Prototype and window.onload

We’ve known for some time that simply assigning to window.onload is bad. Simon Willison created addLoadEvent a long time back to work around the problem. But I’m in Rails, and we have prototype. So what’s the correct idiom?

Well, after a bit of playing, it seems to be this:

  <% content_for("page_scripts") do %>
    Event.observe(window, 'load',
      function() { $('username').focus() }
  <% end %>

I do love that $() function.

This assumes that you have a layout that looks something like this, in order to insert bits of JavaScript into the head.

  <script type="text/javascript">
    <%= @content_for_page_scripts %>

Anyway, the solution seems a little more verbose than addLoadEvent(), but not disastrously so.


Singleton Rows in PostgreSQL

A common technique in Unix is the lock file. This is often followed up by having the script that creates the lock file run a command and automatically remove it afterwards. It’s a convenient way of enforcing serialisation.

Unfortunately, I’ve got a number of jobs to serialise across two different machines. They’re all database related tasks. So, I thought that I’d try to put the lock inside PostgreSQL itself as it’s what they share in common (never use NFS for locking!).

What we need is a table that can only ever contain one single row. Initially I thought of something like this:

  pid integer not null,
  hostname text not null,
  command text not null,
  created_at timestamp with time zone default now(),
  -- zero or one rows only in this table
  CHECK (count(*) < 2)

Sadly, you’re not allowed aggregate functions in a CHECK constraint. I’m not sure why, but I’ll have to work around it. So, instead, I came up with this:

  active boolean DEFAULT true NOT NULL UNIQUE,
  pid integer NOT NULL,
  hostname text NOT NULL,
  command text NOT NULL,
  created_at timestamp with time zone DEFAULT now(),
  CONSTRAINT no_inactive_mutex CHECK (active)

This has a boolean column, no nulls allowed, and with a unique index. So no more than two rows. Then, we add an extra constraint to say that no false values are allowed. This gets us to where we want to be—only zero or one rows allowed in this table.

The next bit is wrapping all this up in a script, robustly. But that’s relatively trivial compared to the SQL. The main bit is ensuring that you use an END{} block (in Perl) or trap '...' EXIT (in a shell script) to ensure that you always remove the mutex on exit. Of course, it can never be as simple and reliable as a call to unlink, but it’s good enough for me.

Update 2018-09-12: Bono Stebler points out that there is a more idiomatic PostgreSQL solution, creating an index on a constant value:

CREATE UNIQUE INDEX ON table_name ((true));

Perl & Ajax

Well, my article on Using Ajax from Perl has finally been published. It’s about using CGI::Ajax and how it makes it really, really simple to add some of that Ajax magic to your own scripts. I’d love to have a look at HTML::Prototype next, as I’ve been looking quite jealously at all the lovely Prototype features. But I need to have a look at how it integrates into Perl.


Cygwin / Apache

A quick note about running Apache under cygwin. If you just run httpd2 on it’s own, you get an error about “invalid system call”. Very annoying.

A quick bit of googling reveals that you have to have something called cygserver running. You have to set it up by running cygserver-config the first time. After that, you just need to ensure that cygserver is running.

Then, you can start Apache by saying CYGWIN=server httpd2. And it all works!