Blog Entries

use autouse ... or not

Due to my cursory reading in the Perl Cookbook, 2nd Edition, earlier this week, I've been investigating the use autouse pragma, to see if it will indeed solve my issue of wanting to use different modules based on the current situation. Unfortunately, I cannot find any documentation on it in perldoc.

I remember seeing something about wrapping this stuff into a BEGIN block, but that would require knowing certain information immediately, and I might need the code to work through some steps before getting there.

Fortunately, this node just appeared on Perl Monks today, and I got to see other ways of doing it:

  • The if module lets you do something like use if $type eq 'x', "Some::Module"; However, $type must be known at compile time (i.e., it's based on system info or on @ARGV); this probably wouldn't work in a web-based application.
  • Use require and import instead: if $type wq 'ex') { require Some::Module; Some::Module->import if Some::Module->can("import"); } If your module doesn't export anything, you can even omit the call to import.
  • Use an eval: if ($type eq 'x') { eval "use Some::Module"; } This gets around the import problem, but could possibly run into other compile time issues.

So, basically, I already had the tools to do the job; just needed to examine the problem more.

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More CGI::App research... Try the manual!

So, I'm a bit of an idiot... it's been so long since I looked at CGI::App, and yet I felt I had such a grasp on it, that I overlooked the obvious step: look at the manual!

In particular, there's a whole series of methods that are used to tailor CGI:App to your particular needs, and these include cgiapp_init(), cgiapp_prerun(), and cgiapp_postrun().

  • cgiapp_init() is used to perform application specific initialization behaviour, and is called immediately before the setup() method. It can be used to load settings from elsewhere; if it were called only from a superclass from which other modules inherited, it would then provide common settings for all modules.
  • cgiapp_prerun() is called immediately before the selected run-mode. If it were called only by your superclass, you could perform items such as authorization or even form validation; this would then be standard for all your applications. (You can use the $self->prerun_mode('mode') call to to override the selected run-mode, for instance, thus allowing you to redirect to a different mode if a user isn't permitted there.)
  • cgiapp_postrun() is called after the run-mode has returned its output, but before http headers have been generated or anything sent to the web browser. Again, if defined in a superclass, it means that you could then place the run-mode output in a specific place within a larger template, and even call other routines to fill in other parts of the main template. You could even check to see if certain parameters were passed to the page, and change the type of output you send back (XML, PDF, image, etc.), allowing you to have a common query element that changes the output type (e.g., a 'print' parameter that returns a PDF or a stripped down template).

In addition, you could specify in the superclass that you're using CGI::Simple for the query object (using the cgiapp_get_query method), or you could rewrite the load_tmpl() method to use Template::Toolkit or some other templating system, etc.

Doesn't look so crazy anymore...

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CGI::Application Research

I've been wanting to redevelop my home website for some time using CGI::Application. The last time I rewrote it from PHP to perl, I developed something that was basically a subset of the things CGI::App does, and those things weren't done nearly as well.

The problem I've been running into has to do with having sidebar content, and wanting to run basically a variety of applications. I want to have a WikiWikiWeb, a photo gallery, some mail forms, and an article database/blog; CGI::App-based modules for each of these all exist. But I want them all to utilize the same sidebar content, as well -- and that sidebar content may vary based on the user.

My interest got sparked by this node on Perl Monks. The author tells of an acquaintance who goes by the rule that a CGI::App should have 10-12 states at most; more than that, and you need to either break it apart or rethink your design. And all CGI::Apps inherit from a common superclass, so that they share the same DB connections, templates, etc.

So, I've been investigating this problem. One node on PM notes that his ISP uses CGI::App with hundreds of run modes spread across many applications; they created a module for session management and access control that calls use base CGI::Application; each aplication then calls use base Control, and they all automatically have that same session management and access, as well as CGI::Application.

Another node mentions the same thing, but gives a little more detail. That author writes a module per application, each inheriting from a super class: UserManager.pm, Survey.pm, RSS.pm, Search.pm, etc. You create an API for that super class, and each CGI::App utilizes that API to do its work.

This also seems to be the idea behind CheesePizza, a CGI::App-based framework for building applications. (All pizzas start out as cheese pizzas; you simply add ingredients.) The problem with that, though, is that I have to learn another framework on top of CGI::App, instead of intuiting my own.

But how do I write the superclass? Going back to the original node that sparked my interest, I found a later reply that described how you do this. The big key is that you override the print method -- this allows you to customize the output, and from here you could call functions that create your sidebar blocks, and output the content of the CGI::App you just called in a main content area of your template.

Grist for the mill...

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robots.txt

One thing I've wondered about is the syntax of the robots.txt file, where it's placed, and how it's used. I've known that it is used to block spiders from accessing your site, but that's about it. I've had to look into it recently because we're offering free memberships at work, and we don't want them indexed by search engines. I've also wondered how we can exclude certain areas, such as where we collate our site statistics, from these engines.

As it turns out, it's really dead simple. Simply create a robots.txt file in your htmlroot, and the syntax is as follows:

User-agent: *
Disallow: /path/
Disallow: /path/to/file

The User-agent can specify specific agents or the wildcard; there are so many spiders out there, it's probably safest to simply disallow all of them. The Disallow line should have only one path or name, but you can have multiple Disallow lines, so you can exclude any number of paths or files.

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More SSH tips: Tunnelling

I wrote up a short tutorial today on the IT wiki about SSH tunneling. What I didn't know is that you can start a tunnel after you've already ssh'd to another machine. Basically, you:

  • Press Enter
  • Type ~C

and you're at an ssh> prompt. From there, you can issue the tunnel command of your choice: -R7111:localhost:22, for instance.

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IT hiring principles

I was just reading an article about the Dean campaign's IT infrastructure, and there's an interesting quote from their IT manager, Harish Rao:

"I believe in three principles", he said. "First I always make sure I hire people I can trust 100%. Second, I always try to hire people who are smarter than I am. Third, I give them the independence to do as they see fit as long as they communicate about it to their other team members. We've had a lot of growing pains, a lot of issues; but we've been able to deal with them because we have a high level of trust, skill and communication."

I know for myself that when I (1) don't feel trusted, and/or (2) am not given independence to do what I see as necessary to do my job, I don't communicate with my superiors about my actions, and I also get lazy about my job because I don't feel my work is valued.

Fortunately, I feel that in my current work situation, my employers followed the same principles as Rao, and I've felt more productive and appreciated than I've felt in any previous job.

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PHP standards ruminations

I've been thinking about trying to standardize the PHP code we do at work. Rob and I follow similar styles, but there are some definite differences. It would make delving into eachother's code much easier if we both followed some basic, agreed upon, guidelines.

One thing I've been thinking about is function declarations. I find that I'm often retooling a function to make it more general, and in doing so either need to decrease or increase the number of arguments to it. This, of course, breaks compatability.

So I propose that we have all functions take two arguments: $data and $db. $data is a hash which can then be extract'd via PHP. To change the number of arguments, you can simply set defaults for arguments or return meaningful errors for missing arguments.

Another thought going through my mind deals with the fact that we reuse many of our applications across our various sites, and also export some of them. I think we should try and code the applications as functional libraries or classes, and then place them somewhere in PHP's include path. We can then have a "demo" area that shows how to use the libraries/classes (i.e., example scripts), and to utilize a given application, we need simply include it like: 'include 'apps/eventCalendar/calendar.inc';'. This gives us maximum portability, and also forces us to code concisely and document vigorously.

I was also reading on php.general tonight, and noticed some questions about PHP standards. Several people contend that PEAR is becoming the de facto standard, as it's the de facto extension library. In addition, because it is becoming a standard, there's also a standard for documenting projects, and this is phpdocumenter. The relevant links are:

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Making RCS a little easier...

One thing I noticed today when using RCS is that it isn't terribly user friendly -- you need to checkout a file to make edits. Often, I make edits, and then want to commit my changes.

So I wrote a wrapper script called revise. It makes a temporary copy of the file you've been editing, checks it out of RCS with locking, makes it writeable, moves the temporary copy to the permanent name, checks it in and unlocks it (which prompts for a log message), and then makes the file writeable for the user and group again. The script is outlined here:

    #!/bin/bash
    FILE=$1
    cp $FILE $FILE.new
    co -l $FILE
    chmod u+w $FILE
    mv $FILE.new $FILE
    ci -u $FILE
    chmod ug+w $FILE

Being the ROX-Filer centric person I am, I also wrote a quick perl script called rox-revise that I can then put in my SendTo menu. It parses the file's path, changes to that directory, and then calls the revise script on the filename, from within a terminal. This script follows:

    #!/usr/bin/perl -w
    use strict;

    use vars qw/$path $file $TERMCMD $REVISE $ZENITY/;

    # Configurable variables
    $TERMCMD = "myTerm";    # What terminal command to use; must be xterm compliant
    $REVISE  = "revise";    # What command to use to revise (i.e. rcs ci) the file
    $ZENITY  = "zenity";    # The zenity or dialog or xdialog command to use

    # Grab the filename from the command line
    $path = shift;
    $file = $path;

    # If no file given, raise a dialog and quit
    if (!$path || ($path eq '')) {
        system(
            $ZENITY, 
            '--title=Error', 
            '--warning', 
            "--text=No path given to $0; rox-revise quit!"
        );
        exit 0;
    }

    # Get the path to the file and switch to that directory
    if ($path =~ m#/#) {
        $path =~ s#^(.*)/.*?$#$1#;
        if ($path !~ m#^/#) { $path = "./$path"; }
        chdir $path or die "$path not found!n";
    } else {
    # Or else assume we're in the current directory
        $path = './';
    }

    # Get the filename
    $file =~ s#^.*/(.*?)$#$1#;

    # Execute the revise statement
    my $failure = system($TERMCMD, '-e', $REVISE, $file);
    if ($failure) {
        # on failure, raise a dialog
        system(
            $ZENITY, 
            '--title=Error', 
            '--warning', 
            "--text=Unable to revise $file"
        );
    }

    1;

Now I just need to check out Subversion, and I can have some robust versioning!

Continue reading...

SSH tips and tricks

In trying to implement some of the hacks in Linux Server Hacks, I had to go to the ssh manpage, where I discovered a number of cool tricks.

  1. In order to get key-based authentication (i.e., passwordless) working, the $HOME/.ssh directory must be mode 0700, and all files in it must be mode 0600. Once that's setup properly, key-based authentication works perfectly.
  2. You can have a file called config in your $HOME/.ssh directory that specifies user-specific settings for using SSH, as well as a number of host-specific settings:
    • Compression yes turns on compression
    • ForwardX11 yes turns on X11 forwarding by default
    • ForwardAgent yes turns on ssh-agent forwarding by default
    • Host-based settings go from one Host keyword to the next, so place them at the end of the file. Do it in the following order:
          Host nickname
          HostName actual.host.name
          User username_on_that_host
          Port PortToUse
                  
      This means, for instance, that I can ssh back and forth between home using the same key-based authentication and the same ssh-to script (more below) I use for work servers -- because I don't have to specify the port or the username.

I mentioned a script called ssh-to earlier. This is a neat little hack from the server hacks book as well. Basically, you have the following script in your path somewhere:

    #!/bin/bash
    ssh -C `basename $0` $*

Then, elsewhere in your path, you do a bunch of ln -s /path/to/ssh-to /path/to/$HOSTNAME, where $HOSTNAME is the name of a host to which you ssh regularly; this is where specifying a host nickname in your $HOME/.ssh/config file can come in handy. Then, to ssh to any such server, you simply type $HOSTNAME at the command line, and you're there!

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RCS quickstart

Gleaned from Linux Server Hacks

  • Create an RCS directory
  • Execute a 'ci -i filename'
  • Execute a 'co -l filename' and edit as you wish.
  • Execute a 'ci -u filename' to check in changes.

The initial time you checkout the copy, it will be locked, and this can cause problems if someone else wishes to edit it; you should probably edit it once and put in the version placeholder in comments somewhere at the top or bottom:

$VERSION$

and then check it back in with the -u flag to unlock it.

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