properly rendering 404 errors from inside a rails application

I just migrated a site that had a bunch of links that have been in in the search engines for a while. Oddly it seems that the only thing hitting those links seem to be the crawlers themselves. I needed a way to invalidate those links, since I couldn’t create a proper redirect because of changing IDs.

/records/show/12345 used to be valid, but has been replaced with the RESTful version /records/00123. The ID is now also meaningful instead of a MySQL generated id.

My first attempt was to just redirect to the 404 page.
record = Record.find(params[:id]
rescue ActiveRecord::RecordNotFound
redirect_to("/404.html")

But as I watched the logs, I noticed that this really wasn’t right since it was still returning a 302 (redirect) and the a 200 (OK) code for those links. The crawlers were getting the instruction that you should just display the 404 page for those links. That might seem OK, but really I wanted them to get the 404 immediately and remove the page from their databases.

record = Record.find(params[:id]
rescue ActiveRecord::RecordNotFound
render(:file => "#{RAILS_ROOT}/public/404.html", :layout => false, :status => 404)

By rendering the 404.html directly and including the 404 status code, it should help to fix the situation.

shared host or small vps

I started looking into moving off of dreamhost because I’ve had some issues with responsiveness on my applications. For $20 a year, I could put up with it. Now that I’m paying $100, it’s a bit more annoying since there are other options out there at that price point.

I’m considering slicehost.com, linode.com and webfaction.com.

I guess the other big reason is that I want to play with MongoDB and each of these gives me that option.

bundler times two

I wish they would have made a bigger deal about this, but it seems that bundler now has two different gems.

bundler08 is for bundler 0.8.4 and such, which plays really well with rails 2.3.5

bundler is for bundler 0.9.x and beyond which plays well with rails 3 (and rails 2.3.5 if you can get it to work…)

This is a really good thing because you can now install both of them at the same time and the warning that you must un-install all previous versions of bundler is now moot. Really helpful if you’re running on dreamhost with mixed rails 2/3 sites.

instance_variable_set is your friend

If you’ve got some code like this that sets an instance variable, it can be a problem to test.

def self.amqp
  @amqp ||= open_connection
end

If you mock open_connection, you would hope that it would avoid making the call, but if that variable is set, it’s going to ignore the mock. This is more of a problem when running the tests on the command line (instead of by itself in Textmate).

  describe "amqp" do
    it "should create a new amqp" do
      a = mock("amqp")
      Messaging.should_receive(:open_connection).and_return(a)
      Messaging.amqp.should == a
    end
  end

I had another test that was setting @amqp and it would cause my current test to fail. I needed to find a way to make sure that @amqp was nil every time the test was run.

describe "amqp" do
    it "should create a new amqp" do
## reset the @amqp variable to make sure we have it in a known state
      Messaging.instance_variable_set(:@amqp, nil)
##
      a = mock("amqp")
      Messaging.should_receive(:open_connection).and_return(a)
      Messaging.amqp.should == a
    end
  end

instance_variable_set will let you reset that variable without trying to mess around, which I had been doing for a while. This works really well when you’re trying to test cached things as well.

How to kill a job in progress

I’m building a web application the analyze large data sets. The simplified process is: upload a data set, split it into multiple chunks, process the chunks and then zip the results together.

While you can kill a job in progress right now, all it’s really doing is removing it from the database. The queue is still clogged full of tasks that need to complete for a job that doesn’t exist anymore. I’m referring to this as a push model, since I’ve pushed all the tasks onto the queue and the workers consume them as fast as they can. The problem lies in the fact that to remove the job messages from the queue, you need to kill the queue. (Using beanstalkd right now) This is fine if you have a single job on the queue, and you can ssh into the server, but it’s still a pain.

After some thought, I’m going to try to impliment a pull model. Each worker will announce it’s available to the head node when it starts up. The head node will note it’s existence in a ‘workers’ table, with the status of available. When a job gets submitted, the head node looks to see if any workers are available. If so, it drops the message onto the worker queue. It doesn’t matter if the worker that was available gets the job, just that there was one available. When the worker pulls the task off the queue, it sends a message back to the head queue saying that it’s now busy. The process continues once we have a series of tasks backing up on the head node, where the head will see if we have available workers, and if so, drop a task onto the worker queue.

What we gain from this is the ability to kill the job, and all associated tasks on the head node before they’re put into the worker queue. The tasks that are in process will still complete since we can’t go in and stop them, but that’s ok. Once they’ve all finished, we clean up the working files, and remove the job and other valid jobs can continue on without any issues.

Another gain is the ability to pause jobs, or better assign priorities. We don’t want a job that’s 95% done to be trumped by a higher priority job, since the system would think it’s stuck.