Class: Lhm::LockedSwitcher

Inherits:
Object
  • Object
show all
Includes:
Command, SqlHelper
Defined in:
lib/lhm/locked_switcher.rb

Overview

Switches origin with destination table nonatomically using a locked write. LockedSwitcher adopts the Facebook strategy, with the following caveat:

"Since alter table causes an implicit commit in innodb, innodb locks get
released after the first alter table. So any transaction that sneaks in
after the first alter table and before the second alter table gets
a 'table not found' error. The second alter table is expected to be very
fast though because copytable is not visible to other transactions and so
there is no need to wait."

Instance Attribute Summary collapse

Instance Method Summary collapse

Methods included from SqlHelper

#annotation, #idx_name, #idx_spec, #version_string

Methods included from Command

#run

Constructor Details

#initialize(migration, connection = nil) ⇒ LockedSwitcher

Returns a new instance of LockedSwitcher.



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# File 'lib/lhm/locked_switcher.rb', line 25

def initialize(migration, connection = nil)
  @migration = migration
  @connection = connection
  @origin = migration.origin
  @destination = migration.destination
end

Instance Attribute Details

#connection ⇒ Object (readonly)

Returns the value of attribute connection.



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# File 'lib/lhm/locked_switcher.rb', line 23

def connection
  @connection
end

Instance Method Details

#statements ⇒ Object 



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# File 'lib/lhm/locked_switcher.rb', line 32

def statements
  uncommitted { switch }
end

#switch ⇒ Object 



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# File 'lib/lhm/locked_switcher.rb', line 36

def switch
  [
    "lock table `#{ @origin.name }` write, `#{ @destination.name }` write",
    "alter table `#{ @origin.name }` rename `#{ @migration.archive_name }`",
    "alter table `#{ @destination.name }` rename `#{ @origin.name }`",
    'commit',
    'unlock tables'
  ]
end

#uncommitted(&block) ⇒ Object 



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# File 'lib/lhm/locked_switcher.rb', line 46

def uncommitted(&block)
  [
    'set @lhm_auto_commit = @@session.autocommit',
    'set session autocommit = 0',
    yield,
    'set session autocommit = @lhm_auto_commit'
  ].flatten
end

#validate ⇒ Object 



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# File 'lib/lhm/locked_switcher.rb', line 55

def validate
  unless @connection.table_exists?(@origin.name) &&
         @connection.table_exists?(@destination.name)
    error "`#{ @origin.name }` and `#{ @destination.name }` must exist"
  end
end