I couldn’t find may examples of zeroMQ usage in Ruby so here is a basic sender/consumer I made to test it. First install the ‘zmq’ gem:

gem install zmq --no-ri --no-rdoc

Now start a worker, you can start as many as you want:

Now stick some messages on the queue:

You should get messages distributed to all the worker processes you started up. Pretty simple!

To map-reduce across the collection I need to define a map and reduce function in javascript then run the on the collection:

I can roll this into my VisitorSession model:

This obviously makes it easier to call:

>> VisitorSession.first(:conditions => {:project_id => '2f5178'}).visits_for_project
=> 1

This caused the tests so fail with validation errors as test data was getting re-inserted, and a validates_uniqueness_of validation was complaining each time, note the second failing test, this test should have passed but failed with a “Validation failed: Name has already been taken” error:

It turns out that what was actually happening was that an exception raised in the after(:each) block was causing the rollback to fail and the failing test was just masking the error. This was my after(:each) block:

This code fails when there is no directory to remove and though this worked fine in the older version of rspec (rspec just ignored the error) with rspec2 it caused the issues discussed above. The solution (because I don’t care if the file exists or not before I delete it) is as simple as adding a condition:

$ echo ”flush_all” | nc localhost 11211

However I almost never use the actual shell version of this, mostly I do the equivalent in Ruby by opening up a socket and communicating through that. Here’s a simple example:

socket = TCPSocket.new( '127.0.0.1', 11211 )
socket.write( "flush_all\r\n" )
result = socket.recv(2)
puts "Success!" if result == 'OK'
socket.close

The performance problem

I’ve been very happy with the responsiveness of the Recycling Group Finder, so just out of interest, just to see what it would tell me, I installed the NewRelic RPM plugin and activated the free Bronze account available to EngineYard customers. The results were pretty satisfying as my average response time for the most popular page was 163ms maximum with the second most popular page at 90ms. Those are good response times and fall well within the 37signals response time rule:

Our general rule of thumb is that most pages should render their HTML on the server in less than 200ms and almost all in less than 500ms.

Suspicious looking

One of the great things about data visualisations is it can make it really easy to spot patterns. Take this New Relic graph for example:

Recycling Group Finder - before optimisation

The yellow on the graph represents time spent in the database, the blue is time spent in Ruby, ie. rendering, controllers etc. Memcached accesses are on there too but they’re so fast they hardly appear. This graph looked suspicious to me, I’d normally expect database time to be a much smaller proportion of the overall request time. So it looks like there may be some optimisation that can be done, but in order to optimise I first need to know what to optimise.

The hunt

Google for “rules of optimisation“. Most rules are something like this:

1. Don’t optimise yet
2. If you need to optimise, profile first.

I’m never going to be able to optimise my code unless I know what to optimise. If I trawl through looking for places that might be slow and trying to make them faster the chances are I’m going to spend hours changing code for no benefit. I might even make it slower. I need to know exactly where the bottleneck is, I need to profile my code.

There are a bunch of ways of finding out where your code is slow and I’ve personally used ruby-prof before with good results. However I know that the issue here is in the database, and I know that Rack::Bug will show me SQL queries that have run for an action, and importantly how long they took, so that’s what I’m going to try first. I install the plugin, configure it and load it up. The issue is immediately obvious:

Recycling Group Finder - Rack::Bug SQL queries

Almost all of the SQL that is executed is under 0.5ms per query, there are a few queries at ~4ms but he one query that really stands out is the third one down. At 44.75ms it is more than half of the overall SQL time. Bingo! Now I know what is slow I need to know why it is slow. Time to break out the query analyser.

Fixing it

I needed to dig deeper into that SQL statement to see what it was doing, so I opened up a postgres shell and ran an explain analyse on the query:

The issue seems pretty clear. There is a Sequential scan on groups:

Seq Scan on groups (cost=0.00..626.75 rows=4885 width=363) (actual time=0.038..26.495 rows=5126 loops=1)

A Sequential scan on a large table is going to sink performance. I can see that the sequential scan is definitely the issue in this case as the cost and time taken are significant proportions of the overall query time. I need to eliminate it. Here’s the code that generates that query:

@groups = Group.find(:all, :include => :group_page, :origin => [@location.lat, @location.lng], :limit => 30, :conditions => ["defunct = false AND lat is not null and lng is not null and full_address is not null and full_address != '' and country_code = ?", @location.country_code], :order => 'distance ASC, num_members DESC')

I wrote this code ages ago and re-reading it now I can see that although I am limiting the returned results to 30 rows the query will have to hit every row in the table to determine which rows are in the returned 30 as there are no conditions to the query. Whoops. Looking over the Geokit docs I see there’s a :within condition so I added a :within => 100 to the find. Testing the resultant query in the postgres shell using explain analyse again and the query has dropped to 10ms. Not bad but it’s still using a sequential scan. Adding an index on the conditions speeds up the query further to ~1.2ms:

Not bad when it started out at nearly 45ms. Here is the result reflected in the New Relic graph:

Recycling Group Finder - after optimisation

I deployed the new code approximately in the middle of the graph, it should be pretty obvious where.

Conclusion

Before you can optimise your Ruby on Rails app (or your app in any other framework/language for that matter) you need to know know where to optimise. Tools like Rack::Bug and NewRelic allow you to do this effectively and easily allowing you to direct your attention only on those parts of your app that need the attention.

On the Recycling Group Finder I cut response times drastically in about half an hour. Without knowing exactly where to make the change I would have been left guessing and may never have made the optimisation I did.

There are Ruby on Rails jobs out there

Or that’s what is seems like from talking to people at the NWRUG and Geekup meetings I go to and by the phone calls I get from recruitment agents. I know of companies worried about using Ruby and Rails because of concerns over the number of developers available. These companies need you and they need you to write web applications for them in Ruby on Rails! These are companies who want to use Ruby on Rails and they will hire you if you learn it.

I’m fine sticking with $some_other_language but thanks anyway

That’s fine, there are lots of jobs available using your programming language. Well, maybe not if that language is Coldfusion. But if you expand your horizons, teach yourself something new and can prove to others that you’re interested in and capable of learning then you’re going to be a more valuable asset. That’s going to translate into more pay and a more fulfilling job using a language as expressive as Ruby and a framework as labor-saving as Ruby on Rails.

Worst case scenario is that you learn Ruby on Rails and you can write your own web-apps a whole lot faster (you do write your own web-apps right?), your CV looks better and you have more time for the dull stuff that you fit around programming. Watching Buffy or something. You know, programmer social life stuff.

I tried Ruby and Ruby on Rails already but I prefer Python…

Weirdo.

I tried Ruby and Ruby on Rails already but I prefer Cobol!

You don’t exist, go away.

You were thoroughly convincing, I’m sold

This post is so convincing that when I proof read it I nearly went and learned ruby on Rails myself, even though I already know it. If you want to you learn you can start here, and there’s going to be a local Ruby user in your area somewhere, sign up to their mailing list, we’re a pretty helpful bunch.

If you’re anywhere near Manchester, UK then come along to the next NWRUG meeting, it’s this Thursday and there’s free pizza. You need to sign up to attend.

Coding often feels like this.

git push

in your current branch to push it to a remote:

pleb:funpop will$ git push
warning: You did not specify any refspecs to push, and the current remote
warning: has not configured any push refspecs. The default action in this
warning: case is to push all matching refspecs, that is, all branches
warning: that exist both locally and remotely will be updated. This may
warning: not necessarily be what you want to happen.
warning:
warning: You can specify what action you want to take in this case, and
warning: avoid seeing this message again, by configuring 'push.default' to:
warning: 'nothing' : Do not push anything
warning: 'matching' : Push all matching branches (default)
warning: 'tracking' : Push the current branch to whatever it is tracking
warning: 'current' : Push the current branch
Counting objects: 37, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (20/20), done.
Writing objects: 100% (21/21), 4.58 KiB, done.
Total 21 (delta 11), reused 0 (delta 0)
To git@github.com:wjessop/Funpop.git
e9ca06a..5507c19 master -> master

You can get rid of this by just doing:

git config --add push.default current

and now git will work like it did before, without the noise.

Email me or leave a comment to sign up.

Jokes-o-matic

Has a larger proportion of Firefox users than Internet Explorer users due to the high level of traffic from some social bookmarking sites, meaning a more web-savvy crowd who are more likely to be using alternative browsers:

jokes-o-matic.com browser breakdown

The level of IE usage is still pretty high though, but even so only a single hit from IE 5.5 recently:

jokes-o-matic.com IE versions

Recycling Group Finder

A much more broad and ‘average’ user base with Internet Explorer dominant:

Recycling Group Finder browser breakdown

Still, only 6 Internet Explorer hits recently, a tiny proportion of overall visits:

Recycling Group Finder IE versions

What does this mean?

Yahoo.com as seen in Internet Explorer 5.5

These statistics, and others I have seen suggest that almost no-one uses Internet Explorer 5.5 or below any more. The small minority that do must be used to the internet breaking in unique and interesting ways due to the lack of support for more recent developments in CSS and really bad Javascript support (see right for yahoo.com as seen in Internet Explorer 5.5). I predict that a fair number of sites just don’t work in any usable way on anything less than Internet Explorer 6.

Time is money

Designing for multiple browsers takes time. The more browsers you need to support the more time you need to spend supporting them.

A recent project I worked on required a redesign of the site from the ground-up (aswell as a bunch of other stuff) and making the site work in any version of Internet Explorer lower than 6 just wasn’t worth the time, and therefore money, put into it. The return just wasn’t there and I decided that IE 5.5 was gone, dead, obsolete and I wasn’t going to try and mangle the design to work in it. Rather than just display a really crappy looking site to the user I decided I would let them know via a short and to-the-point message that their browser was not up to displaying the site and directing them to upgrade to a new version of their browser.

Give them the good news

Message displayed to all users visiting project in anything below IE 6

But how to do this? The site was developed in Ruby on Rails and I’d heard of a neat gem called rack-noie6 that provides a piece of Rack middleware that blocks Internet Explorer 6 and below, but that isn’t going to work in most cases. Hard as it may be to make your site look good in Internet Explorer 6 you can see from the stats above there’s still a large proportion of users using it and to turn them away would be crazy, at least while the pool of IE 6 users out there is still so large.

So a quick fork and patch later (github we <3 you so much) and the new noie6 was accepting an option for a minimum version of Internet Explorer it should accept instead of just rigidly blocking version 6 and below. Configuration is really simple:

Now any users visiting the site in anything below Internet Explorer 6 gets the message above helpfully directing them to update their browser. Not bad for a two line config update.