Unable to rescue from Redis connection refusal - ruby

Question

I am attempting to write a function which tries to connect to Redis using default TCP settings, and if that fails, tries to connect to Redis through a unix socket. My intent is to have a single connection script that works on all my systems, some of which use TCP and others which use sockets.
However, I can't seem to rescue from the failed TCP connection. Here is my test script.
require "redis"
def r
begin
$redis ||= Redis.new
rescue
$redis = Redis.new(:path => "/tmp/redis.sock")
end
end
puts "You have #{r.keys.count} redis keys"
The rescue block never gets executed and instead an exception is raised. Here is the output of this script.
/usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:236:in `rescue in establish_connection': Connection refused - Unable to connect to Redis on 127.0.0.1:6379 (Errno::ECONNREFUSED)
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:222:in `establish_connection'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:23:in `connect'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:247:in `ensure_connected'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:137:in `block in process'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:206:in `logging'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:136:in `process'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis/client.rb:46:in `call'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis.rb:246:in `block in keys'
from /usr/local/rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/monitor.rb:201:in `mon_synchronize'
from /usr/local/rvm/gems/ruby-1.9.2-p290/gems/redis-2.2.2/lib/redis.rb:245:in `keys'
from scripts/redis.rb:11:in `<main>'
I have verified that Redis.new(:path => "/tmp/redis.sock") works as expected. I have tried to be more specific with my rescue block by using rescue Errno::ECONNREFUSED to no avail. I'm not sure why I cannot catch this exception.
Any ideas?

Answer 1

It turns out the exception is not being thrown when calling Redis.new. The exception doesn't get thrown until certain methods (in this case, Redis#keys) on the connection object are called. This revised connection function appears to do the trick.
require "redis"
def r
begin
$redis ||= Redis.new
$redis.inspect # needed to know if connection failed
rescue
$redis = Redis.new(:path => "/tmp/redis.sock")
end
$redis
end

Answer 2

I found that $redis.inspect didn't actually exercise the REDIS connection. I replaced it with $redis.keys and that correctly threw the exception. Note, am running on Heroko and it passes in the environment variable REDISTOGO_URL. I then have a constant REDIS that I use throughout the application.
In my config/initializers/redis.rb:
uri = URI.parse(ENV['REDISTOGO_URL'])
begin
redis ||= Redis.new(:host => uri.host, :port => uri.port, :password => uri.password)
redis.keys # needed to know if connection failed
REDIS = redis
rescue
puts("Redis not loaded on #{uri.port}")
REDIS = nil
end