Add field to all the tables using migration

def change
    ActiveRecord::Base.descendants.each do|_model|
      add_column _model.table_name.to_sym, :lock_version, :integer, :default => 0 unless [Model name for which don't want to add the field].include? _model.name
    end
  end

Setup the Memcached server to accept the connection other than localhost

In Multiserver Architecture sometimes we require to store some information in a centralized location for some time, in this situation Memcached comes in picture. In Ubuntu you can setup the Memcached by

sudo apt-get install memcached

Memcached has its own configuration file (/etc/memcached.conf). Here you find a option -l 127.0.0.1 you can the change it to the ip of the system and thats it all the other machine now can make the connection to Memcached machine.

Open mysql to accept connection other then localhost

My current project have a multi layer architecture in which mysql is running on a machine and all the other machines have to take connection on that.

By default Mysql allow connection to localhost(127.0.0.1) only. So i need to open for others as well. I need to change the configuration file of mysql

we can found that file at /etc/mysql/my.cnf (in ubuntu)

We need to change a property(bind address)

# Instead of skip-networking the default is now to listen only on
# localhost which is more compatible and is not less secure.
#bind-address        = 127.0.0.1
  bind-address        = 0.0.0.0

Now restart the Mysql service

sudo service mysql restart

It may be possible that after doing all this you might not be able to login from different machine. For make that working you have to reset the root user password by

UPDATE mysql.user SET Password=PASSWORD('MyNewPass') WHERE User='root';
FLUSH PRIVILEGES;

Multiple operation on pdf in ubuntu using pdftk package

Pdftk package provides a lot of options  to work on Pdf, like concatenation, bursting, bursting  selected number of pages, Encryption , Decryption

 Examples:-
     

• Decrypt a PDF
  pdftk secured.pdf input_pw foopass output unsecured.pdf

• Encrypt a PDF using 128-bit strength (the default), withhold all per-       missions (the default)
  pdftk 1.pdf output 1.128.pdf owner_pw foopass

• Same as above, except password 'baz' must also be used to open output       PDF
  pdftk 1.pdf output 1.128.pdf owner_pw foo user_pw baz

• Join in1.pdf and in2.pdf into a new PDF, out1.pdf
  pdftk in1.pdf in2.pdf cat output out1.pdf
  or (using handles):
  pdftk A=in1.pdf B=in2.pdf cat A B output out1.pdf
  or (using wildcards):
  pdftk *.pdf cat output combined.pdf

• Remove 'page 13' from in1.pdf to create out1.pdf
  pdftk in.pdf cat 1-12 14-end output out1.pdf
  or:
  pdftk A=in1.pdf cat A1-12 A14-end output out1.pdf

• Join two files, one of which requires the password 'foopass'. The out-       put is not encrypted.
  pdftk A=secured.pdf 2.pdf input_pw A=foopass cat output 3.pdf

• Uncompress PDF page streams for editing the PDF in a text editor (e.g.,       vim, emacs)

• Burst a single PDF document into pages and dump its data to doc_data.txt
  pdftk in.pdf burst

• Burst a single PDF document into encrypted pages. Allow low-quality printing
  pdftk in.pdf burst owner_pw foopass allow DegradedPrinting

• Rotate the first PDF page to 90 degrees clockwise
  pdftk in.pdf cat 1E 2-end output out.pdf

• Rotate an entire PDF document to 180 degrees
  pdftk in.pdf cat 1-endS output out.pdf

Ruby clone, dup vs Rails deep_dup

In Ruby:

dup and clone only duplicates the main object and leaves the inner object same.

for example

a= {'key1' => 1, 'key2' => {'key3' => 3, 'key4' => 4}}
b = a.dup

a.object_id == b.object_id # false

a['key2'].object_id == b['key2'].object_id #true


This means if i do something like

a['key2']['key5'] = 5

it will also add in b
b['key2']['key5']  # 5

In Rails:

for example

a= {'key1' => 1, 'key2' => {'key3' => 3, 'key4' => 4}}
b = a.deep_dup

a.object_id == b.object_id # false

a['key2'].object_id == b['key2'].object_id #false


This means if i do something like

a['key2']['key5'] = 5

it will not added in b
b['key2']['key5']  # nil

large parallel network file system using Glusterfs on ubuntu

To set up a high-availability storage with two storage servers (Ubuntu 11.10) that use GlusterFS. Each storage server will be a mirror of the other storage server, and files will be replicated automatically across both storage servers. The client system (Ubuntu 11.10 as well) will be able to access the storage as if it was a local filesystem. GlusterFS is a clustered file-system capable of scaling to several peta-bytes. It aggregates various storage bricks over Infiniband RDMA or TCP/IP interconnect into one large parallel network file system. Storage bricks can be made of any commodity hardware such as x86_64 servers with SATA-II RAID and Infiniband HBA.

1 Preliminary Note

• server1: IP address 192.168.0.100 (server)
• server2: IP address 192.168.0.101 (server)
• client1: IP address 192.168.0.102 (client) 

All three systems should be able to resolve the other systems' hostnames. If this cannot be done through DNS, you should edit the /etc/hosts file so that it looks as follows on all three systems:

vi /etc/hosts

127.0.0.1       localhost.localdomain  
192.168.0.100   server1.example.com    
192.168.0.101   server2.example.com    
192.168.0.102   client1.example.com    

2 Setting Up The GlusterFS Server

Setup GlusterFs on both machine. GlusterFS is available as a package for Ubuntu, therefore we can install it as follows:

sudo apt-get install glusterfs-server

test by

glusterfsd --version

make the both servers peer with each other

on server2
sudo gluster peer probe server1

on server 1
sudo gluster peer probe server2

Check the status of peer by
sudo gluster peer  

we create the share named testvol with two replicas (please note that the number of replicas is equal to the number of servers in this case because we want to set up mirroring) on server1 and server2 in the /gluster-fs-directory directory (this will be created if it doesn't exist)
 
sudo gluster volume create testvol replica 2 transport tcp server1:/gluster-fs-directory server2.example.com:/gluster-fs-directory

Start the volume 
sudo gluster volume start testvol

You can check the status of the volume with the command

sudo gluster volume info

By default, all clients can connect to the volume. If you want to grant access to client1= 192.168.0.102) only, run:

sudo gluster volume set testvol auth.allow 192.168.0.102

Please note that it is possible to use wildcards for the IP addresses (like 192.168.*) and that you can specify multiple IP addresses separated by comma (e.g. 192.168.0.102,192.168.0.103). 

Now you are done with glusterfs server

Mysql dump for a single table with data only, no drop and table creation commands

mysqldump -uroot -ppassword --compact -t papaya_development users > users.sql


for more detail type mysqldump --help command on terminal.