay that the processing script waits before open and processing a datafile will depend on how long the cgi script is likely to spend writing to it, but as you will want to respond to the person using the web page with a few seconds, 1 or 2 minutes should be ample.

What's this about a "crooked mitre"? I'm good at woodwork!

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Re: Re: Script run as a Cron Job
by bobrobclob (Initiate) on Aug 23, 2002 at 15:53 UTC
    i like the idea of no.1 how would i go about that?
[reply]

      Take a look at perlman:perlipc with particular reference to the Named-pipe or Unix-Domain TCP Clients and Servers for some simple client and server code samples. A bit further down there is a section on UDP: Message Passing, which would be ideal for your application.

      You could also read the IO::Sockets documentation.

      Essentially, your current cron job script would need to open a socket using the UDP protocol on localhost:someport, (where someport is probably >1024.)

      This would take the place of opening the file in your current script.

      You would then wrap an endless loop around the processing code in your current script and before where you are currently reading from a file you would block waiting for input. When the input arrives, read it, process it and loop back to listen again.

      Where you existing cgi script open and writes to the file, you would connect to localhost:someport (udp), send() the data and close the port.

      I don't recall seeing a UDP server example in perlman:perlipc but its not very different to the TCP server. Maybe there is a good example in IO::Socket docs.

      What's this about a "crooked mitre"? I'm good at woodwork!
[reply]

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