Two phase commit for Couchbase

gistfile1.rb

require 'rubygems'
require 'couchbase'

cb = Couchbase.bucket

karen = {"name" => "karen", "points" => 500, "transactions" => []}
dipti = {"name" => "dipti", "points" => 700, "transactions" => []}

# preload initial documents
cb.set("karen", karen)
cb.set("dipti", dipti)

# prepare transaction document
trans = {"source" => "karen", "destination" => "dipti", "amount" => 100, "state" => "initial"}
cb.set("trans:1", trans)

begin
  # STEP 1: Switch transaction into pending state
  cb.cas("trans:1") do
    trans.update("state" => "pending")
  end

  # STEP 2: Apply transaction to both documents
  cb.cas("karen") do |val|
    val.update("points" => val["points"] - 100,
               "transactions" => val["transactions"] + ["trans:1"])
  end
  cb.cas("dipti") do |val|
    val.update("points" => val["points"] + 100,
               "transactions" => val["transactions"] + ["trans:1"])
  end

  # STEP 3: Switch transaction into committed state
  cb.cas("trans:1") do |val|
    val.update("state" => "committed")
  end

  # STEP 4: Remove transaction from the documents
  cb.cas("karen") do |val|
    val.update("transactions" => val["transactions"] - ["trans:1"])
  end
  cb.cas("dipti") do |val|
    val.update("transactions" => val["transactions"] - ["trans:1"])
  end

  # STEP 5: Switch transaction into done state
  cb.cas("trans:1") do |val|
    val.update("state" => "done")
  end
rescue Couchbase::Error::Base => ex
  # Rollback transaction
  trans = cb.get("trans:1")
  case trans["state"]
  when "committed"
    # Create new transaction and swap the targets or amount sign.
    #
    # The code block about could be wrapped in the method something like
    #
    #   def transfer(source, destination, amount)
    #     ...
    #   end
    #
    # So that this handler could just re-use it.
  when "pending"
    # STEP 1: Switch transaction into cancelling state
    cb.cas("trans:1") do |val|
      val.update("state" => "cancelling")
    end

    # STEP 2: Revert changes if they were applied
    cb.cas("karen") do |val|
      break unless val["transactions"].include?("trans:1")
      val.update("points" => val["points"] + 100,
                 "transactions" => val["transactions"] - ["trans:1"])
    end
    cb.cas("dipti") do |val|
      break unless val["transactions"].include?("trans:1")
      val.update("points" => val["points"] - 100,
                 "transactions" => val["transactions"] - ["trans:1"])
    end

    # STEP 3: Switch transaction into cancelled state
    cb.cas("trans:1") do |val|
      val.update("state" => "cancelled")
    end
  end

  # Re-raise original exception
  raise ex
end

I'm considering that between line 52 trans = cb.get("trans:1") and further transaction updates the transaction document won't change. More robust version will be

trans, flags, cas = cb.get("trans:1", :extended => true)
case trans["state"]
when "committed"
  # ...
when "pending"
  cb.set("trans:1", trans.update("state" => "cancelling"), :flags => flags, :cas => cas)
  # ...
end

Becase #cas is #get + #set

Another point, I found that Bucket#cas operation doesn't replicate flags from original key. http://www.couchbase.com/issues/browse/RCBC-59

Improved version is here: https://gist.github.com/3136027