Axon Flux // A Ruby on Rails Blog

Using this validation method in conjunction with ActiveRecord::Base#save does not guarantee the absence of duplicate record insertions, because uniqueness checks on the application level are inherently prone to race conditions. For example, suppose that two users try to post a Comment at the same time, and a Comment‘s title must be unique. At the database-level, the actions performed by these users could be interleaved in the following manner:

User 1                 |               User 2
 ------------------------------------+--------------------------------------
 # User 1 checks whether there's     |
 # already a comment with the title  |
 # 'My Post'. This is not the case.  |
 SELECT * FROM comments              |
 WHERE title = 'My Post'             |
                                     |
                                     | # User 2 does the same thing and also
                                     | # infers that his title is unique.
                                     | SELECT * FROM comments
                                     | WHERE title = 'My Post'
                                     |
 # User 1 inserts his comment.       |
 INSERT INTO comments                |
 (title, content) VALUES             |
 ('My Post', 'hi!')                  |
                                     |
                                     | # User 2 does the same thing.
                                     | INSERT INTO comments
                                     | (title, content) VALUES
                                     | ('My Post', 'hello!')
                                     |
                                     | # ^^^^^^
                                     | # Boom! We now have a duplicate
                                     | # title!

Something to watch out for as you scale or build out your Rails app. This is the kind of stuff that only bites you when you get bigger.

You may have this error when require ‘xml/mapping’ in your ruby script:

/usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require': no such file to load -- xml/mapping (LoadError)

Solution: go to the gem directory of XML-Mapping (mine is /usr/lib/ruby/gems/1.8/gems/xml-mapping-0.8.1/lib/) and chmod all files in this directory to 644.

Who is to point finger at here ? Rubygem or XML-Mapping authors ?

For some reason the default install has them installed with root-only permissions. I love how this is a Priority 3 bug assigned to nobody, and it blocks the use of this gem entirely.

Here's the fix (on OS X anyway)

find . -type d -exec chmod 755 {} \;

This will recursively search your directory tree (starting at dir ‘dot’) and chmod 755 all directories only.

Similarly, the following will chmod all files only (and ignore the directories):

find . -type f -exec chmod 644 {} \;

Handy.

69% of IE6 users have no choice -- they have to use it. That's just sick. IT'S SICK AND PERVERTED. I want to vomit. I just did, a little, in my mouth.

TradElect runs on HP ProLiant servers running, in turn, Windows Server 2003. The TradElect software itself is a custom blend of C# and .NET programs, which was created by Microsoft and Accenture, the global consulting firm. On the back-end, it relied on Microsoft SQL Server 2000. Its goal was to maintain sub-ten millisecond response times, real-time system speeds, for stock trades.

It never, ever came close to achieving these performance goals. Worse still, the LSE's competition, such as its main rival Chi-X with its MarketPrizm trading platform software, was able to deliver that level of performance and in general it was running rings about TradElect. Three guesses what MarketPrizm runs on and the first two don't count. The answer is Linux.