Rails - Jun 2006 - Separate Read and Write Database

Hello,

I need to access an mirrored database with one read and one write 
cluster. Is there a way to tell a model to use two connections? So that 
all updates etc go to the write database and all read from the read one?

This is a fixed set up I can not use MySQL 5.x stuff and additionally i 
get a token on write, which can be checked on read (and wait for 
replication) if new data is needed.

I saw an open ticket on http://dev.rubyonrails.org/ticket/2041 but it 
seems to be dead.

For me it would be enough to have the possibility to change the 
connection of a model on runtime (e.g. block)

Model.use_read do
# do some stuff

 x= Model.find()
 x.test
end

or

token = Model.use_write do
  x = Model.new
  x.save
end

Any idea how this can be achieved?

Is it possible to use
ActiveRecord::Base.establish_connection(:adapter => "mysql" ,
:host => "localhost" , :database => "railsdb" )
more dynamically?

Maybe creating a inherited model would do it?:

class Order < ActiveRecord::Base
end
---

ActiveRecord::Base.establish_connection(:adapter => "mysql" ,
:host => "localhost" , :database => "railsdb_read" )

class ReadOrder < Order
end

----

ActiveRecord::Base.establish_connection(:adapter => "mysql" ,
:host => "localhost" , :database => "railsdb_write" )

class WriteOrder < Order
end


Any other clues?

-- 
Posted via http://www.ruby-forum.com/.

I just ran into a similar requirement.  Here''s how I solved it:

#First extend ActiveRecord::Base to allow us to override connections
module ActiveRecord
  class Base

    # Sets the active connection to conn and runs the given block.
Active
    # connection is reset to it''s previous value once the block
finishes.
    def self.using_connection(conn)
      old_connection = use_connection(conn)
      ret = yield if block_given?
      use_connection(old_connection)
      ret
    end

    # Sets the active connection to conn, returns the previously active
connection
    def self.use_connection(conn)
      old_active_connection = active_connections[active_connection_name]
      active_connections[active_connection_name] = conn
      old_active_connection
    end

  end
end

class ModelBaseRead < ActiveRecord::Base
  self.abstract_class = true
  establish_connection "whatever_read_#{RAILS_ENV}" if defined?
RAILS_ENV

  @@read_connection = self.connection
  def self.read_connection
    @@read_connection
  end

  def self.use_read_connection
    use_connection(read_connection())
  end

  def self.using_read_connection(&blk)
    using_connection(read_connection, &blk)
  end
end

class ModelBase < ModelBaseRead
  self.abstract_class = true
  establish_connection "whatever_#{RAILS_ENV}" if defined? RAILS_ENV

  @@write_connection = self.connection
  def self.write_connection
    @@write_connection
  end

  def self.use_write_connection
    use_connection(write_connection())
  end

  def self.using_write_connection(&blk)
    using_connection(write_connection, &blk)
  end
end

# Then define some model as
class SomeModel < ModelBase
  # some stuff
end

# Then you can do nifty things like this:
ModelBase.using_read_connection() do
  s = SomeModel.find(:all)
end
# or if you like...
ModelBase.use_read_connection()
s = SomeModel.find(:all)
ModelBase.use_write_connection()

Note that it defaults to the write connection, but you could change that
if you wanted to.

You''ll obviously need to tweak the db config strings as you need.

I know there is a better solution if you make a lot of changes to
connection_specification.rb in activerecord, but I''m a chicken and took
the path of least resistance.

My rails-foo is not great.  I would be very interested to what I true
rails-ninja would do to solve this.

cheers,

benv

-- 
Posted via http://www.ruby-forum.com/.

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