Running with Ruby

Tag: passenger (page 2 of 4)

Clear memcached without restart with Ruby and Capistrano (Rake task)

After successful Capistrano update (cap deploy) if we use Memcached – we should clear it. How can we do it without root access to the server? There are two ways to clear memcached without restarting it:

  • Memcached command: flush_all
  • Rails.cache.clear

One flush to rule them all – flush_all

If we have a dedicated server with one application on it – we can clear whole memcached memory, to do so, we create a rake task (/lib/tasks/memcached.rake):

require 'socket'

namespace :memcached do
  desc 'Flushes whole memcached local instance'
  task :flush do
    server  = ''
    port    = 11211
    command = "flush_all\r\n"

    socket =, port)
    result = socket.recv(2)

    if result != 'OK'
      STDERR.puts "Error flushing memcached: #{result}"



bundle exec rake memcached:flush

It is worth mentioning, that this task doesn’t require the Rails environment to be loaded. If it goes about the server address and port – you can always modify it so it will accept the env settings instead of hardcoding it.

Be aware, that this command will clear out all the data that is stored in Memcached instance, even the data that was used by other applications (other than our). If you want to clear out data used by one of many apps that are using same Memcached server, see the solution presented below.

Clearing single Rails app memcached data – Rails.cache.clear

Apart from flushing all the data that is in Memcached, we can always clear only the Rails cache by creating a really simple rake task (/lib/tasks/memcached.rake):

namespace :memcached do
  desc 'Clears the Rails cache'
  task :flush => :environment do

The execution process is exactly like in the previous case:

bundle exec rake memcached:flush

In this Rake task we do load the Rails environment (because we want to use Rails.cache instance). In multi application environment, this Memcached cleaning method seems way better because we work with our application scope only.

Capistrano task for clearing Memcached

So we have our rake task, but it would mean nothing without a Capistrano hookup:

namespace :memcached do

  desc "Flushes memcached local instance"
  task :flush, :roles => [:app] do
    run("cd #{current_path} && rake memcached:flush")


Now we can use it like this:

bundle exec cap memcached:flush

Or we can hookup it to update process:

after 'deploy:update' do

PhusionPassenger::UnknownError: Psych::SyntaxError – tab character that violate intendation

Yesterday I’ve started (after trying to restart Passenger) to see this exception coming out from Apache error.log:

[ pid=5098 thr=5896300 file=utils.rb:176 time=2012-08-31 19:00:45.293 ]:
*** Exception PhusionPassenger::UnknownError in 
PhusionPassenger::ClassicRails::ApplicationSpawner ((<unknown>): 
found a tab character that violate intendation while scanning a plain scalar at line 49 in 11
(Psych::SyntaxError)) (process 5098, thread #<Thread:0x00000000b3f0d8>):

Passenger ApplicationSpawner was endlessly creating new instances of my app, because the previous onces needed to be shutdown because of this error. It was quite obvious for me, that the error came from invalid yaml file (Psych::SyntaxError), however stack trace looked like this:

from /usr/local/lib/ruby/1.9.1/psych.rb:203:in `parse'
from /usr/local/lib/ruby/1.9.1/psych.rb:203:in `parse_stream'
from /usr/local/lib/ruby/1.9.1/psych.rb:151:in `parse'
from /usr/local/lib/ruby/1.9.1/psych.rb:127:in `load'
from /gems/settingslogic-2.0.8/lib/settingslogic.rb:114:in `initialize'
from /gems/settingslogic-2.0.8/lib/settingslogic.rb:71:in `new'
from /gems/settingslogic-2.0.8/lib/settingslogic.rb:71:in `instance'
from /gems/settingslogic-2.0.8/lib/settingslogic.rb:77:in `method_missing'
from /home/httpd/
from /gems/activesupport-3.2.8/lib/active_support/dependencies.rb:245:in `load'
from /gems/activesupport-3.2.8/lib/active_support/dependencies.rb:245:in `block in load'
from /gems/activesupport-3.2.8/lib/active_support/dependencies.rb:236:in `load_dependency'
from /gems/activesupport-3.2.8/lib/active_support/dependencies.rb:245:in `load'
from /gems/railties-3.2.8/lib/rails/engine.rb:588:in `block (2 levels) in <class:Engine>'
from /gems/railties-3.2.8/lib/rails/engine.rb:587:in `each'
from /gems/railties-3.2.8/lib/rails/engine.rb:587:in `block in <class:Engine>'
from /gems/railties-3.2.8/lib/rails/initializable.rb:30:in `instance_exec'
from /gems/railties-3.2.8/lib/rails/initializable.rb:30:in `run'
from /gems/railties-3.2.8/lib/rails/initializable.rb:55:in `block in run_initializers'
from /gems/railties-3.2.8/lib/rails/initializable.rb:54:in `each'
from /gems/railties-3.2.8/lib/rails/initializable.rb:54:in `run_initializers'
from /gems/railties-3.2.8/lib/rails/application.rb:136:in `initialize!'
from /gems/railties-3.2.8/lib/rails/railtie/configurable.rb:30:in `method_missing'

Ok, bot how to find the file with invalid yaml syntax? In my case, this exception was raised when SettingsLogic could not parse the given yaml file. So I’ve decided to hookup into it, just before the parsing, so I would be able to print out the invalid file. To do so I’ve edited /gems/settingslogic-2.0.8/lib/settingslogic.rb file. If you go to line 114 of this file, you will see something like this:

hash = YAML.load(
if self.class.namespace
  hash = hash[self.class.namespace] or return missing_key("Missing setting '#{self.class.namespace}' in #{hash_or_file}")

So in order to print the invalid file name we just need to add:

p hash_or_file

just before the YAML.load method execution. After this, it was really easy to find an invalid line in my yaml file.

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