Sketchpad - Dan Sketcher's personal blog

Not much of a title, I know, but something for fellow stumped google-wanderers..

I’ve been playing around with deprec for automatically provisioning new servers for my Rails applications. However, I hit a strange problem where certain command-line auto-completions did not work; I’d get an error message like this:

$ ls
temp

$ cd [hit tab to get autocomplete]
-sh: <( compgen -d -- '' ): No such file or directory
-sh: <( compgen -f -X  -- '' ): No such file or directory

The confusing thing was that only the unpriviliged user exhibited this behaviour, not the root user.

I tried reinstalling the bash-completion package to no avail (even an aptitude purge then aptitude install did not work). Eventually I came across this rather long post on bugs.debian.org where someone else has the same problem. Essentially it comes down to the unpriviliged user using /bin/sh as the default shell; under these circumstances autocompletion is supposed to not function (although presumably without error). This is despite /bin/sh being a symlink to /bin/bash

The fix, then, is actually quite simple. Change the default shell for all your unprivileged users to bash:

# usermod -s /bin/bash dansketcher

and then when you make new users, make sure you select bash explicitly:

# useradd newuser -m -s /bin/bash

One of the things that we do at TouchLocal is provide Search Engine Optimised pages that drive traffic to the site through the UK National Directory pages. As you can imagine, there’s quite a lot of data intensive processing that is required to generate these pages, and there are an awful lot of them. As it changes relatively rarely, for a long time we had been caching these pages in a Rails Action cache so that the headers and footers are not cached but the content is.

As we enhanced the caching strategy through the site, and the Rails support for Memcache improved, we changed the Rails cache to use Memcache. This by implication meant that the default cache we had been using for actions (the filestore cache) was no longer used and the infrastructure team could not flush the cache in the same way it previously had.

So that we could use the Rails.cache helper and still have the benefit of the Action cache, we changed our production environment configuration to look like this:

# Namespace for memcache to automatically flush on app version updates.
# APP_REVISION is updated automatically via asking the Version Control System
ns = "dir_#{RAILS_ENV}_#{$persona}_#{APP_REVISION}"

# MC_ADDRESSES constant is put into the root namespace - we use it for other things
::MC_ADDRESSES = ['10.0.0.1:11211', '10.0.0.2:11211', '10.0.0.3:11211']

# Rails.cache is memcache
config.cache_store = :mem_cache_store, MC_ADDRESSES,{:namespace => ns}

# Force the ActionController cache store to be the file one as per the previous default.
config.action_controller.cache_store = :file_store, "#{RAILS_ROOT}/tmp/cache"

Happy days.

At TouchLocal, one of the difficulties in automating deployment was the tight coupling between the app servers that were currently running and what the Load Balancer said was running. The implication of this is that at deployment time a human had to go into the ZXTM interface and update the list of running nodes before and afterwards. To ease this, and allow fully automatic deployment of Ruby on Rails apps running under Phusion Passenger, I built the ZXTMManager class.

This class implements an interface to the Zeus ZXTM API, allowing (in its current state) the ability to programatically change active nodes and manage other details of the commonly changed data in the Load Balancer. On top of that, there is an included deploy.rb file that shows the deployment recipes that can be simply and easily added to your own Capistrano configuration to use this class quickly and easily.

Enjoy.

When we upgraded from Rails 1.x to 2.x, we also had to migrate from Masochism to DataFabric. Unfortunately, the DataFabric way of doing things requires adding a declaration to each of the model classes. For a site like TouchLocal, where there are literally hundreds of model classes, this was a daunting task.

I’ve just gone through extracting the magic that makes it possible to do this programatically, released as DataFabric::Initializer. I hope it helps someone other than me!

Yesterday, I was working on a branch of a branch in a git repository, and I wanted to merge the last branch back to master. Following chapter 3.6 of ProGit, I ran

git rebase --onto master branch1 branch2

Unfortunately for me, I seem to have either done it from the wrong place or done the wrong thing for my situation, because git rewound branch2 to the revision that it was branched from master and all the later commits were inaccessible!

However, I was not going to give up quite so easily, and I figured that once a revision was committed, it had to be somewhere.. Fortunately for me, I manged to find this post on how to recover lost commits in a git repository – I subsequently ran:

git reflog show
# find the commit with the last set of changes in your branch - 
# good commit comments are so useful!
git co b6654bd
git co -b branch2_2

With my branch recovered, I did a simple rebase of the new (old) branch onto master and merged it, and all was well.